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Glasses and glass-ceramics from gels
Journal of Non-Crystalline Solids 73 (1985) 651 660 North-Holland, Amsterdam
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G L A S S E S AND GLASS-CERAMICS FROM GELS Sumio SAKKA Institute for Chemical Research, Kyoto Universi(v, Uji, Kvoto-Fu 611, Japan
The present state and the future of glasses and glass-ceramics synthesized by the sol-gel method from metal alkoxides have been critically reviewed and discussed. Glasses and glassceramics can be prepared in forms of bulk, fiber, sheet, coating film and particulate. Advantages and disadvantages in applying the sol-gel method for each form are discussed. It is shown that this method is very prospective and many materials will be manufactured by this method in the year 2004.
1. Introduction This paper is concerned with the present and future status in the application of the sol-gel method to the preparation of glasses and glass-ceramics. This method is 14 years old, if Dislich's paper [1] is regarded as the start. This is still a new field, although some technological aspects of this method are already known. This might make the prediction of its future easy, if the term future is used in a vague sense. It is not easy, however, to predict the state-of-the-art of the sol-gel method in the year 2004, as is true in all other fields of science and technology. Suppose that several methods are assumed for preparing particular materials. If it is known that one of them is possible at present, that method will be applied to industrial production in a few years instead of 20 years later. Then that method might be replaced by another method in 20 years. Conversely, it is not assured that a difficulty in a method which is not solved at present will disappear in 20 years. The other reason for the difficulty in prediction for the year 2004 is that the method for producing materials is profoundly affected by the progress of the device technology. Considering the above, the only way I can take for prediction is to be based on my own desire for developing the method, hoping that my own desire may be similar to that of people who are engaged in this field. Then, I do not think that this desire may be simply a desire. There are ample examples in which a technology, originally regarded impossible, becomes a practically useful one simply because scientists and engineers try hard to develop the technology with the motivation of desire. The technology of the optical fiber is one of such technologies. However, the presence of embryos in a technology will greatly help us consider the future of the particular technology. Fortunately, there are lots of 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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embryos that have already grown to nuclei. The scientific bases and applications of this method have been discussed in the two International Workshops [2,3] of 1981 and 1983, the Annual Meeting of the American Ceramic Society of 1981, the Gordon Conference on Glass in 1981, the Spring Symposium of the Materials Research Society [4] and other numerous scientific meetings. In this paper embryos and nuclei of the sol-gel method are reviewed to discuss its future. Stress is laid on the method using metal alkoxides as starting materials. Discussion is made on each of the forms of the products: bulk, fiber, thin sheet, coating film and particulate.
2. General features of the sol-gel method using metal alkoxides Generally, the sol-gel method used for producing glasses and glass-ceramics consists of the processes: preparation of a homogeneous solution, change of the solution to a sol, gelation of the sol and conversion of the gel to glass and glass-ceramics. The following advantages of this method emerge from the consideration of each process [5,6]. (1) High purity glasses can be prepared. (2) Multicomponent glasses and glass-ceramics of high uniformity can be expected. (3) Glasses and glass-ceramics can be obtained at relatively low temperatures. Final products may be obtained at low temperatures near room temperature for gels. (4) There are possibilities of producing glasses of new composition, which could not be obtained by conventional melting methods. (5) An ensemble of simple operations without expensive facilities leads to final products. The most important of these are (3) and (4). It is said that low temperature processing is energy-saving. That may be the case when much energy is not needed for preparing the starting metal alkoxides. However, this is the point which has to be considered deliberately. The low temperature processing (3) is of great significance for the following reason. This makes it possible to combine glasses and glass-ceramics with metals and plastic materials at low temperatures into composite materials. Good examples are seen in the coating of the substrate with glass or glassceramics as will be discussed later in this paper. There is another example. At present, the metal and semi-conductor parts are attached to the ceramic substrates in manufacturing multilayered integrated circuits. It may be possible to fire the whole circuit if ceramic substrates can be fired at lower temperatures than 1000°C by using metal alkoxides as starting materials. Another important point in the low temperature processing is that a transition element ion may exhibit a difference valence state in glasses prepared at low temperatures than that in glasses prepared at higher temperatures by melting [7]. The possibility of producing new glasses (4) is also of great importance. As
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an example, glass fibers of ZrO2-SiO 2 compositions containing as much as 48% ZrO 2 have been prepared [8]. This is only possible with the use of the sol-gel method. Preparation by melting of such glasses not only requires extremely high temperatures, but also encounters difficult problems such as liquid immiscibility at the melting temperature and phase separation and crystallization during cooling of the melt. Advantage (4) makes the future of the sol-gel method very significant.
3. Formation of bulk glasses The preparation of bulk glasses at low temperatures is a subject of great interest. However, production of large bulk glasses by the metal alkoxide technique is not an easy task because of large volume shrinkage and the dissipation of large amounts of volatile species during the sol-gel and gel-glass conversions. Crack formation, fracture and swelling occur in various stages leading to glass formation. A long time period is required, which may result in low productivity. These difficulties have been partially solved in forming S i O 2 glass by use of colloidal silica gels [9-11]. As for the metal alkoxide technique, it is difficult and meaningless to produce conventional large products like window glasses and beer bottles by this method. However, it may be valuable to make SiO2 glass plates used as photomasks for integrated circuits and SiO2 and SiO2-based glass rods for drawing optical fibers. Optical fibers have to be drawn from the rod at fairly high temperatures, This decreases the value of the metal alkoxide technique because of increasing heating temperatures, but the simplicity of processing has to be evaluated. Susa et al. [12] made large rods of SiO2 and SiO2-based glasses used for drawing low loss optical fibers. This gives some hope for the future of preparing bulk glasses from metal alkoxides.
