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Sintering studies on ordered monodisperse silica compacts: Effect of consolidation
185
Powder Technology, 69 (1992) 185-193
Sintering studies on ordered effect of consolidation
monodisperse
Muhsin
Ciftcioglu,
and Steven
UNMJNSF
Center
(Received
March 8, 1991; in revised form July 1, 1991)
Douglas
For Micro-Engineered
M. Smith* Ceramics,
University
silica compacts:
B. Ross
of New Mexico, Albuquerque,
NM 87131
(USA)
Abstract Monodisperse silica spheres were synthesized by TEOS hydrolysis, then consolidated by three different processes. Spheres of 270 nm were dispersed in water and centrifuged to form highly ordered compacts with packing efficiencies of 67-68% of the powder density. Dilute sphere suspensions were filtered to form relatively thin dense cakes. Pellets were also prepared from randomly agglomerated powders by uniaxial pressing. Green bodies were sintered at temperatures between 800-l 100 “C for 2-24 h. Pore size distributions of the ordered compacts were determined by mercury porosimetry. No major changes in the pore size distributions were observed for samples sintered between 800 and 950 “C. The sintered ordered samples were not translucent and reached approximately 96-97% of theoretical density at 1000 “C. In contrast to the centrifugally ordered samples, the filtered cakes became translucent at 1000 “C. Temperatures of almost 1 100 “C were required to reach high density for compacts made from agglomerated powders. SEM micrographs of the sintered and ordered compacts show the presence of narrow void regions with sizes on the order of the sphere size. These areas are most likely the remnants of the domain boundaries which existed in the ordered green compact. The results of this study show the interference of the domain boundaries in the green compacts to be detrimental to densification, limiting the achievable final densities and degrading the properties of the sintered ceramics. This is despite the fact that the centrifugally-ordered samples had the highest densities in the green state.
Introduction
The use of monodisperse spherical submicron powders for the preparation of ideal high-density green bodies which can subsequently be densified at relatively lower temperatures have been the subject of considerable research in the past decade. These ordered compacts with high coordination numbers and very uniform pore size distributions are capable of densifying uniformly with the absence of exaggerated grain growth. Thus it should be possible to prepare full-density submicron-grain-size ceramics by densifying these compacts at temperatures several hundred degrees lower than their counterparts. In past work [l-3], monodisperse spherical particles of materials such as TiOz, SiO, and ZrO, have been prepared through novel powder-preparation routes. Alkoxide hydrolysis was one of the most widely used routes which have been developed. Dense green bodies with typically 60-70% of final density have been produced by centrifugal casting or sedimentation. Sintering studies on these high-density compacts have shown that *Author
to whom correspondence
0032-5910/92/$5.00
should be addressed.
they can be densified to 95% and higher at relatively low temperatures. Sacks et al. [4, 51 prepared ordered SiO, compacts and investigated the sintering behaviour of these compacts. The reported green densities of the compacts were low in the first two studies, but the compacts were sintered to full densities at 1000 and 1050 “C. In a later study [6] on the infiltration of these compacts with a fine silica sol, the green densities were higher but the sintered densities remained below those of the earlier studies. A significant increase in the densification rate was observed with the infiltration procedure. In these studies the green densities of the ordered cakes were substantially lower than the theoretically achievable densities. The existence of lower-density packing arrangements in addition to the packing defects was thought to be responsible for the low green densities. In a recent study on the two-dimensional ordering and sintering of spherical latex and glass spheres, it was observed that the number of spheres in the average domain can be as high as 1000 [7]. The boundaries between these domains had pulled apart, leaving large holes and cracks. In contrast, it was shown that the domain sizes for bimodal powders were, on the average,
0 1992 - Elsevier Sequoia.
All rights reserved
186
an order or two lower and there were almost no cracks or defects formed upon densification in these twodimensional bimodal structures. In another study on the densification of two-dimensional arrays of monosized copper spheres by Weiser and De Jonghe [S], it was found that differential densification was the major cause of rearrangement. Pieranski [9] examined the nature of defects in twoand three-dimensional colloid crystals. He reports that the most frequently found colloid crystals are facecentered cubic or body-centered cubic. Colloid crystals are mostly polycrystalline and contain a large number of defects which are easily created. From the photographs of the colloid crystals of this work, the grain boundaries and the dislocation walls can easily be identified. In another study it is suggested that hexagonally packed two-dimensional arrays exist in concentrated suspensions of monosize polyvinyl chloride particles [lo]. Domain sizes of 25 pm and 40 pm were determined for ordered TiO, and SiO, compacts through the adaptation of diffraction line broadening to light diffraction [ll]. It can be concluded from the studies mentioned above that the packing of monosize submicron spheres is likely to create undesirable defects, although relatively high green densities can be achieved. The presence of defects in addition to different packing arrangements may be inevitable. In this study, ordered high-density silica compacts were prepared and the densification behaviour of these compacts was investigated. Changes in the pore structure were followed and SEM analysis of the sintered structures was carried out. The results were compared with those obtained on uniaxially prepared pellets and filter cakes prepared from suspensions with lower sphere concentrations.
