INDUSTRIAL APPLICATIONS

CHAPTER 12 INDUSTRIAL APPLICATIONS Monodispersed particles precisely controlled in size, shape, structure, and composition are not only excellent models for fundamental research, but are also expected to be ideal particulate materials for practical use as highperformance advanced materials. However, they are not necessarily widely used yet in the industry, because of their low productivity or high cost in most cases. Nevertheless, in view of their superb potential functions and the recent remarkable development of simple and inexpensive general methods for their production in large quantities (see sections 7.2.6 and 7.3.1), there seems to be no doubt that a great part of particulate materials will be replaced by monodispersed particles in the near future. In this chapter, industrial applications of monodispersed particles, including their possibilities, will be described.

12.1. Photographic Materials Monodispersed silver halide particles are widely used in the photographic industry, which may be one of the rare examples of practical use of monodispersed particles. They are used in the forms of mixed silver halides, such as AgBrI, AgBrCl, and AgClBrI, nearly pure AgBr and AgCl, or core/shell particles consisting of a core and shell different in composition. The shapes occur in wide variety, including cubes, octahedra, cubooctahedra, spheres, and platelets. Strict control of their shape, size, and composition is essential for the control of their photographic properties, including photosensitivity, spectral sensitivity, image contrast, granularity of the developed images, covering power (= optical density / two-dimensional density of developed silver in film), etc. Indeed, the invention of the controlled double-jet method for the preparation of monodispersed silver halide particles in the 1960s (see sections 1.5.2 and 7.3.2) has made an invaluable contribution to the progress of photographic science and technology.

581

12. INDUSTRIAL APPLICATIONS

Silver halide particles to be used for photographic materials are sensitized to an optimum level of light-sensitivity by surface treatment with chemical sensitizers such as chloroauric acid (HAUCI4) plus sodium thiosulfate or other organic sulfur compounds (gold-sulfur sensitization) after preparation of the particles, but the photographic speed or light sensitivity of photographic materials depends primarily on the grain size of each silver halide particle. Here, the photographic speed is defined as the inverse number of incident photons per unit area required for making a certain fraction of the light-exposed particles developable. For understanding the importance of grain size for photographic speed, the knowledge on the concept of quantum sensitivity may be needed. The quantum sensitivity is defined as the minimum number of absorbed photons (quanta) per grain to create one developable silver cluster in a grain. Hence, the smaller the value of the quantum sensitivity is, the higher is the quantum sensitivity of the grain. The minimum number of absorbed photons per grain is normally defined by the absorbed photons per grain necessary for the development of a half of the light-exposed particles. Figure 12.1 shows the relationship between

^=0-5

A Emulsion No. I O D X

+

0 0 h .s o 0-5

o O 10

o

1-5 I- o lA

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< o

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Fig. 12,1. Relationship between iogio(grain volume) and Iogio(absorbed quanta/grain) required for making a half of the grains developable. (From Ref. 1.)

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logioCgrain volume) and log^oCabsorbed quanta/grain) for a half of each sizeclass to be developable in various photographic emulsions 1 - 6.^ This figure shows that the quantum sensitivity of a particle in each size class is almost constant in a given photographic emulsion, irrespective of the grain size in a practical size range up to ca, 1 fim^. Since a silver halide grain in a properly sensitized photographic emulsion is almost constant, regardless of the grain size up to a certain level, and since the number of absorbed photons per grain is known to be almost proportional to the grain volume, the photographic speed of an optimally sensitized photographic material of monodispersed grains is approximately proportional to the mean volume of a grain. As a consequence, strict control of the size of each grain in a system is of vital importance for designing a high-performance photographic emulsion. For this purpose, monodispersed silver halide particles are ideal materials. It is also noteworthy that ideally precise assessment of quantum sensitivity is possible only with monodispersed particles. This is the main reason why almost all fundamental studies on quantum sensitivity are performed with well-defined monodispersed particles varied in shape and mean size. As a rule, rather polydispersed particles, or uniform particles of different mean sizes coated in multilayers, are used for color or black and white negative films in order to reserve a wide light-exposure latitude with their low-contrast developed image. On the other hand, uniform particles of a narrow size distribution are used for positive photographic materials, such as color or black and white printing papers, reversal films. X-ray films, microfilms, lithographic films, instant direct-positive materials, etc., to give a high-contrast developed image. Figure 12.2 illustrates general features of optical density of developed images vs log^oClight exposure energy), or D/Log^gE curve, for negative and positive photographic materials. For example, monodispersed octahedral silver bromide particles are actually used in some high-speed instant direct-positive color materials, in which each particle is strongly sensitized at an intended internal position by a gold-sulfur sensitizer, and weakly sensitized at the particle surface as well. The internal sensitization is performed when the particles are grown to a certain size level by the controlled double-jet technique, and then further growth is continued until the sensitized position reaches a certain depth. After being spectrally sensitized to yellow, green, or red light by adsorption of a spectral sensitizing dye, the particles are finally coated in a layer of an instant color material consisting of ca. 20 layers, having specific functions. On exposure to light, one or more silver clusters are formed as internal

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583

Positive u

Q

1 O

logE Fig. 12.2. General features of optical density of developed images vs log^oGight exposure intensity), or D/Log^oE curve, for negative and positive photographic materials. latent images at the internal chemically-sensitized center in each particle exposed to light, through the electronic and ionic processes (see section 11.8), while unexposed particles remain free from internal silver clusters. When they are exposed to an alkaline solution of developer, a nucleating agent (a kind of reducing agent, such as hydrazines, aldehydes, and acetylenes, ordinarily with some adsorptive group to silver halides) which has previously been coated with the silver bromide particles is activated by alkali and injects electrons into both light-exposed and unexposed particles. By this action of the nucleating agent, while the unexposed particles form silver clusters on their weakly-sensitized surfaces, the light-exposed particles form no silver clusters on their surfaces, because the injected electrons are trapped by the internal silver clusters as a much deeper electron trap than the surface sensitivity center. As a consequence, while the lightexposed particles are not developed by developing agent, the unexposed ones are developed to form a positive image. The oxidized developing agent reacts with a dye-releaser in the neighboring layer to make it release and diffuse the anchored dye to a dye-receiving layer across a reflection layer, for forming a positive dye image. Figure 12.3 depicts the action of the nucleating agent. The precise control of the position of the internal

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APPLICATIONS

Weak Chemical Sensitization >^ Strong Chemical Sensitization \ \ Latent Image Speck Nucleating Agent hv

rJ—^

undevelopable Nucleating Agent

developable

Fig. 12.3. The action of the nucleating agent in a silver halide particle of a directpositive photographic material. sensitivity centers and the achievement of the delicate balance between the internal and surface sensitization in the sophisticated "instant" system are impossible unless monodispersed particles are used. Even for low-contrast negative films consisting of polydispersed particles, it may be desirable to design a mixed system of monodispersed particles of different mean sizes optimally sensitized in the individual monodisperse systems, since optimal conditions for chemical sensitization strongly depend on the grain size. In this sense, the multilayered assemblies composed of different monodisperse systems are in accord with this requirement. Silver halides used for photography are originally sensitive to only blue light (k - 450 nm) and ultraviolet light, so they have to be spectrally sensitized to green light (X. - 550 nm) and red light (>^ ~ 650 nm) with sensitizing dyes such as carbocyanines or merocyanines which are adsorbed to the surfaces of silver halide grains. These absorb incident photons of particular wavelength ranges, and the excited electrons in the sensitizers are transferred to silver halide grains to form latent images in the grains. Specifically, for color reproduction in color photography, not only the choice of spectral sensitizing dyes with proper absorption bands, but also the composition of the silver halide particles and their crystal habits are important determinants, since their composition and crystal habits strongly

12. INDUSTRIAL APPLICATIONS

585

affect the absorption spectra of sensitizing dyes (see section 10.9). Thus, strict control of the composition and crystal habit of silver halide particles is needed for this purpose as well. The design of the double-structured core/shell grains with different compositions in core and shell, partly described in section 8.5.3, may be one of the most effective ways for using the best functions of individual silver halides.^"^ Bando et al"^ studied the characteristics of double-structure grains (DSG), AgBro9loi(core)/AgBr(shell), which were monodispersed octahedra of 0.8 |xm in diameter and the silver contents of the core and shell were equal. Figure 12.4 shows characteristic curves of D/log^oE for pure AgBr grains (emulsion A), AgBro95loo5 grains uniform in composition (emulsion B), and DSG (emulsion D) at different development times in a typical color development process, CN-16, after white-light exposure. All the specimens for Fig. 12.4 were optimally sensitized for the grain surfaces with a gold-sulfur sensitizer, and thus latent images in the form of silver clusters were formed exclusively on the grain surfaces. The AgBr grains (emulsion A) and AgBrg 95I0.05 grains (emulsion B) were also monodispersed ones of 0.8 pun in diameter. The results clearly show that the DSG has both the merits of pure AgBr and AgBrI without their respective demerits; /.e., pure AgBr is fast in development, but low in photographic speed owing to the small intrinsic absorption of blue light, whilst AgBrI containing iodide is slow in development and low in image contrast, but high in photographic speed due to the extended absorption edge to the longer wavelength with increasing iodide content.^

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Fig. 12.4. Characteristic curves of (A) AgBr, (B) AgBro.95Io.05, and (D) DSG. (From Ref. 4.)

Moreover, when the DSG grains were spectrally sensitized, a significant

586

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2.0

H.O

Fig. 12.5. Relative photographic speed vs amount of dye (a) added: (A) AgBr (O); (B) AgBro.95Io.05 (^); (D) DSG (G). (From Ref. 4.)

6.0

ADDED DYE (a) (10"* mol / mol silver halide)

effect of the DSG was found against dye-desensitization (= bleaching of surface latent images by dye-holes) for blue radiation, as shown in Fig. 12.5, where varying amounts of a red-sensitizing dye (dye (a)) of the following formula were added to each emulsion. C2H5

r

(CH2)3S03^ (CH2:

(CH2)3S03Na

In order to elucidate this effect, Bando and coworkers'* investigated the ESR signals resulting from the dye-holes for these samples. Table 12.1 shows the relative intensities of light-induced ESR signals of dye (a) adsorbed to the particles of emulsions A, B, and D (8.0 x 10"^ mol/mol Ag), for exposure to monochromatic light of X. = 640 nm (red) and X = 420 nm (blue). The signal intensity, I^^, is proportional to the number of positive holes staying in the dyes after injection of photoelectrons into the silver halide grains by radiation with the red light, while I^iue is proportional to the number of the positive holes trapped by the dyes, but originally generated in the grains by the blue light and transferred to the dyes. The small values of both Ijgjj and \^^^ for emulsion B, as compared to emulsion A, may be due to the hole-trapping of the iodide ions of the AgBr^95I005 grains.^ Also, the low Ibiue/^red ^^^^^ ^^ I^^G (cmulsion D) seems to be due to trapping of the

587

12. INDUSTRIAL APPLICATIONS

Table 12.1. Relative intensities of light-induced ESR signals of the dye adsorbed to (A) AgBr, (B) AgBro.95Io.05, and (D) DSG {Source: Ref. 4) Emulsion (A) AgBr (B) AgBro.95lo (C) DSG

IK,

100 50 100

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positive holes in the A^XQ^\^ cores after their generation by the blue radiation, and to the blocking of the hole transfer from the dyes to the grain by the AgBr shell after generation of positive holes in the dyes by the red radiation. The prevention of the hole transfer from the core to the dye is thought to be the main reason for the inhibited dye-desensitization. As a consequence, they proposed the band-structure model of DSG in Fig. 12.6, by reference to the work of Berry^. The concentration of positive holes in the AgBrI core is in accord with the results of Hirsch^: this was attributed to the effect of the DSG against the dye-desensitization for blue radiation. As has been shown above with a few examples, the fundamental and practical studies on advanced processing of silver halide particles, including precise control of crystal imperfections, heavy metal doping, core/shell control, heterojunction of different silver halides, etc. (see chapter 8), are being actively continued with monodispersed silver halide particles in order to extract the ultimate functions from their inherent potentialities. CONDUCTION BAND

^

o.^ h

VALENCE BAND

Fig. 12.6. Band structure model of the double-structured grain for the efficient charge separation between photoelectrons and positive holes. (From Ref. 4.)

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APPLICATIONS

12.2. Ceramic Materials The scaling law for the sintering model indicates that powders of 10 nm in size will sinter 10^ - 10^ faster than micron-size powders.^ Mazdiyasni et al^'^^ prepared a variety of very small high-purity metal oxide particles of less than 20 nm by thermal decomposition and hydrolysis of metal alkoxides. However, the predicted advantage on the sintering time and temperature was not necessarily realized due to the agglomeration of the smaU particles, since agglomerates are generaUy sintered so fast that they become large particles in the early stage of sintering and create large voids around them.^^"^"* In the meantime, Stober et al}^ prepared monodispersed and unagglomerated silica particles of 0.05-2 |xm by hydrolysis of silicon alkoxides in the presence of ammonia. In this procedure, as the length of the alkyl chains of alcohols, used as the solvent, and silicon esters, as the starting material, becomes larger, the final size increases owing to the slower hydrolysis and thus the lower nucleation rate. Other factors, such as the concentrations of anwnonia and water, and temperature, are also effective in determining the final size (see section 7.2.9). The concentration of silicon alkoxides is less than ca. 0.3 mol dm"^, significantly lower than used in the ordinary sol-gel processing of colloids, in order to avoid aggregation of growing silica particles. On the other hand, Rhodes^^ demonstrated that the alkoxide-derived oxide particles developed by Mazdiyasni et al^'^^ can be sintered at very low temperatures, ca. 1100 °C, as opposed to 1700 ""C for calcined and milled powders, by dispersing, eliminating the agglomerates, and centrifugally compacting the particles. For the achievement of lowtemperature sintering, a highly dense microstructure of the compact is generally required. For this purpose, monodispersed spherical particles are ideal, since they readily form a compact ordered structure of face-centered cubic array if we utilize the electrostatic repulsive forces of their charged surfaces and settling owing to the gravitational or centrifugal forces in polar liquid media (see section 11.6).^^'^^ Bowen and coworkers prepared amorphous Ti02 and Zr02 spheres of narrow size distributions and their doped particles by hydrolysis of the corresponding metal alkoxides and studied this effect extensively. Namely, Barringer and Bowen^^*^" prepared fairly uniform TiOj particles {ca. 30% in relative standard deviation) by hydrolysis of 0.1-0.2 mol dm"^ titanium tetraethoxide at 25 ""C, and investigated the effect of dense packing on the sintering temperature by gravitational or centrifugal settling at appropriate pH values above 9 or below 4, sufficiently away from the isoelectric point (I.E.P.- 5.5)^\ at ionic

12. INDUSTRIAL APPLICATIONS

589

strengths less than 0.01 mol dm'^ They found a significant reduction of sintering temperature to 1050 ''C for >99 % of the theoretical density using the dense compact (green compact) of the uniform Ti02 powder of 0.35 \xm, ca, 300 °C lower than 1300 - 1400 °C necessary for conventional Ti02 powders to sinter to 97 % of the theoretical density. Figure 12.7 shows SEM images of the so-obtained dense and uniform compact of Ti02 and the microstructure sintered to >99 % of the theoretical density in 90 min at 1050 °C.^^ They also found that the sintering temperature was further lowered to 800 °C with a dense compact of Ti02 powder of 0.08 |xm in mean diameter, prepared from titanium isopropoxide. Although some particle aggregation is observed in the TEM of this original powder, the fairly uniform and dense compact and the small particle size may favor the dramatic reduction of the sintering temperature. Similarly, ZrOj spheres of a narrow size distribution were also prepared from zirconium alkoxides.^^"^ Typical parameters were 0.1 mol dm"^ zirconium n-propoxide, 0.50 mol dm'^ H2O, and a reaction time of 2 min at 50 °C. Moon et al^'^'^ prepared monodisperse Zr02 powders with a mean diameter of ca. 0.3 \xm by hydrolysis of a relatively high concentration of zirconyl chloride (ZrOCy such as 0.2 mol dm"^ in 1-propanol or 2-propanol in the presence of water and ca. 0.1 wt % HPC (hydroxypropyl cellulose, MW « 100,000) as a dispersant. They raised the reaction temperature rapidly to the boiling point of the solution by using a microwave oven, which was effective for the formation of the monodisperse particles, probably because of the improved separation between the short nucleation period and the subsequent growth stage.

Fig. 12.7. SEM images of uniform green compact of Ti02 (A) and its sintered structure (B). (From Ref. 19.)

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For uniform spheres of ZrOj (d = 0.2 [im, I.E.P.= 6-7.5) and SiOj (d = 0.4 \xm, I.E.P.= 2-3), the effect of the dense microstnicture of the green compacts, prepared at pH 4 or 9-11 for Zr02 and at pH ^ 5 for Si02, on the sintering temperatures were tested, and the sintering temperatures were found to be 1160 ''C for the Zr02 compact {ca, 1700 °C for conventional milled Zr02 powders) and ca. 1100 ^C for the Si02 compact.^*^^ Doping a host powder with a small amount of different metal ions is a useful technique for the modification of the physical or chemical properties of original ceramic powders. Since TiOj ceramics doped with various combinations of cations have been studied for use as low-voltage varistors,^^ grain-boundary capacitor materials,^° oxygen sensors,^^ current-collecting electrodes in Na-S batteries,^^ etc., singly-doped Ti02 powders with either Ta205 or Nb205 and doubly-doped Ti02 with BaO, CuO, or SrO and either Ta205 or Nb205 were also prepared. The Ti02 powders singly-doped with Ta205 or Nb205 were made by co-hydrolysis of Ti(OC2H5)4 with Ta(002115)5 or Nb(OC205)5, while the doubly-doped samples were prepared by precipitating the carbonates of Ba^^, Cu^^, and Sr^^ onto the once-washed singly-doped Ti02 particles in aqueous solutions of their metal chlorides in the presence of (NH4)2C03. On heating, these carbonates are converted into oxides. For the doping with SrO, co-hydrolysis of Ti(OC2H5)4 and Sr(iso031170)2 were also performed.^^*^^ The contents of these dopants ranged from 0.1 to 1.0 wt % of Ti02. Fegley, Barringer and Bowen"^*^^ concluded, from transmission electron microscopy and X-ray diffractometry on the amorphous spheres, that the dopants, Ta and Nb, were homogeneously distributed in each singly-doped Ti02 particles. However, since the hydrolysis rates of two different alkoxides are generally not equal, and thus there is a high possibility of formation of a compositional double structure in each particle, some other means such as XPS analysis may be recommended for more precise assessment. In order to attain a uniform composition, proper choice of alkoxyl group for each alkoxide, or controlled continuous addition of an active alkoxide to a hydrolysis system of the other less active alkoxide, is required in many cases. For the stabilization of the cubic phase of Zr02 ceramics to avoid the phase transition between the monoclinic and tetragonal phases with the change of temperature and for the reduction of their sintering temperature, yttria-stabilized zirconia (YSZ) is widely used.^"*'^^*^"* Fegley et al}^ prepared Zr02 particles of a narrow size distribution with 0.2 |im mean diameter, doped with 6.3 mol % Y2O3 by co-hydrolysis of ca, 0.08 mol dm"^ Zr(n-C3H70)4 and ca. 0.01 mol dm"^ Y(iso-C3H70)3 at 50 °C. The reaction was instantaneous. The uniformly

12. INDUSTRIAL APPLICATIONS

591

packed compact sintered to >98% of the theoretical density at temperatures as low as 1160 °C, similar to the result for pure ZiQ^?^ Monodispersed SiOj powders doped with a high content of B2O3 (e.^., 1:1 in molar ratio) were also prepared by partially hydrolyzing 81(062115)4, adding B(n-C4H90)3, and then completely hydrolyzing this mixture?^'^^ The uniform compact of the B-doped Si02 (1:1) powder of 0.2 \xm could be densely sintered at temperatures approaching 700 °C, significantly lower than the sintering temperature (-1100 °C) of a uniform compact of a pure monodispersed Si02 powder (d = 0.4 [im) prepared in a similar manner. The chemical and physical properties of uniform ceramic powders prepared by Bowen et al are summarized in Table 12.2.^ In this table, there are striking differences in specific surface area between powders washed with alcohol and with water. For example, the BET specific surface areas of the water-washed Ti02 powders of 0.3-0.7 [im in mean particle diameter are as high as 170 to 200 mVg, which are equivalent to the specific surface areas of a sphere with a diameter of 88 to 75 A. Similar results were obtamed by Kondo et al as well.^ If we assume that the alkoxide remaining only on the external surfaces is hydrolyzed by washing with water to form very small titania particles on the surfaces, the upper limit of the enlarged specific surface area is calculated as only 4.63 times the original surface area, irrespective of the generated particle size, on the assumption of a closest-packed particulate monolayer. Thus, as-prepared amorphous titania particles must retain a considerable amount of unreacted alkoxide in the interiors, and become porous with hydrolysis of the internal alkoxide by permeating water used for washing. Fegley and Barringer^ also proposed the use of porous ceramics, prepared by sintering the uniform compacts of high-purity monosized powders at relatively low temperatures, for filter membranes, chromatography substrates, catalytic substrates, and sensor elements. Unlike the most alkoxide-based particles being amorphous, Heistand and Chia^^ prepared polycrystalline spheres of ZnO, having a narrow size distribution of 0.2 pim in diameter and 15 nm in crystallite size, by hydrolysis of ca. 0.2 mol dm"^ ethylzinc r-butoxide, (C2H5)Zn(r-C4H90), at room temperature, which may be used as a ZnO-based varistor material. Besides the synthesis of uniform ceramic particles by hydrolysis of alkoxides, Nishisu and Kobayashi^^ prepared monodisperse spheres of EujOa-doped Y2O3 (0.1-0.3 \xm), to be used as a red phosphor, by precipitation of the monodispersed mixed carbonate spheres from a dilute solution of mixed chloride salts of Y^^ and Eu^* (totally 1.0 x lO'^mol dm"^)

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and urea (5.0 x 10"^ mol dm"^) at 97 "^C for 60 min and final calcination at temperatures above 650 °C for 60 min after repeated centrifugal washing with deionized water and propanol. The composition of the mixed carbonate can be varied from pure Eu2(C03)3 to pure Y2(C03)3 with excellent monodispersity, and the yield was always more than 99% after aging at 97 °C for 60 mm. The amorphous carbonate particles were converted into crystalline Eu-doped Y2O3 (cubic crystal) with a specific emission at 611 nm by calcination at temperatures above 650 ""C without changing their spherical shape. Figures 12.8 and 12.9 show a TEM of sample A - 1 (Eu:Y = 0.05:0.95) calcined at 850 °C for 1 h and emission spectra of Eu-doped Y2O3 powders depending on the doped content of EU2O3, respectively. In Fig. 12.9, while Y2O3 with no EU2O3 shows no emission, sample A - 1 with 5 mol% EU2O3 demonstrates the strongest emission, and the specific emission is rather lowered with an increasing content of EU2O3 over 5 mol % due to the concentration-extinction. The advantage of the monodispersed particles may be the uniform distribution

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Fig. 12.8. TEM of Eu203-doped Y2O3 particles (Eu:Y = 0.05:0.95) prepared by calcination of the corresponding carbonate particles at 850 ""C for 1 h. (From Ref. 36.)

Fig. 12.9. Emission spectra of Eudoped Y2O3 powders, depending on the doped content of EU2O3. (From Ref. 36.)

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APPLICATIONS

of Eii203 content among particles as well as the sharp size distribution, both of which have a critical influence on the luminescent property of the final product. There is a number of merits to the above uniform particles, including the simple preparative procedures at moderate temperatures, high purity, high yields, and low sintering temperatures. However, if the concentrations of starting materials are limited to ca. 0.2 mol dm'^ and cannot be increased significantly without degrading the uniformity of the products, then the cost of scale-up to large liquid volumes, as well as the rather costly starting materials, may outweigh the advantages.^^ One of the solutions to this problem may be to ust polymeric steric stabilizers to inhibit aggregation.^^'^^ For example, Jean and Ring'*^ found hydroxypropylcellulose (HPC) to be an effective stabilizer in the growth system of TiOj particles by hydrolysis of titanium ethoxide. The unagglomerated powder with HPC formed a uniformly packed compact by sedimentation in ethanol and proved to have the excellent sinterability {e.g,, 99% of theoretical density by sintering at 1050 ''C for 90 min with Ti02 particles of 0.36 pun). Since then, HPC has been used also for the preparation of fairly uniform AI2O3 particles'*^ and ZrOj particles.^^'^^ Although it is not easy, so far, to exceed the concentration level of 0.2 mol dm"^ for achievement of satisfactory monodispersity even with HPC in these systems, the use of polymers or surfactants as stabilizers undoubtedly offers a promising way for the production of highly condensed monodisperse particles. hi addition, it is of great importance to develop new productive methods, such as the gel-sol method, or to extend conventional productive methods, such as the controlled double-jet (CDJ) method (see section 7.3). Recently, Her et al^'^'^'^ have prepared fairly uniform particles of pure and Zr- and Sr-doped BaTiOj particles for use as ceramic dielectrics. These were produced by simultaneous introduction of a mixed solution of BaClj, TiCl4, and the chloride salts of desired dopant metals and a NaOH solution into a vessel containing a NaOH solution, under stirring at 85 °C, based on the double jet method. The total concentration of the used chloride salts in the whole system was ca. 0.37 mol dm'^. Interestingly, the nearly spherical particle products have considerably narrow size distributions, despite the clear evidence of coagulation of the primary particles during the growth of the product particles. Probably, the hydroxide gel network preformed in the highly alkaline media may prevent extensive agglomeration, like the synthetic system of uniform magnetite particles.'^ The dielectric property of barium titanate so-obtained with 20 % Zr substitution and sintered at

595

12. INDUSTRIAL APPLICATIONS

Fig. 12.10. SEM of a Zr-doped BaTiOg powder (Zr/Ti = 0.25) prepared by the double-jet method. (From Ref. 43.) 1275 "^C meets the specifications of Y5V multilayer capacitors. The grain sizes ranged from 1 to 3 (im. Solids with an extremely sharp change in dielectric constant as a function of temperature, which are suitable for application as thermal IR detectors, can be obtained when both Sr and Zr are incorporated as dopants. Figure 12.10 shows a SEM of a Zr-doped BaTi03 powder. Figure 12.11 displays the relative dielectric constant (K) as a function of temperature of Zr-doped BaTiOj powders of different Zr contents; (a) 17%, (b) 20%, and (c) 23% of Ti atoms, sintered at 1275 °C. The peak shift of the maximum dielectric constant to lower temperature. 10000 8000 6000

4000 H 2000

0

50

T (X)

100

150

Fig. 12.11. Relative dielectric constant (K) as a function of temperature of Zr-doped BaTiOg powders of different Zr contents; (a) 17%, (b) 20%, and (c) 23% of Ti atoms, sintered at 1275 °C. (From Ref. 43.)

APPLICATIONS

596

with increasing content of BaZr03, corresponds to the Curie-point shift at which the crystal lattice undergoes a structure change from tetragonal to cubic and exhibits a sharp peak in the dielectric constant. Figure 12.12 is a relation of K V5 T for a BaTi03 powder doubly doped with Zr and Sr, showing a very sharp peak of dielectric constant. This sample was also sintered at 1275 °C. Both BaZr03 and SrTi03 are known as typical Curie point shifters of BaTi03 to control the temperature and the magnitude of the maximum dielectric constant of barium titanate. 40000

30000

K

20000

10000

150

Fig. 12.12. Kvs T for a BaTi03 powder doubly doped with Zr and Sr, sintered at 1275 ^C [(Ba+Sr)/ (Ti+Zr) = 0.996; Sr/Ba = 0.061; ZryTi = 0.184]. (From Ref. 43.)