4. Formation of glass fibers 4.1. Fibers made by drawing from alkoxide solutions at low temperatures Fibers prepared by heating after drawing from alkoxide solutions at near room temperature are considered. The maximum temperature of heating required in this technique is much lower than that required for fibers drawn from a bulk glass rod. Thus the advantage of low temperature processing is expected. Two examples of silica-alumina fibers [13] and silica fibers [14] will be mentioned, in order to show the advantage of the metal alkoxide technique and discuss its future. Conventional silica-alumina refractory fibers have been prepared by centrifugal spinning of the melt poured out of a rotating cylindrical container through the orifice. Fibers produced in this way are not continuous and
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accordingly cannot be made into textiles. With the use of the sol-gel method, continuous silica-alumina fibers have been produced for the first time [13]. The fibers are drawn from the viscous alkoxide solution near room temperature and are then heated at several hundred degrees. Precipitation of a controlled amount of A1203 crystals gives a high Young's modulus of about 150 GPa, which cannot be achieved by oxide glasses of any composition. Such high Young's modulus fibers are useful for strengthening plastics and metals. The future problem will be to design many other compositions which are supposed to give various excellent characteristics. 4.2. D&cussion of opticalfiberformation by drawing at room temperature
In order to make silica fiber, silicon tetra-ethoxysilane Si(OC2H5)4, for example, is used as the starting alkoxide. When an Si(OC2Hs)4-H20-HC1CzHsOH solution of pertinent composition is kept at temperatures near room temperature, hydrolysis-polycondensation reactions increase the viscosity of the solution [14,15]. Fibers can be drawn from the solution at viscosities higher than about 10 P. Heating at 500-900°C change the gel fibers to the silica glass fibers. The cross-section of the fibers thus obtained is circular or non-circular, which can be controlled by the composition of the starting alkoxide solution. The diameter of the fiber can be varied between 10-100/~m by controlling the viscosity of the solution at fiber-drawing and the drawing rate. In the future it will be possible to make optical fibers of SiO2-based glasses for the reasons described below. (1) It should be easy to modify the composition of the fiber into those of the SiO2-TiO2, SiOz-GeO2, SiO2-B203 and other systems by adding alkoxides of Ti, Ge, B and other metallic elements to the Si(OC2 H s)4 solution. Glass fibers of different refractive indices will be thus obtained. (2) Coating of the fibers with a glass of desirable composition will be carried out by dip-coating, in order to achieve the core-clad structure. (3) Heating the gel fibers in C12 or SOCl2 atmosphere will remove OH groups, resulting in low optical loss fibers. At present, the strength of gel-derived silica fibers is about 50% lower than that of silica fibers prepared by conventional methods at high temperatures. The reason for this is not known yet. However, the problem will, it is hoped, be overcome by some means in the future. It is known that Mahler and Bechtold [16] produced high strength silica glass fibers from gel fibers prepared by the unidirectional freezing of gel. Briefly, the metal alkoxide technique will prove a very useful technique for producing various kinds of fibers in the future. 5. Thin glass sheets
The sol-gel method starting from metal all'oxides presents the possibility of obtaining thin glass sheets of thicknesses ranging from several tens to several
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hundreds of microns [17]. The method consists of forming thin gel sheets from metal alkoxide solutions near room temperature and converting them to glass sheets by heating to, say, 500-800°C. For instance, thin SiO 2 glass sheets can be obtained at lower temperatures than 1000°C. The following three technologies have been proposed in order to make gel sheets from metal alkoxide solutions. (1) Scooping off the viscous solution by using a wire ring. (2) Drawing the viscous solution into a thin sheet through a slit. (3) Forming a thin sheet by applying the viscous solution on a plastic plate and then peeling off the plate. An explanation will be made of the preparation of S i O 2 glass sheets. In any of the a b o v e t h r e e m e t h o d s , the c o m p o s i t i o n of the s t a r t i n g Si(OCzH5)4-H20 HC1-C2HsOH solution must be in a limited compositional region. The requirements of composition of the starting solution for sheet formation is very similar to those for fiber drawing; the water content must be relatively low so that linear alkoxide polymers may be formed in the course of the hydrolysis-polycondensation reaction [15]. It is true that the above three techniques give a possibility of producing thin glass sheets, but there are many difficulties. The most serious difficulty common to the three techniques arises from an enormous shrinkage in volume and area on solidification of the solution, which may cause deformation, crack formation and fracture of gel sheets. In order to avoid this, it would be necessary to expose both the surfaces of a solidifying gel sheet to the ambient atmosphere at the same time, so that the reaction and drying might proceed at similar rates on both surfaces of the sheet. At present this problem is not yet fully solved. I hope, however, this problem will certainly be solved in the future. Then the thin sheet formation will be a very useful technique. It should also be noted that techniques (2) and (3) out of the three will make possible the production of continuous glass sheets, that is, glass ribbons.
6. Coating films It is already known that the dip-coating or the spin-coating of glasses, metals and plastics using metal alkoxides solutions is very useful for modifying the properties of substrates with a large surface area and provide substrates with new active properties [18]. In the years around 1970, dip-coating was applied by Schroeder of Schott Company in Germany [19] to modify the optical properties of glasses and plastics. In recent years the dip-coating technique has again attracted much attention as the demand for new electronic materials has increased. It is expected that the dip-coating is useful for protecting particular electrical characteristics of the substrate and providing the substrate with particular electrical and magnetic properties, and so on, in addition to a modification of optical properties and an increase in chemical durability.
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In the future, dip-coating will be widely applied. The present status and the future of this technique will be discussed in the following.
6.1. Dip-coating using metal alkoxide solutions The dip-coating consists of three steps: dipping, withdrawal and heating. The spin coating consists of two steps: spinning and heating. The following concerns dip-coating, but spin coating shares most of the advantages and disadvantages of dip-coating. In order to form glass and glass-ceramic coating films which firmly adhere to the substrate, heating of the coated substrate to a certain temperature is required. It is assumed that heating forms chemical bonds between the film and substrate. Since relatively low heating temperatures are sufficient, it is not required that the substrate be highly heat-resistant. A temperature of 500°C is often sufficient [6,20], except when the coating films have to be converted to fully crystallized ceramics. Another advantage of the dip-coating is that coating of the substrate of a large surface can be easily attained, compared with sputtering, vacuum vapor deposition and spraying. A characteristic of this technique is the thickness of the film. The film successfully prepared in one application is thin at thicknesses of between 50 to 300 nm or thinner. The film is peeled off under the coating condition of high concentrations of the coating oxide in the solution, high viscosities of the solution and high rates of withdrawal, all of which might give a thick film in one application. A small thickness is usually an advantage for films of electronic activity, while it is usually a disadvantage for protecting coatings. Thicker films can be obtained by increasing the number of applications. It is essential to repeat the whole coating process including heating to form a uniform coating film which firmly adheres to the substrate. Since heavy repetition is laborious, however, this technique is not so suitable for films of several microns or thicker films. Various coating films which have already been prepared are reported by Dislich [6] with references. Table 1 lists examples of the present and future possible applications of the dip-coating technique. In table 1, the purpose of (a) is to increase the scratch resistance of metals and plastics by applying a coating of hard oxides. In plastics the bonding between the film and the substrate has to be accomplished chemically at low temperatures instead of heating up to 500°C, because plastic materials are not so heat-resistant. In the case of metal substrates, the chemical bond between the film and the substrate can be formed by heating at 400-500°C. It is assumed that the bonding may be formed owing to the presence of a thin oxidized layer on the surface of metals. (b) aims at the chemical protection of the substrate, such as the protection from oxidation and increase in acid resistance of metals. Passivation of silicon semiconductors by SiO2 coating can be included in (b). An increase of protection ability by the introduction of nitrogen into SiO 2 film has been attempted [32].