The recovered powders were dispersed in distilled water using ultrasonic forces. The pH of the suspensions was kept between 7 and 10. These suspensions, which were -5-10 wt.% solids, were centrifuged to form high-density ordered compacts. After the clear top liquid layer was removed, the ordered compacts were air dried at room temperature for several days and then dried at 383 K overnight. Scanning electron microscope photographs (Hitachi S-450 SEM) of the compact fracture surfaces were taken in order to examine the nature of the ordering. A typical SEM micrograph of the fracture surface of a 270 nm ordered sphere compact is given in Fig. 1. Ordered compacts of 270 nm spheres show spectacular colors in both the wet and dry states. In addition to the ordered compacts, thin high-density silica compacts were prepared by directly filtering the precipitate solution through a Buchner funnel. The weight percent of solids in the precipitate solutions was -5 times lower than the solids content of the suspensions used for preparing ordered cakes. These solutions were very basic and well dispersed. In addition to solution processing techniques, compacts were prepared by uniaxially pressing powders prepared by grinding the filtered and dried cakes. Pellets were prepared at 70 MPa using 9.5 mm diameter dies. Sintering experiments used an air atmosphere under isothermal conditions at temperatures in the range of 800-l 100 “C. Isothermal hold periods varied between 2 and 24 h. Mercury porosimetry (Autoscan 33 Mercury Porosimeter, Quantachrome) over the pressure range of 12-33 000 psia was used for the determination of the total pore volume and pore size distribution (contact angle assumed to be 140”). Some heat-treated samples
Experimental Monodisperse submicron silica spheres were synthesized by the method described by Stober et al. [12]. Tetraethyl orthosilicate, TEOS (Aldrich Chemical, technical grade), in ethyl alcohol (0.28 M) was hydrolyzed by the addition of an N&OH-ethyl alcohol solution (1.33 mol NH, per liter of ethanol and 2.94 mol H,O per liter of ethanol for the precipitation of 270 nm spheres). Precipitation of monodisperse silica spheres was completed by stirring overnight. The powder was recovered by filtering the precipitate solution and drying in an oven at 383 K. Before drying, a sample was taken from the sphere slurry for size analysis. The sample was pipetted onto a carbon 8lm and micrographs of the spheres were obtained with a TEM (JEOL JEM2000 FX TEM). The mean sphere size was obtained from image analysis of the TEM micrographs.
Fig. 1. Scanning electron micrograph ordered cakes of 270 nm spheres.
of the fracture
surface
of
187
were broken and SEM micrographs microstructure analysis.
Results
were taken for
and discussion
The precipitated particles are spherical and monosize. The SEM photograph of the fracture surface of the ordered cakes given in Fig. 1 shows that the order is long-range, dominating the whole of the compact rather than regional (the layers close to the surface are most likely to be ordered, which was not the case in this study). The presence of hexagonal ordered planes of spheres is clearly visible although the arrangement of these planes is not. In a previous study on the mercury intrusion and extrusion to these ordered compacts, it was observed that the intrusion pore sizes were very close to the opening sizes expected for several high density sphere packings (rhombohedral, tetragonal and body-centered cubic) [13]. It was concluded in that study that extrusion pore sizes were controlled mainly by the defects in these ordered structures. The extrusion pore sizes were significantly higher than expected pore body sizes for these ordered sphere packings. The intrusion pore size distribution in the ordered compact of 270 nm spheres is shown in Fig. 2. The pore size distribution is unimodal and the pore peak in these compacts is located between 30-40 nm. Table 1 gives
the intruded pore volumes for all the samples investigated in this study. From the intruded pore volumes of these ordered compacts and the density of the material heat treated at 110 “C, it was observed that the green compacts were about 66-69% dense. Although the pore peak for the filtered green compact is located slightly below 30 nm (Fig. 3), the intruded pore volume is slightly higher than the ordered compact. In another study conducted by the authors on the determination of the coordination number in these compacts [14], it was observed that the coordination number varied between 10-12. From these studies and earlier studies of other researchers, it can be concluded that these compacts do contain a combination of different packing arrangements and they are representative of the densest monosize powder compacts practically possible. When these silica compacts are isostatically pressed at 210 MPa, the intruded volume decreases to less than 0.2 cm3 g-‘, forming dense compacts which are only 2-3% below the theoretically possible 74% limit for rhombohedral packing. The pore size distributions obtained on some of the sintered samples by mercury porosimetry are given in Figs. 4 through 8 and the intruded volumes are given in Table 1. The location of the pore size peak and the pore size distribution is very similar for ordered green compact and for compact heat-treated for 2 h at 800 “C with a slight decrease in the intruded volume.
1.6
0.6
100
Radius, nm Fig. 2. Pore size distribution
of ordered
compact
of 270 nm spheres.