12.3. Magnetic Recording Materials For understanding the importance of monodispersed particles as magnetic recording media, it seems necessary to have some knowledge on the fimdamentals of the mechanism of magnetism and the effects of particle shape and size. For this purpose, the fundamental aspects of magnetism will be surveyed first. 12.3.1. Fundamentals of Magnetism a) Magnetic Force and Its Unit Systems In the COS system of units, the magnetic force, F, between magnetic poles of respective strengths, nij and m2, separated by r in a vacuum is given by

12. INDUSTRIAL APPLICATIONS

597

F= !!lf!l

(12.3.1)

where the magnitude of pole strength, m, is defined as 1 emu, when m =mj = m2, r = 1 cm, and F = 1 dyne. On the other hand, the corresponding equation in the SI unit system is given by F= —^—^ ,

(12.3.2)

471 l l /

where Mo is a constant to yield F in newton (N), when rtij and m2 are given in weber (Wb), and r in meter (m). Since l/4jt emu cm"^ = 1 gauss = lO""* Wb m"^ or Wb = (l/4n) x 10^ emu as the definition of Wb unit, and since emu^ cm'^ = dyne = 10"^ N, F in N unit for m^ = m2 = 1 Wb and r = 1 m is given from Eq. (12.3.1) as JO^^

(12.3.3)

On the other hand, from Eq. (12.3.2) F=-^N.

(12.3.4)

Comparison between Eqs. (12.3.3) and (12.3.4) yields Mo = 471x10-1

(12.3.5)

If B and H denote the magnetic flux density and magnetic field, respectively, at magnetic pole 2 under the influence of pole 1, Eq. (12.3.2) may be rewritten as F = - ^ - — • m , = B- — • m , = i f - w , , where

(12.3.6)

598

APPLICATIONS

^=_^!L. =A

(12.3.7)

5=-^.

(12.3.8)

and

471 r^

In the SI unit system, A m'^ is used as a unit of the magnetic field, if, instead of the Wb m"^ unit used for the magnetic flux density, B, and the constant \x^ is redefined as a physical quantity with a unit, henry m'^ As a consequence, ^ = 4JI x 10"^ henry m'\ and there are the following relationships among these units: A m"^ = N Wb'"\ Wb m'^ = (henry m"^)(A m-'), and N = (Wb m-^)(m henry-')Wb = (A m-')Wb. If we place a magnetic substance in a magnetic field H (A m"^), where the dimensions of the magnetic substance in all directions perpendicular to the magnetic field are virtually infinite, the magnetic flux density, 5, in the magnetic substance is given by B=n^^+/,

(12.3.9)

where / (Wb m"^) is the magnetization, equivalent to the magnetic moment per unit volume. Magnetic flux density, B in Wb m"^, corresponds to the external magneticfield,H^^ in A m'\ as 5 = Mo^«- Magnetization, / in Wb m'^, corresponds to demagnetizing field, H^ in A m'\ a magnetic field induced by H^ in the direction opposite to H^^, related as / = \Xffld' Hence, H is the effective magnetic field in the magnetic substance, given by H = H^^ - H^, which is an alternative expression to Eq. (12.3.9). In order to characterize a magnetic substance, (x (magnetic permeability), ft (relative permeability), % (magnetic susceptibility), and x (relative susceptibility) are defined as

and

12. INDUSTRIAL APPLICATIONS

X =^ ;

599

X =^ =- ^ -

(12.3.11)

There are the following relationships between x and |i, and between x and fl: |Ji = tio"'X'

ii = l + X-

(12.3.12)

In particular, Ho is called the vacuum permeability according to the definition of magnetic permeability of a magnetic substance, \i (= B/H). Basically, SI units are used in this book unless specified otherwise, but it seems convenient to refer to the relations between the units in SI and CGS systems, in view of many available data of magnetic properties given by CGS units. For example, the equation in the CGS unit system corresponding to Eq. (12.3.9) is B = H^4nI,

(12.3.13)

where the unit of B and 4jt/ is gauss (G), and the unit of H is oersted (Oe), but the magnitudes in gauss and oersted are equal (G = Oe). If 5 = b Wb m"^ H = h A m"\ Mo = KQ henry m"^ (KQ = 4jr X 10'^), and / = z Wb m"^ in Eq. (12.3.9) in the SI system, whilst B =fc'G,H = A'Oe, and 4jt/ = 4jt/'G in Eq. (12.3.13) in the CGS system, it holds from the equality between the absolute magnitudes of the corresponding terms in Eqs. (12.3.9) and (12.3.13) that 6 Wb m-2 = b' G, (KQ henry m'')(h A m'') = h' Oe, and i Wb m"^ = 4m' G, wherefe,h, KQ, Z,fc',A', and V are the dimensionless multipliers of the respective units. Using G = Oe = 10"^ Wb m"^ and Wb = henry A, we obtain b = IQ-^fc', h = 10"^/I7KO, and / = 4jt x 10"^/', so that G = 10-^ Wb m-2 Oe = (1/47C) X 10^ Am"'

(b' = 1, fo = 10"^) (A' = 1, A = IQ-^/KQ)

G' = 4jt X 10-" Wb m-^

(/' = 1, / = 4JCX10^)

where G' represents a gauss unit for magnetization in the CGS system, which is 4jt times greater than the gauss unit G for magnetic flux density, 5, (G = 10"^ Wb m"^). It may also be noted that the emu unit for magnetic moment is equal to emu (for magnetic pole strength) x cm; z.e., emu (magnetic moment) = emu

APPLICATIONS

600

(pole strength) x cm = 4jt x 10"^° Wb m. If we use the emu unit for magnetic moment, then emu cm'^ = G' = 4jr x 10"* Wb m'^ for (volumic) magnetization, corresponding to magnetic moment per unit volume, while emu g'^ = 4jt x 10'^ Wb m kg"^ for mass magnetization, corresponding to magnetic moment per unit mass. In addition, the relative permeability in the SI unit system, pT, is equal to the magnetic permeability in the CGS unit system, while the relative susceptibility % in the SI system is 4jt times the magnetic susceptibility in the CGS system. h) Classification of Magnetisms All substances are more or less magnetized when they are in a strong magnetic field. In this sense, all materials are magnetic substances, but their magnetic properties vary widely according to the characteristics of their Table 12.3. Classification of magnetisms Magnetism Ferromagnetism (X = 10^-10^)

Subgroup Ferromagnetism

Fe, Co, Ni, Gd, Cu2MnAl, MnBi, MnSb, Cr02, CrTe, CrBrg, EuO, EuS

Ferrimagnetism

M^Fe^^'O^, Y-FeA. RgFejOj^, MO-6Fe203, FeTiOj, FeCr204

Parasitic Ferromagnetism

a-Fe203

Metamagnetism Paramagnetism (X = 10-^-10-^)

FeCl2 Paramagnetism

Sc Ti, V, Cr, Pd, Pt, Al, O2, N2O

Antiferromagnetism Diamagnetism

Example

FeO, MnO, MnF2 most other substances

(X = -io-*~io-^) Note: M"Fe2"^04 = spinel-type ferrites (M": divalent metals); R3Fe50i2 = garnettype ferrites (R: ianthanides from Sm to Lu, or Y(III)); MO'6Fe203 = magnetoplumbite-type ferrites (M: Ba, Sr, Pb).

12. INDUSTRIAL APPLICATIONS

601

atomic magnetic moments. They may be classified as in Table 12.3. Ferromagnetism The subgroups of the ferromagnetism family, such as ferromagnetism in a narrow sense,/emmagneri^m, dis\di parasitic ferromagnetism are classified according to the ways of alignment of a material's atomic magnetic moments or spins. Ferromagnetism in a narrow sense is characterized by the alignment of all spins in the same direction in the individual magnetic domains of a crystal. The relative susceptibility, x M very large - in the range of 10^ to 10^. However, % is not constant against the increase of //, but shows a saturation magnetization, /^, and a hysteresis in the magnetization, as shown in Fig. 12.13. Here, H^ is the coercive force or coercivity, which is the reverse magnetic field to return / to zero, and /^ is the residual magnetization or remanent magnetization when H is reduced to zero from a suflFiciently high positive value. The quantities H^, /^, /^, and IJI^ (squareness) are important parameters for characterizing the magnetic properties of ferromagnetics. Above the Curie temperatures they become paramagnetic. Most metals and their alloys of strong magnetism belong to this group. Typical ferromagnetic substances (Curie temperatures) are Fe (1043K), Co (1388K), Ni (627K), Gd (292K), CujMnAl (710K), CrO^ (352K), CrTe (339K), CrBr3 (37K), MnBI (630K), MnSb (587K), EuO (77K), EuS (16.5K), etc. Ferrimagnetism is characterized by the alternate antiparallel alignments of spins of magnetic atoms on different lattice points, A and B, in each r B

/. —:> c

/y

-He

JD 0

E

H

}.

•

H

Fig. 12.13. Hysteresis of magnetization of a ferromagnetic material.

602

APPLICATIONS

crystal unit cell. The strong magnetization is due to the great difference in magnitude of the magnetic moments at the points, A and B. Magnetic properties of ferrimagnetism are almost the same as those proper to the above ferromagnetism. The relative susceptibilities of ferrimagnetic materials are similarly in the range from 10^ to 10^, and they are also widely used as magnetic materials. Typical ferrimagnetic substances are ferrites, including spinel-type ferrites (M"Fe2^°04, M° = divalent metals), garnet-type ferrites (R3Fe50i2, R = lanthanides from Sm to Lu, or Y(III)) and hexagonal magnetoplumbitetype ferrites (MO'6Fe203, M = Ba, Sr, Pb), and compounds other than ferrites, such as ilmenite (FeTIOj), or chromite (FeCr204). Above all, inverse spinel-type ferrites, represented by maghemite (Y-Fe203), cobalt ferrite (CoFe204), and magnetite (Fe304) are important as magnetic recording materials or their precursors. Spinel-type ferrites in general consist of a close-packed cubic lattice of oxygen ions with interstitial cations occupying A and B sites, the former positions being surrounded tetrahedrally by four oxygen ions, the latter by six oxygen ions placed octahedrally. The number of oxygen ions in a unit cell is 32, and there are 8 A-sites and 16 B-sites in a unit cell. For the regular spinel, the 8 A sites are occupied by divalent cations and the 16 B-sites by trivalent cations such as Fe^^; e.g, ZnFe204. For the inverse spinel, the 8 A-sites are occupied by trivalent cations, and the 16 B-sites randomly by 8 divalent cations and 8 trivalent cations; e.g., CuFe204, MgFe204, CoFe204, and Fe304. Although maghemite (Y-Fe203) is a kind of inverse spinel, the 16 B-sites are occupied only by 13Vi Fe^* ions, and the remaining 2% B-sites are vacant. Parasitic ferromagnetism is a very weak magnetism due to equivalent antiparallel spins at A and B lattice sites, but with partly reversed spins at the B sites. A typical example is hematite (a-Fe203). The magnetic behavior is very close to antiferromagnetism to be referred to below. Metamagnetism, Paramagnetism^ and Diamagnetism Metamagnetism, first found in FeCl2 crystals, is characterized by the paramagnetic behavior within a low magnetic field {I ^ H) and a sharp increase in / to a relatively high saturation magnetization when H reaches a certain high level around 2 - 3 x 10^ A m"\ However, in contrast to normal ferromagnetism, no hysteresis is observed. The sharp lift of / is believed to be due to the transient spontaneous magnetization caused by a high magnetic field breaking the antiferromagnetic spin aligiunent.

12. INDUSTRIAL APPLICATIONS

603

Paramagnetism is a very weak magnetism with / in proportion to H (x = 10"^ - 10'^). In most cases, the relation of 1/x to T is a straight line passing through the origin, which suggests that the magnetic spins are randomly oriented by thermal oscillation. Transition metals^ such as So, Ti, V, Cr, Pd, and Pt, and their compounds, aluminum (Al), oxygen (O2), nitrous oxide (N2O), etc. belong to this group. Antiferromagnetism is also a weak magnetism classified into a kind of paramagnetism in a broad sense, but the weak magnetism is due to the almost completely antiparallel alignment of spins at A and B lattice sites. Above a certain transition temperature (the Neel temperature), the alignment becomes random by thermal oscillation. With elevation of temperature, 1/x is lowered and turns to increase linearly above the Neel temperature, while normal paramagnetic substances show only a linear increase of 1/x with increasing temperature from 0 K. Typical antiferromagnetic substances are FeO, MnO, and MnF2. Diamagnetism is also a very weak magnetism of negative x whose absolute value is of the order of 10'^ or less, owing to Larmor's precession of electronic orbitals of the atoms. A majority of all substances, including most gases, metals, salts, and organic compounds, belong to this category. Practically, only the ferromagnetic and ferrimagnetic substances are used as magnetic materials. According to conventional usage, we hereafter call them simply, "ferromagnetic substances," without distinction between them, unless it is required. c) Size Effects of Ferromagnetic Particles If we place a ferromagnetic solid in a magnetic field, H, the magnetization is dependent on the shape of the solid and the direction of the principal axes of the anisotropic form relative to the magnetic field. If the demagnetizing factor in the direction of the magnetic field, H, is denoted by A^, the magnetic flux density, B, is generally given by B=\i^^NI, where / is the magnetization. obtains

(12.3.14)

From H - H^^ - H^ and B = m/^^^, one

H.^N—.

(12.3.15)

If N^, Ny, and N^ denote the demagnetizing factors of the solid in the

APPLICATIONS

604

directions of the tliree principal axes nonnal to each other, it generally holds that (12.3.16)

N^^N^^N^^h

If the length in the direction of jc axis is virtually infinite as compared to the others, then A^^ = 0 because the distance between the negative and positive poles is infinitely large. Hence, if the magnetic material is a plate with its flat plane peq^endicular to the magnetic field in the direction of the x axis, then A^ = 1, corresponding to Eq. (12.3.9), because N = N^=^ 1 and Ny = N^ = 0. If the plate is set with its flat plane parallel to the direction of the magnetic field, or if a rod-like material is set with its principal axis parallel to the magnetic field, then A^ = 0 in both cases, because N =^ N^ = Ny = 0 and N^ = I in the former case, and N = N^ =^ 0 and Ny = N^ = 1/2 in the latter. Similarly, N = 1/2 when the magnetic field is perpendicular to the principal axis of a rod, and A^ = 1/3 when the solid is a sphere. If a magnetic solid is a prolate spheroid with a half length, a, and a half width 6, the demagnetizing factors along the polar axis, N^ is given by N^

1

a

a^-1

^M

ln(a+ya^-l)-l

(12.3.17a)

where a = a/b. For an oblate spheroid with a half length of the polar axis a and equatorial radius fe.

P^

-sm ^ 2 (P2.1)3/2-

(12.3.17b) p

p2.i

where p = fc/a. Since A^^ + 2Nf, = 1 in both cases, A^^, for a prolate spheroid and N^ for a oblate spheroid can be calculated as well. In the CGS unit system, each A^ value is 47C times the corresponding A^ value in the SI unit system; e.g., N^ -^ Ny -¥ N^ = 4jt, and H^ = NL In the absence of external magnetic field, a large ferromagnetic crystal with uniaxial magnetocrystalline anisotropy such as cobalt is divided into many magnetic domains separated by parallel domain walls. The spins in a magnetic domain are oriented in the same direction, but the directions in adjacent domains are opposite, to minimize the magnetostatic energy. If we

12. INDUSTRIAL APPLICATIONS

605

consider a single-domain prolate spheroid with uniaxial magnetocrystalline anisotropy along the polar axis in the absence of external magnetic field, the magnetic field, //, is only the demagnetizing field of its own, so that H=-—L,

(12.3.18)

Hence, if the volume of the single-domain particle is denoted by v, its magnetic energy is only the magnetostatic energy of the particle, U^, given by r2

(12.3.19)

U=-1HIV^!^V.

"

2 ^ 2|io

If the particle is a sphere {N = 1/3), t/=-£-u. m

(12-3.20)

6^0

If the spherical particle is changed into a double-domain particle with two magnetic domains of magnetic moments antiparallel to each other, the magnetostatic energy is reduced to a half of the original, but a domain wall energy with a specific waU energy, y, is generated (y = 1 ~ 5 mJ m'^)."*^ Hence, if the effect of the thickness of the domain wall is neglected for simplicity, the critical radius, r^, at which the magnetic energies of a single domain particle and a double domain particle are equal is given from

Ur^^'wlH as

rK ' cJ

(12.3.21)

606

APPLICATIONS

9|io

rr-^yIs

(12.3.22)

The relative weight of the domain wall energy in the total magnetic energy increases with size reduction of a magnetic particle, because the waU energy is proportional to r^, while the magnetostatic energy is proportional to r^. Hence, the particle initially consisting of many magnetic domains reduces the total area of the domain wall with decreasing particle size, and finally it is switched to a single domain particle with no domain wall at a certain critical radius, as given by Eq. (12.3.22), in order to minimize the total magnetic energy. This treatment was applied to platelet-type particles as well with uniaxial magnetocrystalline anisotropy."*^ In the case of a ferromagnetic particle of a cubic crystal with easy directions for magnetization in the principal axes of the cubic crystal such as iron, the magnetic spins may be arrayed in a squire flux closure with triangular domains or rectangular flux closure with triangular and trapezoidal domains. Although the magnetostatic energy is zero in this domain structure due to the internal flux closure, the domain wall energy and magnetocrystalline anisotropy energy are present, so that there is also a critical size for transition to a single-domain particle."*^ If the magnetocrystaUine anisotropy is small in a multidomain particle, the domain structure may be represented by ring-shaped domains in which the spins are arrayed in a circular configuration, so that there is no magnetostatic energy due to the internal flux closure without poles. However, the total of the exchange energies between adjacent spins in each ring per unit volume increases with size reduction, since the exchange energy between adjacent spins in a circular flux increases with increasing curvature of the circularflux.'^^'*'''*^Hence there is also a critical size for transition from the circular domain structure to a single domain one with parallel spin alignment."*^ However, as the single domain particle becomes still smaller to a particle volume level at which the magnetic energy is close to the thermal energy, kT, the atomic magnetic moments start to be randomly oriented. In this size range, the particles are readily magnetized to the saturation magnetization with a relatively weak magnetic field, but show no hysteresis in the magnetization curves, and thus the coercive force is zero. This characteristic magnetism is called superparamagnetism. As a consequence, the coercive

607

12. INDUSTRIAL APPLICATIONS

Multidomain

o U

Particle Volume Fig. 12.14. Coercivity as a function of the mean volume of ferromagnetic particles. force, //^, has a peak around r^ for transition to a single domain particle and it declines with size reduction until, finally, the particle becomes superparamagnetic {H^ = 0), as shown in Fig. 12.14.^° The critical diameter for transition of particles of isotropic shape from multidomain to single domain is of the order of 10 to 100 nm,"*^ while the size range of superparamagnetic particles is ca. 10 nm or less.^^ According to the theoretical calculation on the basis of the circular multidomain model, the critical particle volume of transition from a multidomain particle to a single-domain particle increases with increasing aspect ratio of elongated ellipsoidal particles (aspect ratio = ratio of length to width)."*^'"*^ Figure 12.15 shows the critical length (= 2a) between multidomain and single-domain structures, as a function of the bla ratio, for ellipsoidal particles of maghemite (Y-Fe203) and magnetite (Fe304), calculated by Morrish and Yu'*^ Here a and fe denote the half length and equatorial radius of an ellipsoid, respectively. Obviously, the critical length for ellipsoidal particles of magnetite greatly increases with increasing aspect ratio. For maghemite the critical size is somewhat larger. Moreover, the critical size increases significantly with increasing particle density, owing to the reduction of the magnetostatic energy in each particle, caused by magnetic interaction among the particles.^^ It is also known that the critical size drastically decreases as the degree of orientation of ellipsoids is

608

APPLICATIONS

Fig. 12.15. The critical singledomain length (= 2«) as a function of the bla ratio for ellipsoidal particles of maghemite (7-^6203) and magnetite (FcgO^). (From Ref. 49.)

lowered .^^" The single-domain particles of relatively large coercive force are used for magnetic recording materials, while much smaller superparamagnetic particles with no hysteresis in magnetization are mainly used for magnetic fluids, etc. d) Effects of the Magnetic Anisotropics The magnetic properties of ferromagnetic single domain particles depend strongly on their magnetic anisotropics. For example, if they have no magnetic anisotropy, they may behave like superparamagnetic particles which show neither hysteresis nor coercive force in their magnetization curves. Hence significant magnetic anisotropy is required for ferromagnetic particles used for recording media. Major magnetic anisotropics are shape anisotropy, magnetocrystalline anisotropy, and strain anisotropy."^^ Shape Anisotropy Let us consider a particularly important particle as an magnetic recording element, an ellipsoidal single-domain particle, whose aspect ratio a/b (a = half-length of polar axis, b = equatorial radius) ranges from 0 to 00. Figure 12.16 shows a prolate spheroid {a/b > 1) in a magnetic field, H, with its

12. INDUSTRIAL APPLICATIONS

609

^H

Fig. 12.16. A prolate spheroid {alh > 1) in a magnetic field, //, with its polar axis tilted by 9 from the positive direction of H. The angle between the directions of the saturation magnetization, 7^, and the polar axis of the particle and that between /^ and H are al; and (|), respectively. (From Ref. 48.)

polar axis tilted by 6 from the positive direction of H. The angle between the directions of the saturation magnetization, 7^, and the polar axis of the particle and that between 7^ and 77 are \p and cj), respectively."*^ There is the following relationship between them: <|) = e-M|f.

(12.3.23)

For a general ellipsoid, the shape-anisotropy energy per unit volume associated with the demagnetizing field is

' (N^al^N/^^N/;).

(12.3.24)

''-2H where cx^, tty, a^ are the direction cosines of 7^ with respect to the principal axes, and N^, Ny, N^ are the demagnetizing factor along the principal axes, where JV^ + A^^ + iV^ = 1 (see Eq. (12.3.16)). In addition, the energy per unit volume due to the applied field is Ejj= -ffl^coscj).

(12.3.25)

Using N^ = A^^, Ny^N^= N^, a^ = cos \|), a^ = sin \p, and a^ = 0, E^ and the variable part of the total energy £ (= JE^ + E^) may be written as (12.3.26)

and

610

APPLICATIONS

E=Ksm^-HIcos<^,

(12.3.27)

where NrN.

A:=--^—^//.

(12.3.28)

2^0

When the ellipsoid is a prolate spheroid {a > b\ N^ > N^ and K^ > 0. K^ is a magnetic anisotropy constant representing the magnitude of the shape anisotropy. Using \p = (]) - 6, and treating /^ and 6 as fixed, cj) values which give the minima, inflection points, and maxima of E^ are obtained fi-om BE - ^ = ^,sin2( = 0

a<|)

(12.3.29)

At the minima, inflection points, and maxima of E^, the following d^Eld^^ values are positive, zero, and negative, respectively: — = 2^^cos2(<|)-e)+/f/^cos(|). d(|)

(12.3.30)

As H is changed from +» to -oo via zero along the H axis at a constant 6, (|) may increase from zero to reach 6 when / / = 0, and then jump from an angle greater than 6 to an angle between 180"" and (180° + 6) when H reaches a certain negative value, as readily expected from Eq. (12.3.29). Finally, ^ may return to 180° with further increase of H in the negative direction. The jump of cj) occurs when H reaches a certain negative value at which the branch of the energy minimum is united with that of an energy maximum to form an inflexion point, and E is allowed to go down to another energy minimum branch located at ^ between 180° and 180° + 6 at the same H, Figure 12.17 shows the relations between h (= HIJ2K^) as a dimensionless magnetic field and cos (j) (=//4) as a dimensionless magnetization, at different 6 values (in degrees). In the case of an oblate spheroid (a < h) in Fig. 12.18, E is also given by Eq. (12.3.27), but ^ , < 0 since N^ < A^^. As H is changed from +« to 00 via zero along the H axis, ^ changes from zero to reach (6 - 90°) (< 0°) when / / = 0, and then the magnetization vector, /,, rotates around the polar axis on the equatorial plane to make (j) = 6 + 90° with a slight increase of

611

12. INDUSTRIAL APPLICATIONS

1

1

-L

0 .

1

.

1

I 1

[

1

1

1

^^^=^'' _

45

-

-

/ /io /io

0,10,90

1

1

^.''cos ^0 ,

1

1

L

7/ /

\

J A

AS

\ 45.JL^

-j

^y//

V

10 0

L ,,.

1 0,10.90

'

10

\cosf5o y COS (j>

1

0

_____^ f

i

l

l

1 ,1

Fig. 12.17. The relationships between a dimensionless magnetic field, h (= HIJTK^, and a dimensionless magnetization, cos (j) (^I/Qy for a prolate spheroid at different 0 values on the curves (in degrees). The dotted curves give cos ^Q and cos ct)'o, where ^Q and (|)'o are the angles made with the positive field direction by the magnetization vector at the beginning and the end of the discontinuous change at the critical value, /IQ, of the field. (From Ref. 48.)

^H

Fig. 12.18. An oblate spheroid (a/b < 1) in a magnetic field, H.

H in the negative direction, since no additional energy is required for this rotation of the /, vector. Finally, the /, vector turns toward the negative direction of H with increasing ^ to 180°. Hence, cos ^ plumnniets from a positive value to a negative value of the same absolute magnitude at / / = 0, so that the magnetization curves show no hysteresis and thus no coercive force.

APPLICATIONS

612

If we consider an assembly of similar spheroids with their polar axes oriented at random in a magnetic field, //, the mean resolved value of cos (|) in the positive field direction of the magnetization is given by X ^H r^/2 COS(|)= — = / (cossin6J6,

(12.3.31)

where I„ is the mean resolved value oil, in the field direction. Figure 12.19 shows mean values of cos (|) as a function of h (= HIJIK^) for prolate spheroids (solid lines) and oblate spheroids (broken lines) at random orientation, as obtainedfi-omthe numerical integration of Eq. (12.3.31) using (j) as a function of 0 in Eqs. (12.3.29) and (12.3.30) at each h. As has been predicted for a single oblate spheroid, the mean value of cos ^ for an assembly of randomly oriented oblate spheroids falls from a positive value to a negative value of the same absolute magnitude at A = 0 and thus the coercive force H^ = 0.

cos ^

Fig. 12.19. Mean values of cos (j) as a function of h (= HIJIK^) for prolate spheroids (solid lines) and oblate spheroids (broken lines) at random orientation. (From Ref. 48.)

For an assembly of prolate spheroids at random orientation,"*^

12. INDUSTRIAL APPLICATIONS

613

Coercive force:

H = 0.479 — ^ — ^ 1^0

Initial susceptibility:

2V.0

[dHL

Residual magnetiz0ion:

3(N,-N)

/^= —/^

Magnetocrystalline Anisotropy Magnetocrystalline anisotropy energy is an excess magnetic energy due to orientation of the magnetization vector away from the easy crystal axes for magnetization. The magnetocrystalline anisotropy energy, E^, for uniaxial anisotropy, apart from constant terms, is given by E^ = K^,sm^^ -^ JS:^sin>,

(12.3.32)

where xj) is the angle between the direction of magnetization and the primary axis of symmetry of the anisometric crystal .^^ If K^^ + K^2 > 0, E^ is minimum at ip = 0°. If X„^ ^K^2<^^ ^c is minimum at ij) = 90°. \fK^iK^2 < 0 and (-KJ2K^2y^ < 1, £, is minimum at \p between 0° and 90° or between 90° and 180°. As a typical ferromagnetic material of uniaxial anisotropy, the hexagonal crystal of cobalt has K^j = 4.3 x 10^ J m'^ and K^2 = 1.2 X 10^ J m'^.^^ As an example of magnetic materials with K^^j + ^2^ < 0 having easy magnetization directions in a plane of \|)= 90°, the so-called ferroxplana such as Ba3Co2Fe2404i is known.^^ Also, some ferroxplanas are known to have easy magnetization directions in a conical plane of \p ?« 0°, 90°. The magnetocrystalline anisotropy energy for cubic crystals with isometric principal axes x, y, and z is given, apart from constant terms, by £, = ^ i ( a ' a ^ a ' a ^ a^a^)+^2«^a;a',

(12.3.33)

where ot^, a^, a^ are direction cosines of /^ with respect to the cubic axes.^^ For the <100> directions of a-iron (body-centered cubic crystal), Kj = 4.2 X 10"* J m"^ and K2 = 1.5 x 10"* J m'"'.^^ In this case, the easiest direction

614

APPLICATIONS

for magnetization are the <100> axes, including [100], [010], and [001]. The most difficult magnetization directions are <111>, and the intermediate ones are <110>. For the <100> directions of nickel (face-centered cubic crystal), K^ = -5.1 x 10^ J m"^ and iC^ " 0 J ^'^^^ Hence, for nickel, the [100], [010], and [001] axes are the difficult magnetization directions, while the <111> axes are the easiest directions. On the other hand, representative magnetic oxides, maghemite (Y-Fe203) and magnetite (FejOJ, of inverse spinel stmcture in the cubic crystal group have negative magnetocrystalline anisotropy constants such as K^ = -4.64 X 10^ J m-^ for Y-FePs and K^ = -1.10 x 10^ J m"^ for YtfiJ^'' and thus their easiest magnetization directions are collinear with the body diagonals or <111> axes. If the magnetocrystalline energy of uniaxial anisotropy in Eq. (12.3.32) is approximated by £^ = A:„sin^i|;,

(12.3.34)

the variable part of the total energy of a spherical single-domain particle with magnetocrystalline anisotropy in a magnetic field, //, is written in a manner similar to Eq. (12.3.27) as E^K^sm^

'HIfos^,

(12.3.35)

where (j) is the angle between the directions of H and 7^. Hence, E in Eq. (12.3.35) for magnetocrystalline anisotropy can be related in the same way as that in Eq. (12.3.27) for shape anisotropy. If an ellipsoidal particle has a uniaxial magnetocrystalline anisotropy as well, with easy directions of magnetization along the polar axis or on the equatorial plane, the variable part of the total energy may be written as £:=^sin^i|r-^/^cos
(12.3.36)

K^K^^K^.