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Table 1 Examples of application of the dip-coating using metal alkoxide solutions Objective
Example
(a) Mechanical protection protection of plastics of substrate and metals (b) Chemical protection increaseof durability, of substrate protection of Si (c) Films of particular colored and absorbing optical characteristics film[6,21,22] reflecting film [23] anti-reflection film [24] photoconductive film (light, X-ray) (d) Ferroelectric f i l m capacitor [15,25,26] (e) Electrically conducting electronic conductor [27,28] film ionic conductor [29] (f) Donation of magnetic verticallymagnetizing property film (g) Photocatalytic f i l m generation of H 2 [30] (h) Analysis and catalyst ISFET [31] catalyst
Possibility Exampleof composition future
SiO2
yes
SiO2
yes
TiO 2-SIO2, SiO 2 -R mOrt ,
yes yes future
oxide of Fe, Cr, Co In 203 - S n O 2 Na20-B203-SiO 2
yes yes
BaTiO3, KTaO3, PLZT In203-SnO2, SnO2-CdO
yes future
B-alumina -
future yes future
glass
As shown in (c) of table 1, there are various investigations concerning the modification of optical properties of the substrate and providing the substrate with new optical characteristics. The formation of a photoconductive layer is cited as an example of a target for the future. For instance, the coating of substrates of large surface area with photoconductive films sensitive to light or X-rays will be quite useful for in situ display. Dielectric coating films shown in (d) have already been investigated to some extent so far. Capacitor films of high dielectric constant can be expected. The electrically conductive films shown in (e) are important and are being investigated fairly intensively. A serious problem is found in the latter case. It is k n o w n that I n 2 0 3 - S n O 2 films prepared by dip-coating, for example, show conductivities lower by an order of magnitude than those prepared by other techniques like chemical vapor deposition. This problem has to be solved, in order to make this technique useful. The magnetic films in (f) are listed as examples for the future. In this case it is the key point whether in the dip-coating the orientation of crystals and magnetic anisotropy can be controlled or not. F o r m a t i o n of photocatalytic and catalytic films cited in (g) and (h), respectively, will possibly emerge as an important technology in the future. It is expected that thin films will work well for these purposes.
6.2. Future applications of the dip-coating The dip-coating technique is already at the stage of practical application for some fields. However, there are several technical problems to be considered for further progress of particular films, as follows.
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(1) (2) (3) (4) (5)
Adherence of the film with the substrate. Defects such as chipping and cracks in the film. Reaction of the coating film and the substrate. Small thickness of the film prepared by one application. Porous structure of the film. Problems (1)-(3) can and should be solved. For (1) and (2) the mechanism of adherence between the coating film and substrate should be investigated. As for problem (3), one layer of SiO 2 prepared by the dip-coating is usually enough to protect the coating film from reacting with the substrate. The small thickness of the film (problem (4)) has already been discussed. Dip-coating is not effective, however, for the mechanical protection of the substrate, because it needs films thicker than several microns at least. Next, problem (5) concerning the porous structure of the film will be considered. Glasses and glass-ceramics formed through the gelation of metal alkoxide solutions may retain the structure of the gel when they are not heated at sufficiently high temperatures. They may be an agglomerate of round solid particles or a three-dimensional network containing fine, continuous pores, that is fine channels. If the former is the case, the electrical conductivity, for instance, would be lower than expected for a continuous structure. The lower conductivity of ITO films mentioned in the explanation of table 1 might be attributed to this structure. In this case the porous structure is not favorable and an investigation for modifying a discrete structure into a continuous one has to be carried out. It should be noted, however, that the porus structure should be more desirable than non-porus structures for certain applications such as photocatalysts and other catalysts. It was shown in a study at Mie University [30] that dip-coated TiO 2 films on a soda-lime glass sheet exhibit large photocurrents, that is, higher efficiencies in hydrogen generation than conventional TiO 2 ceramics and single crystals do. This might be attributed to larger available surface areas resulting from the porous structure of the film. For the same reason, the dip-coated films should be suitable as catalysts for other purposes. The next topic will be the application of the film to ISFET (ion-sensitive field effect transistor) shown in (h) of table 1. ISFET's used for detecting H ÷, Li +, Na +, and other ions selectively have a TazO 5 or other film at the extremities of the probe [31]. Since metal and silicon semiconductor elements are incorporated in the probe, it is desirable that the oxide film can be formed at as low temperatures as possible. Then it is obvious that dip-coating using metal alkoxides is especially suitable. The indications are that dip-coating will be a very important technique in the future.