188 TABLE 1. Sintering results on ordered of 270 nm silica spheres Nature of sample
Sintering (“C, h)
ordered compact filtered compact ordered compact ordered compact ordered compact ordered compact ordered compact ordered compact filtered compact
--800,2 900,2 950,24 1000,2 1000,24 1050,2 1000,19
temperature,
and filtered
time
compacts
Pore volume (cm’ g-‘) 0.2430 0.2722 0.2270 0.1970 0.1817 0.0158 opaque 0.0181 translucent
A similar trend is seen in the sample heat-treated at 900 “C for 2 h with a slight decrease in pore volume. Figure 6 shows the pore size distribution in the ordered compact heat-treated at 950 “C for 24 h. The pore volume is still high, showing the absence of significant densification, and the sintered compact has only 70-71% of theoretical density. The pore peak size is under 30 nm, most probably due to closing of the openings through neck growth. Significant densification starts with the 1000 “C sample heat-treated for 2 h. There are two groups of pore sizes in this sample, as can be seen in Fig. 7. The first population has sizes greater than the original pores in the green compact and the second 3 SIN-6
2.5
2 s :: i .!?
1.5
s 6 1
0.5
Radius, nm Fig. 3. Pore size distribution
of filtered cake of 270 nm spheres.
population has a wide, finer pore size distribution. Although there is still some microporosity below 32 A, which is the lower limit of the porosimeter used, even with the measured intruded pore volume the sample has less than 97% of theoretical density. Increasing the holding time at this temperature yielded opaque samples, showing that the pore phase has similar characteristics. Increasing the sintering temperature to 1050 “C did not increase the density and gave more of the bigger pores, as seen in Fig. 8. Although densification is relatively rapid above 950-975 “C, the compacts did not densify completely and had >3% porosity in the samples. Despite small spots on the compacts which were transparent, the compacts were predominantly opaque. Contrary to the above observations, the green compacts prepared through filtering the precipitate solutions yield translucent pieces when sintered at or above 1000 “C. The SEM micrographs of the fracture surfaces of translucent pieces obtained from sintering filtered cakes are shown in Fig. 9. The low magnification photograph shows a uniform fracture surface free from pores and defects. The grains are about the same size as the original spheres, and due to densification the partial ordering has been replaced by almost perfect ordering of the grains. The completely dense planes of the grains can easily be detected. There are no apparent cracks or large voids in the sintered body.
189
SIN-l
2.5
2 9 :: _ & 9
1.5
s 4 1
0.5
0
1
lb
Radius, Fig.
4.
Pore size distribution
nm
of ordered
compact
of 270 nm spheres
of ordered
compact
of 270 nm spheres
heat-treated
at 800 “C for 2 h.
heat-treated
at 900 “C for 2 h.
3 SIN-2
2.5
Radius,
Fig. 5. Pore size distribution
nm
190 1.E
1.6
1.4
1.2
9 8
1
i E 5
0.8
Y -0 0.6
SIN-5
0.4
0.2
0
Fig. 6. Pore size distribution
of ordered
Radius, nm of 270 nm spheres
compact
heat-treated
at 9.50 “C for 24 h.
0.01t
0.014
0.011
0.010 9 8 6 E 2
0.008
P 0.006
0.004
0.002
0.000
I
I
I
I
I
I
5
10
15
20
25
30
-I 3:
Pressure, kpsi Fig. 7. Mercury porosimetry intruded treated at 1 000 “C for 2 h.
and extruded
volume VS. pressure
curves for the ordered
compact
of 270 nm spheres
heat.
191
SIN-4
0.000
0
I
5
I
10
I
15
I
20 *
I
25
I
30
5
Pressure, kpsi Fig. 8. Mercury porosimetxy intruded treated at 1050 “C for 2 h.
and extruded
volume vs. pressure
The SEM micrographs of the fracture surfaces of sintered ordered pieces at 1000 “C contain a series of defects, as shown in Fig. 10. There are cracks in the pictures, some of which may have been created during sample preparation for the SEM work. The second and the most important defects are seen as bright lines in the micrographs. These bright lines are Xl-100 pm in length, and some of them are connected while some are isolated. Depressed and elevated regions are seen between these lines or boundaries. They are not local, seem to be parallel to each other and most probably lie perpendicular to the particle motion during centrifugation. The lines surrounding less dense areas are likely to be remnants of the original grain boundaries. During sintering these domains densified rapidly, leaving some of the domain boundaries as pore deposition areas. At the end, with the driving force of densification diminishing, these low density areas remained in the body, limiting the final density to 95-97% of theoretical density. This dramatic difference in microstructure, which is accompanied by a small difference in sintered densities (l-5%), can only be traced back to the powder-consolidation step. During preparation of the ordered compacts from suspensions with 5-10 wt.% solids content through centrifugation, it was always observed that the process of incorporation of individual spheres into
curves for the ordered
compact
of 270 nm spheres
heat-
the dense cake at the bottom of the tubes is not a single-step process. We always found a fluid-like dense colloid on top of the ordered cakes in the tube when the centrifugation process was stopped prematurely. This fluid-like colloid has a much higher solids content and the spheres are relatively closer to each other than in the suspension. It is well known that dispersion stability is a sensitive function of particle concentration. It is probable that despite the electrostatic repulsive forces between particles, ordered planes and domains form in this colloid state and incorporate with the cake afterwards. The dimensions of these domains must be a function of the surface charges and suspension characteristics, in addition to other factors such as tube size and extent of centrifugal forces. In other words, the ordering process is not accomplished by individual spheres but by ordered domains of groups of spheres. This creates a structure somewhat similar to the case in which an agglomerated powder is processed. The undesirable effects of agglomerates in powder processing are well known [15]. The ordering process, in contrast to dry powder processing where agglomerates already exist, creates agglomerates in situ just before consolidation is achieved. So, although suspension processing is generally used to eliminate these agglomeration effects, in the particular case of centrifugal ordering, it does not prevent them from forming.