(12.3.37)

where

Strain Anisotropy For a single-domain spherical particle of negligible magnetocrystalline

12. INDUSTRIAL APPLICATIONS

615

anisotropy, and isotropic in respect of the saturation magnetization coefficient, X, subject to uniform tension, a, the dependence of the magneto-strain energy on the angle, \|), between 4 2uid a is given apart from constant terms by^^ E =A:sin^i|r,

(12.3.38)

where K^lxa. ^ 2

(12.3.39)

If K^ is substituted for K^ in Eq. (12.3.27), then £ = i:^sin^t|f-ff/^cos
(12.3.40)

and thus the result obtained for shape anisotropy can be applied to strain anisotropy. If "k and a are of the same sign (e.g., nickel under compression and iron under tension), the magnetic characteristics are similar to those of a single domain prolate spheroid. If X. and a are of opposite sign, the magnetic characteristics are similar to those of an oblate spheroid. 12.3.2. Relations between the Magnetic Anisotropies and Coercive Force a) Shape Anisotropy If we neglect the contribution of the magnetocrystalline anisotropy for randomly oriented prolate spheroids with aspect ratio a/b « », then A^^ - N^ = 1/2 and thus, if we assume the Stoner-Wohlfarth modeU^^ the coercive force H^ for various magnetic materials can be calculated from H^ = 0A79(N^'N,)IJ[iQ = 1.906 x 10' /, (A m'^). For example, Fe: /, = 2.154 Wb m"^ (1714 emu cm"^); H, « 4.1 x 10' A m"' (-- 5200 Co: 4 = 1.787 Wb m'^ (1422 emu cm"^); //, - 3.4 x 10' A m"' {^ 4300 Ni: /, = 0.608 Wb m'^ ( 484 emu cm''); H, « 1.2 x 10' A m'^ (-- 1500 Y-FejOj: /, = 0.52 Wb m-\414 emu cm"^); //, « 1.0 x 10' A m-X^1200

Oe) Oe) Oe) Oe)

If these acicular particles are uniformly oriented in the direction of the magnetic field, H, is given by H^ = IJ2\i^ = 3.98 x 10' /^ (A m"^), which is greater than H^ at random orientation by a factor of 2.09 (= 1/0.479).

616

APPLICATIONS

Moreover, even if we assume a much smaller aspect ratio such as 10 or 5, the reduction of (iV^ - N^) is rather small; e,g., N^ - N^ = 0.470 for a/b = 10; and N^, - N, = 0.416 for a/b = 5. b) Magnetocrystalline Anisotropy The coercive force of an assembly of randomly oriented single domain spheres with uniaxial magnetocrystalline anisotropy may be calculated from A=///y2/^=0.479;i.e., fl^ =

0.958^, -^

For example, if we assume spherical single-domain particles of hexagonal cobalt and barium ferrite (BaO •6Fe203), their H^ values at random orientation may be calculated as Co:

4 = 1.787 Wb m'^ (1422 emu cm"^) i „ = 4.3 X 10^ J m~^ (4.3 x 10^ erg cm'^) H^ « 2.3 X 10^ A m"' (^ 2900 Oe)

BaO-6Fe203:

/, = 0.45 Wb m'^ (358 emu cm"^) K^ = 3.3 X 10' Jm"' (3.3 x 10' erg cm"^) 7/, « 7.0 X 10' A m-' (^ 8800 Oe)

If thin platelet-type particles of barium sulfate are coated parallel to the substrate film, the coercive force due to the crystalline anisotropy in the direction perpendicular to the flat basal planes may be estimated as H^ « 1.5 X 10' A m"^ (- 18000 Oe) from /i = 1 and thus from //, = IKJI^, In this case, the contribution of shape anisotropy to the coercive force is about H^ = -3.6 X 10' A m-^ (~ - 4500 Oe) from H^ = (N^-N,)IJ\IQ with iV^, = 0 and A^, « 1. Hence, totally, H, - 1.1 x 10' A m"^ (- 14000 Oe). For the treatment of cubic crystals, we must use Eq. (12.3.33), in place of Eq. (12.3.32) for uniaxial crystalline anisotropy, and the coercive force, i/^, is equal to oKJI^ with a depending on the direction, but having a maximum value of 2.^^ This allows us to estimate the maximum coercive forces of cubic crystals due to crystalline anisotropy. For example,

12. INDUSTRIAL APPLICATIONS

617

Fe(^c)max = 2x4.2x10^/2.15 « 3.9 x 10' A m"' (~ 490 Oe) Ni: (^c)max = 2x5.1x10^/0.61 « 1.7 x 10' A m"^ (^ 210 Oe) Y-Fe^Gj: (i/J^,, = 2x4.6x10^/0.52 « 1.8 x 10' A m"^ (~ 220 Oe) The coercive forces, due to crystalline anisotropy, for randomly oriented particles would be considerably less. c) Strain Anisotropy If we assume a high pressure, 2 x 10^ kg m"^ (= 1.96 x 10^ J m"^), to estimate the maximum coercive forces due to strain anisotropy from h = 2KJHJ^ = 3ka/HJ, = 1, they may be calculated as'^ Fe: Ni: Co:

X = 1.8 X 10-^ /, = 2.15 Wb m'^ //, = 4.9 x 10' A m"' (- 620 Oe) X = -3.3 X 10-^ /, = 0.61 Wb m'^ if, = 3.2 x 10" A m"' (- 4000 Oe) X = -0.4 -- -2.0 x 10^^ depending on direction, /, = 1.8 Wb m"^; H^ = 1.3 ~ 6.5 X 10' A m-^ (160 ~ 820 Oe) depending on the direction.

d) Some Reductive Factors for Coercive Force The observed coercive forces of acicular particles of Fe, Co, and yFe203, mainly due to shape anisotropy, are nomGially in the following ranges. Fe: H, « 3.2 x 10' ~ 1.4 x 10^ A m"' (400 - 1800 Oe) Co: H, « 3.2 X 10' ~ 8.0 x 10' A m"' (400 - 1000 Oe) Y-Fe^Gj: H, « 1.6 x 10' ~ 3.6 x 10' A m"' (200 ~ 450 Oe) Obviously, the experimental coercive forces are much smaller than the theoretical ones, particularly for those based on shape anisotropy. This is mainly because the above Stoner-Wohlfarth model is based on the assumption of only coherent rotation of atomic magnetic moments in the reversal of magnetization, and because the effect of the magnetic interaction among particles is disregarded in the theory. Also, the significant size dependence of the coercive force is inexplicable by this model. Hence, in addition to the coherent rotation, another uniform reversal mode of spins, fanning (chain-of-spheres asymmetric fanning), and non-uniform reversal modes, such as curling and buckling, have been proposed.^'"^^ In the latter two non-uniform reversal modes, H^ is dependent on particle radius, as proportional to r'^ for curling, and to r"~^ for buckling, when crystalline anisotropy can be ignored. The dominant reversal mode out of these is

618

APPLICATIONS

different according to size ranges. Although the reversal modes other than coherent rotation are useful to some extent for explaining the low coercive forces and their size dependence, the actual coercive forces are still lower. One of the other major reasons for the rather small coercive forces may be the magnetic interaction in an assembly of magnetic particles, mainly observed in the coercivity due to shape anisotropy. As pointed out by Neel,^^ a linear reduction in the coercive force, H^, with increasing powder density, d, is usually observed for single domain particles whose coercive force is governed by shape anisotropy. This is a characteristic of the coercive force due to shape anisotropy, in contrast to the coercivity by magnetocrystalline anisotropy, showing almost no dependence on particle density."^^"^^ Figure 12.20 displays some examples of H^, depending on d/dg (d = powder density in binder; d^ = density of the bulk material), for acicular y-Fe203 and FCjO^ (1 jmi in mean length; 7 ~ 8 in aspect ratio) and cuboidal Fe304 (1.5 ^mi in mean diameter), where the acicular Y-Fe203 and Fe304 are thought to be single domain particles, while the cuboidal Fe304 particles are likely to be multidomain ones from the theoretical calculation of Morrish and Yu,"*^ shown in Fig. 12.15. The intercepts at 1 d/dg = 0 in Fig. 12.20 may correspond to H^ values due only to crystalline anisotropy.

9^\J y \

^ '

^/

a O hi

/A

••>* -^^

.—. 240 CO o

y

/

tan

/

y

/

,

/

o Ui

>

A

/

-^-3-1

bi O

"

80

0 0

.2

.6

.8

(l-i)

LO

Fig. 12.20. Dependence of H^ on dldg (d = powder density in binder; dg = density of the bulk material), for acicular y-F^Os and ¥^^0^ (1 \xm in mean length; 7 ~ 8 in aspect ratio) and cuboidal Fe304 (1.5 \im in mean diameter). (From Ref. 49.)

12. INDUSTRIAL APPLICATIONS

619

12.3.3. Magnetic Particles Used for Recording Media It is now obvious that the size and shape are decisive factors for the fundamental properties of a given ferromagnetic particulate material used for recording media. Hence narrow distributions in size and shape are strongly desired for these materials. In addition, a properly high coercivity, large saturation magnetization, high dispersibility, high chemical stability, sufficient mechanical strength, etc. are also required.^^ As magnetic materials used for recording media in audio and video systems, computer memory systems, magnetic cards, etc., maghemite (Y-Fe203), cobaltmodified maghemite, iron, chromium dioxide, and barium ferrite are well known. Except for barium ferrite, almost all of them are used in the form of acicular particles in order to achieve a high coercive force by shape anisotropy, because of their relatively low magnetocrystalline anisotropy. Barium ferrite, as a hexagonal crystal, having a strong uniaxial crystalline anisotropy in the direction perpendicular to the c-faces is used in the form of small platelets of ca. 0.08 \Jim in diameter with flat basal planes of cfaces. a) Maghemite Elongated maghemite (Y-Fe203) articles 0.2-0.5 \im in length and ca, 10 in aspect ratio are manufactured mainly from acicular goethite (a-FeOOH) particles without changing the original shape of a-FeOOH in a sequential process of hydration from a-FeOOH to a-Fe203 at ca. 300 °C, reduction from a-Fe203 to Fe304 with hydrogen gas at 300 - 400 °C, and oxidation from Fe304 to Y-Fe203 at ca, 250 °C.^'^^ For the precipitation of a-FeOOH precisely controlled in size and aspect ratio, ferrous hydroxide precipitated from a solution of ferrous salt, such as ferrous sulfate or ferrous chloride, with alkali is oxidatively converted into a-FeOOH seeds with air, which are then grown further to a desired size and aspect ratio. In order to keep the original shape of a-FeOOH, the surfaces of a-FeOOH are treated with silica^^ or phosphate^^ prior to the thermal treatment. It is also possible to produce elongated Y-Fe203 by reduction-reoxidation of ellipsoidal a-Fe203 prepared from Fe(0H)3 ^" *he presence of organic phosphonates^"* or forced hydrolysis of ferric ions in a dilute ferric chloride solution (- 0.02 mol dm" ^) in the presence of inorganic phosphates.^^ The Y--Fe203 thus-obtained has an advantage of being pore-free over Y-Fe203 particles prepared from a FeOOH, since it is difficult to completely remove pores generated during the dehydration process of a-FeOOH. Amdt^^ prepared tin-doped Y-Fe203 particles from ellipsoidal a-Fe203, obtained by thermal transformation from

620

APPLICATIONS

Fe(0H)3, to improve the magnetic properties of Y-Fe203. b) Cobalt-Modified Maghemite Cobalt ferrite of inverse spinel structure, which can form solid solutions with y"Fe203 and Fe304 in the same crystal system, has a magnetocrystalline anisotropy constant X^ = 2 x 10^ J m"^, much higher than K^ = -4.64 x 10^ J m"^ for Y-Fe203 and K^ = -1.10 x 10^ J m"^ for Fe304, by a factor of 10 or more in absolute value. However, Co-doped Y~F^203 of a high coercivity prepared on the basis of this nature of cobalt ferrite^^ was not stable enough in magnetic properties such as magnetization and coercive force, against the change of temperature and pressure.^^**^^ This problem has been resolved by depositing CoO and FeO solely onto the surfaces of acicular Y-Fe203 particles, through adsorption of Co^^ and Fe^^ ions followed by addition of an alkaline solution^^*^*"^° or by growing CoFe204 on the surfaces of Y-Fe203 particles.^^*^^ These procedures for surface modification yielded magnetic particles of a high coercive force of ca. 4.5 x 10"* A m'^ (- 570 Oe), with a high thermal stability and small demagnetization with repeated playback, comparable to stable pure Y-Fe203, whose maximum coercive force achieved by now is, however, practically ca, 3.6 x 10"* A m"^ ir- 450 Oe). c) Iron Pure a-Fe has a saturation magnetization as high as 2.74 x 10"^ Wb m kg'^ (= 218 emu g"^) at room temperature, so it is an ideal material for high density recording systems and is actually widely used. Iron powder is normally manufactured from acicular particles of goethite (a-FeOOH) by dehydration and subsequent gas-phase reduction with hydrogen without extensive change of the original shape.^*^"* In order to prevent the sintering of the iron powder during the thermal treatments, the starting material, a FeOOH, is pretreated with silicone oil, silver or cobalt compounds, silica, or alumina.^^'^^ Also, for preventing corrosion of the iron powder, the surfaces are carefully oxidized to form a thin but dense oxide layer.^^'^^ Although the saturation magnetization of thus-prepared iron powders is reduced to ca. 60 - 70 % of that of pure iron, a high coercive force such as 1.2 X 10^ A m'^ (- 1500 Oe) or more can readily be attained. One of the key aspects of this technology may be the achievement of a high resistivity to oxidative corrosion. d) Chromium Dioxide

12. INDUSTRIAL APPLICATIONS

621

Cr02 is an artificial compound of strong ferromagnetism developed by the Du Pont Company in 1961. The acicular single-crystal particles of Cr02 (tetragonal crystals) is prepared by a dissolution-reciystaUization process from CrOg at a high pressure and high temperature in the presence of Sb203 or RUO2 as a catalyst, and a small amount of water.^^ Since it is easy to achieve a high coercivity, such as 4.0 ~ 6.4 x 10"* A m"^ (500 - 800 Oe) with Cr02 particles, they were widely used as high-quality magnetic material.^^ However, as magnetic media with cobalt-modified Y-Fe203 have prevailed, Cr02 appears to be less used, probably because of some disadvantages such as somewhat low thermal stability in coercivity and in residual magnetization, due to the relatively low Curie temperature around 386 K, high production cost, etc. e) Barium Ferrite Barium ferrite (BaO*6Fe203) belongs to the hexagonal magnetoplumbite-type ferrites with the easy direction for magnetization coUinear with the c-axis perpendicular to the flat basal plane. It has widely been used for permanent magnets because of its high coercive force such as 3.2 - 4.8 X 10^ A m"^ (4000 ~ 6000 Oe) due to the high magnetocrystalline anisotropy. Although the coercive force was too high for ordinary recording materials, it attracted special attention because of its potentiality as a highdensity recording material proper to a perpendicular recording system with the platelet-type particles coated parallel to substrate films.^^ Since it is possible to reduce the coercive force of barium ferrite to a proper level by partly substituting the Fe^^ ions with Co'^ and Ti'^^^^'^^ Kubo et al}^^^ prepared platelet-type barium ferrite particles of mean diameter 80 nm and thickness 20 nm with H^ = 7.2 x 10"* A m"^ (~ 900 Oe) and saturation magnetization a^ = 6.9 x 10'^ Wb m kg'^ (~ 55 emu g"^), using a newly developed glass crystallization method, in which the constituents of barium ferrite were heated to above 1300 °C to be dissolved in a melt of a glass forming substance such as B2O3, quenched to be fixed in the glass matrix, and reheated above 700 °C to form crystals of barium ferrite. Finally, the solid matrix was removed with weak acid. Ba-ferrite-coated tapes demonstrated a recording density higher than a tape of Co-modified maghemite and even higher than a Fe-coated tape in a wavelength range shorter than 1 \xm}^ Platelet-type particles of barium ferrite can also be prepared by coprecipitation followed by calcination above 850 °c,^^'^^ hydrothermal synthesis,^^'^^ the sol-gel process followed by calcination,^" coprecipitation

622

APPLICATIONS

in W/0 microemulsions followed by calcination,^^ the stearic acid gel method,^ etc. As it is difficult to coat the magnetic platelets so as to be oriented completely parallel to the film base like acicular particles, it has been proposed to prepare acicular polycrystalline barium ferrite particles, each of which consists of small barium ferrite subcrystals arrayed along its long axis, with each c-axis of the subcrystals perpendicular to the long axis.^^*^ The polycrystalline acicular barium ferrite particles were obtained by adding NaHC03 to a BaCl2 solution containing acicular a-FeOOH particles to deposit BaCOj onto the a-FeOOH particles, and subsequent calcination at 780 - 840 °C with a small amount of either B2O3, P2O3, or 31363 after washing the precipitate with water and drying. During the thermal treatment, Ba^^ ions diffused into the acicular a-FeOOH particles to convert them into the polycrystalline acicular barium ferrite particles of the unique structure.^^ Although the saturation magnetization turned out to be ca, 4.0 X 10'^ Wb m kg"^ (~ 32 emu g"^), corresponding to ca. 60 % of the saturation magnetization of ordinary platelet-type barium ferrite, it is noteworthy as a new approach of interest. The magnetic properties of the main ferromagnetic particles are summarized in Table 12.4.^^'^^'^^ Besides the above materials, iron nitride (mainly Fe4N), prepared by reducing dehydrated goethite particles in the presence of ammonia,^^ is known for its high saturation magnetization and high coercivity, comparable to iron. However, it does not appear to be used practically as a recording medium, at present, because of some problems in magnetic stability. Acicular magnetic particles are dispersed in a polymer solution and coated on a film base. The wet coating is then passed through an oriented magnetic field to align the coated particles with their polar axis in the running direction of the film base, dried, and calendered to smooth the surface. 12.3.4. Monodispersed Particles for Magnetic Recording Media Strict control of the size and shape of a given magnetic material is of vital importance for recording media, since its magnetic properties are mostly determined by the size and shape of the individual particles. Hence, monodispersed particles with a sharp distribution in size and shape are the ideal materials. In this case, there may be two different roles for monodispersed magnetic particles.

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12. INDUSTRIAL APPLICATIONS

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624

APPLICATIONS

The first is their role as ideal models for fundamental studies on definite relations between the moq)hological, dimensional, and structural characteristics and the magnetic properties. These data may be compared directly to the theoretical predictions. For example, Ozaki and Matijevic^^ obtained uniform ellipsoidal particles of y-FCjOj through a reduction-reoxidation process of uniform a-Fe203 of the same shape prepared by forced hydrolysis of dilute FeCl3 solutions in the presence of phosphate ions. The coercive force increases with reducing particle size and increasing aspect ratio in this size range, as expected from theories.'*^''*^'^^ While one may need more systematic data for concluding the optimum size and shape of maghemite particles as a recording material, the highest coercive forces are observed when the particle length is less than ca, 0.4 |xm with the aspect ratio of 5 - 6, and thus the upper size limit of single-domain particles is somewhat smaller than the theoretical prediction, ~ 0.9 |xm at the aspect ratio of 5, suggested by Morrish and Yu."*^ Also, while studies on the mechanisms of the reversal of magnetization of individual particles, in comparison with the theoretical models, are in progress,^^"^^^ monodispersed particles are expected to be useful for such experiments. For example, isolated uniform ellipsoidal particles of y-FcjOj converted directly from a Fe203 of the same shape were found by Lorenz microscopy to have a much higher coercive force of about 9.5 x 10"* A m"^ (~ 1200 Oe) for Y-Fe203 than in their close assembly.^°° The other role of monodispersed magnetic particles is, of course, to be actually used as the most advanced magnetic materials in the future. In fact, though we cannot produce uniform particles in condensed systems by using the conventional forced hydrolysis of metal salts, it is possible to produce monodisperse a-Fe203 particles systematicaUy controlled in size, shape, and internal structure, in large quantities, by use of the recently developed gelsol technique }^^ Studies on the systematic control of monodispersed yFcjOj particles and relevant magnetic particles, converted from precisely controlled a-Fe203 prepared by the gel-sol method, are now in progress, as partly presented earlier.^^^"^^ Such a systematic approach may lead us to the final conclusion on the ideal shape, size, and internal structure of the magnetic recording materials. Figure 12.21 shows some typical TEM images of uniform ellipsoidal maghemite of different sizes at a constant aspect ratio, converted from the corresponding hematite particles prepared by the gel-sol method from a solid precursor, p-FeOOH, containing phosphate ions in the interior of each particle.^^^ Figure 12.22 shows the relationships between the volume of a maghemite particle with different

12. INDUSTRIAL APPLICATIONS

625

0.2|im

Fig. 12.21. TEMs of monodispersed ellipsoids of maghemite (Y-Fe203) systematically controlled in size and shape. (From Ref. 104.)

aspect ratios and the coercive force. This result suggests that the maximum coercive force is located at a particle volume in the range of 7 x 10"^ - 1 X 10"^ ptm^, irrespective of the aspect ratio, and that the upper limit of the aspect ratio that gives the maximum coercive force is close to seven. If the aspect ratio is 5, the length of the principal axis giving the maximum coercive force is ca, 0.36 [im, which is close to a result of Ozaki and Matijevic,^ who found a maximum coercivity with ellipsoidal particles of ca. 0.4 \xm long with aspect ratios 5 - 6 . Meanwhile, the particle volume giving the peak of coercivity corresponds to the critical particle volume between the single-domain and multi-domain particles, which is expected to increase significantly with increasing aspect ratio according to the

APPLICATIONS

626 450 Aspect ratio = 7

400

Aspect ratio = 6 Aspect ratio = 5

350

Aspect ratio = 4

300 Aspect ratio = 3

O

--250

2 CD 200 "^

Aspect ratio = 2

O

O

150 Aspectratio= 1

100 50 h

1x10"^

1x10-5

1x10-^

1x10*3

1x10-2

1x10-^

Particle volume (^m^) Fig. 12.22. Relationships between the volume of amaghemite particle with different aspect ratios and coercivity (powder density in the gelatin binder = 2.26 g cm"^). (From Ref. 104.)

theoretical calculation based on the circular multidomain model (see Fig. 12.15).'^^ However, the above experimental results may imply that the actual dependence of the critical particle volume on the aspect ratio is, at least, much smaller than the theoretical prediction, while it is true that the critical size is a rather complex function not only of the aspect ratio, but also of the particle density and particle orientation. On the other hand, maghemite particles converted from nearly monocrystalline hematite particles are superior to those from polyorystalline hematite in coercivity. This fact may imply that the original crystal imperfections are retained in the final product, although XRD analysis revealed that the final particles, even if originated from monocrystalline hematite particles, became polycrystalline maghemite without changing their external shape during the reduction-reoxidation

12. INDUSTRIAL APPLICATIONS

627

Table 12.5. Magnetic properties of monodispersed y-FejOj particles controlled systematically in mean particle volume and aspect ratio Particle Length (^m)

Particle ^ Volume (^m )

1.0 1.0 1.0 1.0 1.0 1.0

0.03 0.05 0.08 0.11 0.14 0.20

2.70X10-^ 1.25X10"^ 5.12X10-^ 1.33X10"^ 2.74X10-^ 8.00X10-^

126 133 143 140 134 126

0.35 0.36 0.34 0.36 0.38 0.36

2.5 2.4 2.4 2.0 2.1 2.0

0.04 0.09 0.16 0.20 0.24 0.32

7.46X10-^ 5.56X10-^ 3.12X10'* 8.95X10-* 1.40X10-^ 3.67X10-^

195 225 256 204 174 131

0.39 0.47 0.54 0.51 0.50 0.49

2.7 2.9 2.7 2.6

0.10 0.18 0.22 0.29

6.71X10*^ 3.10X10-* 6.35X10r* 1.61X10"^

255 285 301 232

0.58 0.61 0.62 0.61

3.9 3.9 3.8 3.9 4.0

0.16 0.20 0.29 0.36 0.52

1.11X10-* 2.18X10"* 6.81X10* 1.32X10-^ 3.63X10*^

285 301 338 308 230

0.61 0.64 0.71 0.68 0.69

5.2 4.9 5.1 5.0 5.1 5.3 5.2

0.11 0.20 0.23 0.36 0.43 0.54 0.62

2.08X10'^ 1.41X10-* 1.98X10-* 7.91X10^ 1.29Xia^ 2.37X10-^ 3.73X10-^

275 305 325 379 322 236 236

0.58 0.65 0.68 0.71 0.69 0.67 0.68

5.8 5.9 6.0 5.8

0.15 0.26 0.36 0.46

4.24X10-^ 2.13X10-* 5.47X1 a* 1.22X10-^

290 347 392 312

0.62 0.69 0.72 0.69

6.7 6.9 6.8 6.8 6.8

0.24 0.36 0.42 0.52 0.68

1.30X10-* 4.13X10-* 6.75X10-* 1.28X10-^ 2.87X10-^

355 412 344 276 239

0.67 0.73 0.71 0.69 0.70

Aspect Ratio

Coercivity (Oe)

Squareness

628

APPLICATIONS

process. Main magnetic properties of these maghemite particles are summarized in Table 12.5. In response to the growing potential demand for much higher recording density and signal/noise ratio, the finer particles of excellent inherent properties, with sharp distributions in size and dimensional ratio, are required.

12A Catalysts One of the most important applications of ultrafine metal particles may be their use as catalysts, indispensable for selective and efficient production of intermediates or final products in the chemical industry. The properties of metal catalysts depend strongly on their particle size. There seems to be three main reasons for this: (1) Effect of specific surface area With increase m specific surface area (surface area per unit weight), the catalytic activity per unit weight generally increases. The increase in catalytic activity with size reduction down to ca, 10 nm or less is mainly due to this effect. (2) Effect of active sites With a reduction of particle size the relative proportion of active sites, such as comers and edges, per unit area of the metal particles increases, so that much higher catalytic activity or a highly selective reactions may be expected, particularly in a size range below 10 nm. (3) Quantum size effect As the particle size becomes less than 5 nm, the continuous electronic band structure of a metal starts to be separated into discrete energy levels of the molecular orbitals of a cluster consisting of a finite number of metal atoms (the quantum size effect).^°^ In this size range, the original properties of the bulk metal are lost, and it is not rare to find some new functions as the metal clusters become significant. Metal catalysts are used in the form of well dispersed particles in liquids or on some supports, and thus they may be classified into two categories. One is the dispersed catalysts in liquids, and the other is the supported catalysts on solids or solvated polymers.