7. Particulates
It is well known [33,34] that oxide powders made by precipitation from metal alkoxides are excellent as starting materials for sintered polycomponent
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ceramics, because they are uniform in chemical composition and very fine in particle size. No further explanation of this is made here, but the use of particulates as catalyst carriers will be considered. Gels prepared from metal alkoxide solutions are porous with a very large surface area and so are excellent candidates for catalyst carriers. It was found by Karatani et al. [35] that glucose oxidase on silica gel prepared from tetraethyl ethoxysilane by hydrolysis-polycondensation exhibits a higher enzymatic activity by more than a factor of two than that immobilized on conventional porous glass. The reason for the high activity is attributed to the higher density of silanol groups on the alkoxy-derived gel. As a matter of fact, the gel was treated with fluorine to replace a portion of the OH group by F. In any case, this example indicates the importance of the sol-gel method for preparing catalyst carriers. The following example is concerned with the preparation of the catalyst and the carrier at the same time. Fine particles of Ni, Co, Pt and other metals act as catalysts for the hydrogenation or decomposition of organic substances such as ethylene, propionaldehyde, cyclopropane and benzene. The catalytic behavior and activity depend on the particle size and accordingly, its control is very important. Ueno et al. [36] prepared a catalyst system of N i / S i O 2 by hydrolysing a mixed solution of Si(OCzHs) 4 and ethylene glycol solution of nickel hydroxide. It was found that metal particles are highly dispersed and the average particle size can be varied between 30 and 120 A with sharp particle size distributions. With these catalyst systems, it was clearly shown in the hydrogenation of propionaldehyde CH3CHzCHO that the catalytic activity regularly varies with the average particle size of Ni. This indicates that the sol-gel method is very important for the scientific study of catalytic action as well as for the development of efficient catalyst systems.
8. Summary and conclusion The present status and the future of the sol-gel method using mainly metal alkoxides as starting materials have been critically reviewed and discussed, in order to consider its state-of-the-art in the year 2004. It has been shown that this method has a great potential in producing glasses, glass-ceramics and amorphous materials with excellent performance in various applications. The potential of this method is based on the fact that materials can be made in the forms of bulk, fiber, sheet, coating film and particulates at relatively low temperatures and that new materials which could not be made by conventional methods. Among the various forms, coating films have been discussed in somewhat more detail. This method started recently and there are a number of problems to overcome. However, it is expected that the method which is new at present will have been routine or not special in the years around 2004 in many aspects, as a result of much progress.
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References [1] H. Dislich, Angew. Chem. 83 (1971) 428. [2] Proc. Int. Workshop on Glasses and Glass-Ceramics from Gels (October 9-10, 1981) Padova, Italy; J. Non-Crystalline Solids 48 (1982) 1. [3] Proc. 2nd Int. Workshop on Glasses and Glass-Ceramics from Gels (July 1-2, 1983) W~rzburg, Germany; J. Non-Crystalline Solids 63 (1984) 1. [4] Proc. Symposium on Better Ceramics through Chemistry, Materials Research Society (February 27-29, 1984), Albuquerque, NM, USA (1984) p. 1. [5] S. Sakka, Treatise on Materials Science and Technology 22, Glass III, eds., M. Tomozowa and R. Doremus (Academic Press, New York, 1982) p. 129. [6] H. Dislich, J. Non-Crystalline Solids 57 (1983) 371. [7] S. Sakka, K. Kamiya, K. Matusita and Y. Yamamoto, J. Mater. Sci. Lett. 2 (1983) 395. [8] K. Kamiya, S. Sakka and Y. Taternichi, J. Mater. Sci. 15 (1980) 1765. [9] G.W. Scherer and J.C. Luong, J. Non-Crystalline Solids 63 (1984) 163. [10] E.M. Rabinovich et al., J. Amer. Ceram. Soc. 66 (1983) 683; 688; 693. [11] E.M. Rabinovich et al., J. Non-Crystalline Solids 63 (1984) 155. [12] K. Susa, I. Matsuyama, S. Satoh and T. Suganuma, Electromcs Lett. 18 (1982) 499. [13] 3M Company, Catalogue of Nextel and private communication. [14] S. Sakka and K. Kamiya, Mat. Sci. Res. 17: Emergent Process Methods for High Technology Ceramics (1984) p. 83. [15] S. Sakka, Better Ceramics through Chemistry, Materials Research Society Syrup. Proc. 33 (1984) 91. [16] W. Mahler and M.F. Bechtold, Nature 285 (1980) 27. [17] S. Sakka, K. Kamiya, K. Makita and Y. Yamamoto, J. Non-Crystalline Solids 63 (1984) 223. [18] H. Dislich and E. Hussmann, Thin Solid Films 77 (1981) 129. [19] H. Schroeder, Physics of Thin Films, Vol. 5 (Academic Press, New York, 1969) p. 87. [20] Y. Yamamoto, K. Kamiya and S. Sakka, Yogyo-Kyokai-Shi 90 (1982) 328. [21] Y. Yamamoto, K. Makita, K. Kamiya and S. Sakka, Yogyo-Kyokai-Shi 91 (1983) 222. [22] F. Geotti-Bianchini, M. Guglielmi, P. Palato and G.D. Soraru, J. Non-Crystalline Solids 63 (1984) 251. [23] J.J. Arfsten, J. Non-Crystalline Solids 63 (1984) 243. [24] S.P. Mukherjee and W.H. Lowdermilk, Appl. Optics 21 (1982) 293; J. Non-Crystalline Solids 48 (1982) 177. [25] M.I. Yanovskaya, E.P. Trevskaya, N.Ya. Turova, A.Y. Novoselova, Yu.N. Venetsev and E.M. Soboleva, Inorg. Mater. 17 (1981) 221. [26] E. Wu, K.C. Chen and J.D. Mackenzie, Better Ceramics through Chemistry, Materials Research Society Symp. Proc. 33 (1984) 169. [27] S. Ogiwara and K. Kinugawa, Yogyo-Kyokai-Shi 90 (1982) 157. [28] H. Dislich and P. Hinz, J. Non-Crystalline Solids 48 (1982) 11. [29] B.E. Yoldas and D.R. Partlow, Ceram. Bull. 59 (1980) 640. [30] T. Yoko, K. Kamiya and S. Sakka, to be published. [31] S. Oka, Shimadzu-Seisakusho Company, private communication. [32] R.K. Brow and C.G. Pantano, ibid., ref. 26, p. 361. [33] L.M. Brown and K.S. Mazdiyasni, J. Amer. Ceram. Soc. 55 (1972) 541. [34] Y. Ozaki, Kogyo-Zairyo (Industrial Materials), Japan, 29[5] (1981) 85; 29[6] (1981) 101 (in Japanese). [35] H. Karatani, H. Minakuchi and S. Oka, J. Chem. Soc. Japan, No. 11 (1983) 1577 (in Japanese). [36] A. Ueno, H. Suzuki and Y. Kotera, J. Chem. Soc., Faraday Trans. 79 (1983) 127.