192
(a)
(b)
(b)
Fig. 9. Scanning electron micrographs of the fracture surface of translucent samples prepared from 270 nm spheres. (a) Low magnification, (b) high magnification.
Fig. 10. Scanning electron micrographs of the fracture surface of sintered and ordered samples treated at 1000 “C for 24 h. (a) Low magnification, (b) high magnification.
During filtering of the precipitated sphere suspensions, the consolidation process is accomplished without the formation of secondary particles. The direction of movement of the filtering medium and the particles is the same, and the solids content of the suspension is much lower. The spheres are likely to incorporate into the cake one by one without forming particle assemblages. Although the cake may be less dense and the order may not be as apparent, the results obtained in this work show that the filtering process produces defectfree compacts.
Sintering results obtained on uniaxially pressed pellets are given in Table 2. These results clearly show the advantages of suspension processing. The sintered density was only 67% of theoretical density at 1000 “C, whereas samples near theoretical density were obtained through suspension processing. Similar densities can be achieved only at about 100 “C higher sintering temperatures. A brief experiment on the effect of sphere size was conducted using an ordered compact of 130 nm spheres. As shown in Table 3, decreasing the size by half does
193 TABLE 2. Sintering results on pellets pressed uniaxially at 70 MPa from random powder agglomerates of 270 nm spheres Sintering (“C, h)
temperature,
Density (%)
time
66.9% 90.4% 97.2%
1000,24 1050,24 1100,24
TABLE 3. Effect of sphere compacts
size on the sintering
forward and that defects can easily be introduced. If the advantages of these compacts are to be realized, methods to eliminate these defects will need to be developed. Acknowledgement Support for this work has been provided by Sandia National Laboratories (#OS-5795).
of ordered
References Sphere
size
270 nm 270 nm 130 nm
Sintering (“C, h) 900,2 950,24 950,20
temperature,
time
Pore volume (cm’ g-‘) 0.1970 0.1817 0.1450
not result in significant densification at 950 “C. This observed effect on the densification is consistent with a small temperature difference between 950-l 000 “C and may not be a size effect at all.
Conclusion
1 E. A. Barringer,
R. Brook and H. K. Bowen, in G. C. Kuczynski, A. E. Miller and G. A. Sargent (eds.), Sinfeting and Heterogenous Catalysis, Materials Science Research, Vol. 16, Plenum Press, New York, 1984, pp. 1-21. 2 B. Fegley and E. A. Barringer, in C. J. Brinker, D. E. Clark and D. R. Ulrich (eds.), Better Ceramics Through Chemistv, Materials Research Sociew Symposia Proc., Vol. 32, North Holland, New York, 1984, pp. 187-197. 3 E. A. Barringer, N. Jubb, B. Fegley, R. L. Pober and H. K. Bowen, in L. L. Hench and D. R. Ulrich (eds.), Uitrastruchtre Processing of Ceramics, Glasses, and Composites, Wiley, New York, 1984, pp. 315-333. M. D. Sacks and T. Y. Tseng, L Am. Ceram. Sot., 67 (1984) 526. M. D. Sacks and T. Y. Tseng, /. Am. Ceram. Sot., 67 (1984) 532.
The suspension processing of ordered green bodies of submicron spherical powders for the preparation of defect-free fine grain-sized ceramics has its disadvantages, in addition to the apparent advantages discussed in the ceramics literature for the last decade. The creation of defects may not be easy to control in these ordered dense compacts. This study has shown that densification is halted by the presence of defects formed during the consolidation step. Although the green bodies met all the expected requirements for good sintering behaviour, i.e. uniform and fine pore size distribution and high coordination number, the boundaries of domains forming during consolidation kept the density 3-5% below the theoretical limit. The results of this study show that submicron-monosize sphere ordering for the formation of dense green bodies is not straight-
M. D. Sacks and S. D. Vora, J. Am. Ceram. Sot.,
71 (1988)
245.
E. Liniger and R. Raj, J. Am. Ceram. Sot., 70 (1987) 843. M. W. Weiser and L. C. de Jonghe, _I.Am. Ceram. Sot., 69 (1986)
10 11 12 13
14 15
822.
P. Pieranski, in M. Kleman, R. Balian and J. P. Poirier (eds.), Physics of Defects, North Holland, Amsterdam, 1980, pp. 183-200. R. L. Hoffman, Trans. Sot. Rheol., 16 (1972) 155. S. Y. Chang and T. A. Ring, Langmuir, 4 (1988) 1128. W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. M. Ciftcioglu, D. M. Smith, S. B. Ross, Mercury Porosimetry of Ordered Sphere Compacts: Investigation of Intrusion and Extrusion Pore Size Distributions. Powder Technol., 55 (1988) 193. M. Ciftcioglu, D. M. Smith and S. B. Ross, in preparation. M. Ciftcioglu, M. Akic and L. E. Burkhart, Am. Ceram. Sot. Bull., 6.5 (1986) 1591.