12. INDUSTRIAL APPLICATIONS

629

12.4.1. Preparation and Uses of Metal Catalysts a) Dispersed Catalysts Condensation in Gas Phases When a metal is evaporated by heating with an electric resistance heater, plasma heater, high-frequency inductive heater, electronic beam heater, or arc heater in inert gases such as He, Ar, or Xe at pressures of 0.01 to a few hundred Torr, the evaporated metal atoms are condensed as ultrafine metal particles in the gas phase by collision with the inert gas molecules. They are collected in organic solvents or trapped in a frozen organic medium. For example, ultrafine metal particles of Be, Mg, Al, V, Cr, Mn, Fe, Co, Ni, Fe-Ni, Fe-Co, Cu, Zn, Ga, Se, Ag, Cd, In, Sn, Te, Au, Pb, Bi, etc. have been prepared by this method.^°^ In pure inert gases, they are mostly single-crystal particles with characteristic shapes. The mean size depends strongly on the kind of inert gas and the gas pressure. Figure 12.23 shows the relationships between the mean diameter of Al or Cu particles generated by condensation in gas phases and the pressure of an inert gas, such as He, Ar, or Xe.^°^ Obviously, the mean diameter diminishes in the order Xe, Ar, and He, and is proportional to the cube-root of the gas pressure, irrespective of the kind of the inert gases. In order to obtain particles with a relatively narrow size distribution, the temperature of the molten metal for evaporation

200

2

< 50

20h

z

< 0.5

1

2

5 INERT

GAS

to

20

50

100

200

PRESSURE [torr]

Fig. 12.23. Relationships between the mean diameter of Al or Cu particles generated by condensation in gas phases and the pressure of an inert gas, such as He, Ar, or Xe. (From Ref. 107.)

630

APPLICATIONS

must be kept constant. The mean diameter ranges from 10 to 200 nm. Similarly, it is possible to deposit metal atoms or clusters in a frozen trap of organic solvent from a gas phase, and obtain metal particles suspended in the solvent matrix by melting the frozen solvent. The mean size of the particles, ranging from a few nm to 1500 nm, depends on the kind of solvent and the degree of dilution.^°^ Thus-obtained Ni particles showed highly selective activity in the hydrogenation of 1,3-cyclooctadiene to cyclooctene^^^ and in the dehydrogenation of 2-propanol to acetone^^^ Ultrafine composite particles, consisting of Cu hosts of 15 to 45 nm with still smaller ZnO guests of 2 to 3 nm on each Cu host, were obtained by mild oxidation of bimetallic particles of Cu-Zn deposited from an inert gas phase, and showed almost the same activity and selectivity in the synthesis of methanol from CO and H2 as catalysts prepared by the conventional coprecipitation method.^^^ Precipitation in Liquid Phases A wide variety of metal catalysts are prepared in liquid phases, because it is rather easy to realize conditions for the preparation and stabilization of fairly uniform nanoparticles in liquid media. For example, metal particles of ca. 15 imi or less can be obtained readily by heating metal salts in an organic solvent such as tetrahydrofiiran with metallic potassium as a reducing agent.^^^ However, metal particles are normally lyophobic, and thus it is often necessary to use some protective colloid to stabilize them in liquid media. In aqueous solutions, hydrophilic polymers, such as poly(N-vinyl-2pyrrolidone) (PVP), polyvinyl alcohol (PVA), polyacrylamide, or polyacrylic acid, are used widely as protective colloids in the forms of their homopolymers or copolymers. Cationic surfactants such as dodecyltrimethylammonium chloride (DTAC), or tetradecylpyridinium bromide, anionic surfactants such as sodium dodecylsulfate (SDS), or sodium dodecylsulfonate, amphoteric surfactants such as N-dodecyl-N,N-dimethylbetaine, or N tetradecyl-2-aminopropionic acid, and nonionic surfactants such as hexaoxyethylene dodecyl ether orpolyoxyethylene(20) sorbitan monolaurate, are also used as anticoagulants. Metal ions are normally reduced in the presence of one of these protective agents. For reduction of metal ions, hydrogen gas, sodium borohydride (NaBHJ, citric acid, or alcohol, are used. In particular, when alcohol is used as a mild reducing agent in the presence of PVP or PVA, small and fairly uniform noble metal particles, such as Pd,

12. INDUSTRIAL APPLICATIONS

631

Rh, Pt, Ir, or Os, of mean diameter ranging from 1 to 6 nm, can be obtained.^^^ Also, irradiation with visible light plus ultraviolet rays^^"^ for Pt^*, with ultraviolet rays^^^'^^^ for Ag% Au^, or Rh^*, or with y-rays^^'^^'^^^ for Ir^* or Pt"** is useful for reduction of these metal ions to produce the corresponding still smaller particles. SpecificaUy, y-rays can be used for the preparation of base metal particles as well, such as Co, Ni, Zn, Pb, or Cu-Pb alloys.^^^ It is known that y-rays can generate solvated electrons as a reducing agent of metal ions through ionization of solvent molecules. If we apply NaBH^ to iron, cobalt, or nickel ions, the respective ultrafine metal boride particles are obtained.^^ Platinum particles of ca. 2 nm prepared in a solution of DTAC by irradiation with UV-VIS light from a super-high-pressure mercury lamp showed a much higher activity than those prepared by reduction with alcohol or hydrogen gas in hydrogenation of vinyl acetate, ^^"^ Dispersed Rh particles have specially high activity for the hydrogenation of olefins, so they selectively hydrogenate only the double bond of olefins when other kinds of double bonds, such as aromatic, carbonyl or nitro groups, or C-C triple bonds coexist with an aliphatic double bond in a compound.^^^ This is in contrast to Rh metal particles supported by active carbon which hydrogenate not only olefins but also a large proportion of other kinds of double bonds.^^ Dispersed Rh particles of very small size, such as 1 nm or less, are particularly effective in the hydrogenation of inner aliphatic double bonds rather than terminal ones, while this is normally difficult for supported Rh particles or large dispersed particles, owing to the steric hindrance.^^ NijB particles dispersed in liquid are also active in the hydrogenation of olefins, but are particularly effective in the hydrogenation of terminal methylenes.^^ For example, D-limonene and a-methylstyrene are hydrogenated only at their isopropenyl group by NiB catalyst. Pd particles of mean diameter 2.5 nm, prepared by reduction of palladium chloride with alcohol in a solution of polyvinylpyrrolidone (PVP), hydrogenate only one C-C double bond of the two double bonds of cyclopentadiene, 1,3-cyclooctadiene, or 1,5-cyclooctadiene, while commercially available Pd/active carbon catalysts catalyze the total hydrogenation of these dienes at the same time.^^ Bimetallic nanoparticles of Pd-Pt and Pd-Au, prepared in a similar manner, showed higher catalytic activity than pure Pd particles in the selective hydrogenation of cyclic dienes.^^'^^^ The selective hydrogenation of cyclic dienes can be achieved also with Ni particles prepared by deposition from inert gas phases.^^^ Pd

632

APPLICATIONS

particles dispersed in PVP solution are also active in selective hydrogenation of one double bond of two in a linoleic acid molecule,^^ and those suspended in PVA or gum Arabic solution were found to be active in the hydrogenation of the nitro group, with no effect on aromatic and carbonyl double bonds and carbon-halogen bonds, in nitrobenzene, halogenated nitrobenzene, and nitroacetophenone.^^^ Ir particles suspended in PVA or gum Arabic solution have a similar selective activity in the hydrogenation of nitro groups.^^^ Cu particles of ca, 10 nm suspended in PVP, PVA, or dextrin solution can be used as a catalyst to hydogenate only the cyano group of unsaturated nitriles, such as acrylonitrile and methacrylonitiile, without forming ethylene cyanohydrin.^^° b) Supported Catalysts Although the uniformly dispersed catalysts have the general advantages of high efficiency in catalytic reactions owing to the free diffusion of the catalyst particles and the prompt dispersion of the reaction heat and products, there are some difficulties in handling for repeated use and in the complete prevention of coagulation during reaction and recovery of catalysts. Also, they are unsuitable as catalysts for gas-phase reactions. Hence, many catalysts are used practically with some appropriate supports or carriers. In some cases, the carriers play a decisive role in the catalytic reaction in consort with the metallic parts. A great variety of methods are known for fixing metal particles on solid supports, such as silica, alumina, silica/alumina, hematite, titania, zirconia, active carbon, or zeolite. For instance, metal atoms or clusters deposited in a frozen solvent at 77 K from a gas phase can be transferred uniformly onto oxide supports.^^^'^^^ Also, metal ions are fixed on the extemal and/or internal surfaces of solids, by soaking the solids in solutions of the metal salts, and then reduced by some reducing agents.^^^ In this case, if organometallic compounds are in contact with solid supports from gas or liquid phases and fixed on the supports by heat treatment for reduction or oxidation, we often obtain fairly uniform metal or metal oxide nanoparticles for catalysts.^^ It is also possible to fix highly dispersed metal particles on an oxide support by coprecipitation of hydroxides of different metal ions for catalyst and support in an alkaline range, followed by dehydration and heat treatment with hydrogen gas for reduction of the metal ions. However, in the preparation of gold catalysts supported on oxides such as a-FejOj (hematite), C03O4, or NiO, Au particles of less than 10 nm

12. INDUSTRIAL APPLICATIONS

633

of fairly narrow size distribution can be obtained only by heating a dried mixed precipitate of auric and ferric hydroxides at ca, 400 °C in air.^^^ Ueno et al prepared highly dispersed ultrafine Ni on Si02^^^*^"^'^^ and Co on TiOj^^^^^^ by hydrolyzing Si- or Ti-alkoxides with Ni(N03)2 or Co(N03)2 in ethylene glycol at 80 °C, followed by calcination in air and then in hydrogen atmosphere. The formation processes were probed through EXAFS spectroscopy.^^^^^^'^^^ In the preparation of photocatalysts such as Pt/TiOj, ultrafine Pt particles can be deposited on TiOj particles by UVirradiation to a Ti02 sol containing H2PtCl6, in which Pt(IV) ions are reduced by photoelectrons to Pt nanoparticles on the surfaces of Ti02 particles.^^^ In place of these inorganic solids, one may use polymer gels^^^ or ion-exchange resins^^^*^'*^ as supports. Instead of direct support of metal particles by polymer matrices, one may indirectly fix metal particles to support polymers by forming complexes or chemical bondings between support polymers and protective polymers holding metal particles.^"*^"^"*^ Rh-Fe,'^^'^ Rh by pyrolysis of Rh carbonyl clusters,'^^^ Rh-Zr,'^^" RhZr-Mn^'*^'' catalysts supported on SiOj, ZrOj, TiOj, etc. are used for production of C2H5OH from CO and H2 in the gas phase. Similarly, we use Rh-Mn,^"'^ Rh-Mn-Na,'"'" Rh-Mn-Mg,^"'^ and Rh-V-Li^'^' on Si02 for production of CH3CHO, and Rh-MgClj^^^ on SiOj for production of CH3COOH fi:om CO and H2. Rhodium on oxide supports is used as a catalyst for reduction of NO^ in the exhausts of automobiles.^^^ On the other hand, Rh particles fixed on hydrophilic polyacrylamide gel showed relatively high activity in the hydrogenation of olefins with hydrophilic groups. Particularly, the activity of the resin-supported Rh catalyst was much higher than dispersed Rh catalysts for hydrogenation of olefins with carboxyl groups, such as acrylic acid and 3-butenoic acid, having a strong affinity to the amino groups of the polymer gel.^"*^ The catalytic activity was kept almost constant even after repeated use and recovery. Resin-supported Pd catalyst particles of 1 - 6 nm in diameter showed a high activity in the selective hydrogenation of cyclooctadiene to cyclooctene, like dispersed Pd particles.^^ RU/AI2O3 catalyst shows an especially high activity in the FischerTropsch synthesis for formation of CH4 from CO and H2, while most transition metals have more or less activity in this reaction.^^^ In addition, Fe, Ni, and Co on AljGj,^^^ and Ni on T\0^^^ and on c-FejN^^^ are wellknown catalysts for the same purpose. Also, Ni/SiOj catalyst prepared from nickel ethylene glycoxide and tetraethyl silicate by the sol-gel procedure showed a highly selective activity

634

APPLICATIONS

in the hydrogenation of propionaldehyde to l-propanol with drastically lowered probability of decarbonylation, when the mean diameter was less than 5 nm.^^^(^)'^^^ Cii/oxide is used as a catalyst for the formation of H2 from CH3OH and Ag/oxide is used for the oxidation and dehydrogenation of CH3OH to produce HCHO when methanol is in excess, while MoOj-FCjOj/oxide catalyst is used when oxygen is in excess.^^^ Au nanoparticles of less than 10 nm on oxides such as a-FCjOj were found to be specifically active in the oxidation of CO.^^^ The specific activity of Au/a-Fe203 catalyst for oxidation of CO was explained in terms of the acceleration of the oxidation of CO on a-Fe203, whose rate-limiting step is the adsorption process of CO, by the Au parts acting as adsorption sites for CO. Co/SiOj catalyst, prepared by deposition of Co clusters with solvent vapor and subsequent contact with Si02 supports, is used as a catalyst for the hydrogenation of 1-butene. The activity was found to be raised about 100 times by the addition of a small amount of Mn, probably due to the zero-valent cobalt clusters highly dispersed by the presence of Mn, as revealed by EXAFS analysis.^^^^^'^"> Pt catalysts supported on nylon were found to be powerful in the hydrogenation of benzene solely to cyclohexane.^^^ Pt catalysts supported on polymers through covalent bondings with protective colloids were proved to have a few tens-of-times higher activity than conventional Pt catalysts supported on active carbon in the hydrogenation of olefins, such as cyclohexene and ethyl vinyl ether.^"*^ Pt/AljOs catalysts are also used for the oxidation of CO and paraffins. The composition of the exhausts of automobiles is normally controlled to be nearly in stoichiometric balance between gases to be oxidized (hydrocarbons, CO, and H^ and oxidizing gases (O2 and NO^^), so that we can minimize hydrocarbons, CO and NO,^ using some catalysts to promote the reactions between the reactive gases, such as a triple catalyst consisting of Pt, Pd, and a small amount of Rh on AI2O3. Both Pd and Pt particles are useful for oxidation of hydrocarbons and CO, but Pd is particularly effective in the oxidation of CO, olefins, and methane, while Pt oxidizes more efficiently paraffins of C3 or higher. There is almost no difference between Pd and Pt in activity for the oxidation of aromatic hydrocarbons.^^^'^^^ A smaU amount of Rh is indispensable for reducing NO^.^"*^ Pt/TiOjCa)^^ and Pt/RuGj/TiOjCa)^^^ are known as typical catalysts for

635

12. INDUSTRIAL APPLICATIONS

the photolysis of water, where (a) indicates anatase-type Ti02 particles. The Pt parts of the composite particles of Pt/Ti02 may play the role of active sites for electron transfer from the conduction bands of the Ti02 particles to protons. A high quantum yield such as 30 % has been reported for a system of Pt/Ru02/Ti02(a) in a HCl solution of pH 1.5.^^^ The photocatalysts are also useful for the photolysis of water and alcohols, such as CH3OH, C2H5OH, n-C^HjOH, /-C3H7OH, and AZ-QH^OH, to produce H2 and CO2, where the reactivities of these alcohols are in this order.^^^ In the photolysis of alcohol with H2O, it is not difficult to attain a quantum yield of 40 % or higher.'^^ 12.4.2. Size Effects According to the definition of the turnover frequency (TOP) as the number of reacted molecules per unit number of surface metal atoms and unit time, metal catalysts may be classified into the following four groups: (1) stmcture-insensitive catalysts of a TOP constant with change of particle size; (2) structure-sensitive catalysts of TOP decreasing with increasing particle size; (3) those of TOP increasing with increasing particle size; (4) those of a maximum TOP at some particle size, as illustrated in Pig. 1224}^ Even if the TOP is constant with the change of the size of metal

\ \

\

\

/

/

^^ / \

\

y

/ \

(JU.

0 H

/

/

\ 4 \

\

\

\

"1 / y 1 / / /

\

-3 *»«-* •v^

-1 - 2

Size

Fig. 12.24. Typical four patterns in TOP changing as a function of particle size. (Prom Ref. 164 (c).)

636

APPLICATIONS

particles, the specific surface area of the metal particles, or the surface area per unit weight, is inversely proportional to the mean particle diameter, so that the activity of a catalyst per unit weight is always more or less dependent upon the particle size. Also, it is not rare to find size-dependent selectivity in catalytic reactions. In this sense, the strict control of a mean size and a sharp size distribution are strongly desired for a given catalytic material. a) Size Control of Metal Particles Although the size distributions of most nanoparticles of metal catalysts are not sharp enough to clear the criterion of monodisperse particles - that the coefficient of variation of the size distribution must be 10 % at the most - strenuous efforts have continued to approach the utmost level. As referred to eariier, rather mild reduction conditions are adequate for producing fairiy uniform metal nanoparticles with mean diameters of 6 nm or less; e.g,, reduction of metal ions by alcohols^^^ or still milder reduction by irradiation with UV-rays^^^'^^^ or y-rays^^"^'^^^, in the presence of protective colloids such as PVP, PVA, or DTAC. The mild conditions and the use of protective colloids are in accord with the general requirements for the formation of uniform particles; z.e., (1) complete separation between the nucleation and growth stages, and (2) the inhibition of coagulation during the growth stage. Occlusion of metal ions in a rather rigid gel-matrix of silicon hydroxide by the sol-gel procedure and subsequent reduction with hydrogen in the dried gel,^^^ or uniform dispersion of metal particles on the surface of a metal oxide support at a relatively low temperature on melting a frozen organic solvent such as «-hexane containing metal clusters previously trapped from a gas phase at 77 K,^^^'^^^ are also known as methods giving well dispersed and relatively uniform catalyst particles. Coagulative growth is minimized in both cases. It is also possible to control particle size using the interaction of metal ions or complexes with the surface of the support. For example, it is hardly necessary to say that the mean size of metal particles on a support generaUy becomes smaller with increasing specific surface area of the support when the metal particles are formed by gas-phase reduction of metal ions impregnated in a dry support powder.^^ Moreover, if the surface charge of a support is negative over a wide range of pH, as with silica, one may be able to deposit a positive complex of metal ions more uniformly than a negative one, and thus minimize the agglomeration of metal ions during calcination in air prior to the reduction with hydrogen. For example,

12. INDUSTRIAL APPLICATIONS

637

Dorling et al}^^ obtained significantly smaller Pt particles on silica gel, when they used Pt(NH3)/^ complex in place of PtCl/", to be adsorbed to the silica gel support in the first process. On the other hand, if we use porous supports of different but definite pore sizes, such as many kinds of zeolite, we will be able to control the size of metal particles formed in the pores according to the pore size and the total surface area of the support, including the interiors. For instance, if we use Y-type zeolite as a template for the formation of Pt particles, then Pt agglomerates of less than 1 nm are formed in the supercages, owing to the limitation of the size of the supercages, while Pt crystallites of 1.5 to 2.0 nm are formed on the external surfaces of the zeolite.^^^ b) Size Effects on the Electronic States of Metal Particles As the size of a metal particle is reduced, the ionization potential, electron affinity, Fermi level, polarizability, magnetic susceptibility, etc. are changed, reflecting the change of the electronic structure from the bulk metal. From the extended Hiickel- and CNDO molecular orbital calculations for Agn clusters, it has been shown that the energy levels of the LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) vary in a zigzag manner with cluster size, indicating that clusters with even numbers of atoms have greater ionization potentials than those with odd numbers of atoms, as illustrated in Fig. 12.25a.^^^ The sawtooth behavior, or the odd-even effect, can actually be observed in the experiment of Kr^ sputtering on Ag metal.^^^ The calculated energy gap between the HOMO and LUMO of Ag clusters decreases gradually with increasing cluster size, as shown in Fig. 12.25b.^^^ The odd-even effect is due to the presence of two electrons in the HOMO for Ag clusters with even numbers of atoms and an unpaired electron in the HOMO for clusters of odd numbers of atoms. Hence, the odd-even effect is observed for Na^ clusters as well,^^° but not for Pd clusters consisting of atoms with two valence electrons. If photoelectrons are ejected from inner shells, the positive holes generated are screened by neighboring electrons in a large metal particle. However, this extra-atomic effect is not fully effective in a small metal cluster owing to the discrete molecular orbitals, and thus the binding energy of the inner-sheU electrons of the latter is greater than that of a bulk metal.^^^ For example. Table 12.6 shows the effect of particle size on the binding energy of the 4fj^ level of Au particles on amorphous carbon.

638

APPLICATIONS

a

s

d

Stoles

Stotes

Number of otoms

Fig. 12.25. (a) HOMO and LUMO of Ag as a function of cluster size; (b) Energy gap between HOMO and LUMO of Ag as a function of cluster size. (From Ref. 168.) relative to that of the bulk metal, from X-ray photoelectron spectroscopy (XPS).^^^ The kinetic energy of the conesponding Auger electrons is hence reduced with reducing particle size.^"^ If electron transfer from metal particles to the support occurs, the binding energy of the valence electrons of the metal particles is also expected to increase. Figure 12.26 shows X-ray photoelectron spectra of the 4f-

639

12. INDUSTRIAL APPLICATIONS

Table 12.6. Effect of particle size on the binding energy of the XPS Ai-^,^ level of Au particles on amorphous carbon relative to that of the bulk metal {Source: Ref. 172) Sample

Coverage (10*' atoms/cm^)

Average Area Diameter

Width of Area Distribution

(A)

(A)

0

4f7^ Level Shift (eV)

A

0.45

15

8

+ 0.7

B

1.1

25

15

+ 0.2

C

3.5

60

30

+ 0.2

D

5.1

100

45

+ 0.12

(o)

^^-"

•••w

••~.^_^__^ j "

(b)

*-.

[.....„>•.• '

'

-

-

:

'

-

•

•

'^^^^•'•'-^•--—A-1

4-

1

1

75

73

71

Binding energy (eV)

Fig. 12.26. XPS of the 4f-electrons of Pt foil (a) and Pt/Si02 (b). (From Ref. 173.) electrons of Pt foil and Pt nanoparticles (1.2 - 2.0 nm) on SiOj.'^^ The 1.5 eV higher binding energy of Pt on Si02 than of Pt foil was elucidated in terms of the electron transfer from Pt to Si02. Figure 12.27 shows ultraviolet photoelectron spectra (UPS) of Pd nanoparticles of various sizes on Si02.^^'* We can detect the spectrum proper to discrete Pd atoms at ca. 2.9 eV below the Fermi level, when the surface density of Pd atoms reaches the level of 0.3 x 10^^ cm"^ (b). The characteristic spectrum of Pd metal around 2 to 3 eV becomes detectable from ca, 2.0 X 10^^ cm"^ {ca, 2 - 3 nm) (d), and becomes almost the same as that of the bulk metal at 14.0 x 10^^ cm"^ (h). This figure may suggest that the metallic properties of paUadium become significant as the size exceeds ca, 2 - 3 nm.

640

APPLICATIONS

- 1 — I — I — I — r

5 ^ 3 2 1 Ep Energy below Ep (eV)

Fig. 12.27. UPS of Pd nanoparticles of various sizes on Si02 in terms of the number of Pd atoms per cm': (a) 0, (b) 0.3, (c) 0.7, (d) 2.0, (e) 3.3, (f) 6.8, (g) 9.8, (h) 14.0 X 10^^ (From Ref. 174.)

Greegor and Lytle^^^ inferred the shapes and sizes of metal particles of less than 2 nm on different oxide supports from EXAFS data on the average coordination numbers of the metal atoms, around 7 - 10 - considerably smaller than the value 12 for closely packed atoms, as shown in Table 12.7. The low coordination number, corresponding to a large number of unsaturated surface atoms, is directly relevant to the catalytic activity of these particles. As has been described above, there are two types of Pt particles on the Y-type zeolite; Le,, agglomerates of less than 1 nm and crystallites of 1.5 - 2 nm. It is known that the IR stretching absorption of NO molecules adsorbed to the Pt agglomerates is at 1828 cm~\ while that adsorbed to Pt crystallites is at 1825 cm'\ suggesting a cationic nature of the small agglomerates owing to electron transfer from the agglomerates to the zeolite support. When electron transfer from the small Pt agglomerates to the Jtorbitals of NO is reduced due to the lowered electron density in the agglomerates, the frequency of the IR stretching absorption of NO generally becomes higher.^^^ The increasing frequency of IR absorption with

641

12. INDUSTRIAL APPLICATIONS

Table 12.7. Shapes and sizes of metal particles on Si02 or AI2O3 supports, estimated from EXAFS data on their coordination numbers and mean diameters (Source: Ref. 175) Catalyst

Average radius of Metal Particles (A) Disks (minihedra)

Os on Si02 Ru on Si02 Cu on Si02 Pt on Si02^ Pt on Al203^ IT on Si02 IT on AI2O3

Cubes (minihedra)

Spheres (maxhedra)

7 ± 2 (6)"

7±3 5±2

6±2 5±2

14 ± 7 (18)" 19 ± 5 5±2 4±3 6±2 6±3

T h e values in parentheses indicate weighted average sizes taken from electron microscopy. ^ The Shape trend is uncertain and may indicate distribution of shapes.

2 3 Diameter (nm) Fig. 12.28. IR spectra of NO molecules adsorbed to Pt particles on alumina. (From Ref. 167.)

