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Morphological evolution of ZrO2-SiO2 composite gel and stability of tetragonal ZrO2
November
1995
Materials Letters 25 (1995)
ELSEVIER
151-155
Morphological evolution of ZrO,-SiO, composite gel and stability of tetragonal ZrO, SW. Wang *, J.K. Guo, X.X. Huang, B.S. Li Shanghai Institute of Ceramics, Chinese Academy of Sciences, I295 Ding-Xi Road, Shanghai 200050, China Received 20 March 1995; revised 26 June 1995; accepted 1 July 1995
Abstract Composite ZrO,-SiO, gels were prepared using fumed silica and ZrOCl, solution as starting materials. Morphological evolution of ZrO, particles formed during heat treatment, and the particle sizes of the composites were investigated by transmission electron microscopy. The morphology of the ZrO, particles experiences several changes during heat treatment, i.e. the initially loose sponge-like body changes to dense sponge-like agglomerates, then to spherical particles which are embedded in the silica matrix, and finally to larger spherical particles. The stability of tetragonal zirconia in the gels is discussed in the light of the particle-size effect. The martensitic transformation of ZrOz (to the monoclinic phase) was found to occur in situ, when irradiated by a high-energy electron beam.
1. Introduction Silica-zirconia materials are widely studied because of their superior chemical resistance to alkaline attack [ 1] and fracture toughness [ 21. The toughness of the ZrO*--SiO, based ceramics is associated with the tetragonal to monoclinic transformation of ZrOz particles. The stabilization of the tetragonal phase up to very high temperatures appears possible by the control of microstructure using appropriate processing methods. In recent years, ZrO*-SiO, glass fibres [ 31, glass coatings [ 41, monolithic glasses [ 51 and composite powders [6] have been prepared by sol-gel techniques using alkoxides as starting materials. We are interested in preparing ZrO,-SiO, composite powders by a coprecipitation approach using fumed silica and zirconyl chloride as precursors and in studying the stability and morphology of Zr02 particles in the silica matrix. In * Corresponding 0167-577x/95/$09.50
author. 0 1995 Elsevier Science B.V. All rights reserved
SSDIOl67-577x(95)00168-9
this paper, we present the morphological features of the ZrOz-SiO;? composite powders and the interpretation of the stability of tetragonal zirconia ( t-ZrOz) .
2. Experimental
procedure
range .xZrO, composition in the Gels - (lOO--x)SiO,, x= 10, 20, 30, and 40 ~01% were prepared from fumed silica and zirconyl chloride (made into a standard solution by dissolving in water) as starting materials. The fumed silica and ZrOCl, solution were mixed thoroughly to obtain a homogeneous slurry. The gels can be obtained by adding NH, aqueous solution to the slurry under vigorous stirring. The resulting gels were filtered and washed with distilled water and finally dried at 120°C overnight. The as-dried gels were then heat-treated in air for 2 h at 500, 700, 900, 1100 and 135O”C, respectively. The phase composition was determined by use of an X-ray diffractometer (Rigaku Denki RAX- 10) with Cu
152
S. W. Wmg et cd. / Materids
Ka radiation. Fine particles of the gel powders variously heat treated were held in porous carbon grids for the observations of morphology, particle size and structure by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) (JEM2OOCX, JEOL)
3. Results and discussion Fig. 1 shows the morphological evolution of the ZrO*-SiOz composite powder (20 ~01% ZrO,, 80 ~01% SiO,, denoted as 20ZR) as a function of temperature. The fumed silica dispersed in water and then dried was found to be about 20 nm in size (Fig. 1a). The particles
Letters 2.5 (IYYS) IS/-15.5
grow larger with increasing temperature. The silicapartitle size in the heat-treated composite gel is about 0.50.7 km after being heat treated at 1350°C for 2 h (Fig. If. the black phase encapsulated in the Si02 particle is zirconia, which will be discussed below). The results of IR spectroscopy [ 71 revealed that the formation of Si-0-Si bonds among different SiOz particles, deduced from the intensity of the 800 cm- ’ band, becomes stronger with the increase of temperature, and is due to the growth of silica particles with temperature. From Fig. 1, it is easily seen that the morphological evolution of ZrO, is more complicated than that of SiO,. Fig. lb is the morphology of the 20ZR powder calcined at 5OO”C, and we can see the loose spongelike agglomerates which consist of small grains and
Fig. I. Transmission electron micrographs of fumed silica (a) and 20ZR gel (20 ~01% ZrOz, 80 ~01% SiO,) after heat treatments at: (b) 5OO”C, CC) 700°C (d) 900°C. (e) 1100°C and (f) 1350°C. The black round phase in (e) or (f) are zirconia particles.
S. W. Wang et al. /Materials
(a)
(b)
(4
Fig. 2. Schematic illustrations of the morphological evolution of ZrO,: (a) sponge-like body in a loose state, (b) the dense spongelike body, (c) spherical particles (black) of tetragonal zirconia (tZrO,) embedded in silica matrix, (d) the growth of t-ZrO, spherical particles.