Powder Technology, 69 (1992) 185-193
Sintering studies on ordered effect of consolidation
monodisperse
Muhsin
Ciftcioglu,
and Steven
UNMJNSF
Center
(Received
March 8, 1991; in revised form July 1, 1991)
Douglas
For Micro-Engineered
M. Smith* Ceramics,
University
silica compacts:
B. Ross
of New Mexico, Albuquerque,
NM 87131
(USA)
Abstract Monodisperse silica spheres were synthesized by TEOS hydrolysis, then consolidated by three different processes. Spheres of 270 nm were dispersed in water and centrifuged to form highly ordered compacts with packing efficiencies of 67-68% of the powder density. Dilute sphere suspensions were filtered to form relatively thin dense cakes. Pellets were also prepared from randomly agglomerated powders by uniaxial pressing. Green bodies were sintered at temperatures between 800-l 100 “C for 2-24 h. Pore size distributions of the ordered compacts were determined by mercury porosimetry. No major changes in the pore size distributions were observed for samples sintered between 800 and 950 “C. The sintered ordered samples were not translucent and reached approximately 96-97% of theoretical density at 1000 “C. In contrast to the centrifugally ordered samples, the filtered cakes became translucent at 1000 “C. Temperatures of almost 1 100 “C were required to reach high density for compacts made from agglomerated powders. SEM micrographs of the sintered and ordered compacts show the presence of narrow void regions with sizes on the order of the sphere size. These areas are most likely the remnants of the domain boundaries which existed in the ordered green compact. The results of this study show the interference of the domain boundaries in the green compacts to be detrimental to densification, limiting the achievable final densities and degrading the properties of the sintered ceramics. This is despite the fact that the centrifugally-ordered samples had the highest densities in the green state.
Introduction
The use of monodisperse spherical submicron powders for the preparation of ideal high-density green bodies which can subsequently be densified at relatively lower temperatures have been the subject of considerable research in the past decade. These ordered compacts with high coordination numbers and very uniform pore size distributions are capable of densifying uniformly with the absence of exaggerated grain growth. Thus it should be possible to prepare full-density submicron-grain-size ceramics by densifying these compacts at temperatures several hundred degrees lower than their counterparts. In past work [l-3], monodisperse spherical particles of materials such as TiOz, SiO, and ZrO, have been prepared through novel powder-preparation routes. Alkoxide hydrolysis was one of the most widely used routes which have been developed. Dense green bodies with typically 60-70% of final density have been produced by centrifugal casting or sedimentation. Sintering studies on these high-density compacts have shown that *Author
to whom correspondence
0032-5910/92/$5.00
should be addressed.
they can be densified to 95% and higher at relatively low temperatures. Sacks et al. [4, 51 prepared ordered SiO, compacts and investigated the sintering behaviour of these compacts. The reported green densities of the compacts were low in the first two studies, but the compacts were sintered to full densities at 1000 and 1050 “C. In a later study [6] on the infiltration of these compacts with a fine silica sol, the green densities were higher but the sintered densities remained below those of the earlier studies. A significant increase in the densification rate was observed with the infiltration procedure. In these studies the green densities of the ordered cakes were substantially lower than the theoretically achievable densities. The existence of lower-density packing arrangements in addition to the packing defects was thought to be responsible for the low green densities. In a recent study on the two-dimensional ordering and sintering of spherical latex and glass spheres, it was observed that the number of spheres in the average domain can be as high as 1000 [7]. The boundaries between these domains had pulled apart, leaving large holes and cracks. In contrast, it was shown that the domain sizes for bimodal powders were, on the average,
0 1992 - Elsevier Sequoia.
All rights reserved
186
an order or two lower and there were almost no cracks or defects formed upon densification in these twodimensional bimodal structures. In another study on the densification of two-dimensional arrays of monosized copper spheres by Weiser and De Jonghe [S], it was found that differential densification was the major cause of rearrangement. Pieranski [9] examined the nature of defects in twoand three-dimensional colloid crystals. He reports that the most frequently found colloid crystals are facecentered cubic or body-centered cubic. Colloid crystals are mostly polycrystalline and contain a large number of defects which are easily created. From the photographs of the colloid crystals of this work, the grain boundaries and the dislocation walls can easily be identified. In another study it is suggested that hexagonally packed two-dimensional arrays exist in concentrated suspensions of monosize polyvinyl chloride particles [lo]. Domain sizes of 25 pm and 40 pm were determined for ordered TiO, and SiO, compacts through the adaptation of diffraction line broadening to light diffraction [ll]. It can be concluded from the studies mentioned above that the packing of monosize submicron spheres is likely to create undesirable defects, although relatively high green densities can be achieved. The presence of defects in addition to different packing arrangements may be inevitable. In this study, ordered high-density silica compacts were prepared and the densification behaviour of these compacts was investigated. Changes in the pore structure were followed and SEM analysis of the sintered structures was carried out. The results were compared with those obtained on uniaxially prepared pellets and filter cakes prepared from suspensions with lower sphere concentrations.