642

APPLICATIONS

decreasing particle size is also observed in NO molecules adsorbed to Pt particles on AljOa,^^^ as shown in Fig. 12.28. These results may suggest that NO molecules can be used as a probe of the electronic state of metal catalysts. c) Size Effects on Activity and Selectivity

TERRACE MONATOMIC STEP STEP-ADATOM

TERRACE VACANCY

Fig. 12.29. Schematic model of a heterogeneous solid surface. (From Ref. 176.) Somoqai^^^'^^^ showed, using single crystals of Pt with surfaces of different lattice indices, that the activity of Pt for hydrogenation of hydrocarbons associated with the scission of C-C bonds was proportional to the number of kinks, or steps, of crystal surfaces of Pt (see Fig. 12.29), while the activity for dehydrogenation associated only with scission of C~H bonds was independent of the number of kinks or steps. Since the kinks and steps may correspond to the comers and edges of small particles, hydrogenation with scission of C-C bonds or dehydrogenation with cyclization is expected to be enhanced with decreasing size of metal catalysts, owing to the dramatic increase in the fractions of comer atoms and edge atoms on the surfaces, as illustrated in Fig. 12.30,^^^ while dehydrogenation or hydrogenation without scission or formation of C-C bonds is independent of particle size in many cases. On the other hand, since sufficiently large metal particles consisting of an ample number of metal atoms are required for dissociation of some adsorbed molecules such as C=0 and N=N, the activity of catalysts in terms of TOF (tumover frequency) is generally lowered with reducing particle size in the hydrogenolysis of these triple bonds for ammonia and FischerTropsch syntheses with Ni or Fe catalysts.^^^^ Catalytic activity is often reduced, when the particle size is so small that the metaUic properties are

12. INDUSTRIAL APPLICATIONS

643

i ;

20

25

Fig. 12.30. The numbers of comer atoms (€4^'^°), edge atoms (Cj^, and plane atoms (Cg^) of an octahedral metal cluster, relative to the total surface atoms, as a function of the dimensionless cluster diameter, d^^i (= diameter of a sphere of the same volume relative to the atomic volume). (From Ref. 178.)

lost by electron transfer to the support, or when the space for a dynamic process of large molecules on a metal cluster is too small. Typical examples of the four groups of catalytic reactions in Fig. 12.24 are as follows. (1) Reactions of a Constant TOP with Size Change Oxidation of Hj to H p with Ft/Si02'^^'^ Hydrogenation of ethylene to ethane with Pt/Al203^^^ Hydrogenation of benzene to cyclohexane with Pt/Al20

179

Dehydrogenation of cyclohexane to benzene with Pt/Al20

180

Hydrogenation of methylcyclopentane to n-hexane with Pt/Al203^^^ (2) Reactions of an Increasing TOF with Size Reduction Hydrogenation of ethane to methane with Ni/Si02-Al203^^^

644

APPLICATIONS Hydrogenation of pentane to CH4, €2^^ and CjHg with Rh/Al203^^^ Dehydrocyclization of n-heptane to toluene with Pt/Al203^^ Hydrogenation of propionaldehyde to 1-propanol with Ni/Si02^^ 185 Isomerization of2-methylpentane to 3-methylpentane withPt/Al20 L2v^3

(3) Reactions of a Decreasing TOF with Size Reduction Oxidation of CO with Pt/Si02^^^ Synthesis of CH4 from CO and H2 with Ni/Si02^^^ Synthesis of C^H^ from CO and H2 with Ru/Al203^^^ or Co/Al203^^^ Synthesis of C2H5OH from CO and H2 with Rh/Si02^^° Synthesis of NH3 from N2 and H2 with Fe/MgO^^^ (4) Reactions of a Maximum TOF with Size Change Isotope exchange between H2 and D2 with Pd/C, Pd/Si02 (1.3 nm)^^Hydrogenation of benzene to cyclohexane with Rh/Si02 (1.8 nm)^^^ Hydrogenation of benzene to cyclohexane with Ni/Si02 (1.2 nm)^^"* Here, the figures in parentheses for the reactions in group (4) indicate the sizes at the respective maximum activities. The size-dependent catalytic activity leads to a size-dependent catalytic selectivity. For example, the decarbonylation of propionaldehyde with the Ni/SiOj catalyst is dramatically suppressed, and 1-propanol is produced selectively when the size of the Ni becomes less than 5.0 nm.^^ This has been explained by the high activity of Ni atoms at the comers and edges for hydrogenation of aldehydes. Further, it is known that cyclization and structural isomerization become. predominant in the hydrogenolysis of whexane with an ultrathin membrane Pt catalyst, when the size of the Pt

12. INDUSTRIAL APPLICATIONS

645

crystallites becomes less than 2 nm. This is also believed to be a consequence of the specific action of the comer atoms and edge atoms of Pt crystallites.^^"^ 12.4.3. The Roles of the Support Supports for ultrafine metal catalysts may have been employed originally to permit the use of unstable and elusive nanoparticles repeatedly in a well dispersed state. However, the roles of supports for heterogeneous catalysts are not limited only to these basic aspects, but they also play decisive roles in the dramatic expansion of the versatile functions of catalysts. For example, they act as controllers of mean size and size distribution of metal crystallites in their preparation, stabilizers of the metal particles, adsorption media in catalytic reaction, electronic-state controllers, cooperating catalysts, etc. a) Controller of the Mean Size and Size Distribution of Metal Particles The factors of supports, responsible for controlling the mean size and size distribution of metal particles, may be summarized as follows. (1) Effect of the Specific Surface Area Inorganic solids and organic polymers or resins, used as supports for metal catalysts, normally have large specific surface areas (surface areas per unit weight) to load as many highly dispersed metal crystallites as possible. The final size of the metal particle is generally reduced as the specific surface area of the support increases, as long as the preparation conditions are otherwise the same and the nucleation of the metal particles occurs on the surface of the support, because of the enlarged surface area for nucleation of a fixed amount of metal. If the metal particles are formed in a liquid in the presence of a support, this characteristic may be used as evidence of heterogeneous nucleation of metal particles on the surface of the support, since, otherwise, the particle size must be independent of the surface area of the support. If precursor metal ions impregnated in a dry support powder are reduced by hydrogen gas, the size of the metal particles must reduce with increasing specific surface area of the support, since the nucleation of metal particles undoubtedly occurs on the surface of the support. Figure 12.31 shows an example of the effect of the specific surface area of a silica support on the mean particle size of Pt particles prepared on the support by reduction of Pt ions impregnated in silica powders of different specific surface areas.^^^

APPLICATIONS

646

200

400 SILICA. AREA (fn2/q)

bOO

Fig. 12.31. Effect of the specific surface area of a silica support on the mean crystallite size of Pt prepared by reduction of Pt ions impregnated in silica powders of different specific surface areas: sizes from CO chemisorption (A), X-ray data (v), and average of both measurements (o). (From Ref. 166,)

(2) Effect of the Surface Roughness The effect of the surface roughness of a support may be difficult to distinguish from the effect of the specific surface area when the individual particles of a support powder are partly sintered to form a porous agglomerate. However, unevenness of the order of the size of metal particles may have a substantial effect on the stability of the metal particles, since concave parts of a rough surface may hold and greatly stabilize the particles. For example, the gold and platinum particles prepared on platelet-type hematite particles with smooth surfaces in Figs. 7.44 and Fig. 12.42 are particularly large as compared to those on the other supports with rough surfaces. Similar result was also obtained with nickel particles on different hematite supports.^^^ Fig. 12.32 shows the effect of the surface roughness of hematite supports on the catalytic activity of Ni particles for hydrogenation of 1octene. Here the nickel particles were prepared by reducing nickel acetylacetonate with NaBH4 in the presence of spindle-type, pseudocubic, or platelet-type hematite particles (Ni/hematite molar ratio = 1/20) in 2 propanol at its boiling temperature, 355 K, for 10 min. The mean diameter of the Ni particles on the spindle-type or pseudocubic hematite particles with rough surfaces was ca, 4 nm, while it was ca, 20 nm for Ni particles on the platelet-type particles with smooth surfaces proper to their single crystal structure. The nickel particles on smooth-surface platelet-type hematite are not only originally low in catalytic activity, because of their large particle size, but also lose their catalytic activity during the hydrogenation reaction, because of their further growth. (3) Effect of the Pore Size

647

12. INDUSTRIAL APPLICATIONS 20

T—I—I—I—I—\—I—I—I—r O

:Spindle

A

'.Pseudocube

•

:Platelet

J

0

20

40

60

80

100

Time on Stream (min.)

i

L

120

Fig. 12J2. Effect of the surface roughness of hematite supports on the catalytic activity of Ni particles for hydrogenation of 1-octene in 60 cm^ of 2-propanol. Reaction conditions: 5.0 x 10'^ mol of Ni; molar ratio of [Ni]/[a-Fe203] = 1/20; 3.2 X 10"^ mol of 1-octene; flow rate of H2 = S.OxlO"^ mol h'\ temperature = 82 ®C. (From Ref. 195.)

If Rh^^ ions are incorporated in the supercages of NaY zeolite (internal diameter = 1.3 nm; entrance diameter = 0.7 nm) by ion exchange, and reductively carbonylated with CO and H2, a Rh-carbonyl cluster, Rh6(COX6, of ca. 1 nm in diameter can be formed in the supercages ("ship-in-bottle synthesis" - see section 7.3.10).^^^ One can obtain corresponding Rh^ nanoparticles in the supercages by oxidizing the Rh carbonyl clusters with O2 to remove the coordinated CO, followed by reduction with Hj.^^^ If Pt ions are introduced into a NaY zeolite and treated with CO plus H2O, one can obtain a cluster, [Pt3(CO)6]/- (n = 3, 4), corresponding to the pore size of the NaY zeolite.^^ Also, if we use FSM-16 (folded-sheet mesoporous material - 16), having a honeycomb structure with pores of 2.7 and 4.7 nm, in place of NaY zeolite, carbonyl clusters, [Pti5(CO)3o]^', are formed in the pores.^^^ These Pt carbonyl clusters can be reduced to the corresponding Pt nanoparticles with hydrogen.^^^ When the Pt ions incorporated in FSM--16 were irradiated with light or y-rays in the presence of 2-propanol and water, Pt nanowires were found to grow along the channels of the cylindrical pores.^° These Pt nanoparticles and nanowires are particularly active catalysts in the gas shift reaction of CO + H2O -* CO2 + H2. Figure 12.33 shows a scheme for the formation of the nanowires and nanoparticles of platinum in FSM-16.^°°^> If we use these kinds of templates of a definite pore size, we will be able to obtain ideally uniform nanoparticles of a predetermined size. In these systems, however, the size of the internal catalyst particles is invariably determined by the pore size of a given support. Hence, if we can control

648

APPLICATIONS

evacuated at 473K -CO

Nano Particles

Fig. 12.33. Scheme of the formation of the nanowires and nanoparticles of platinum in FSM-16. (From Ref. 200 (b).)

the relative size of the internal metal particles, independently of the pore size to some extent, for example, by changing the size of the ligands of the cluster complexes in the pores, we could use this technique for the preparation of a tailor-made catalyst with an optimum particle size and reaction space suitable for each specific reaction. (4) Effect of the Affinity to Metal Particles Cusumano et al}^ investigated the relationship between the catalytic activity of Pt catalysts for the dehydrogenation of cyclohexane and the degree of dispersion of the Pt particles on the support, using Pt/AljOj and Pt/SiOj AIjOj catalysts. They found that Pt particles on aluminum were dispersed to a much greater extent than on silica-alumina, from the measurement of chemisorption of hydrogen. The mean sizes of these Pt particles were calculated to be ca, 1 nm or less and 8.5 nm, respectively, on the assumption of a cubic shape. However, from the comparison of the degree of dispersion with the catalytic activity for these catalysts, they concluded that the TOP was essentially unaffected by differences in the supports used. They prepared the Pt catalysts by impregnation of alumina of specific surface area 295 mVg or silica-alumina (13% AI2O3, 87% Si02) of s. s. a. 450 m^/g with aqueous chloroplatinic acid solution, followed by drying and subsequent calcination in air for 1 h at 540 °C. As the specific

12. INDUSTRIAL APPLICATIONS

649

surface area of the former is rather smaUer than the latter, the higher dispersion of Pt particles on alumina may be attributed to the greater affinity of Pt to the alumina than to the silica-alumina. (5) Effect of the Affinity to Metal Complexes Figure 12.34 shows the relationships between their content (wt %), F, and the specific surface area, 5, for Pt particles on a silica gel support prepared by reducing Pt ions with hydrogen in gas phase after deposition of Pt ions by ion exchange with [PtCNHj)^^* or by impregnation with PtCl^^".^^^ As mentioned earlier (section 12.4.2), the positively charged [Pt(NH3)4]^* complex is adsorbed uniformly to the negatively charged silica gel surface. Hence, the great difference between the two cases in the behavior of the total surface area of Pt particles changing with Pt content may reflect the great difference in the state of deposited Pt ions before reduction with hydrogen. That is, in the case of ion exchange with [Pt(NH3)4]^^, the apparently linear relationship between S and V may suggest that the Pt particles were formed from uniformly dispersed Pt ions adsorbed on the negative sites of the support. On the other hand, in the case of impregnation of PtClg^", the downward deviation from linearity of S vs V may suggest direct deposition of aggregated clusters of the metal salt formed in the course of condensation of the covering solution with drying, because the metal ions are not adsorbed to the support beforehand. As a rule, the random coagulation of clusters in a liquid phase, which is dramatically enhanced by condensation, results in large agglomerates with a broad size distribution.

4

6

PLATINUM CONTENT (wt «/o)

Fig. 1234. Surface area of R particles per unit weight of catalyst, 5, against Pt content (wt %), K, for the ion-exchange and impregnation methods. (From Ref. 166.)

650

APPLICATIONS

If the final number of metal particles per unit area of support is proportional to the surface density of metal ions, the mean particle volume is kept constant with the change of the surface density of metal ions. In this case, the surface area of the metal particles per unit weight of catalyst, 5, is proportional to the total metal content, V. Thus the linear relationship for ion exchange in Fig. 12.34 indicates that the number density of the Pt particles was proportional to that of Pt ions. Hence, the downward deviation of 5 vs F for impregnation means that the number density of Pt particles was lower than expected from the proportionality. If one considers the state of Pt ions in the case of impregnation, one may readily understand the result in Fig. 12.34 for this case. However, the surface density of Pt particles, proportional to the loaded Pt ions in the case of the ion exchange, may need some theoretical elucidation. As we cannot find this in the original literature, some theoretical approach to this aspect has been attempted for this book in particular, in the Appendix entitled Growth of metal particles on supports by stepwise coalescence (see the last part of section 12.4). From this theoretical approach, if the vapor pressure of the bulk metal at the given temperature is sufficiently low in a reducing atmosphere, one may ascribe the linear relationship in Fig. 12.34 to particle growth by a stepwise coalescence process of the smallest mobile metal clusters with clusters of the same class or with higher immobile clusters. In this growth model, the difftisivity of the smallest clusters at an arbitrary stage is assumed to be much higher than those of the other higher classes, so that the latter are regarded to be virtually immobile during the coalescence process of the smaUest clusters in each step. Another important conclusion of the theoretical approach is that for obtaining the highly dispersed metal particles, the reduction of metal ions should be done as quickly as possible, for example, with a high pressure of hydrogen at a relatively low temperature, in order to achieve a migration-controlled particle growth by the mobile metal clusters, rather than a reduction-controlled growth by a low concentration of hydrogen. In the theoretical approach, it is presumed that the maximum mobile cluster size is determined as a fimction of temperature. This assumption may be supported by experiment as shown in Fig. 12.35, where the particle size in terms of dispersity (H/Pt) by adsorption of H2 becomes constant after a certain time of aging in the presence of hydrogen at a level proper to each temperature, regardless of the content of Pt.^°^ As referred to in the Appendix, the final mean particle size being determined only by temperature, irrespective of the initial density of metal ions, is equivalent to the final particle density being in proportion to the initial

12. INDUSTRIAL APPLICATIONS

651

H/Pt ATOMIC RATIO (FRACTION OF METAL ATOMS EXPOSED)

o

Pi, Wt% CATALYST

ae

0.4 04 0.8 2.0

E 0 A B C

Fig. 12.35. H/Pt values changing with aging in hydrogen at different temperatures for Pt particles on Pt/Ai203 catalysts different in Pt content. (From Ref. 201.) density of the metal ions at a given temperature. However, it should be noted that the proportionality of the final particle density to the initial ion density holds only when the starting metal ions are uniformly distributed without agglomeration. Hence, this condition will not be attained with an excessively high initial surface density of metal ions, even if the metal complex has a strong affinity to the support.

IMPREGNATION 300

d 200

200

300

400

FIRING TEMPERArURE(°C)

500

Fig. 12.36. Effect of the pretreatment temperature in air prior to hydrogen reduction on the final particle size of Pt (2.5 wt% Pt) on silica gel (Davison 70) for the ionexchange method with [Pt(NH3)4] CI2 and for the impregnation method with H2PtCl6. (From Ref. 166.)

652

APPLICATIONS

Figure 12.36 shows that the highly dispersed Pt particles on Si02 by ion exchange of the cationic complex are stable in heat treatment in air before reduction with hydrogen, as compared to those from impregnation of anionic complex.^^^ This result may again be readily accounted for in terms of the affinity of the metal complexes to the support. As would be expected, the uniformity in size distribution of the Pt particles on Si02 prepared by the ion exchange method is superior to that from the impregnation method. Figure 12.37 shows the size distributions of Pt particles prepared by these two different methods at varying temperatures for the heat treatment in air prior to the reduction in hydrogen.^^

-

^

30 -

1 ES-i soo'c

1

T'

I

1

\

\

ES-iv 770°C

IS-i -

j ^ _ ^ ^

soo'c

rj-'

IS-ii

1

-- 600°C 1

—^-^—,

1 1 8

1

_a~l~~^ 1 12 16 20

*"•—«—,__^ 1 24

CRYSTAL DIMENSION. A

1

1

Fig. 12.37. Effect of the pretreatment temperature in air prior to the hydrogen reduction on the size distribution of the Pt particles prepared by the ion-exchange method (ES) or the impregnation method (IS). (From Ref. 202 (a).)

b) Stabilizer Ultrafine metal particles for catalysts are liable to coagulate, so one of the most important roles of their supports is as stabilizers against coagulation during catalytic reaction. Most of the above-mentioned features of supports for controlling the mean size and size-distribution of metal particles in their preparation can also be applied to the stabilization of metal particles on supports. For example, we should choose for the support a material which is characterized by a large specific surface area, a high

12. INDUSTRIAL APPLICATIONS

653

surface roughness of the order of the size of the metal particles, a strong affinity to the metal particles, and a strong affinity to the metal complexes or ions, in order to obtain a thermally stable catalyst. The change of the specific surface area of metal particles on a support, S, is generally given by dS ^-KSP, dt where X is a constant and the exponent p depends strongly on the atmosphere in some case; e.g., p ranges from ca, 2 to 16 for Pt on y-AIjOj.^"^ For example, /? = 2 in air,^^^^^'^^ while /? = 6 ~ 15 in hydrogen, increasing with temperature.^°^ In addition, oxygen drastically accelerates the particle growth of Pt, as shown in Fig. 12.38.^°^ 240 220 200 180

~r

T

1

1

r

700»C

| - A Air atmosphere, Wynblatt a Gjostein • 2 % O 2 1 N 2 atmosphere, Wynblatt a Gjostein •Oxidizing atmosphere (unspecified), Somorjoi

-160 |Q= 140

10

20

30 t(h)

40

.50

60

70

Fig. 12.38. Effect of oxygen on the growth of Pt particles on y-AloOg at 700 °C. (From Ref. 206.)

Ruckenstein et al.'^^ attributed the growth of Pt particles to coalescence of the metal clusters on the support, and explained the acceleration of the growth of Pt caused by oxygen in terms of increased diffusion coefficients of the metal clusters by adsorption of oxygen affecting the interfacial energies of Pt/gas and Al203/gas interfaces, tadeed, the bondmg strength between Pt and AI2O3 may be lowered with decreasing contact area between them, since the surface areas of Pt/gas and Al203/gas interfaces may be increased due to the reduction of the Pt/gas and Al203/gas interfacial

654

APPLICATIONS

energies by adsorption of oxygen. Hence, they accounted for the effect of oxygen on p by the shift of the rate-determining step in the whole coalescence process from the surface migration step of the clusters to the inter-fusion step of the clusters in collision. In the meantime, Wanke et aL^'^ proposed an atomic migration model in which metal particles are assumed to grow by a kind of Ostwald ripening. Hence, in their model, atomic monomers are intermediates in the process, instead of the direct migration of clusters. It is possible to assume two growth modes in this model as well; i,e., diffusion-controlled Ostwald ripening and reaction-controlled Ostwald ripening (see section 4.2), corresponding to the migration-controlled coalescence and the fusioncontrolled coalescence, respectively, in the above coalescence model. The two different growth mechanisms may be distinguished on the basis of the difference in the shape or skewness of the resulting size distributions. If a regular size scale is taken as the abscissa, the modal diameter is expected to shift leftward from the distribution center in the coalescence process, as often represented by the log-normal distribution, while being shifted rightward from the distribution center in the Ostwald ripening process.^°^ Figure 12.39 represents typical size distributions of Pt particles prepared by reducing Pt ions on an AI2O3 support with hydrogen at different temperatures:^^^ the changes in surface area with time have been shown in Fig. 12.35. This figure suggests that Pt particles on AI2O3 in a reducing

NUMBER OF PARTICLES

o

©

1.0

2.0

3.0

4.0

1.0

2.0

3.0

4.0

1.0

U 2.0

I 3.0

I 4.0

10

i I 1 2.0 3.0 4.0 PARTICLE SIZE, nm

Fig. 12.39. Effect of temperature in hydrogen reduction of Pt ions on the size distribution of the resulting Pt particles [Pt(0.8 wt%)/Al203 system]: (I) 500 "*€, 110 h; (II) 550 ^C, 60 h; (III) 600 °C, 60 h; (IV) 675 °C, 10 h. (From Ref. 201.)

12. INDUSTRIAL APPLICATIONS

655

atmosphere with hydrogen are grown by the coalescence process. However, if we consider the exceedingly slow migration of metal clusters^°^ as compared to the very fast interfusion of clusters in contact^^° (e.g,, 3300 A in 5 h for migration of a Pt cluster of 25 A on AI2O3 at 600 ''C; 1.3 msec for complete fusion of Pt clusters of 25 A in contact on AI2O3 at 600 °C), the fusion-controlled growth in the coalescence model seems unlikely even if the migration of the clusters is accelerated to a great extent by oxygen. Moreover, there may be no special reason for attributing the reduction of the Pt/gas and Al203/gas interfacial energies only to the adsorption of oxygen, since hydrogen is also a very strong adsorbate for platinum. Hence, the significant role of oxygen may be ascribed to some other reasons, such as the formation of volatile Pt02 which may intermediate the Ostwald ripening of the metal particles through diffusion via the gas phase (and the surface of the support).^^^^^'^^^^^' ^^' ^^^^^ In this case, the contribution of the surface diffusion of Pt02 may not be negligible if its surface density, in equilibrium with the vapor, and the diffusivity on the support are sufficiently high. If this Ostwald ripening process becomes predominant in the growth of a metal catalyst, it is hard to completely exclude this process only from the design of the support. c) Adsorption Medium Supports for metal catalysts, such as active carbon, silica gel, and zeolites, may work as adsorption media to concentrate reactants near the metal sites. As stated eariier, a Rh catalyst supported by polyacrylamide resin with amino groups showed a definite selectivity in hydrogenation of olefins according to their degree of hydrophilicity.^"*^ For example, when the activity of the resin-supported catalyst was assessed in terms of activity relative to that of a comparable dispersed Rh catalyst, the resin-supported catalyst displayed much higher relative activity for the hydrogenation of hydrophiUc olefins, such as ethyl vinyl ether and allyl alcohol, than for the hydrogenation of hydrophobic olefins, such as cyclohexene and 1-pentene. The result was explained by the difference in effective density of reactants near the metal particles. In particular, the relative activity of the resinsupported catalyst was especially high for olefins bearing carboxyl groups. This effect may be accounted for in terms of the condensation of these olefins around the Rh particles by the electrostatic attractive force between their negatively charged carboxyl groups and the positively charged amine groups of the support.

656

APPLICATIONS

d) Controller of the Electron Density of Metal Particles Figure 12.40 shows a relationship between the electronegativity of oxide supports, or additives to a AI2O3 support, of Ru/support catalysts and TOF (turnover frequency) for synthesis of ammonia from N2 and H2. The figure suggests that the catalytic activity for the synthesis of ammonia is reduced with increasing electronegativity of the support or increasing electron transfer from metal to the support.^^^'^^^

Fig. 12.40. Relationship between the electronegativity of oxide supports, or additives to a Ai203 support, of Ru/support catalysts and TOF (turnoverfrequency)for synthesis of ammonia from N2 and H2. (From Ref. 211.)

Taylor et al}^^ found that the specific activity of Ni on SiOj, in terms of TOF for hydrogenolysis of ethane to methane, was particularly high relative to Ni on AI2O3 or on SiOj-AlzOa (Si02:Al203 = 87:13). It may imply some electronic interaction between Ni and the supports. Similarly, Vannice and Garten^^^ found a Ni/TiOj catalyst to have a specially high specific activity for the hydrogenation of CO to produce methane and higher molecularweight hydrocarbons, as compared to the conventional Ni/Si02, Ni/Ti-Al203, and Ni/a-Al203 catalysts for the selective production of methane in the same reaction. Resasco and Haller^^"* reported that the specific activity of a Rh/Ti02 catalyst prepared by reduction at a high temperature such as 773 K was drastically lowered by three orders of magnitude for the hydrogenolysis of ethane, relative to the same catalyst prepared by reduction at a lower temperature such as 523 K, while the specific activity for the dehydrogena-

12. INDUSTRIAL APPLICATIONS

657

tion of cyclohexane was unaffected by the reduction temperature. They attributed these effects partly to migration of reduced species of TiOj onto Rh metal particles during the reduction at the higher temperature, and partly to the localized electron-transfer from Ti02 to Rh at the higher temperature, in contrast to the delocalized electron-transfer from Rh to Ti02 at the lower temperature. e) Synergistic Catalyst Pt nanoparticles supported on styrene-divinylbenzene copolymer beads with sulfonic acid groups can produce acetone from a equimolar mixture of propylene and water.^^^ In this system, propylene is first converted to 2 propanol by the hydrated sulfo groups as a catalyst, and then the generated 2-propanol is converted into acetone by the Pt particles as a catalyst for dehydrogenation. Thus the support works as a first-step catalyst in this two-step process. The a-Fe203 support of the aforementioned Au/a-FcjOj catalyst for the oxidation of CO in section 12.4.1 is believed to work as a main catalyst assisted by Au particles as adsorption sites for CO in the reaction of CO with O2 adsorbed on the a-Fe203 surfaces.^^^ The supports of photocatalysts such as Pt/TiOjCa), Pt/RuGj/TiOjCa), Pt/TiOjCr), and Pt/SrTiOj are also rather main catalysts for the photolysis of water into H2 and O2 gases: (a) and (r) indicate anatase and rutile, respectively. The catalytic activity is in the order, Ti02(a) > Ti02(r) > SrTiOj.^^^ The activity of anatase being higher than that of rutile in the generation of hydrogen is due to the negatively higher conduction band level of the former, coming from the greater band gap by 0.2 eV. Although the conduction band level of the single crystal of SrTi03 is close to that of anatase-type Ti02 and the catalytic activity of the single crystal is rather higher than that of rutile-type Ti02,^^^ the catalytic activity of SrTiOj powder is even lower than that of rutile-type Ti02, because of the cryst^d imperfections of SrTi03 powder as electron traps and the dissolution of Sr^^ during catalysis.^^^ The photocatalytic activity is generally enhanced with size reduction of the semiconductor support.^^^ This size effect may be explained in terms of the electropotential floating effect?^'^^ Namely, when generated positive holes react with H2O or organic compounds, the energy levels of the semiconductor particles including the conduction-band level are lifted in the negative direction, so that the activity of the photoexcited electrons for the reduction of H2O is elevated. This effect of potential floating is pronounced

658

APPLICATIONS

as the particle size becomes smaller owing to the decreasing electric capacitance.^^^ In addition, the activity of the positive holes for the photooxidation of H2O is also raised with the size reduction. This effect may further enhance the potential floating effect. 12AA. Application of Monodispersed Particles to Catalysts As has been seen in the preceding sections, the particle size of metal catalysts is a decisive factor for catalytic activity and selectivity (see section 12.4.2). Also, as would be expected from the specific roles of the individual surface sites of Pt crystals in selective reactions of hydrocarbons,^^^'^^^ the crystal habit of metal particles may be another important factor to be controlled. For supports of heterogeneous catalysts, the external size, pore size, surface roughness, composition, etc. are the main factors to be controlled in order to attain the utmost functions in a composite catalyst system (see section 12.4.3). In this sense, monodispersed particles, well controlled in mean size, crystal habit, and internal structure, are the ideal particulate materials for both the catalysts themselves and their supports. However, for enjoying the numerous benefits of monodispersed particles as catalyst materials for practical use, we have to clear many barriers in the technology for the preparation of monodispersed nano- and micro-particles, fully controlled in mean size, crystal habit, and external and internal structures, and in large quantities, to achieve a low cost. The general principles for the preparation of monodispersed particles, i.e., the complete separation between nucleation and growth periods and the complete inhibition of coagulation during the growth period are valid even for the synthesis of monodispersed nanoparticles as well. However, the achievement of these conditions becomes harder with decreasing particle size, particularly in the range of nanometers, because the extensive nucleation needed for the synthesis of ultrafine particles make it difficult to separate the nucleation stage from the growth stage, and because the high particle density is liable to lead to drastic coagulation due to both kinetic and energetic requirements. Hence, even the use of powerful protective colloids such as polyvinylpyrrolidone, polyvinyl alcohol, and various surfactants is normally unsatisfactory for the synthesis of homogeneous dispersions of monodispersed nanoparticles of less than 10 % in the coefficient of variation of the size distribution. One of the most promising methods known for the preparation of monodispersed nanoparticles in dispersion may be the synthesis in W/0 microemulsions, as represented by the early work of Boutonnet et al for the

12. INDUSTRIAL APPLICATIONS

659

preparation of monodispersed nanoparticles (d = 3 - 5 nm; standard deviation « 10 %) of metals in the platinum group, including Pt, Rh, Pd, and Ir (see section 7.3.4).^^ Figure 12.41 shows a size distribution of Rh particles thus prepared. This method was further developed by Nagy and coworkers for the preparation of uniform nanoparticles (d = 2 - 7 nm) of FeB as a catalyst, in particular, for anmionia and Fischer-Tropsch syntheses,^^ NijB for selective hydrogenation of the C=C double bond,^^"*-^^ and COjB having activity for the hydrogenation of C=0 and C=0 bonds.^^ The secret of success in the achievement of monodispersity of nanoparticles seems to lie in the strict limitation in final size to be reached, by the strong adsorption of exceedingly high concentrations of surfactants in the microemulsions, in addition to the effect of suppressed coagulation by the condensed surfactants. Probably, the condensed surfactants play a key role in the formation of the uniform particles in this system. The major issue for this method in future may be the systematic control of the mean size and crystal habit without degrading the uniformity.

67»/.

20»/.

2.5

Fig. 12.41. A size distribution of Rh particles prepared in a microemuision. (From Ref. 222.)

In the control of the size and stability of heterogeneous metal catalysts with solid supports, such as metal oxides, silica, or active carbon, the supports play a key role, as described in the above section. Except for some special cases, such as tht photo-plating of Pt particles onto TiOj particles.