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G L A S S E S AND GLASS-CERAMICS FROM GELS Sumio SAKKA Institute for Chemical Research, Kyoto Universi(v, Uji, Kvoto-Fu 611, Japan
The present state and the future of glasses and glass-ceramics synthesized by the sol-gel method from metal alkoxides have been critically reviewed and discussed. Glasses and glassceramics can be prepared in forms of bulk, fiber, sheet, coating film and particulate. Advantages and disadvantages in applying the sol-gel method for each form are discussed. It is shown that this method is very prospective and many materials will be manufactured by this method in the year 2004.
1. Introduction This paper is concerned with the present and future status in the application of the sol-gel method to the preparation of glasses and glass-ceramics. This method is 14 years old, if Dislich's paper [1] is regarded as the start. This is still a new field, although some technological aspects of this method are already known. This might make the prediction of its future easy, if the term future is used in a vague sense. It is not easy, however, to predict the state-of-the-art of the sol-gel method in the year 2004, as is true in all other fields of science and technology. Suppose that several methods are assumed for preparing particular materials. If it is known that one of them is possible at present, that method will be applied to industrial production in a few years instead of 20 years later. Then that method might be replaced by another method in 20 years. Conversely, it is not assured that a difficulty in a method which is not solved at present will disappear in 20 years. The other reason for the difficulty in prediction for the year 2004 is that the method for producing materials is profoundly affected by the progress of the device technology. Considering the above, the only way I can take for prediction is to be based on my own desire for developing the method, hoping that my own desire may be similar to that of people who are engaged in this field. Then, I do not think that this desire may be simply a desire. There are ample examples in which a technology, originally regarded impossible, becomes a practically useful one simply because scientists and engineers try hard to develop the technology with the motivation of desire. The technology of the optical fiber is one of such technologies. However, the presence of embryos in a technology will greatly help us consider the future of the particular technology. Fortunately, there are lots of 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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embryos that have already grown to nuclei. The scientific bases and applications of this method have been discussed in the two International Workshops [2,3] of 1981 and 1983, the Annual Meeting of the American Ceramic Society of 1981, the Gordon Conference on Glass in 1981, the Spring Symposium of the Materials Research Society [4] and other numerous scientific meetings. In this paper embryos and nuclei of the sol-gel method are reviewed to discuss its future. Stress is laid on the method using metal alkoxides as starting materials. Discussion is made on each of the forms of the products: bulk, fiber, thin sheet, coating film and particulate.
2. General features of the sol-gel method using metal alkoxides Generally, the sol-gel method used for producing glasses and glass-ceramics consists of the processes: preparation of a homogeneous solution, change of the solution to a sol, gelation of the sol and conversion of the gel to glass and glass-ceramics. The following advantages of this method emerge from the consideration of each process [5,6]. (1) High purity glasses can be prepared. (2) Multicomponent glasses and glass-ceramics of high uniformity can be expected. (3) Glasses and glass-ceramics can be obtained at relatively low temperatures. Final products may be obtained at low temperatures near room temperature for gels. (4) There are possibilities of producing glasses of new composition, which could not be obtained by conventional melting methods. (5) An ensemble of simple operations without expensive facilities leads to final products. The most important of these are (3) and (4). It is said that low temperature processing is energy-saving. That may be the case when much energy is not needed for preparing the starting metal alkoxides. However, this is the point which has to be considered deliberately. The low temperature processing (3) is of great significance for the following reason. This makes it possible to combine glasses and glass-ceramics with metals and plastic materials at low temperatures into composite materials. Good examples are seen in the coating of the substrate with glass or glassceramics as will be discussed later in this paper. There is another example. At present, the metal and semi-conductor parts are attached to the ceramic substrates in manufacturing multilayered integrated circuits. It may be possible to fire the whole circuit if ceramic substrates can be fired at lower temperatures than 1000°C by using metal alkoxides as starting materials. Another important point in the low temperature processing is that a transition element ion may exhibit a difference valence state in glasses prepared at low temperatures than that in glasses prepared at higher temperatures by melting [7]. The possibility of producing new glasses (4) is also of great importance. As
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an example, glass fibers of ZrO2-SiO 2 compositions containing as much as 48% ZrO 2 have been prepared [8]. This is only possible with the use of the sol-gel method. Preparation by melting of such glasses not only requires extremely high temperatures, but also encounters difficult problems such as liquid immiscibility at the melting temperature and phase separation and crystallization during cooling of the melt. Advantage (4) makes the future of the sol-gel method very significant.
3. Formation of bulk glasses The preparation of bulk glasses at low temperatures is a subject of great interest. However, production of large bulk glasses by the metal alkoxide technique is not an easy task because of large volume shrinkage and the dissipation of large amounts of volatile species during the sol-gel and gel-glass conversions. Crack formation, fracture and swelling occur in various stages leading to glass formation. A long time period is required, which may result in low productivity. These difficulties have been partially solved in forming S i O 2 glass by use of colloidal silica gels [9-11]. As for the metal alkoxide technique, it is difficult and meaningless to produce conventional large products like window glasses and beer bottles by this method. However, it may be valuable to make SiO2 glass plates used as photomasks for integrated circuits and SiO2 and SiO2-based glass rods for drawing optical fibers. Optical fibers have to be drawn from the rod at fairly high temperatures, This decreases the value of the metal alkoxide technique because of increasing heating temperatures, but the simplicity of processing has to be evaluated. Susa et al. [12] made large rods of SiO2 and SiO2-based glasses used for drawing low loss optical fibers. This gives some hope for the future of preparing bulk glasses from metal alkoxides.