The recovered powders were dispersed in distilled water using ultrasonic forces. The pH of the suspensions was kept between 7 and 10. These suspensions, which were -5-10 wt.% solids, were centrifuged to form high-density ordered compacts. After the clear top liquid layer was removed, the ordered compacts were air dried at room temperature for several days and then dried at 383 K overnight. Scanning electron microscope photographs (Hitachi S-450 SEM) of the compact fracture surfaces were taken in order to examine the nature of the ordering. A typical SEM micrograph of the fracture surface of a 270 nm ordered sphere compact is given in Fig. 1. Ordered compacts of 270 nm spheres show spectacular colors in both the wet and dry states. In addition to the ordered compacts, thin high-density silica compacts were prepared by directly filtering the precipitate solution through a Buchner funnel. The weight percent of solids in the precipitate solutions was -5 times lower than the solids content of the suspensions used for preparing ordered cakes. These solutions were very basic and well dispersed. In addition to solution processing techniques, compacts were prepared by uniaxially pressing powders prepared by grinding the filtered and dried cakes. Pellets were prepared at 70 MPa using 9.5 mm diameter dies. Sintering experiments used an air atmosphere under isothermal conditions at temperatures in the range of 800-l 100 “C. Isothermal hold periods varied between 2 and 24 h. Mercury porosimetry (Autoscan 33 Mercury Porosimeter, Quantachrome) over the pressure range of 12-33 000 psia was used for the determination of the total pore volume and pore size distribution (contact angle assumed to be 140”). Some heat-treated samples
Experimental Monodisperse submicron silica spheres were synthesized by the method described by Stober et al. [12]. Tetraethyl orthosilicate, TEOS (Aldrich Chemical, technical grade), in ethyl alcohol (0.28 M) was hydrolyzed by the addition of an N&OH-ethyl alcohol solution (1.33 mol NH, per liter of ethanol and 2.94 mol H,O per liter of ethanol for the precipitation of 270 nm spheres). Precipitation of monodisperse silica spheres was completed by stirring overnight. The powder was recovered by filtering the precipitate solution and drying in an oven at 383 K. Before drying, a sample was taken from the sphere slurry for size analysis. The sample was pipetted onto a carbon 8lm and micrographs of the spheres were obtained with a TEM (JEOL JEM2000 FX TEM). The mean sphere size was obtained from image analysis of the TEM micrographs.
Fig. 1. Scanning electron micrograph ordered cakes of 270 nm spheres.
of the fracture
surface
of
187
were broken and SEM micrographs microstructure analysis.
Results
were taken for
and discussion
The precipitated particles are spherical and monosize. The SEM photograph of the fracture surface of the ordered cakes given in Fig. 1 shows that the order is long-range, dominating the whole of the compact rather than regional (the layers close to the surface are most likely to be ordered, which was not the case in this study). The presence of hexagonal ordered planes of spheres is clearly visible although the arrangement of these planes is not. In a previous study on the mercury intrusion and extrusion to these ordered compacts, it was observed that the intrusion pore sizes were very close to the opening sizes expected for several high density sphere packings (rhombohedral, tetragonal and body-centered cubic) [13]. It was concluded in that study that extrusion pore sizes were controlled mainly by the defects in these ordered structures. The extrusion pore sizes were significantly higher than expected pore body sizes for these ordered sphere packings. The intrusion pore size distribution in the ordered compact of 270 nm spheres is shown in Fig. 2. The pore size distribution is unimodal and the pore peak in these compacts is located between 30-40 nm. Table 1 gives
the intruded pore volumes for all the samples investigated in this study. From the intruded pore volumes of these ordered compacts and the density of the material heat treated at 110 “C, it was observed that the green compacts were about 66-69% dense. Although the pore peak for the filtered green compact is located slightly below 30 nm (Fig. 3), the intruded pore volume is slightly higher than the ordered compact. In another study conducted by the authors on the determination of the coordination number in these compacts [14], it was observed that the coordination number varied between 10-12. From these studies and earlier studies of other researchers, it can be concluded that these compacts do contain a combination of different packing arrangements and they are representative of the densest monosize powder compacts practically possible. When these silica compacts are isostatically pressed at 210 MPa, the intruded volume decreases to less than 0.2 cm3 g-‘, forming dense compacts which are only 2-3% below the theoretically possible 74% limit for rhombohedral packing. The pore size distributions obtained on some of the sintered samples by mercury porosimetry are given in Figs. 4 through 8 and the intruded volumes are given in Table 1. The location of the pore size peak and the pore size distribution is very similar for ordered green compact and for compact heat-treated for 2 h at 800 “C with a slight decrease in the intruded volume.
1.6
0.6
100
Radius, nm Fig. 2. Pore size distribution
of ordered
compact
of 270 nm spheres.