660

APPLICATIONS

metal catalysts are nonnally prepared on their supports by reduction of the ions previously deposited on the supports by impregnation or ion exchange. However, even if the metal ions are uniformly distributed over the support surfaces by the ion-exchange method, broadening of the size distribution is inevitable during the reduction of metal ions by hydrogen, since the growth of the metal particles proceeds basically through coalescence or Ostwald ripening, as referred to in the above section. The situation is unchanged even if the deposition of metal particles is performed by direct deposition of vaporized metal atoms onto a support from the gas phase. Hence, if possible, the deposition of metal particles should be carried out in a liquid phase, in which the mechanism of a definite two-step process of nucleation and subsequent growth is in effect. Recently, a new method, selective deposition method, has been developed for the preparation of fairly uniform and densely dispersed nanoparticles of noble metal catalysts (Au, Ru, Rh, Pd, Ir, and Pt) on metal oxide supports (see section 7.3.8).^^^ As the supports, monodispersed metal oxide particles controlled in size, shape, and surface structure were used. The selective deposition method is based on a very simple procedure of aging a solution of a hydroxide precursor complex of metal ions to be selectively deposited onto coexisting support particles at 100 °C. In the case of Au particles, the deposited hydroxide precursor is catalytically reduced on the supports without addition of reducing agents.^^^*^'^^ But, for metals in the platinum group, such as Ru, Rh, Pd, Ir, and Pt, hydrogen reduction was needed for the selectively deposited hydroxide particles on supports.^^^^""^ This method for the preparation of the metal catalysts in the platinum group differs from the ion-exchange method, described in section 12.4.3(a) in principle, since the former is based upon the selective deposition of hydroxide complex associated with the hydrolysis of metal ions at a high temperature such as 100 ^^C, resulting in an extremely high loading of metal particles on supports

Fig. 12.42. High-resolution TEM images of some representative nanoparticles of noble metals prepared by the selective deposition method on metal oxide supports, different in composition and surface roughness: (a) Rh/a-Fe203(E); (b) Ir/a-FczOj (E); (c) Pt/a-Fe203(E); (d) Pt/a-Fe203(P); (e) Pt/ZrOj; (f) PtmOj. Here, a Fe203(E) is poiycrystalline ellipsoidal hematite with a rough surface; a-Fe203(P) is monocrystailine platelet-type hematite with a smooth surface; Zr02 is monocrystalline spherical zirconia with a microscopically rough surface; TiOj is monocrystailine ellipsoidal titania with a microscopically rough surface.

661

12. INDUSTRIAL APPLICATIONS (a) Rh/a-FcjOsCE)

(c) Pt/a-FejOjCE)

(b) Ir/a-Fe203(E)

(d) Pt/a-Fe^OjCP)

10 nm

662

APPLICATIONS

(« 20 wt %) in a high dispersity comparable to or even better than those by the ion-exchange method (< 5 wt %). Figure 12.42 shows some representative TEM images of so-prepared nanoparticles of noble metals deposited on metal oxide supports, different in composition and surface roughness. Above all, Pt/TiOjCa) catalyst showed an excellent performance as a catalyst for hydrogenation of l-octene.^^^""^ Fortunately, monodispersed well-defined metal oxide particles, such as hematite, titania, and zirconia, precisely controlled in size and surface structure are becoming available in large quantities (see section 7.3.1), and thus we may say that we are ready to develop practical heterogeneous catalysts with metal nanoparticles of a sharp size-distribution, on monodispersed support particles strictly controlled in size and surface structure. Zeolite is a kind of composite oxide mainly of Si, Al, Na, and Ca. Its natural minerals or synthetic products are widely used as inorganic molecular sieves in many fields, because of its unique structures with regular micro- or meso-pores of different sizes. In the field of catalysis, it is used as elementary catalysts or as supports or templates of catalysts, because of its excellent performance as an adsorbent and the well-defined pore structures. In general, synthetic zeolite is prepared by a hydrothermal treatment of reactive alkaline aluminosilicate gels at relatively low temperature (100 °C) and pressure, as introduced by Milton in the late 1940s.^^^'^^ Addition of cationic surfactants to the systems for the synthesis of zeolite results in a marked reduction of crystal size to submicrometer order.^^^ Meanwhile, Ueda et al^^ synthesized several types of zeolite from homogeneous solution systems. Nowadays a wide variety of zeolite particles of submicrons or less are synthesized and their size distributions are controlled in a fairly narrow range. For detailed information on this field, the extensive studies of Schoeman et al on the synthesis of zeolite particles and their growth mechanisms are available.^^'^^ There seems to be no doubt that the advancement in the field of zeolite synthesis, including the perfect control of the size distribution, mean size, and crystal habits, as well as the pore size, would make a great contribution to the development of the science and technology of catalysts. In this book, the text on catalysts has mostly been limited to metal catalysts, because precise control of the size and size-distribution directly governs the activity, selectivity, and stability of a catalyst. However, even if the topics are limited only to heterogeneous catalysts, non-metallic heterogeneous catalysts are widely used in various chemical industries. They may cover a great part of heterogeneous catalysis, comparable to - or

12. INDUSTRIAL APPLICATIONS

663

even greater than - the territory of metal catalysts. However, from a scientific viewpoint, they do not look easy to approach, and little is known of their essential natures as catalysts, probably because of their ill-defined characteristics and the complex mechanisms in catalysis. In this sense, the field of non-metallic catalysts appears to be full of fascinatmg scientific subjects to be explored. The use of monodispersed particles, including metal oxides, metal sulfides, and metal phosphates, for practical nonmetallic catalysts may be a future issue. However, there seems no doubt that if a wide variety of general and practical methods of low cost for the preparation of monodisperse particles become available, many non-metallic catalysts will be replaced by monodispersed catalysts ideally controlled in size, shape, external and internal structures, and chemical composition, since the importance of weU-defined particles is unchanged for non-metallic catalysts as well. Such attempts are already in progress, as represented by the early work of Hamta et al on the preparation of monodispersed spheres of molybdenum sulfide and cobalt sulfide as catalysts for hydrodesulfurization.^^

Appendix: Growth of Metal Particles on Supports by Stepwise Coalescence This appendix concerns the theoretical approach to the mechanism of formation of metal particles from the metal ions uniformly dispersed on a support, as represented by Pt/Si02 prepared by ion exchange of cation complexes such as Pt(NH3)4^^ followed by drying and reduction with hydrogen. Prior to the derivation of the theory, we make the following assumptions. (1) Small clusters of metal can move on a support, but the mobility of clusters of each size class, defined by the number of the constituent metal atoms, is so discrete that the coalescence of mobile clusters of the nth size class with clusters of the same or higher size classes virtually starts after the exhaustion of all clusters of the n-7th class. This assumption of stepwise coalescence is based on the idea that the diffusion coefficient of a cluster may be inversely proportional to the number of its atoms in contact with the support. (2) The maximum size of mobile metal clusters is determined by the temperature of the growth process on the support in the gas phase. This assumption is supported by some experimental studies.^^ (3) Dissociation of each metal cluster does not occur. In this case, the

664

APPLICATIONS

coalescence is governed only by the probability of collision of mobile clusters with mobile or immobile clusters, in which the clusters in collision are assumed to fuse together with a certain probability up to unity. This assumption is reasonable when the surface density of the metal monomers, in equilibrium with the vapor pressure of the bulk metal at the given temperature, is extremely low as compared to their actual surface density in all stages of the growth process. From assumption (3), the change of the surface density of the nth clusters consisting of n atoms, c^, is given by

where k^^ denotes the rate constant of coalescence between a monomer and an nth cluster. If one totals the left- and right-hand sides for n greater than unity, one obtains: ^

= k c"

(2)

where AZ2 is the total surface density of the dimers and still higher clusters,

From the mass balance of generation of metal monomers by reduction of metal ions and consumption for the formation of clusters larger than the monomer, the change of density of the monomers may be written as

where a is the surface density of metal ions during the reduction by hydrogen and Jto is the rate constant. Here the term, -Tk^^^c^^, on the righthand side corresponds to the consumption rate of monomers for forming dimers whose formation rate is k^^c^^. Since the rate of generation of the metal monomers is equal to the decay rate of the ions, it holds that

12. INDUSTRIAL APPLICATIONS

665

da

,

Hence, if the initial value of a is denoted by GQ, then a is given by a = aoexp(-V).

W

The rate constants, k^^, may increase with increasing n because of the increasing positive factors, such as the capture cross-section of mobile clusters, van der Waals attraction, and stability in terms of surface energy. But, for the present, one may assume '^l,!

'^l^

'^1,3

^ '^V

for simplicity, since the essential feature seems unaffected by this assumption. Hence, Eq. (3) may be rewritten as —- = k^ - Ik^c^ - k^c^n^. dt

(^)

Here, we consider two cases. One is that the migration of the metal monomers is the rate-determining step for the growth of metal particles (^0^0 » Ik^a^ or k^ » Tk^a^, The other is that the reduction of metal ions is the rate-determining step (Jc^ « ^^a^. Case 1. Migration-controlled growth (^Q » 2k^aQ) In this case, all metal ions are promptly reduced to metal monomers, and then the metal monomers migrate to coalesce with each other and with cumulatively generated higher clusters. When all the monomers are depleted in this first step for growth, the much slower coalescence of dimers with themselves and still higher clusters will follow as the second step. In this way, further steps for the growth of metal particles will follow at the expense of the smallest clusters at each step, until finally the minimum size of the remaining clusters exceeds the maximum mobile size proper to the temperature.

666

APPLICATIONS

1st step A s ^0^ = 0 in Eq. (5), it reduces to

(6)

Eliminating t from Eqs. (2) and (6),

dcj

(7)

2ci+n2

Now let us consider the behavior of monomers {n = 1) and still higher clusters (n ^ 2) in the first step. For n = i If we use, instead of n^, the total surface density of all clusters including the smallest clusters (monomers in this case), 2, given by

we obtain from Eq. (7) that

"At

.(0)

= z(°)exp|

,(0)

^K^\

(8)

where the symbol (0) indicates the initial values of the first step, and thus 2<°> = c/°^ = AQ, KS^^ = a^e'^. If c^ is written as a function of 2, JJ<0) Cj = - z l n

(9)

The total surface density of all clusters at the end of the first step, z^^\ is given from Eq. (8) with c^ = 0 as

667

12. INDUSTRIAL APPLICATIONS

zW = z<»)exp|

^1 (0)

K^^'^^a^e-K

(10)

\ z

For n > 2 From Eqs. (1) and (2), dc

c ^'C n-1

n _

n

(11)

If one uses z and c^ in Eq. (9), this is transformed as ^n-l

dz

(12)

z

Moreover, if one defines A^ and y in place of c„ and z, respectively, as c

r(0)

Eq. (12) reduces to

dy

= A.n-l

(13)

Since A^ is given as a function of y from Eq. (9) and the definitions of A^ and y by A = -L = -In = -y, z z

(14)

and since the y values at the start and end of the first step are -1 and 0, respectively, A^ at the end of the 1st step, A}^\ for n ^ 2 is given by the following multiple integration repeated n-l times as

668

APPLICATIONS

c'''

""'-^s-nv-i^)*-**-

<^=>

For example,

4^>=1, ^

A1^>=I, A1^>=1,

2

^

3

^

8

^<^>=J-,^

(16)

30

Although we have assumed that y is virtually zero at the end of the first step, this must be much higher than the level expected from the vapor pressure of the bulk metal.

2nd step For w = 2 Equations corresponding to Eqs. (2), (6), and (7) for the behavior of the surface density of the clusters of the minimum size, c„, and the total surface density of clusters higher than the smallest clusters, n^^p hold not only for the first step, but also for the second and further steps. Namely, for the mth step when the number of atoms of a minimum cluster is m, the following equations hold.

dt

^

= -2Vm-*»V«.l .

and

If 2 in the mth step is defined as

^^^>

12. INDUSTRIAL APPLICATIONS

669

^^^^n^^m.l

in a similar manner, the relationship between 2 and c^ during the mth step is given by "C

(m-l)\

zexp -^l = z<'"-^>exp| V^ \ ^

s j^m-1)

(20)

/

Hence, 2 at the end of the mth step, z^'^\ is given from Eq. (20) with c^ Oas

= z^-^>exp(-^l^-^>).

2(m) = ^(m-l)^^p| \

.T(»»-1)

(21)

I

At the end of the second step, r' is given by W^ = = ao« ..-3/2 z^^J = zWexp(-xn

(22)

For « = 3 Since coupled dimers jump directly into tetramers, the change of c-^ is given by (23)

Hence, using Eq. (18), (24) If we use Cj = -zln^^V^, ^3 = c-Jz, and y = IniKP'Vz), this equation reduces to

670

APPLICATIONS (25)

—^ = 0. dy Since A^ is constant during the second step, A _ ^ ( 2 ) _ .(1)_

1

(26)

For n = 4 The change of c^ is given by (27) and hence dc^ c^-c^ dn^

(28)

Cj

Similarly Eq. (28) reduces to dA^ dy

(29)

A,= -[\ydy^A'^'=-^y'^^

(30)

^f=i.

(31)

Thus

and

4 For n = 5 A^ during the 2nd step and A^^'^^ are given by

12. INDUSTRIAL APPLICATIONS

671

^5 = / . i ^ ' f y ^ ^ 5 ' ' 4 3 ' + ^ ^3 ' ' 3' 5

(32)

and

f>=l.

(33)

^' " 5 For n ^ 4, in general, /I _

n-2

n

(34)

and dA ''=.4 , . 'dy

(35)

Therefore, for n = 2r (n = even number; r ^ 2)

^S^ = /-l/'i •••/.V-)')'^-^'^"^2^ 2 "2

(36a)

For n = 2r+l (« = odd number; r ^ 2)

^-'=/.i/.V"/.i3^-'^^^^-'2 2

(36b)

2

3rd step z(3) = z(2)exp(-4'^) = V-"/«.

(37)

672

APPLICATIONS

AfKAfKl;

^f) = A f = l . 5

(38)

z<^) = z % x p ( - A f ) = aoC-25/12.

(39)

Af = 4^> = A f = l

(40)

4

4th step

5th step z<5> = z^%xpi-Af)

= a,e''''f''.

(^l)

Generally, the total number of clusters at the end of the mth step, z^°'\ is given from Eq. (21) as zW = aoexp(-Exf-'V

(42)

1=1

where ^(1-1) ^ f i _ ^0-1) • For example,

(43)

12. INDUSTRIAL APPLICATIONS j(0)_i

Al

jU). 1

673 A2)_ 1

- l, A2 ~ T ' -^3 " T '

2 4(3)_ 1 ^W- 1 . . .

3

(44)

and

j,(4)=

-25/12^^(5)^

-137/«

...

As a consequence, the total surface density of the metal particles at the end of an arbitrary step is proportional to the initial surface density of the metal ions, ^0, at least under the condition of an equal rate constant for coalescence of mobile minimum clusters with different clusters of the same or higher classes in a given step, Lc, k„ = k„^^ = k^^,, = k^^^,^ -• With the progress of the stepwise coalescence, the initial number fraction of the minimum clusters for each step, Af'^^ is lowered, and the difference in diffusivity between the minimum clusters and those of neighboring classes becomes small, so that these small clusters of several classes will proceed to move at the same time in a group. Nevertheless, the proportionality of the final particle density to the initial ion density at a given temperature will be retained, since the situation is unchanged as long as the smallest group behaves in the same way as the smallest discrete class. General features of Case 1 If we assume size-dependent rate constants of coalescence between the minimum clusters and the other clusters of different sizes at a given step, the treatment for the change of total particle density at each step may be much more complex. However, even if we assume size-dependent rate constants in the first step, it follows from Eqs. (1), (2), and (3) with a = 0 that

(Kn^ ^^"2 [ * U j <^1 1*1.1 J and

674

APPLICATIONS

dCy

-2-

ik \ k

f3_ <^1

••• .

<^1

Since it is evident that the change of nJaQ can be described as a function only of cja^ from these equations with the initial conditions of nja^ = 0 and CJQQ = 1, HJUQ at the end of the first step {cja^ = 0) is a dimensionless constant without a^. In other words, n^ at the end of the first step is proportional to a^. It can be shown likewise that the decrease in ^2 ^it the end of the second step is also proportional to ^2 at the end of the first step. Consequently, n^ at the end of the second step is proportional to a^. In this manner, the total surface density of the metal particles at the end of an arbitrary step is shown to be proportional to a^. If the rate constant of coalescence of the mobile minimum clusters with the higher clusters increases with increasing size of the latter, the decay rate of the minimum clusters is more accelerated with the generation of the higher clusters than in the case of an equal rate constant. In this case, the increase of n^^^ in this step becomes the smaller due to the more rapid reduction of c^, and thus the increase in mean particle size in this step is the greater. These are obvious if one considers the decay rate of monomers and the increasing rate of AI2 in the first step in Eqs. (2) and (3) with a = 0. For further insight into this effect, it seems useful to simulate Eq. (3) at a = 0 with the following much simpler equation for the change of c^ during the first step: a 2-a

(46)

where a is a dimensionless parameter. This equation always agrees with Eq. (3) with A = 0 at the start of the 1st step (c^ = a^), regardless of a. Using Eq. (2), one obtains (47)

Since ^2 = 0 at c^ = a^, n^^\ corresponding to «2 ^^ the end of the first step (c^ = 0), is given by

12. INDUSTRIAL APPLICATIONS

/,W. "2

675

^0 . 2(o + l)

(48)

If *ii = )fci2 = ^13 = ••% then «2<^Vao = e'^ - 1/2.72 so that a = e/2 - 1 0.36. If ifc,, < ^1,2 "^^ ^1,3 '^ ***» ^'^^^ ^ TDLVL^X be a > 0.36. If the decay rate of c^ is simulated with a relatively large a, such as 2, until c^ is close to zero, then n^^^^la^ = 1/6, corresponding to the increase of mean particle volume by a factor of 6 in the first step. As the factor of increase in particle volume for each step is progressively lowered, the factor of 6 may be close to the upper limit for the proportion of the volume increase in one step. It is now evident that the normalized size distribution of the metal particles after the same number of times of repetition of this procedure is identical regardless of the initial surface density of metal ions, and that the particle number is proportional to the initial surface density of the metal ions. Since the maximum size of mobile clusters is assumed to be determined by the temperature, the number of times for the repetition of this procedure at a given temperature is fixed, and thus the final particle number at a given temperature is proportional to the initial surface density of the metal ions. In other words, the final particle size is determined only by temperature, irrespective of the initial concentration of the metal ions. In this case, a linear relationship is obtained for S vs V. Case 2. Reduction-controlled

growth (ICQ « ^k^a^)

In this case, the metal monomers are in a quasi-steady state of the generation fi:om the metal ions and the consumption for the growth of the metal particles, so one may obtain the following relationship for the first step from Eq. (5) with dc^/dt « 0:

Eliminating c^ from Eqs. (2) and (49), one obtains

676

APPLICATIONS

dx __ / f c i a a y ^ ^ 4 ^ e x p ( - y ) - y ^ x "idt' 2

(50)

where x = HJUQ, Although dxidt is independent of a^ as dxidt = ^oexp(-V) in the early stage of the first step, it becomes inversely proportional to a^ with time. Hence, n^^^ as n^ at the end of the first step must be lower than expected from the initial proportionality to a^, so that the plot of 5 as a function of V at the end of the first step will deviate downward from a linear increase. Therefore, a linear relationship between S and V cannot be expected in Case 2, even at a later stage of the stepwise procedure.

12.5. Pigments Pigments are particulate color materials relatively insoluble in solvents and used in paints, varnishes, cosmetics, etc. mainly for coloring something, but also for coating to protect the substrates from harmful irradiation, corrosion, or contamination, or to hide original colors of the substrates, or to impart to the coated materials some specific functions such as magnetism or electrical conductivity. Since they are used in the form of dispersions in liquid media, they are distinguished from dyes, which are used similarly as coloring materials, but normally in the form of molecules dissolved in solvents. Although pigments are generally classified into inorganic and organic groups, we will mainly deal with the former in this book, because the studies on the effects of the size and shape of individual particles upon the optical properties of colored pigments have been performed mainly for inorganic pigments using monodispersed particles. Nonetheless, the general theory of the optical natures of colloidal particles, based on light scattering and absorption, is common to both groups. As examples of inorganic pigments according to their colors, the most typical ones may be as follows. White: Black: Yellow: Orange: Red: Blue:

TiO^, ZnO, ZnS Ytp^, C PbCr04-PhSO^, PbCrO^, a-FeOOH, TiO,-NiO-Sb^, CdS 25PbCr04 •4PbMo04 •PbS04, PbCr04-PbO a-ftp^, Cd(S,Se) Na^,Si,2.A4-NayS,, KFe[Fe(CN)J, CoG-wAlA

12. INDUSTRIAL APPLICATIONS

Green: Violet:

677

CT2O2, CoO-nZnO Co3(P04)2

In addition to the above pigments, colorless powders of CaC03, BaSO^, AI2O3 •nH20, Al2Si205(OH)4, etc. of relatively low refractive indices, around 1.5-1.7, are used as extender pigments (diluents of colored pigments) or white pigments. Although the color of a pigment is primarily determined by the substance or its chemical composition, the size and shape of the particles have significant effects on the color. Hence, if these colored pigments are prepared as monodispersed particles precisely controlled in mean size and shape, we will be able to obtain, ideally, pure-colored pigments which are not yet achieved, but may be used as pure single pigments or, if necessary, as elementary components of mixed pigments. 12.5.1. Relationship between Particle Size and Color The optical properties of spherical particles can be fully quantified if their refractive index and diameter are known.""^ For experimental studies on the optical properties of uniform colloidal particles, extinction spectra in visible and UV ranges of non-absorbing polymer latices,^^^*^^^ of highly absorbing metal particles such as gold and silver,~^^ and of moderately absorbing dielectric particles such as hematite^^"^"*^ have been measured and compared with calculations based on the Lorenz-Mie theory?'^^ For calculation of extinction spectra of light-absorbing particles, the most fundamental parameter is the complex refractive index, m, defined by m = n(l-id),

(12.5.1)

where n is the real part of the refractive index given by the ordinary vacuum refractive index divided by the refractive index of the medium, and K is the absorption index. For the determination of the absorption index, K, studies on slightly absorbing particles of atmospheric aerosols,^"*^ highly absorbing metals,^"^ and pigments^^'^"^^ are known. In particular, Kerker et alr'^ determined the absorption index, K, of hematite (a-Fe203) as a function of wavelength, using Rayleigh scatterers {rfk < 0.05) which require no information on the particle size. Namely, the extinction cross-section of a single particle, C^^, is written as the sum of the cross-sections for scattering, Q^^, and absorption, C^^,:

678

APPLICATIONS

c^=c at

+c.. sea

(12.5.2)

abs

The corresponding relationship for a colloidal dispersion is !^fir)C^{r)dr = [^mC^a(r)dr^f^Ar)C^mr,

(12-5.3)

where/(r)rfr is the number of particles per unit volume between radius r and r+dr. This relation may be expressed alternatively as (12.5.4)

€=T+A,

where the extinction e, turbidity x, and absorption A represent the total power attenuated, scattered, and absorbed per unit volume of dispersion for unit incident irradiation. If the particle sizes are sufficiently small for the Rayleigh approximation to be applied, T and A are given by T =

12871- m^-l

3k'

(12.5.5)

w^+2 Jo

and A=

Im X

m^-\

|/;/(r)r'*.

(12.5.6)

{m^-^2

where Im denotes the imaginary part of the complex number. On the other hand, the Rayleigh ratio RJJd), representing the radiance per unit volume of the dispersion per unit solid angle scattered into direction 6 for unit unpolarized incident irradiation, is related to the turbidity, T, for Rayleigh scatterers as 3(l^W0)^_ 16n

(12.5.7)

As a consequence, dividing through Eq. (12.5.4) by the particle concentra-

679

12. INDUSTRIAL APPLICATIONS tion in g/cIn^ c, given by _ 4K p

llmPdr,

(12.5.8)

one obtains

c

16K

^«(6)

6K

3(l+cos^0)

c

Xp

Im

m^-1

(12.5.9)

ym^-^l)

where p is the sohd density of the particles. This equation is used to evaluate K from the measurement of the specific extinction, T/C, and the specific Rayleigh ratio, Rj[&)/c, at dilutions sufficiently high to avoid multiple scattering, along with data of the real refractive index, n, obtained by separate experiments of ellipsometry^"*^ or an immersion technique.^'*^'^'*^ Figures 12.43 and 12.44 show the thus-obtained, K, and the real refractive index, respectively, as functions of wavelength - the latter of which Kerker et al obtained using compiled data of other reports.^^ The numerical values

0.05

Q02

0.005

0.002

0.001

400

500

X . (nm)

600

700

Fig. 12.43. Absorption index, K, of hematite as a function of vacuum wavelength. (From Ref. 238.)

APPLICATIONS

680

nsCC

75o~ X^ (nm)

Fig. 12.44. Real part of the refractive index, w, of hematite as a function of vacuum wavelength. (From Ref. 238,)

Table 12.8. Complex refractive indices, m = n{l-Ki\ of hematite particles at various wavelengths at 25 °C (Source: Ref. 239) ^(nm) 350 375 398 420 436 455 479

1 500

n

K

\)(nm)

n

1.53 1.85 2.02 2.12 2.18 2.25 2.30 2.32

0.68 0.52 0.41 0.33 0.27 0.21 0.17 0.144

525 546 560 579 600 625 650

2.34 2.34 2.33 2.31 2.30 2.27 2.24

K

0.104 0.072 0.051 0.029 0.017 0.010 0.006

of K and n are summarized in Table 12.8.^^ Once K and n are given as functions of wavelength, the extinction spectra and thus the chromaticities of monodispersed spherical particles of a given mean radius can be calculated for single scattering at an arbitrary scattering angle by the Lorenz-Mie theory.^'^^'^^ For concentrated suspensions, the

12. INDUSTRIAL APPLICATIONS

681

reflection spectra can be calculated from the Kubelka-Munk multiple scattering theory^^^r:^ -R=l

K

+ --

ik .+. \s^ s

(K^

,1/2

(12.5.10)

with

iS^

(12.5.11)

and

^^9C,,(l-)^

(12.5.12)

where the symmetry factor is cosd weighted by the scattering irradiance.^'^'*^'^^ Figures 12.45 and 12.46 show calculated chromaticities of monodispersed hematite particles for single scattering at 90"" to the incident irradiation of white light and for reflection based on the KubelkaMunk theory, as a function of the particle diameter from 20 to 3000 nm, respectively.^^ The curves within the horseshoe-shaped spectrum diagrams are the loci of the chromaticity coordinates with varying particle diameters from 20 to 3000 nm as indicated. One may recall that the intercept on the spectrum diagram of the line drawn from the source point, S, through the chromaticity coordinates gives the dominant wavelength. Also, the length of the segment from the source point to the chromaticity coordinates, relative to the total line length, gives the purity of the color. A chromaticity coordinate position lying within the triangle ASAB, formed by the lines from S to the coordinate positions of spectrum stimuli 380 nm. A, and 770 nm, B, and their joint connecting straight-line AB, has a complementary wavelength given by the intercept on the spectrum diagram of the line passing through the chromaticity coordinate position and source point, S. The purity of the complementary color is given by the length of S to the chromaticity position, relative to the full length extended to the joint connecting line between 380 and 770 nm. In Fig. 12.45 the color of 20 nm-diameter particles is blue (k = 475 nm) of about 50 % purity. As the colors swing around the spectrum with increasing size, they become nearly

APPLICATIONS

682 1.0 r 52(

1^

^ 54 0

' piO ^60

\^ Y

J-aoc

J .58 1

1

2oa _

0.4 r\\

0.2r

f\ L

isop V

/

\^ \ \

^0

\

^^1

s

ir

V

7 1&

-T" ^6

^^

r 620

^

p^7<5

21

^

1^ 02

^ 99 0.4

0.6

0.8

Fig. 12.45. Calculated chromaticities of monodispersed hematite particles for single scattering at 90° to the incident irradiation of white light as a function of the particle diameter from 20 to 3000 nm. (From Ref. 238.)

Fig. 12.46. Calculated chromaticities of monodispersed hematite particles for reflection based on the Kubelka-Munk theory, as a function of the particle diameter from 20 to 3000 nm. (From Ref. 238.)