4. Formation of glass fibers 4.1. Fibers made by drawing from alkoxide solutions at low temperatures Fibers prepared by heating after drawing from alkoxide solutions at near room temperature are considered. The maximum temperature of heating required in this technique is much lower than that required for fibers drawn from a bulk glass rod. Thus the advantage of low temperature processing is expected. Two examples of silica-alumina fibers [13] and silica fibers [14] will be mentioned, in order to show the advantage of the metal alkoxide technique and discuss its future. Conventional silica-alumina refractory fibers have been prepared by centrifugal spinning of the melt poured out of a rotating cylindrical container through the orifice. Fibers produced in this way are not continuous and
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accordingly cannot be made into textiles. With the use of the sol-gel method, continuous silica-alumina fibers have been produced for the first time [13]. The fibers are drawn from the viscous alkoxide solution near room temperature and are then heated at several hundred degrees. Precipitation of a controlled amount of A1203 crystals gives a high Young's modulus of about 150 GPa, which cannot be achieved by oxide glasses of any composition. Such high Young's modulus fibers are useful for strengthening plastics and metals. The future problem will be to design many other compositions which are supposed to give various excellent characteristics. 4.2. D&cussion of opticalfiberformation by drawing at room temperature
In order to make silica fiber, silicon tetra-ethoxysilane Si(OC2H5)4, for example, is used as the starting alkoxide. When an Si(OC2Hs)4-H20-HC1CzHsOH solution of pertinent composition is kept at temperatures near room temperature, hydrolysis-polycondensation reactions increase the viscosity of the solution [14,15]. Fibers can be drawn from the solution at viscosities higher than about 10 P. Heating at 500-900°C change the gel fibers to the silica glass fibers. The cross-section of the fibers thus obtained is circular or non-circular, which can be controlled by the composition of the starting alkoxide solution. The diameter of the fiber can be varied between 10-100/~m by controlling the viscosity of the solution at fiber-drawing and the drawing rate. In the future it will be possible to make optical fibers of SiO2-based glasses for the reasons described below. (1) It should be easy to modify the composition of the fiber into those of the SiO2-TiO2, SiOz-GeO2, SiO2-B203 and other systems by adding alkoxides of Ti, Ge, B and other metallic elements to the Si(OC2 H s)4 solution. Glass fibers of different refractive indices will be thus obtained. (2) Coating of the fibers with a glass of desirable composition will be carried out by dip-coating, in order to achieve the core-clad structure. (3) Heating the gel fibers in C12 or SOCl2 atmosphere will remove OH groups, resulting in low optical loss fibers. At present, the strength of gel-derived silica fibers is about 50% lower than that of silica fibers prepared by conventional methods at high temperatures. The reason for this is not known yet. However, the problem will, it is hoped, be overcome by some means in the future. It is known that Mahler and Bechtold [16] produced high strength silica glass fibers from gel fibers prepared by the unidirectional freezing of gel. Briefly, the metal alkoxide technique will prove a very useful technique for producing various kinds of fibers in the future. 5. Thin glass sheets
The sol-gel method starting from metal all'oxides presents the possibility of obtaining thin glass sheets of thicknesses ranging from several tens to several
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hundreds of microns [17]. The method consists of forming thin gel sheets from metal alkoxide solutions near room temperature and converting them to glass sheets by heating to, say, 500-800°C. For instance, thin SiO 2 glass sheets can be obtained at lower temperatures than 1000°C. The following three technologies have been proposed in order to make gel sheets from metal alkoxide solutions. (1) Scooping off the viscous solution by using a wire ring. (2) Drawing the viscous solution into a thin sheet through a slit. (3) Forming a thin sheet by applying the viscous solution on a plastic plate and then peeling off the plate. An explanation will be made of the preparation of S i O 2 glass sheets. In any of the a b o v e t h r e e m e t h o d s , the c o m p o s i t i o n of the s t a r t i n g Si(OCzH5)4-H20 HC1-C2HsOH solution must be in a limited compositional region. The requirements of composition of the starting solution for sheet formation is very similar to those for fiber drawing; the water content must be relatively low so that linear alkoxide polymers may be formed in the course of the hydrolysis-polycondensation reaction [15]. It is true that the above three techniques give a possibility of producing thin glass sheets, but there are many difficulties. The most serious difficulty common to the three techniques arises from an enormous shrinkage in volume and area on solidification of the solution, which may cause deformation, crack formation and fracture of gel sheets. In order to avoid this, it would be necessary to expose both the surfaces of a solidifying gel sheet to the ambient atmosphere at the same time, so that the reaction and drying might proceed at similar rates on both surfaces of the sheet. At present this problem is not yet fully solved. I hope, however, this problem will certainly be solved in the future. Then the thin sheet formation will be a very useful technique. It should also be noted that techniques (2) and (3) out of the three will make possible the production of continuous glass sheets, that is, glass ribbons.
6. Coating films It is already known that the dip-coating or the spin-coating of glasses, metals and plastics using metal alkoxides solutions is very useful for modifying the properties of substrates with a large surface area and provide substrates with new active properties [18]. In the years around 1970, dip-coating was applied by Schroeder of Schott Company in Germany [19] to modify the optical properties of glasses and plastics. In recent years the dip-coating technique has again attracted much attention as the demand for new electronic materials has increased. It is expected that the dip-coating is useful for protecting particular electrical characteristics of the substrate and providing the substrate with particular electrical and magnetic properties, and so on, in addition to a modification of optical properties and an increase in chemical durability.
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In the future, dip-coating will be widely applied. The present status and the future of this technique will be discussed in the following.
6.1. Dip-coating using metal alkoxide solutions The dip-coating consists of three steps: dipping, withdrawal and heating. The spin coating consists of two steps: spinning and heating. The following concerns dip-coating, but spin coating shares most of the advantages and disadvantages of dip-coating. In order to form glass and glass-ceramic coating films which firmly adhere to the substrate, heating of the coated substrate to a certain temperature is required. It is assumed that heating forms chemical bonds between the film and substrate. Since relatively low heating temperatures are sufficient, it is not required that the substrate be highly heat-resistant. A temperature of 500°C is often sufficient [6,20], except when the coating films have to be converted to fully crystallized ceramics. Another advantage of the dip-coating is that coating of the substrate of a large surface can be easily attained, compared with sputtering, vacuum vapor deposition and spraying. A characteristic of this technique is the thickness of the film. The film successfully prepared in one application is thin at thicknesses of between 50 to 300 nm or thinner. The film is peeled off under the coating condition of high concentrations of the coating oxide in the solution, high viscosities of the solution and high rates of withdrawal, all of which might give a thick film in one application. A small thickness is usually an advantage for films of electronic activity, while it is usually a disadvantage for protecting coatings. Thicker films can be obtained by increasing the number of applications. It is essential to repeat the whole coating process including heating to form a uniform coating film which firmly adheres to the substrate. Since heavy repetition is laborious, however, this technique is not so suitable for films of several microns or thicker films. Various coating films which have already been prepared are reported by Dislich [6] with references. Table 1 lists examples of the present and future possible applications of the dip-coating technique. In table 1, the purpose of (a) is to increase the scratch resistance of metals and plastics by applying a coating of hard oxides. In plastics the bonding between the film and the substrate has to be accomplished chemically at low temperatures instead of heating up to 500°C, because plastic materials are not so heat-resistant. In the case of metal substrates, the chemical bond between the film and the substrate can be formed by heating at 400-500°C. It is assumed that the bonding may be formed owing to the presence of a thin oxidized layer on the surface of metals. (b) aims at the chemical protection of the substrate, such as the protection from oxidation and increase in acid resistance of metals. Passivation of silicon semiconductors by SiO2 coating can be included in (b). An increase of protection ability by the introduction of nitrogen into SiO 2 film has been attempted [32].