188 TABLE 1. Sintering results on ordered of 270 nm silica spheres Nature of sample
Sintering (“C, h)
ordered compact filtered compact ordered compact ordered compact ordered compact ordered compact ordered compact ordered compact filtered compact
--800,2 900,2 950,24 1000,2 1000,24 1050,2 1000,19
temperature,
and filtered
time
compacts
Pore volume (cm’ g-‘) 0.2430 0.2722 0.2270 0.1970 0.1817 0.0158 opaque 0.0181 translucent
A similar trend is seen in the sample heat-treated at 900 “C for 2 h with a slight decrease in pore volume. Figure 6 shows the pore size distribution in the ordered compact heat-treated at 950 “C for 24 h. The pore volume is still high, showing the absence of significant densification, and the sintered compact has only 70-71% of theoretical density. The pore peak size is under 30 nm, most probably due to closing of the openings through neck growth. Significant densification starts with the 1000 “C sample heat-treated for 2 h. There are two groups of pore sizes in this sample, as can be seen in Fig. 7. The first population has sizes greater than the original pores in the green compact and the second 3 SIN-6
2.5
2 s :: i .!?
1.5
s 6 1
0.5
Radius, nm Fig. 3. Pore size distribution
of filtered cake of 270 nm spheres.
population has a wide, finer pore size distribution. Although there is still some microporosity below 32 A, which is the lower limit of the porosimeter used, even with the measured intruded pore volume the sample has less than 97% of theoretical density. Increasing the holding time at this temperature yielded opaque samples, showing that the pore phase has similar characteristics. Increasing the sintering temperature to 1050 “C did not increase the density and gave more of the bigger pores, as seen in Fig. 8. Although densification is relatively rapid above 950-975 “C, the compacts did not densify completely and had >3% porosity in the samples. Despite small spots on the compacts which were transparent, the compacts were predominantly opaque. Contrary to the above observations, the green compacts prepared through filtering the precipitate solutions yield translucent pieces when sintered at or above 1000 “C. The SEM micrographs of the fracture surfaces of translucent pieces obtained from sintering filtered cakes are shown in Fig. 9. The low magnification photograph shows a uniform fracture surface free from pores and defects. The grains are about the same size as the original spheres, and due to densification the partial ordering has been replaced by almost perfect ordering of the grains. The completely dense planes of the grains can easily be detected. There are no apparent cracks or large voids in the sintered body.
189
SIN-l
2.5
2 9 :: _ & 9
1.5
s 4 1
0.5
0
1
lb
Radius, Fig.
4.
Pore size distribution
nm
of ordered
compact
of 270 nm spheres
of ordered
compact
of 270 nm spheres
heat-treated
at 800 “C for 2 h.
heat-treated
at 900 “C for 2 h.
3 SIN-2
2.5
Radius,
Fig. 5. Pore size distribution
nm
190 1.E
1.6
1.4
1.2
9 8
1
i E 5
0.8
Y -0 0.6
SIN-5
0.4
0.2
0
Fig. 6. Pore size distribution
of ordered
Radius, nm of 270 nm spheres
compact
heat-treated
at 9.50 “C for 24 h.
0.01t
0.014
0.011
0.010 9 8 6 E 2
0.008
P 0.006
0.004
0.002
0.000
I
I
I
I
I
I
5
10
15
20
25
30
-I 3:
Pressure, kpsi Fig. 7. Mercury porosimetry intruded treated at 1 000 “C for 2 h.
and extruded
volume VS. pressure
curves for the ordered
compact
of 270 nm spheres
heat.
191
SIN-4
0.000
0
I
5
I
10
I
15
I
20 *
I
25
I
30
5
Pressure, kpsi Fig. 8. Mercury porosimetxy intruded treated at 1050 “C for 2 h.
and extruded
volume vs. pressure
The SEM micrographs of the fracture surfaces of sintered ordered pieces at 1000 “C contain a series of defects, as shown in Fig. 10. There are cracks in the pictures, some of which may have been created during sample preparation for the SEM work. The second and the most important defects are seen as bright lines in the micrographs. These bright lines are Xl-100 pm in length, and some of them are connected while some are isolated. Depressed and elevated regions are seen between these lines or boundaries. They are not local, seem to be parallel to each other and most probably lie perpendicular to the particle motion during centrifugation. The lines surrounding less dense areas are likely to be remnants of the original grain boundaries. During sintering these domains densified rapidly, leaving some of the domain boundaries as pore deposition areas. At the end, with the driving force of densification diminishing, these low density areas remained in the body, limiting the final density to 95-97% of theoretical density. This dramatic difference in microstructure, which is accompanied by a small difference in sintered densities (l-5%), can only be traced back to the powder-consolidation step. During preparation of the ordered compacts from suspensions with 5-10 wt.% solids content through centrifugation, it was always observed that the process of incorporation of individual spheres into
curves for the ordered
compact
of 270 nm spheres
heat-
the dense cake at the bottom of the tubes is not a single-step process. We always found a fluid-like dense colloid on top of the ordered cakes in the tube when the centrifugation process was stopped prematurely. This fluid-like colloid has a much higher solids content and the spheres are relatively closer to each other than in the suspension. It is well known that dispersion stability is a sensitive function of particle concentration. It is probable that despite the electrostatic repulsive forces between particles, ordered planes and domains form in this colloid state and incorporate with the cake afterwards. The dimensions of these domains must be a function of the surface charges and suspension characteristics, in addition to other factors such as tube size and extent of centrifugal forces. In other words, the ordering process is not accomplished by individual spheres but by ordered domains of groups of spheres. This creates a structure somewhat similar to the case in which an agglomerated powder is processed. The undesirable effects of agglomerates in powder processing are well known [15]. The ordering process, in contrast to dry powder processing where agglomerates already exist, creates agglomerates in situ just before consolidation is achieved. So, although suspension processing is generally used to eliminate these agglomeration effects, in the particular case of centrifugal ordering, it does not prevent them from forming.