683

12. INDUSTRIAL APPLICATIONS

achromatic in a narrow size range round 150 nm, and then range through almost pure yellows to reds from about 200 to 1000 nm diameter. At still larger sizes the colors become increasingly achromatic and shift toward the violet. In Fig. 12.46 for the locus of the chromaticity coordinates on the Kubelka-Munk multiple scattering model, the chromaticity is yellowish to reddish for all sizes from 20 to about 1000 nm diameter. The scattering light becomes progressively achromatic for sizes larger than 1000 nm diameter. These calculated chromaticities are quite consistent with the observed ones. Experimental and calculated Mie extinction efficiencies, corresponding to the extinction cross-sections, for nearly spherical hematite particles of various diameters are shown as a function of the wavelength of incident light in Fig. 12.47.^^^ The concentrations of the dispersions used in this measurement were less than 10"^ g/g to avoid multiple scattering. As the size increases from 0.10 to 0.16 |xm, the color of the dilute dispersions gradually changes from yellowish orange to reddish, owing to the expansion of the absorption spectmm region from near-violet to blue-green. It is noteworthy that a very small variation in particle size, even a difference of

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APPLICATIONS

684

the order of 0.01 [jim, causes a dramatic change in the extinction spectram. The measured peaks are shifted somewhat toward the longer wavelengths, as compared to the calculated values, which may be due to the lessweighted contribution to the extinction of a small number of larger particles as well as to a small deviation from the exact values of the complex refractive indices.^^ It is of interest to compare the spectra of colloidal hematite as a representative of dielectric particles with those of colloidal gold as a representative of metal particles. Figure 12.48 shows the calculated absorption efficiencies of hematite and gold sols at various wavelengths as a function of particle diameter.^^ For the calculation of the spectra of gold sols, previously reported refractive index values^^^^*^^^ were used. For hematite particles, the absorption shifts toward the longer wavelengths with increasing particle size, corresponding to the observed color change from yellow to orange, red, and dark red. The sharp resonances in the absorption efficiency, as typically observed with dielectric particles especially at high wavelengths with small values of K, are likely to be due to surface waves,^^ as suggested in ref. 239. Generally, the resonances are observed for

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12. INDUSTRIAL APPLICATIONS

685

dielectric particles with a positive dielectric constant and with a high real refractive index, n, and a low absorption index, K « 1. On the other hand, the color of colloidal gold dispersions changes from ruby red through purple and violet to pale blue as the particle size increases. The great difference in color change from that of hematite sols is mainly due to the strong absorption by surface plasmons^^*^"* with a maximum at X^ = 510 nm (Frolich mode) for 0.05 |im < D < 0.1 \xm, which occurs for metal particles when the real part of the dielectric constant is negative. As a rule, metal sols give strong surface-plasmon absorption in the visible/near-UV region, whereas the phonon vibrations, corresponding to the virtual modes^^ of the dielectric particles, shift to the IR region.^^'^^ 12.5.2. Shape Effect on Optical Properties Figure 12.49 shows the calculated scattering and absorption crosssections of ellipsoidal hematite particles of: (a) / = 335 nm, w = 220 nm; (b) / = 467 nm, w = 216 nm; (c) Z = 556 nm, w = 180 nm.^^^ The calculated

0.5

Scattering and absorption cross sections EUlpsoldal particles

0.0 400

500

600

700

X/nin Fig. 12.49. Calculated scattering and absorption cross-sections of ellipsoidal hematite particles of: (A) / = 335 nm, w = 220 nm; (B) / = 467 nm, w = 216 nm; (C) / = 556 nm, w = 180 nm. (From Ref. 241.)

686

APPLICATIONS

spectra were in good agreement with the experimental ones. 12.5.3. Hiding Power Hiding power is one of the most important characteristics of pigments used for hiding the colors of the substrates or for shielding the substrates from harmful irradiation such as UV rays. It is a function of the relative refractive index of the pigment particles, the absorption index, volume fraction, and particle size. If we consider a white pigment of a relative refractive index, m {m = n = fi/Mt)j K = 0, where \i and \x^ are the refractive indices of the pigment particles and the medium in vacuum, respectively.), the hiding power increases with (/n^-l)/(/n^+2), as would be expected from Eq. (12.5.5) for the turbidity of Rayleigh scatterers. Hence, whiteners must have high refractive indices; e.g,, Ti02 (\i « 2.7 for rutile; \i « 2.5 for anatase); ZnO (\i « 2.0); ZnS + BaSO^ (\i « 1.7-2.2). The refractive indices of media such as linseed oil, castor oil, or resins are normally in the range of 1.45 - 1.60. When the particle diameter, d, is much smaller than the wavelength (d « X), the scattering efficiency is proportional to d^fk"^ (see Eq. 12.5.5), but when d » X, it is proportional to l/d. Thus, there is a maximum in the scattering efficiency and thus hiding power at a certain particle size. With respect to the subject of optimum particle sizes for hiding power, many studies are known.^^*^^ For example, Mitton^^^ calculated the optimum diameters of various white pigments in linseed oil for blue, green, and red light as summarized in Table 12.9, using the following equation, and reported that the calculated values were in good agreement with the diameters actually measured. rf=

^ ^^"^ . 1.414|ioTr m^'l

(12.5.13)

The hiding power of white pigments is due to the reflection by light scattering, whereas not only the scattering but also light absorption contributes to the hiding power of colored pigments. Thus, color pigments are normally higher in hiding power, and the optimum sizes to give maximum hiding powers are considerably smaller than those of white pigments. This principle also applies to white pigments with UV absorption. For example, ultrafine powders of rutile titania of ca. 30 nm diameter, having a strong absorption band in the UV range, are particularly useful for the foundations of cosmetics to screen harmful UV radiation.^^ Here, the UV absorbance of a coating of the above titania of ca. 30 nm diameter per

12. INDUSTRIAL APPLICATIONS

687

Table 12.9. Calculated optimum diameters of white pigments in linseed oil for blue, green, or red light (Source: Ref. 259) Pigment

Blue Light

Green Light

Red Light

Rutile Titania

0.140 ^m

0.192 fim

0.205 iLim

Anatase Titania

0.158

0.215

0.230

Zinc Flower

0.275

0.389

0.416

Barium Sulfate

0.070

1.30

1.36

Calcium Carbonate

1.44

1.74

1.84

unit surface density was about three times higher than that of rutile titania powder of ca. 300 nm diameter usually used as a whitener. In any case, monodispersed particles precisely controlled in size and shape are ideal for controlling the hiding powers of both white and color pigments. 12.5.4. Composite Pigments and Tlieir Optical Properties Since the color of particles is quite sensitive to even a small change of particle size, it is essential to make the colored matter as uniform as possible and to control strictly the mean size. Since most pigments cannot be prepared as weU-defined dispersions at present, it may be convenient to combine dyes with uniform spheres which can be prepared by conventional techniques. An organic dye, 3 Mordant Blue, was incorporated into uniform amorphous spheres of aluminum hydroxide by heating dilute solutions of aluminum sulfate in the presence of the chelating dye.^^^ The resulting colored aluminum hydroxide spheres were 0.23 to 0.73 \xm in diameter, and the maximum content of the dye was as high as 30 mol % of Al in the solid phase. However, with increasing incorporation of the dye, the size distribution was considerably broadened as 0.46 in coefficient of variation at a molar ratio of dye/Al = 0.29. Similarly, uniform silica spheres are used as a matrix for incorporated dyes or as a core of hybrid particles covered with a dye layer.^^^*^^ For this purpose, colloidal silica is particularly useful because of its sphericity, uniformity, and low refractive index. For example, Hsu et al^^ incorporated Methylene Blue or Azure A in silica particles by coprecipitation, and coated Ethyl Violet or Thioflavine T by adsorption on the silica surface.

APPLICATIONS

688

Figure 12.50 shows reflectance spectra of powders consisting of aminomodified silica particles coupled with different dyes, such as Flavazin L, C. 1. Acid Red 183, Violamin R, and C. 1. Acid Blue 45.^^

Fig. 12.50. Reflectance spectra of powders consisting of amino-modified silica particles (a) and those coupled with different dyes: (b) Flavazin L, (c) C. 1. Acid Red 183, (d) Violamin R, (e and f) C. 1. Acid Blue 45. (From Ref. 263.)

On the other hand, as the optics of particles consisting of concentric spheres is fuUy understood,^ it is possible to design dispersions for specific use, such as whiteners. Figures 12.51 - 12.53 show a model of a coreshell particle of Ti02/Si02, the calculated scattering efficiency, Q^^^, of silica cores covered with different thicknesses of titania, as a function of the optical size, 2jrr/X, (r = particle radius; X = wavelength of the light), in a cellulose matrix, and the reflectance spectra of the core-shell particles.

CELLULOSE, n„ = 1.45

Fig. 12.51. Model of a core-shell particle of Ti02/Si02 in a cellulose matrix. (From Ref. 264.)

12. INDUSTRIAL APPLICATIONS

689

TITANIA COATED SILICA

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i

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1.3 1.0 0.8 0.5

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700

WAVELENGTH (nm)

8a

Fig. 12.53. The reflectance spectra of the core-shell particles, consisting of the same silica particles of 1 fxm diameter coated with different amounts of titania (a) and of silica particles of different sizes coated with the same percent amount (40%) of titania (b). The heavy solid lines are for a commercial titania sample (RLP2), (From Ref. 264.)

consisting of the same silica particles of 1 |iin diameter coated with different amounts of titania (a) and of silica particles of different sizes coated with the same percent amount (40 %) of titania (b), respectively.^^ 12.5.5. Other Properties Required for Pigments When pigments are used in paints, coatings, cosmetics, fillers, etc., the properties required are not limited only to color, whiteness, and hiding power. For cosmetics, for example, appropriate gloss, brightness, agreeable

690

APPLICATIONS

touch, smooth spreading, and affinity to the human skin are important properties, which are closely related to the crystal structure, particle shape, mean size, and size distribution of pigments used including colored and extender pigments. Extender pigments are uncolored powders mixed with general colored pigments or whiteners to control their final properties. For this purpose, they are required to have no absorption band in the visible range and a low refractive index around 1.5 - 1.7, close to those of the medium called the vehicle such as vegetable oils and resins, so that they become transparent when dispersed in the vehicles. Thus, materials widely used as extender pigments are clay minerals (pi = 1.5-1.6), such as kaolinite (Al2Si205(OH)4), halloysite (Al2Si205(OH)4 •2H2O), and talc (Mg3Si40io(OH)2), common mica (KAl2(AlSi3)Oio <0H)2; pi = 1.55-1.59), CaCOj (pi = 1.5-1.66), SiO^ (pi - 1.45), SiO^fiH^O ((x - 1.45), AXfi^nH2O (|x = 1.7-1.8), and BaS04 (pi - 1.64). For example, talc has been used as an extender pigment to improve the spread of cosmetics over the human skin, because of its softness by virtue of the very weak bonding between he crystal layers. The kinetic friction coefficient of talc is lowered from 0.46 to 0.24 with particle diameter increasing from 2.3 to 9.2 pun,^^ showing that the spreading property is improved with increasing size. However, as the particle size exceeds a few tens of microns, it becomes to feel rough, and thus there is some optimum size. Recently, however, monodispersed spheres of nylon, polymethyl methacrylate, polystyrene, etc. of a few microns to a few tens of microns in diameter have also become popular as extenders in cosmetics, because of their smooth spread over the skin. This is a consequence of their low kinetic friction coefficient, probably caused by rolling of these elastic spheres (see Table 12.10). As an application of spherical latices, some hybrid pigments such as nylon spheres covered with ultrafine titania particles, prepared by milling a mixture of the components, are in practical use as face masks or foundations of cosmetics.^^ Such hybrid pigments have several advantages in whiteness, UV screening, and smooth spreading, at the same time. The affinity of cosmetics to the human skin nomially increases as the size of the extender such as talc is reduced. This is one of the important factors of cosmetics to be considered. The gloss is also an important property. Since it is caused by regular reflection of light by flat particles, mica particles are often used as foundations for giving a proper gloss to the skin. The gloss of mica particles is known to increase with decreasing diameter in the range of 2 to 10 pmi.^° As the gloss also depends strongly upon the aspect ratio of

12. INDUSTRIAL APPLICATIONS

691

Table 12.10. Kinetic friction coefficients of various powders (Source: Ref. 260) Powder Species

Mean Diameter (^ni)

Talc Mica Kaoline Titania Titania (ultrafine) Zinc Flower Silica spheres Alumina spheres Nylon spheres Polystyrene spheres Polymethylmethacrylate spheres

3-10 3^-5 3-5 0.2-0.3 0.02-0.03 0.4-0.6 4-10 5-10 3-5 5-10 5-10

Kinetic Friction Coefficient 0.27-0.33 0.42-0.47 0.54-0.59 0.49 0.80 0.60 0.28-0.32 0.29 0.33 0.26-0.30 0.29

platelet particles and the smoothness of their surfaces, the shape and structure control of cosmetic powders is also strongly desired, in addition to the precise size control. To increase the brightness of color pigments, ultrafine barium sulfate powder of ca. 50 nm diameter is used widely as an extender pigment for cosmetics. It dramatically improves the affinity to the skin as well, owing to its small particle size. Ultrafine barium sulfate powders are also used as extender pigments for coating automobiles to give a gloss and improve the smoothness and anticorrosion properties. Rheological properties are also of importance for the practical use of paints, printing inks, cosmetics, etc. In order to increase the viscosity of aqueous and nonaqueous dispersions, ultrafine silica powders of the order of 10 to 20 nm in diameter are used as extenders. In this case, the size control of silica powder is decisive for providing functions mentioned in the above applications of barium sulfate particles.

12.6. Biological and Medical Uses Diverse functional particles called "microspheres" are widely utilized in biology and medicine for mechanistic studies of endocytosis of phagocytes, cell separation, diagnostic assays, remedial uses as represented by drugdelivery systems, etc.

692

APPLICATIONS

Except for the studies of some non-specific endocytosis of phagocytes, the biological and medical uses of microspheres are mostly based on the specific reactions between microspheres and some specific cells, such as antigen-antibody, saccharide-lectin, and enzyme-substrate reactions. For this purpose, microspheres are functionalized by conjugation with some functional groups such as antibodies, lectins, or enzymes to be reacted with antigens, saccharides, substrates, etc. on specific viruses or cells such as bacteria, or malignant tumor cells. In addition to the discriminative function for specific cells and other organisms, the microspheres are provided simultaneously with many devices for a variety of functions, such as labeling of the bound cells with markers, collection of the bound cells with loaded ultrafine magnets, or timed release of drugs to the bound tumor cells. In this section, we shall survey the biological and medical uses of functional microspheres, including their application to cytology and diagnostic examination in vivo, applications to cell separation and diagnostic assays, and remedial uses. Finally, the roles of monodispersed particles in this field are referred to. 12.6.1. Applications to Cytology and Diagnostic Examinations in Vivo a) Specific Staining of Proteins in Cells As an application of ultrafine particles, ultrafine gold particles ranging from 4 to 40 nm in mean diameter have been used to visualize specific target proteins in cells for light microscopy and electron microscopy, based on the immunological reactions between an antigen protein and an antibody linked to a gold particle. The procedure called the IGS (Immuno Gold Staining) method was introduced by Faulk and Taylor^^ in 1971, and have been developed extensively by De May et al'^^''^^ The characteristics of this method are as follows. 1) Clear transmission- and scanning-electron-microscope images of antigens by virtue of the high electron density of gold and the strong radiation of the secondary electrons. 2) Easy double staining for different antigens in each cell, using two kinds of gold particles of different sizes carrying different antibodies corresponding to the respective antigens. 3) High detection sensitivity, greatly enhanced by the deposition of metaUic silver onto the gold particles in a development process called the IGSS {Immuno Gold-Silver Staining) method.

12. INDUSTRIAL APPLICATIONS

693

4) The inert electrostatic adsorption of gold to bulky protein molecules such as antibody molecules, exerting no influence on the activity of the protein. 5) The long-term preservability of the stained samples. The advantages of gold particles are: 1) The easy preparation of fairly uniform particles with different mean sizes. 2) The high electron density necessary for clear electron microscope images. 3) A strong affinity to proteins. As is obvious from the purpose of this method, the size distribution of the gold particles must be as narrow as possible. Commercially available gold particles are prepared by reduction of HAUCI4 in dilute homogeneous solutions normally with a diethyl ether solution saturated with yeUow phosphorus for those of 3 to 5 nm, and with sodium borohydride or salts of ascorbic acid or citric acid under different conditions for larger ones varying in mean diameter up to 40 nm. Although there are two methods, the direct and indirect methods, for the IGS procedure, we employ more frequently the indirect method in which a secondary antibody, previously combined with a gold particle, is linked with a specific antigen through the intermediacy of a primary antibody bound directly with the antigen. As a rule, the indirect method is characterized by its high sensitivity and broad applicability to almost all antigens using a limited number of typical secondary antibodies combined with gold particles. Figure 12.54 shows the principle of the indirect immuno gold staining method. Protein A, which is a kind of protein of molecular weight 42,000 as a cell wall component of staphylococcus aureus, is often used in Gold Particle

Secondary Antibody

Primary Antibody Antigen

Fig. 12.54. The principle of the indirect IGS (immuno gold staining) method.

694

APPLICATIONS

Fig. 12.55. (a) Electron micrograph of a cell (PTK2 cell) stained with antitubulin and 5-nm gold particles linked to antibody, goat anti-rabbit/IgG, clearly illustrating labeled microtubules and mitochondria, (b) Light micrograph of PTK2-cell stained with antitubulin and 20-nm gold particles linked to goat antirabbit/IgG. The microtubules are stamed in red. In the anaphase cell, the interzonal fibers and remaining microtubules are clearly resolved. (From Ref. 268.) place of the secondary antibodies, because of its nature to link with the Fc part of immuno-globulins (IgG), as primary antibodies reacted previously

695

12. INDUSTRIAL APPLICATIONS

with specific antigens, and to physically adsorb gold particles. Some examples of IGS of a cell is shown in Fig. 12.55.^^^ The IGS and IGSS methods are also applied to the detection of extremely small amounts of specific proteins separated in a striped pattem on a cellulose membrane, prepared by transferring the same pattem of the polyacrylamide-gel electrophoresis through blotting (blotting method)?^^ Non-specific staining with simple gold particles for all proteins separated in a striped pattem on a cellulose membrane is often combined with the specific-staining blotting method for the accurate identification of each protein. Figure 12.56 is an example of non-specific staining of polypeptide bands of a protein blot on a nitrocellulose sheet with 15-nm gold particles, showing the effect of pH during the incubation for staining the blot with the gold sol on the staining intensity.^^^

k I

pk

'-

r

M

H

I !'

I

feB

C

D

E

Q

H

Fig. 12.56. Effect of pH during the mcubation for staining of a blot of a HMW protein standard, consisting of known amounts of proteins, with a 15-nm gold sol overnight: A, 3.15; B, 4.06; C, 5.10; D, 6.18; E, 7.07; F, 7.81; G, 8.45; H, 10.06. Maximal staining was obtained at pHs 3.15 and 4.06. The original color of the gold stain is purple. (From Ref. 272.)

b) Magnetopneumography The lung has a function of clearing dusts settled in itself, through endocytosis of some cells, called macrophages, in the puhnonary alveoli and their migration to the trachea at the end of their lives. The diagnostic examination of this dust-clearance function of the lung is conducted by inhaling ca, 1 mg of magnetite particles {- 2.8 (im) into the lungs, followed

696

APPLICATIONS

by checking every month the magnetic moment of the remaining magnetite particles with a highly sensitive magnetic flux meter, the so-called SQUID (Superconducting Quantum Interference Device), inmiediately after application of a magnetic field to the lungs to cause the magnetic orientation of the remaining magnetite particles.^^^ For this purpose, strict control of the mean size with a sufficiently narrow size ditribution is desired for the magnetite particles, since the retention time in the lungs is strongly dependent on the particle size; Le., the smaller the particle, the longer is the retention time.^^"* Cohen et al}'^^ reported that the amount of magnetic particles remaining after one year for non-smokers was found to reduce to ca. 10 % of the initial value on average in a first-order decay, while the corresponding value for smokers remained as high as 50 %. In the SQUID measurement, the relaxation of the residual magnetic moment is so fast - of the order of seconds due to the active microscopic motion of alveolar macrophages^^^ - that the measurement must be done at a fixed time immediately after magnetization. In other words, if we measure the relaxation of the residual magnetic moment as a function of time within a few minutes, it is possible to quantify the activity of the macrophages.^^^ Nowadays, this technique is known to be applicable to the measurement of the activity of general cells having some affinity and endocytic nature to foreign bodies. c) Mechanistic Studies of Endocytosis Although a wide variety of cells have some non-specific endocytic nature, as shown above, it is possible to provide cells with specificity in their endocytic activity by modifying the surfaces of solid particles with socalled biological ligands to be coupled with some specific receptors on the cells.^^^ For example, Sato et al}''^ modified the surfaces of ultrafine ferrite particles (10-20 nm) by coating bovine serum albumin (BSA) and subsequently conjugating the BSA with biological ligands, asialoglycopeptide (ASGP), having an ability to form complexes with specific receptors on Kupffer and parenchymal cells in the rat liver. Both for coating ferrite particles with BSA and for conjugating the BSA-coated ferrite particles with ASGP, glutaraldehyde was used as a cross-linking agent and a conjugating agent, respectively. The magnetic microspheres having the biological ligands were introduced into rat livers by perfusion or by injection in vivo through the tail vein. The magnetic particles were enclosed as a number of vesicles through endocytosis into each Kupffer cell and parenchymal cell, as observed by transmission and scanning electron microscopy on ultrathin

12. INDUSTRIAL APPLICATIONS

697

sections stained with uranyl acetate and lead nitrate. Meanwhile, the endocytic vesicles (endosomes) and lysosomes fused with endosomes, bound by the magnetic microspheres, were isolated using the HGMS (HighGradient Magnetic Separation) technique^ from rat liver homogenates. This technique may be applied to mechanistic studies of many processes associated with the endocytic activity of cells, including ligand-receptor complexation, the invagination of foreign bodies as vesicles, subsequent splitting of the vesicles, transport and fusion of the endosomes with lysosomes, recycling of the receptors, etc.^^ 12.6.2. Applications to Cell Separation and Diagnostic Assay As has been referred to in the introduction to this section 12.6, specific discrimination of cells or other organisms is required, in the first place, for their separation or labeling.^^^ For this purpose, we utilize various specific reactions, such as antigen-antibody,^^^'^^^ saccharide-lectin,^^^ and different reactions of receptors on cells with Fc part of antibodies, complements, saccharides, etc.^^^ If we fix an antibody or lectin to some support such as an agarose gel and use it as a column for cell affinity chromatography, particular cells with a specific kind of antigens, receptors, or saccharides on their surfaces can be captured and isolated selectively by the affinity chromatography. In the meantime, colloidal particles - polymer microspheres in particular - are also used widely in place of the fixed support for the labeling or separation of cells and for diagnostic assays. This is because of the high efficiency in separation, and thus high detective sensitivity, owing to the active motion of fine particles, and the fitness for small scale separation, as compared to affinity chromatography.^^^ In the cell separation with particles, however, an additional procedure is normally needed for the separation of cells bound by reactive particles from free cells, by natural or centrifugal settlement, electrophoresis,^°° magnetic separation,^° etc., on the basis of differences in specific gravity, electric charge, magnetic moment, etc.^^ If needed, the condensed cells bound by the reactive particles are isolated and purified by recovering the cells from the particles binding them. Figures 12.57 and 12.58 schematically illustrate the principle of the cell separation using reactive particles and a high-gradient separator.^^ When the magnetism of reactive microspheres is utilized for the separation of cells, the magnetic particles incorporated singly or multiply in each polymer microsphere are normally of ferromagnetic materials, but in a size range of superparamagnetism to avoid self-agglomeration among the

APPLICATIONS

698

cells

oo ooo

magnetic labeled antibody

I Oo

Ooo

magnetic separation

oo

ooo

Fig. 12.57. The principle of the cell separation by the use of inmiunoreaction between magnetic labeled antibody and antigen on cells with a high-gradient magnetic separator.

Suspension of Cells Labeled with Magnetic Micro^heres

Rinse

\ \ / /

Fig. 12.58. Scheme of a magnetic cell-separation apparatus.

microspheres; e.g., 10-50 mn for magnetite. The detection or quantitative evaluation of condensed or labeled cells is performed by light microscopy, electron microscopy,^^^^^^ turbidimetry, cohrimetry,^'^'''fluorometry,''' radiometry^''^''^''''^^''^'^ luminometry,'''-''^ etc.