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Table 1 Examples of application of the dip-coating using metal alkoxide solutions Objective
Example
(a) Mechanical protection protection of plastics of substrate and metals (b) Chemical protection increaseof durability, of substrate protection of Si (c) Films of particular colored and absorbing optical characteristics film[6,21,22] reflecting film [23] anti-reflection film [24] photoconductive film (light, X-ray) (d) Ferroelectric f i l m capacitor [15,25,26] (e) Electrically conducting electronic conductor [27,28] film ionic conductor [29] (f) Donation of magnetic verticallymagnetizing property film (g) Photocatalytic f i l m generation of H 2 [30] (h) Analysis and catalyst ISFET [31] catalyst
Possibility Exampleof composition future
SiO2
yes
SiO2
yes
TiO 2-SIO2, SiO 2 -R mOrt ,
yes yes future
oxide of Fe, Cr, Co In 203 - S n O 2 Na20-B203-SiO 2
yes yes
BaTiO3, KTaO3, PLZT In203-SnO2, SnO2-CdO
yes future
B-alumina -
future yes future
glass
As shown in (c) of table 1, there are various investigations concerning the modification of optical properties of the substrate and providing the substrate with new optical characteristics. The formation of a photoconductive layer is cited as an example of a target for the future. For instance, the coating of substrates of large surface area with photoconductive films sensitive to light or X-rays will be quite useful for in situ display. Dielectric coating films shown in (d) have already been investigated to some extent so far. Capacitor films of high dielectric constant can be expected. The electrically conductive films shown in (e) are important and are being investigated fairly intensively. A serious problem is found in the latter case. It is k n o w n that I n 2 0 3 - S n O 2 films prepared by dip-coating, for example, show conductivities lower by an order of magnitude than those prepared by other techniques like chemical vapor deposition. This problem has to be solved, in order to make this technique useful. The magnetic films in (f) are listed as examples for the future. In this case it is the key point whether in the dip-coating the orientation of crystals and magnetic anisotropy can be controlled or not. F o r m a t i o n of photocatalytic and catalytic films cited in (g) and (h), respectively, will possibly emerge as an important technology in the future. It is expected that thin films will work well for these purposes.
6.2. Future applications of the dip-coating The dip-coating technique is already at the stage of practical application for some fields. However, there are several technical problems to be considered for further progress of particular films, as follows.
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(1) (2) (3) (4) (5)
Adherence of the film with the substrate. Defects such as chipping and cracks in the film. Reaction of the coating film and the substrate. Small thickness of the film prepared by one application. Porous structure of the film. Problems (1)-(3) can and should be solved. For (1) and (2) the mechanism of adherence between the coating film and substrate should be investigated. As for problem (3), one layer of SiO 2 prepared by the dip-coating is usually enough to protect the coating film from reacting with the substrate. The small thickness of the film (problem (4)) has already been discussed. Dip-coating is not effective, however, for the mechanical protection of the substrate, because it needs films thicker than several microns at least. Next, problem (5) concerning the porous structure of the film will be considered. Glasses and glass-ceramics formed through the gelation of metal alkoxide solutions may retain the structure of the gel when they are not heated at sufficiently high temperatures. They may be an agglomerate of round solid particles or a three-dimensional network containing fine, continuous pores, that is fine channels. If the former is the case, the electrical conductivity, for instance, would be lower than expected for a continuous structure. The lower conductivity of ITO films mentioned in the explanation of table 1 might be attributed to this structure. In this case the porous structure is not favorable and an investigation for modifying a discrete structure into a continuous one has to be carried out. It should be noted, however, that the porus structure should be more desirable than non-porus structures for certain applications such as photocatalysts and other catalysts. It was shown in a study at Mie University [30] that dip-coated TiO 2 films on a soda-lime glass sheet exhibit large photocurrents, that is, higher efficiencies in hydrogen generation than conventional TiO 2 ceramics and single crystals do. This might be attributed to larger available surface areas resulting from the porous structure of the film. For the same reason, the dip-coated films should be suitable as catalysts for other purposes. The next topic will be the application of the film to ISFET (ion-sensitive field effect transistor) shown in (h) of table 1. ISFET's used for detecting H ÷, Li +, Na +, and other ions selectively have a TazO 5 or other film at the extremities of the probe [31]. Since metal and silicon semiconductor elements are incorporated in the probe, it is desirable that the oxide film can be formed at as low temperatures as possible. Then it is obvious that dip-coating using metal alkoxides is especially suitable. The indications are that dip-coating will be a very important technique in the future.