192
(a)
(b)
(b)
Fig. 9. Scanning electron micrographs of the fracture surface of translucent samples prepared from 270 nm spheres. (a) Low magnification, (b) high magnification.
Fig. 10. Scanning electron micrographs of the fracture surface of sintered and ordered samples treated at 1000 “C for 24 h. (a) Low magnification, (b) high magnification.
During filtering of the precipitated sphere suspensions, the consolidation process is accomplished without the formation of secondary particles. The direction of movement of the filtering medium and the particles is the same, and the solids content of the suspension is much lower. The spheres are likely to incorporate into the cake one by one without forming particle assemblages. Although the cake may be less dense and the order may not be as apparent, the results obtained in this work show that the filtering process produces defectfree compacts.
Sintering results obtained on uniaxially pressed pellets are given in Table 2. These results clearly show the advantages of suspension processing. The sintered density was only 67% of theoretical density at 1000 “C, whereas samples near theoretical density were obtained through suspension processing. Similar densities can be achieved only at about 100 “C higher sintering temperatures. A brief experiment on the effect of sphere size was conducted using an ordered compact of 130 nm spheres. As shown in Table 3, decreasing the size by half does
193 TABLE 2. Sintering results on pellets pressed uniaxially at 70 MPa from random powder agglomerates of 270 nm spheres Sintering (“C, h)
temperature,
Density (%)
time
66.9% 90.4% 97.2%
1000,24 1050,24 1100,24
TABLE 3. Effect of sphere compacts
size on the sintering
forward and that defects can easily be introduced. If the advantages of these compacts are to be realized, methods to eliminate these defects will need to be developed. Acknowledgement Support for this work has been provided by Sandia National Laboratories (#OS-5795).
of ordered
References Sphere
size
270 nm 270 nm 130 nm
Sintering (“C, h) 900,2 950,24 950,20
temperature,
time
Pore volume (cm’ g-‘) 0.1970 0.1817 0.1450
not result in significant densification at 950 “C. This observed effect on the densification is consistent with a small temperature difference between 950-l 000 “C and may not be a size effect at all.
Conclusion
1 E. A. Barringer,
R. Brook and H. K. Bowen, in G. C. Kuczynski, A. E. Miller and G. A. Sargent (eds.), Sinfeting and Heterogenous Catalysis, Materials Science Research, Vol. 16, Plenum Press, New York, 1984, pp. 1-21. 2 B. Fegley and E. A. Barringer, in C. J. Brinker, D. E. Clark and D. R. Ulrich (eds.), Better Ceramics Through Chemistv, Materials Research Sociew Symposia Proc., Vol. 32, North Holland, New York, 1984, pp. 187-197. 3 E. A. Barringer, N. Jubb, B. Fegley, R. L. Pober and H. K. Bowen, in L. L. Hench and D. R. Ulrich (eds.), Uitrastruchtre Processing of Ceramics, Glasses, and Composites, Wiley, New York, 1984, pp. 315-333. M. D. Sacks and T. Y. Tseng, L Am. Ceram. Sot., 67 (1984) 526. M. D. Sacks and T. Y. Tseng, /. Am. Ceram. Sot., 67 (1984) 532.
The suspension processing of ordered green bodies of submicron spherical powders for the preparation of defect-free fine grain-sized ceramics has its disadvantages, in addition to the apparent advantages discussed in the ceramics literature for the last decade. The creation of defects may not be easy to control in these ordered dense compacts. This study has shown that densification is halted by the presence of defects formed during the consolidation step. Although the green bodies met all the expected requirements for good sintering behaviour, i.e. uniform and fine pore size distribution and high coordination number, the boundaries of domains forming during consolidation kept the density 3-5% below the theoretical limit. The results of this study show that submicron-monosize sphere ordering for the formation of dense green bodies is not straight-
M. D. Sacks and S. D. Vora, J. Am. Ceram. Sot.,
71 (1988)
245.
E. Liniger and R. Raj, J. Am. Ceram. Sot., 70 (1987) 843. M. W. Weiser and L. C. de Jonghe, _I.Am. Ceram. Sot., 69 (1986)
10 11 12 13
14 15
822.
P. Pieranski, in M. Kleman, R. Balian and J. P. Poirier (eds.), Physics of Defects, North Holland, Amsterdam, 1980, pp. 183-200. R. L. Hoffman, Trans. Sot. Rheol., 16 (1972) 155. S. Y. Chang and T. A. Ring, Langmuir, 4 (1988) 1128. W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. M. Ciftcioglu, D. M. Smith, S. B. Ross, Mercury Porosimetry of Ordered Sphere Compacts: Investigation of Intrusion and Extrusion Pore Size Distributions. Powder Technol., 55 (1988) 193. M. Ciftcioglu, D. M. Smith and S. B. Ross, in preparation. M. Ciftcioglu, M. Akic and L. E. Burkhart, Am. Ceram. Sot. Bull., 6.5 (1986) 1591.