12. INDUSTRIAL APPLICATIONS

699

For light and electron microscopy, surface-modified reactive ultrafine inorganic particles of a high electron density are usually used as visual markers of specific cells. In this case, separation of the cells is not necessarily needed for observation, as long as the concentration of the target cells is sufficiently high. On the other hand, since reactive particles normally have more than one reactive site each, when they are introduced into a suspension of mixed cells, the particles and target cells may be coagulated and precipitated by the bridging action of each other. But, if we choose appropriate conditions, it is possible to keep the aggregates metastable for a while and to quantify the content of the specific cells by tuibidimetry. If we use inmiunoreactive particles conjugated with chromophores, luminescent groups, fluorescent groups, or ligands containing radioactive elements such as ^H, ^^I, or ^^Co, we can evaluate the quantity of the bound, labeled, and isolated cells by colorimetry, luminometry, fluorometry, or radiometry with high sensitivity. In this case, if the optical groups can be liberated from the particles in the cell-particle aggregates by reaction with some added enzyme or by some alternative reactions after the immunoreaction, we will be able to measure the optical groups without the influence of the light scattering. Moreover, if we use inmiunoreactive particles conjugated with an enzyme in place of the optically active groups and add, after the inmiunoreaction, a substrate of the enzyme having groups which become luminescent on reaction with the enzyme, we can also avoid the influence of the light scattering of the cell-particle aggregates by measuring the luminescence of only the luminescent groups released from the substrate by the enzyme-substrate reaction and separated from the cell-particle aggregates. The advantages of using reactive particles for cell separation and immunoassay may be sununarized as follows. 1) High sensitivity owing to the active motion of fine particles. 2) Efficient separation and collection of specific cells or other organisms by the use of immunoreactive particles with a magnetic moment, electric charge, or high specific gravity. 3) There are numerous possibilities in labeling specific cells, leading to accurate measurement with high sensitivity. Above all, the potential ability of functional particles for the rapid separation and efficient condensation of specific target cells may be most

700

APPLICATIONS

highly appreciated. In particular, magnetic immunoparticles containing magnetite or ferrites are widely used because of the easy procedure of cell separation using a magnet. The original particles, prior to the surface modification with functional groups, consist of magnetic particles and some inorganic or organic polymers as a substrate for surface modification. These are as follows. a) Latices of synthetic polymers, such as polystyrene, poly(methyl methacryiate), polyacrylamide, polyacrylic acid, poIy(2-hydroxyethyI methacrylate), poIy(4-vinylpyridine), and copolymers thereof containing magnetic particles dispersed or as a single core in each polymer particle,^^^"^^^'^^^'^^^"^^^ or being coated with ultrafine magnetic oxide particles.^^^ b) Biopolymer particles such as albumin,^^"*'^^^*^^^"^^^ dextran,^^^ and gelatin,^^^ containing magnetic particles dispersed or as a single core in each polymer particle. c) Magnetic oxide particles covered with a silicon oxide layer having functional groups such as amino, carboxy, or isocyano groups.^^ d) Inorganic particles, such as porous glass or other ceramics, containing dispersed ultrafine magnetic particles.^^^ The composite particles consisting of small magnetic particles and organic or inorganic polymers are called magnetic microspheres. The mean size of magnetic microspheres ranges from tens of nm to tens of \xm according to the objectives. The incorporation of dispersed magnetic particles in each synthetic particle can be achieved by emulsion polymerization or radiation polymerization of monomers in the presence of magnetic oxide particles.^^^" 293,301,313314,316 j ^ ^ incorporation of a magnetic particle as a core in each polymer particle is performed by precipitation of magnetic particles in a solution of hydrophilic polymer such as dextran^°^ which can be adsorbed to the generated magnetic particles, or by polymerization of hydrophilic monomers with persulfates as a initiator in the presence of magnetite particles.^^^ The coating of magnetic particles over the surfaces of polymer particles is carried out by precipitation of magnetic particles in the presence of polymer particles. A silica layer having functional groups can be coated over the surfaces of magnetic oxide particles or general oxide particles containing magnetic particles by treating the oxide particles with silane coupling agents ?^'^^'^ This technique is important for providing oxide particles with reactive radicals on their surfaces, which are necessary for further linkage with additional functional groups. For this purpose, a multitude of silane

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coupling agents can be used preferably, as represented by the general formula, (YR')j^SiR4_n, wherein Y is a member selected from the group consisting of amino, aminophenyl, carbonyl, carboxy, isocyano, diazo, isothiocyano, nitroso, epoxy, and halocarbonyl; R' is a member selected from the group consisting of lower alkyl, lower phenyl, and alkylphenyl; R is a member selected from the group consisting of lower alkoxy, phenoxy, and halo; and « is an integer having a value of 1-3.^^^* The group R, typically a methoxy or ethoxy group, is readily converted into the hydroxy group as a result of hydrolysis by water in an acidic organic solvent such as methanol and ethanol. The silane couplers are then polymerized and at the same time conjugated to the hydroxy groups of metal oxide particles, by condensation with the release of water molecules, as illustrated below.^^^^

Y R'

RO-Si-OR I OR

Y +H2O

R'

•HO-Si-OH OH

-H2O

OH OH

The methods for coupling functional proteins, such as antibodies, lectins, protein A as a connector of antibodies, etc., with magnetic microspheres may be classified into physical methods and chemical methods. The physical methods are based on the physical adsorption through hydrophobic bonding between functional proteins and the hydrophobic surfaces of monodispersed latices such as polystyrene spheres.^^ They have some merits over chemical methods in being a simple procedure with little effect on the activity of the functional proteins. In addition, the dispersive stability of the magnetic microsphere, basically low due to the high hydrophobicity, can be improved with little effect on the hydrophobic bonding by partial copolymerization of hydrophilic monomers such as acrylamide or styrene sulfonate, or by limited coating of hydrophilic matters such as albumin or polyethylene glycol.-'^ However, further studies may be required to clear the essential problems proper to the physical methods, such as the low specificity in hydrophobic bonding and the low bonding stability. The chemical methods are based on chemical bonding between the functional groups of proteins and the particle surfaces,^^^'^^ so they are superior to physical bonding, at least, in specificity and stability. For this

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Table 12.11. Chemical methods for conjugation between microspheres and functional proteins Carbodiimide Method [Al-C-OH + Ri-N=C=N-R2 + H+

•

[Al-C-O-C

H2N-[B] •

[A]-C-NH-[B]

/NHRi ,

/NHRi + 0=C,

+ H*

Cyanogen Bromide Method

[A]

+ BiCN — • [A]^ /C = NH OH O , . x , ™, r^ 0-C-NH-[B] H2N-[B] /O^ / II ^ ^ ! - ? - • [A]^ /C=N-[B] [A] NH ^O ^oH

0-C-NH-[B] ,^/ II [A]^ O ^OH

Glutaraldehyde Method [AJ-NH2 +H-C-(CH2>-C-H+ HJS[-[B] O

•

[A]-N=C-(CH2>-C=N-[B]

O

H

H

Maleimido- Hinge Method

[A]-NH2 +

r-
"

+

HS-[B]

x

o'^ -•

[A]-NH-C-k^^N II

o

^ ^

o

S-[B]

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purpose, hydrophilic polymers with carboxy, amino, and hydroxy radicals, such as polymethacrylic acid, polyacrylamide, poly(2-hydroxyethyl methacrylate), and biopolymers, are used for microspheres. Typical reaction systems are those between carboxyl and amino groups mediated by carbodiimides {Carbodiimide method)^^ between hydroxy and amino groups mediated by cyanogen bromide {Cyanogen bromide method),^^^ between two amino groups mediated by glutaraldehyde {Glutaraldehyde method),^^^'^^^ and between amino and thiol groups mediated by N-(Y-maleimidobutyryloxy)succinimide {Maleimido-hinge method),^^^ as shown in Table 12.11. Here, one of [A] and [B] is a microsphere and the other is a protein. It is, of course, possible to connect [A] and [B] in a way such as linking diamines (H2N-R-NH2) to [A] first by one of the carbodiimide, cyanogen bromide, and maleimido-hinge methods, and then to [B] having amino groups by the glutaraldehyde method. Similarly, diols and dicarboxylates may be usable as a spacer, like the diamines. We shall now review a few examples of magnetic microspheres used for cell separation and diagnostic assays. a) Use of Magnetic Microspheres for Cell Separation Heden^^^ suggested in 1972 the utility of magnetic particles with surfaces modified and conjugated with enzymes, antibodies, and antigens, for the separation of biologically active proteins. Hersh and Yaverbaum^^ also used an immunoadsorbent of anti-digoxin antibodies coupled through an intermediate silane to magnetic iron oxide particles for determining the concentration of digoxin in human plasma by radioimmunoassay with ^^^I on the basis of magnetic separation. The magnetic particles used were ca, 2.5 [im in modal diameter with a size distribution ranging from 1.5 to 10 |im. Nye et al}^^ reported polymer-coated finely divided magnetic iron oxide particles conjugated to antibodies for the immunoradioassay of thyroxine, human placental lactogen, and digoxin, in which they used polymerized m-diaminobenzene as the polymer matrix. After that, Guesdon and Avrameas ^^^ developed polyacrylamide agarose-magnetite microspheres for enzyme-immunoassay for antibodies such as rabbit anti-bovine serum albumin antibodies, sheep anti-rabbit immunoglobulin antibodies, and human immunoglobulins including IgA, IgM, IgG, and IgE. They prepared the polyacrylamide agarose-magnetite beads by radical polymerization of acrylamide monomers in the presence of melted agarose, a cross-linking agent, ammonium persulfate as an initiator, and suspended Fe304 particles. The size distribution of the magnetic beads and the distribution of the

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magnetic oxide content were both rather broad, so they removed the ironfree beads by magnetic separation and excessively large beads by passing them through a sieve of 250 \im mesh to obtain beads of a size distribution approximately from 50 to 100 [xm in diameter. Furthermore, Molday et al}^^ and Rembaum et al?^^ prepared a wide variety of magnetic immunomicrospheres of mean diameter from 10 nm to 8 jmi, containing ultrafine magnetite particles, for cell separation, by radiation polymerization (y-ray) or emulsion polymerization of mixed or pure monomers such as methylmethacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), methacrylic acid (MAA), acrylamide (AA), and 4-vinylpyridine (VP), together with a cross-linking agent such as ethylene glycol dimethacrylate (EGDMA) and bisacrylamide (BAM), in the presence of ultrafine Fe304 particles for magnetic fluids. Then they conjugated antibodies or lectins to the magnetic microspheres through the reaction of the carboxy and/or hydroxy groups on the microspheres with amino groups of the antibodies and lectins using the carbodiimide, cyanogen bromide, or glutaraldehyde methods. According to the report of Molday, Yan, and Rembaum^^'', the magnetic microspheres were prepared by the Co-y irradiation of mixed monomers of 1.6 g MMA, 0.9 g HEMA, 0.3 g MAA, and 0.2 g EGDMA with 1 g of ultrafine FCjO^ particles and 1 g of Triton X-405 (0.2 Mrad at 0 °C for 60 min). Since it is difficult to achieve a uniform distribution of the iron content of each microsphere in this method, they classified the prepared microspheres by density-gradient centrifugation into some classes according to the iron content, and used one of these. The resulting copolymer microspheres containing Fe304 particles were 40 mn in average diameter. As the magnetic microspheres have hydroxyl and carboxyl groups, their hydroxyl groups were derivatized with a spacer, diaminoheptane, by the cyanogen bromide method, andfluoresceinisothiocyanate was tagged to the terminal amino groups for fluorescent labeling, whilst the carboxyl groups were coupled with diaminoheptane by the carbodiimide method, each of whose terminal amino group was then linked with an antibody, such as goat anti-rabbit Ig and goat anti-mouse Ig (Ig = immunoglobulin), by the glutaraldehyde method. For example, they conjugated goat anti-mouse Ig to the fluorescent magnetic microspheres and then mixed them with mouse spleen lymphocytes to separate only B cells having Ig receptors from T cells having no Ig receptors, using a horseshoe magnet, where ca, 97-99 % of the labeled B cells were retained by the magnet. The labeled cells were easily distinguished from unlabeled ones under a fluorescence microscope. Finally, applications of magnetic microspheres as visual markers of SEM

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and TEM and as diagnostic and/or therapeutic agents were also suggested. b) Separation of Cancerous Cells Kronick et al?^^ coated magnetite particles of diameter ca. 50 nm with a copolymer of 2-hydroxyethyl methacrylate and methacrylic acid along with a cross-linking agent, N,N'-methylene-bisacrylamide, by a redox polymerization in which coexisting ammonium persulfate is reduced by a trace of Fe^^ ions dissolved from the magnetite particles and initiates the polymerization by transferring its electrons to the monomers for forming free radicals. Thus, the magnetite particles are efficiently covered with the hydrophilic copolymer to yield magnetic microspheres containing a single magnetite particle in each sphere. The mean diameter of the magnetic microspheres is controllable in a range from tens of nm to tens of ^m. The magnetic microspheres are then conjugated with choleragen (cholera enterotoxin) using the diaminoheptane spacer by the carbodiimide and glutaraldehyde methods. By utilizing the nature of choleragen to link specifically to a glycolipid, ganglioside G^p on neuroblastoma cells (tumor cells of a cancer mainly of children), they isolated the Gj^^-containing neuroblastoma cells, corresponding to 10-20 % of the total neuroblastoma cells, with a purity of over 99 %. As will be shown in the next section 12.6.3, this technique was developed to therapeutic application in the efficient extracorporeal removal of neuroblastoma cells from bone marrow by the use of monoclonal antibodies conjugated to magnetic microspheres,^^^ where the monoclonal antibody is a uniform clone antibody produced from antibody-forming cells by a reaction with a specific antigen. c) Diagnosis of Virus Diseases Mizutani et a/.^°^'^^ developed a high-sensitivity diagnostic method for virus diseases based on an immunospecific reaction of an excessive amount of large non-magnetic immunomicrospheres with viruses, a subsequent reaction of an excessive amount of much smaller magnetic immunomicrospheres with the viruses previously bound by the large non-magnetic microspheres, centrifugal sedimentation of the vimses sandwiched between magnetic and non-magnetic microspheres, and quantitative measurement of the captured viruses by an optical device equipped with a magnet, as illustrated in Fig. 12.59. In this optical measurement they counted the number of interference fringes projected onto a screen by a laser beam incident upon and reflected from a meniscus created by the attractive force of a magnetic pole set slightly above the surface of the sample liquid to lift

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virus o oo

magnetic labeled antibody centrifugation X.J.XX

micro beads

~i^

antibody

magnetic pole laser beam

interference fringe

electromagnet

Fig. 12.59. Procedure for the quantitative measurement of captured viruses by an optical device equipped with a magnet. (From Ref. 303 (b).)

part of the fluid underneath containing concentrated magnetic microspheres holding the viruses together with the non-magnetic large microspheres. Figure 12.60 shows an example of the laser interference fringes for influenza virus EB3. The mean sizes of the EB viruses, the small magnetic microspheres, and the large non-magnetic microspheres were ca, 90, 35, and 1000 nm, respectively. To the large non-magnetic microspheres (-1 \im) and the small magnetic microspheres (-35 nm), purified rabbit IgG antibody was conjugated. Though they used a commercially available latex suitable for the conjugation of the IgG antibody for the large non-magnetic microspheres, the small magnetic microspheres were prepared according to the method of Molday and Mackenzie,^°^ in which an ammoniacal solution was added dropwise to the mixture of ferrous and ferric chloride in a dextran solution up to pH 1 0 -

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707

Fig. 12.60. An example of the laser interference fringes for influenza virus EB3. (From Ref. 303 (b).)

11, while stirring and heating to 60-65 °C for 15 min. The resulting dextran-coated magnetite particles in a size range of 30-40 nm with a magnetite core of ca. 15 nm after purification by gelfiltrationchromatography were quite stable against homocoagulation as well as non-specific heterocoagulation with cells in the physiological buffer. The ferromagnetic dextran microspheres were then oxidized with NaI04 by a modified procedure of Dutton et al?'^^ and covalently coupled with protein A extracted from staphylococcus aureus. The IgG antibody was linked to the protein A - ferromagnetic dextran conjugate through the protein A sites (see example (a) in section 12.6.1). In addition to the simplicity of the procedure, the detection sensitivity for virus EBB was about 4 times higher than the conventional fluoroimmunoassay. Moreover, since the viruses could be condensed up to 100 times the original concentration, it was possible to identify their viruses directly by electron microscopy and thus to make a diagnosis in the early stage of infection. This highly sensitive method may be applied to general immunoassays of dilute cells including many kinds of bacteria, as well as early diagnosis of virus diseases in general including AIDS. But, in the case of cells much larger than viruses, the large non-magnetic particles used as a baUast are not needed, since centrifugal separation of bound and unbound magnetic particles is possible without ballasting particles. d) Chemiluminescent Enzyme-Immunoassay for Tumor Markers Some proteins or carbohydrates in serum, such as carcinoembryonic antigen (CEA), a-fetoprotein (AFP), and tumor-related antigens including CA19-9 and CA125, are well known as markers of malignant tumors such

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APPLICATIONS

as colorectal carcinoma, hepatoma, and pancreatic and ovarian cancers.'^^^ The clinical assays of these antigens as tumor markers are of vital importance for diagnosis of the corresponding tumors, and thus they have been studied extensively. Above all, chemiluminescent enzyme immunoassay may be one of the most sensitive methods for this purpose. Nishizono et al?^'^ successfully conducted highly sensitive enzyme immunoassays for the above tumor markers using a unique chemiluminescent compound, AMPPD, first introduced by Bronstein et al?^^~'^^^ as an ultrasensitive substrate for an enzyme, alkaline phosphatase (ALP). AMPPD is a derivative of 1,2dioxetane phosphate, described as 3-(2'-spiroadamantane)-4-methoxy-4(3"-phosphoryloxy)phenyl-l,2-dioxetane disodium salt, having a 1,2dioxetane moiety to generate the energy for chemiluminescent emission and a phenol phosphate group as a trigger of luminescence by ALV?^^ Enzymatic cleavage of the phosphate group destabilizes and decomposes the AMPPD molecule to produce a light emission at 470 nm. Prior to the immunoassay, monoclonal antibodies to the marker antigens and Fg^. fragments of the antibodies are conjugated to albumin-coated magnetic microspheres and to the enzyme ALP, respectively. Then, both the antibody-magnetic microsphere conjugate and F^^. fragment-ALP conjugate are mixed with a sample containing a marker antigen to form a sandwich complex of A-M-A', where/I is an antibody on a magnetic microsphere, A' is di Fjb' fragment of antibody A linked to ALP, and M is a marker antigen. After collecting the sandwich complexes by high-gradient magnetic separation, AMPPD is added to react with the ALP of the collected complexes, followed by luminometry. Nishizono et al?^^ used a monodispersed polystyrene/polyacrylate copolymer latex coated with 50 wt % ultrafine magnetite particles of overall diameter 0.3 tmi, prepared by partial oxidation of FeCl2 with NaN03 in the presence of the latex particles suspended in a buffer solution. The magnetite-coated particles were further coated with antibodies and then with bovine serum albumin by adsorption. For the conjugation of V^, fragments of antibodies, prepared by pepsin digestion of antibodies, to the ALP of calf intestine, they applied the maleimido-hinge method by using N-(Y-maleimidobutyryloxy)succinimide for modification of ALP,^^^ in which the thiol groups in the hinge region of the Fj^b- fragments of the antibodies are linked to the maleimido groups of the modified ALP. 12.6.3. Remedial Uses The medical application of microspheres is not limited to diagnostic

12. INDUSTRIAL APPLICATIONS

709

assays. They are extensively used as carriers in drug delivery systems^ bioreactors, biocleaners, heat sources in thermotherapy such as hyperthermia, etc. for treatment of a multitude of intractable diseases such as malignant tumors. Strenuous efforts are being continued to explore new therapeutic systems. The sizes of the microspheres are in a very wide range from ca. 10 nm to even a few millimeters if we include large beads as artificial organs ?^^ a) Drug Delivery Systems Drug delivery systems (DDS) applied through intravascular administration or direct infusion into the affected part are particularly useful means for attacking malignant tumors intensively with minimized side effects.^^^ The materials of the microspheres used for drug delivery are mostly chosen from those which are biocompatible and readily subject to biodegradation, such as albumin, fibrinogen, gelatin, starch, polylactides, polyglycolides, and dextran. However, in some cases, materials which are not decomposed in vivo, such as ethylceliulose, are used. Microspheres containing medicines, such as anticancer agents, for DDS are prepared by several methods, such as solvent evaporation, thermal modification, or cross-linking, in the presence of the medicines. For designing microspheres, their size, porosity, drug-content distribution, and the hydrophobicity of the drug and its matrix are important factors to be controlled, since they have a decisive influence on the drug release rate. Thus, the microspheres should be monodisperse, or intentionally poly dispersed under strict control of the size distribution. The size of the microspheres is responsible for their distribution in vital tissues. For example, when microspheres of 0.1 - 3 tmi in mean diameter are infused through an artery or a vein, they may be removed rapidly from blood through endocytosis by the Kupffer cells in the liver or by the spleen tissue. Intravenous administration of microspheres larger than 7 [im may cause embolism of the capillary vessels in the lung, so they may be captured there. If the size exceeds 12 \xm, microspheres injected through an artery are shortly trapped in the tissues downstream. Hence the size of the microspheres is chosen according to the target organs. In this sense, albumin microspheres are particularly convenient, since their size can be changed in a wide range from 0.3 to 500 ^un by controlling some factors in the preparation of the emulsion.^^^"^^^ Since cancerous cells are more active in metabolism than normal cells for their rapid proliferation, their proliferation is suppressed by clogging the

710

APPLICATIONS

arteries which supply routes for oxygen and nutrition: z.e., embolization therapy ?^^ If an effective therapeutic drug is enclosed in the microspheres, the remedial effect is still more pronounced, by virtue of the long-term retention of a high local concentration of the drug in addition to the embolization effect {chemical embolization therapy)?^^ Magnetic microspheres are also used in DDS for more selective targeting. Widder et al}^ prepared albumin microspheres (0.2 - 2 pun) including both an anticancer drug (doxorubicin) and magnetite particles ( 1 0 - 2 0 nm) by heat solidification and administered them through an artery to Yoshidasarcoma-bearing rats on each tail. They observed a total remission rate as high as 75 % for rats having a magnet fixed on the tail, owing to the concentration of the magnetic microspheres in the tumor on the tail, while the remission rate was zero for those without a magnet on the tail. Similar concentration of magnetic microspheres or magnetic emulsion has also been observed for other organs of rats, such as the lung, bladder, and kidney, in the same procedure.^^^ The immunospecific magnetic microspheres, widely used in diagnostic assays, are expected to constitute the most efficient drug delivery systems. For establishing such systems in clinical treatment, we must compile a great deal of data on safety assessments, in addition to the ultimate refinement of the methods including the rapid clearance of the magnetic metal or metal oxide particles, while evading the endocytosis in the liver and spleen, after completion of their missions. b) Bioreactors L-asparaginase has been used extensively for the treatment of acute lymphoblastic leukemia on the assumption that the circulating L-asparagine is essential for leukemic cells which appear to lack the abUity of producing L-asparagine. However, the use of L-asparaginase involves several problems, such as hypersensitivity, antibody formation, rebound phenomenon due to the rapid induction of liver L-asparagine synthetase. To overcome these problems, the enzyme was immobilized in solid drug carriers, such as microspheres and liposomes, giving a higher stability against denaturation and reduced immunogenicity. Also, it was reported that L-glutaminase used together with L-asparaginase reduced the rebound phenomenon. Edman et alr'^^ injected L-asparaginase-L-glutaminaseloaded polyacrylamide microspheres (~1 jmi) directly into the abdominal cavity of a rat, and found that L-glutamine and L-asparagine were both decreased, and kept much lower than the ordinary level for five days, while

12. INDUSTRIAL APPLICATIONS

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the level of L-asparagine was expected to be restored within 24 h if Lglutaminase was not used together. Moreover, they installed such microspheres into outer compartment of a commercial hollow fiber dialyzer and completed an efficient extracorporeal reactor for decomposing circulating Lasparagine and L-glutamine. Hence it has been suggested that the use of such bioreactors may have an potential applicability for the treatment of acute lymphoblastic leukemia. c) Biocleaners Treleaven et al?^^ proposed a therapeutic application of magnetic microspheres in the treatment of the childhood cancer, neuroblastoma, through extracorporeal clearance of the neuroblastoma cells by magnetic separation from harvested bone marrow destined for autologous transplantation. In this system, they prepared monodisperse magnetic polystyrene beads of 3 |xm diameter containing 19.4 wt % of ultrafine magnetite by polymerization of styrene in the presence of divinylbenzene as a cross-linking agent and ultrafine magnetite particles, and subsequent modification to render them hydrophilic. Then sheep anti-mouse Ig was mixed with the magnetic beads to be physically adsorbed thereon. In the meantime, they admixed a mouse anti-neuroblastoma monoclonal antibody with bone marrow harvested from a patient to label selectively the tumor cells of neuroblastoma therein with the monoclonal antibody. Finally, they mixed the magnetic immunospecific microspheres with the so-treated bone marrow and collected selectively only the tumor cells combined with the magnetic microspheres through the immunological reaction between the Ig antibody on the magnetic beads and the monoclonal antibody on the tumor cells using Sm-Co permanent magnets arrayed along a flow tube. They removed more than 99.9 % of the tumor cells by this method. This method may be suitable for the efficient treatment of a small amount of biofluid without loss. Inununomicrospheres are used also as a blood cleaner. For example. Savin et al?^^ obtained a good result in the treatment of adverse digitalis effects by hemoperfusion through a colunm of the antidigoxin antibody fixed to agarose polyacrolein microspheres. d) Hyperthermia Hyperthermia is a modality for cancer therapy. This is a kind of thermotherapy of cancer, in which the whole body, affected part, or blood is wanned up by use of hot water, microwaves, radiofrequency, ultrasound,

712

APPLICATIONS

or some other energy sources, since cancerous cells are mostly more vulnerable to heat than are nomial cells. However, it is difficult to limit the heating only to the tumorous region deep in the body. In this respect, the method of Luderer et al?^^ is noteworthy. These workers used a fenimagnetic glass-ceramic of mean dimensions 1.5 (im, nominally consisting of Li ferrite (--72 wt %), P2O5 (23.7 wt %), SiO^ (3.4 wt %), and AI2O3 (0.4 wt %). The powder was prepared by milling a glass of the above composition, containing ferrimagnetic Li ferrite, crystallized from the melt. They injected mice with the magnetic powder, suspended in a physiological saline solution, into the region of a subcutaneously transplanted mammary carcinoma in the left thigh, and attained a high total regression (-50 %) and a total cure (-12 %) by applying an alternating magnetic field (10 kHz, 500 Oe) for intensive heating up to 43.5 °C only on the tumorous part, based on the energy of the magnetic hysteresis loss. This method is outstanding in its highly efficient local heating of a target deep in the body. Probably because of the large particle size around 1.5 [im, the particles remained virtually inunobile at the site of injection for at least 8 days post-injection. The next development of this method may be inmiunospecific targeting and appropriate mobility, both for targeting and for smooth post-mission excretion. 12.6A. Roles of Monodispersed Particles As is obvious from the survey on the biological and medical use of microspheres, their size control is of primary importance for realizing their ultimate functions, and thus the size distribution must also be strictly controlled to be as sharp as possible or of some designed distribution. Now that some synthetic polymers are available as almost perfectly monodispersed latices with mean sizes over a wide range, at least, the technology in size control of functional microspheres of these non-biopolymers may be close to the ultimate goal. However, once we consider the application of multiftmctional microspheres to remedial purposes by way of administration, they must not only be pathologically powerful against diseases, but also in perfect conformity with all physiological conditions. This seems to be the main reason for the still modest use of fully functional microspheres for clinical treatments, as compared to their remarkably positive use for diagnostic examinations in vitro. Thus it is requisite for their use in vivo to add another function to the microspheres used in vitro; i.e., the readiness for rapid excretion or biodegradation after completion of their missions. For this purpose, it would be desirable to advance the development of new

12. INDUSTRIAL APPLICATIONS

713

technologies for the preparation of monodispersed particles of biopolymers, or fully biocompatible synthetic polymers, perfectly controlled in mean size. It is also important to provide the magnetic particles with some evasive nature from the endocytic processes in the liver and/or the spleen, and to make the mechanistic studies on the clearance of foreign bodies such as artificial polymer latices, magnetic metal oxides, etc. remaining in the human body. The ultrafine magnetic oxide particles used for the functional microspheres are usually magnetite (FejOJ particles with a mean size mostly in a range of 10-50 nm, corresponding to the range of transition from ferromagnetism to superparamagnetism (see section 12.3.1), in order to keep their stability against coagulation due to their magnetic attraction. For the preparation of ideal microspheres of high stability and uniform performance, the distribution in the magnetic oxide content among microspheres must be minimized. This is not necessarily achieved yet even in systems with almost perfectly monodispersed polymer latices prepared by emulsion polymerization in the presence of magnetic oxide particles. For this purpose, we must realize a homogeneous dispersion of the magnetic oxide particles and their uniform incorporation into all latex particles during their growth. An alternative solution of this problem may be the specific deposition of magnetic particles onto the surfaces of monodispersed latex particles, if necessary, followed by additional growth of the latex particles to incorporate the magnetic oxide layer, or selective growth of a polymer shell over a monodispersed magnetic oxide core. If each microsphere consists of a single magnetic oxide core and a polymer shell layer, the use of monodispersed magnetic oxide particles precisely controlled in their mean size is highly desirable, since the magnetic response of each particle to the external magnetic field is quite sensitive to its size, particularly in the transition range of the magnetism. However, the magnetic oxide particles available so far in such a size range, including magnetite particles in the range of 10 - 50 nm, is far from satisfactory in uniformity, even for those obtained in reversed micellar systems (see section 7.3.4). The development of a new technology for the synthesis of monodispersed magnetic oxide particles strictly controlled in mean size within this size range is important for the future. Also important is the shape control to form ellipsoidal magnetic particles of a definite aspect ratio, affecting their movement in blood vessels, their retention time in a target region, and their magnetic performance based on their shape anisotropy (see section 12.3.1).

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APPLICATIONS

In the near future, multifunctional microspheres including magnetic inmiunomicrospheres will be used widely as a powerful measure for clinical treatment including gene therapy. In particularly, it will be necessary to design multifunctional microspheres characterized by a navigation system for accurate targeting, immunospecific affinity to target cells of malignant tumors, intensive release of medicines at a predetermined rate, biodegradation of the matrix polymer following the achievement of the mission, and rapid excretion of the remainders, such as magnetic navigator particles, decomposed fragments of the matrix polymer, etc. The treatment will be performed in combination with embolization therapy and some magnetic recovery system with empty satellite magnetomicrospheres for absorbing the surplus or remaining medicines which are harmful to normal tissues, and/or for capturing the post-mission major microspheres. For realizing these strategies with multifunctional microsphere systems, it is indispensable to continue the comprehensive studies from the total viewpoints of colloid science, including areas such as particle formation, particle interaction, and surface modification, as well as bioscience including genetics, immunology, biochemistry, pharmacy, cytology, bacteriology, physiology, and pathology. This is undoubtedly one of the most fascinating fields in the application of monodispersed particles at present and for the future.

References 1. 2. 3. 4. 5.

6. 7. 8.

9.

G. C. Famell and J. B. Chanter, J. Photogr, Set 9, 73 (1961). E. Klein and E. Moisar, aK Patent 1,027,146 (1966). C. R. Berry, J. Photogr ScL 18, 169 (1970). S. Bando, Y. Shibahara, and S. Ishimam, J. Imaging Set 29, 193 (1985). F. Mosar and R. K. Ahrenkiel, in "The Theory of the Photographic Process," 4th Edn., (T. H. James, Ed.), p. 37. Macmillan, New York, 1977. T. Tani and Y. Sano, J. Photogr ScL 27, 231 (1979). H. Hirsch, 7. Photogr Set 10, 129 (1962); ibid. 10, 134 (1962). C. Herring, J. Appl. Phys, 21, 301 (1950); M. Yan, in "Advances in Powder Technology," (G. Y. Chin, Ed.), pp. 99-133. Am. Soc. Metals (Ohio), 1982. K. S. Mazdiyasni, C. T. Lynch, and J. S. Smith, J, Am, Ceram. Soe. 48,

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