7. Particulates
It is well known [33,34] that oxide powders made by precipitation from metal alkoxides are excellent as starting materials for sintered polycomponent
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ceramics, because they are uniform in chemical composition and very fine in particle size. No further explanation of this is made here, but the use of particulates as catalyst carriers will be considered. Gels prepared from metal alkoxide solutions are porous with a very large surface area and so are excellent candidates for catalyst carriers. It was found by Karatani et al. [35] that glucose oxidase on silica gel prepared from tetraethyl ethoxysilane by hydrolysis-polycondensation exhibits a higher enzymatic activity by more than a factor of two than that immobilized on conventional porous glass. The reason for the high activity is attributed to the higher density of silanol groups on the alkoxy-derived gel. As a matter of fact, the gel was treated with fluorine to replace a portion of the OH group by F. In any case, this example indicates the importance of the sol-gel method for preparing catalyst carriers. The following example is concerned with the preparation of the catalyst and the carrier at the same time. Fine particles of Ni, Co, Pt and other metals act as catalysts for the hydrogenation or decomposition of organic substances such as ethylene, propionaldehyde, cyclopropane and benzene. The catalytic behavior and activity depend on the particle size and accordingly, its control is very important. Ueno et al. [36] prepared a catalyst system of N i / S i O 2 by hydrolysing a mixed solution of Si(OCzHs) 4 and ethylene glycol solution of nickel hydroxide. It was found that metal particles are highly dispersed and the average particle size can be varied between 30 and 120 A with sharp particle size distributions. With these catalyst systems, it was clearly shown in the hydrogenation of propionaldehyde CH3CHzCHO that the catalytic activity regularly varies with the average particle size of Ni. This indicates that the sol-gel method is very important for the scientific study of catalytic action as well as for the development of efficient catalyst systems.
8. Summary and conclusion The present status and the future of the sol-gel method using mainly metal alkoxides as starting materials have been critically reviewed and discussed, in order to consider its state-of-the-art in the year 2004. It has been shown that this method has a great potential in producing glasses, glass-ceramics and amorphous materials with excellent performance in various applications. The potential of this method is based on the fact that materials can be made in the forms of bulk, fiber, sheet, coating film and particulates at relatively low temperatures and that new materials which could not be made by conventional methods. Among the various forms, coating films have been discussed in somewhat more detail. This method started recently and there are a number of problems to overcome. However, it is expected that the method which is new at present will have been routine or not special in the years around 2004 in many aspects, as a result of much progress.
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References [1] H. Dislich, Angew. Chem. 83 (1971) 428. [2] Proc. Int. Workshop on Glasses and Glass-Ceramics from Gels (October 9-10, 1981) Padova, Italy; J. Non-Crystalline Solids 48 (1982) 1. [3] Proc. 2nd Int. Workshop on Glasses and Glass-Ceramics from Gels (July 1-2, 1983) W~rzburg, Germany; J. Non-Crystalline Solids 63 (1984) 1. [4] Proc. Symposium on Better Ceramics through Chemistry, Materials Research Society (February 27-29, 1984), Albuquerque, NM, USA (1984) p. 1. [5] S. Sakka, Treatise on Materials Science and Technology 22, Glass III, eds., M. Tomozowa and R. Doremus (Academic Press, New York, 1982) p. 129. [6] H. Dislich, J. Non-Crystalline Solids 57 (1983) 371. [7] S. Sakka, K. Kamiya, K. Matusita and Y. Yamamoto, J. Mater. Sci. Lett. 2 (1983) 395. [8] K. Kamiya, S. Sakka and Y. Taternichi, J. Mater. Sci. 15 (1980) 1765. [9] G.W. Scherer and J.C. Luong, J. Non-Crystalline Solids 63 (1984) 163. [10] E.M. Rabinovich et al., J. Amer. Ceram. Soc. 66 (1983) 683; 688; 693. [11] E.M. Rabinovich et al., J. Non-Crystalline Solids 63 (1984) 155. [12] K. Susa, I. Matsuyama, S. Satoh and T. Suganuma, Electromcs Lett. 18 (1982) 499. [13] 3M Company, Catalogue of Nextel and private communication. [14] S. Sakka and K. Kamiya, Mat. Sci. Res. 17: Emergent Process Methods for High Technology Ceramics (1984) p. 83. [15] S. Sakka, Better Ceramics through Chemistry, Materials Research Society Syrup. Proc. 33 (1984) 91. [16] W. Mahler and M.F. Bechtold, Nature 285 (1980) 27. [17] S. Sakka, K. Kamiya, K. Makita and Y. Yamamoto, J. Non-Crystalline Solids 63 (1984) 223. [18] H. Dislich and E. Hussmann, Thin Solid Films 77 (1981) 129. [19] H. Schroeder, Physics of Thin Films, Vol. 5 (Academic Press, New York, 1969) p. 87. [20] Y. Yamamoto, K. Kamiya and S. Sakka, Yogyo-Kyokai-Shi 90 (1982) 328. [21] Y. Yamamoto, K. Makita, K. Kamiya and S. Sakka, Yogyo-Kyokai-Shi 91 (1983) 222. [22] F. Geotti-Bianchini, M. Guglielmi, P. Palato and G.D. Soraru, J. Non-Crystalline Solids 63 (1984) 251. [23] J.J. Arfsten, J. Non-Crystalline Solids 63 (1984) 243. [24] S.P. Mukherjee and W.H. Lowdermilk, Appl. Optics 21 (1982) 293; J. Non-Crystalline Solids 48 (1982) 177. [25] M.I. Yanovskaya, E.P. Trevskaya, N.Ya. Turova, A.Y. Novoselova, Yu.N. Venetsev and E.M. Soboleva, Inorg. Mater. 17 (1981) 221. [26] E. Wu, K.C. Chen and J.D. Mackenzie, Better Ceramics through Chemistry, Materials Research Society Symp. Proc. 33 (1984) 169. [27] S. Ogiwara and K. Kinugawa, Yogyo-Kyokai-Shi 90 (1982) 157. [28] H. Dislich and P. Hinz, J. Non-Crystalline Solids 48 (1982) 11. [29] B.E. Yoldas and D.R. Partlow, Ceram. Bull. 59 (1980) 640. [30] T. Yoko, K. Kamiya and S. Sakka, to be published. [31] S. Oka, Shimadzu-Seisakusho Company, private communication. [32] R.K. Brow and C.G. Pantano, ibid., ref. 26, p. 361. [33] L.M. Brown and K.S. Mazdiyasni, J. Amer. Ceram. Soc. 55 (1972) 541. [34] Y. Ozaki, Kogyo-Zairyo (Industrial Materials), Japan, 29[5] (1981) 85; 29[6] (1981) 101 (in Japanese). [35] H. Karatani, H. Minakuchi and S. Oka, J. Chem. Soc. Japan, No. 11 (1983) 1577 (in Japanese). [36] A. Ueno, H. Suzuki and Y. Kotera, J. Chem. Soc., Faraday Trans. 79 (1983) 127.