pores. With the increase of calcining temperature, the agglomerates become denser (from Fig. 1b to Fig. Id). Fig. Id shows the microstructure of the same powder calcined at 900°C for 2 h. The agglomerates appear clearly and are more compacted, and only pores between agglomerates remain. As mentioned above, the SiOz particles grow rapidly with increasing temperature. When the temperature rises to I lOO”C, the dense agglomerates of ZrOz shrink further and evolve into a spherical shape (Fig. le, black round particles). The spherical particles of zirconia are embedded in silica particles, which are of irregular shape (Fig. 1f) . The particle size of ZrO, is about 8-10 nm at 1100°C and finally 50 nm or so at 1350°C. The structure of the agglomerated ZrO,-SiO, fine powder in the loose state during calcining is schematically illustrated in Fig. 2. As we observed, the morphological evolution of ZrO, in the composite powder can be divided into four steps. First, the morphology of ZrO, after calcining at 500°C is the sponge-like body (Fig. 2a), which already existed in the Zr( OH), gels. The sponge-like body (agglomerates) then changes gradually from loose to dense with increasing temperature. The characteristic of the second stage is the coalescence in agglomerates. As argued by Lange [ 81, for agglomerates first sintered and densified, a tensile stress should be produced between combined agglomerates and cause them to break away from the surrounding matrix, leaving gaps between them (Fig. 2b). The third step is that the densified agglomerates of zirconia change into spherical form at a higher temperature ( 1100°C). We can see that the spherical zirconia particles (Figs. 2c and 2d, black phase) are entrapped in the silica particles, which is consistent with the suggestion of Kagawa et al. [ 91, revealed by electrophoretic mobility measurements, that the surfaces of the heated ZrO,-SiO, particles consist predominantly of SiOz. Finally, the particles of ZrO, or Si02 grow larger
Letters 25 (1995) 151-155
153
with increasing temperature. Still, some smaller grains of t-ZrO, can be seen in Fig. lf, which indicates that there are different sizes of the agglomerates in the asprepared gels. It is known that zirconia can exist in three crystalline forms: cubic (c), tetragonal (t) and monoclinic (m) Generally, it is possible to stabilize metastable zirconia at room temperature (t- or c-phase) by doping with MgO, Y,O,, and CeO, or their mixtures. For the ZrO,SiO, composite powders, however, the t-ZrO, can survive to higher temperatures without doping with stabilizers. Many papers [ 10,111 reported that the SiO, matrix has the ability to stabilize zirconia in a tetragonal phase. According to the literature, the stabilizing effects of the tetragonal phase in zirconia are of two kinds: one is the particle-size effect, reported by Garvie [ 121 and the other is the constraint effect, reported by Heuer et al. [ 131. Since the ZrO, particles are dispersed in the silica matrix, grain growth of ZrO, is difficult due to encasement in the SiOz. For example, the grain size of t-ZrO, is about 10 nm after heat treatment at 1100°C. It is reasonable to expect t-ZrO, with smaller grain size to be stable at room temperature on the basis of the particle-size effect. After calcining at 1350°C for 2 h, the grains of t-ZrO, grow larger and the average size of t-ZrO, particles is about 50 nm. Fig. 3a shows that crystallization of the as-prepared gel into the tetragonal form of ZrOz has occurred at 900°C. and no transformation from the tetragonal to the monoclinic phase is detected by XRD (Fig. 3b) when the composite gel was subsequently heat-treated for 2 h at 1350°C. The critical particle size of a t-ZrO, free particle is around 30 nm, while the critical particle size of the
Fig. 3. X-ray diffraction pattern of 20ZR gel after being calcined at (a) 9OO”C, and heated to 1350°C (b) for 2 h, respectively.
154
Fig. 4. (a) High resolution electron micrograph of the spherical t-ZrO? particle (a quarter of a particle) dispersed in the amorphous silica matrix, (b) typical twinning is observed in the encapsulated spherical t-ZrO, particles after the tetragonal to monoclinic transformation is induced by localized electron beam irradiation.
constrained particle, such as a ZrO, particle in an A1203 matrix is larger, typically = 600 nm [ 121. Even though t-ZrO, particles were precipitated in a silica matrix, the critical particle size was closer to that of the free particle [ 21. It is not strange that there is a difference between the critical size of t-ZrO, in the present study and that in the literature [ 21, because the critical size of t-ZrO, depends upon the composition and/or temperature in the same matrix. It is suggested that the critical size of the t-ZrO, grain has different values when the ZrO, grains are dispersed in different matrices or in the same matrix with different composition and/or different heat treatment. Therefore, the stability of 50 nm t-ZrO, grains is ascribed to their smaller particle sizes than the critical size of t-ZrO, in the present system. The high resolution electron micrograph (Fig. 4a) shows the lattice image of the spherical t-ZrO, particle (a quarter of a particle) dispersed in the amorphous silica matrix for the 20ZR sample after being calcined at 1350°C for 2 h. Additionally, an interesting phenomenon has been found by HREM, namely, that some of the tetragonal (t) spherical grains become monoclinic (m) when irradiated with an intense electron beam (which results in heating) in HREM. The typical twinning that occurs during this transformation of the encapsulated tetragonal particles is clearly revealed in Fig. 4b.
4. Conclusion According to the TEM observations, some conclusions may be drawn: ( 1) homogeneously dispersed ZrO*-SiOz composite powders have been obtained by
a coprecipitation approach using fumed silica and zircony1 chloride as precursors, (2) the silica matrix is simply growing with increasing temperature, and gradually encases the ZrO, grains in it, (3) the morphological evolution of ZrO, is complicated, i.e. the loose sponge-like body + the dense sponge-like one (agglomerates) -+ the dense agglomerates change into spherical ZrO, particles (embedded in the silica matrix) + the growth of spherical grains, (4) the smaller size of the t-ZrO, grains contributes to the stability of the tetragonal zirconia, and (5) the transformation from the tetragonal to the monoclinic phase has been induced by irradiation with a high-intensity electron beam.
Acknowledgements The authors would like to thank MS Meiling Ruan for her help in the TEM and HREM experiments.
References [ 11 A. Paul, Chemistry of glasses (Chapman and Hall, London, 1982) p. 139. [ 2 I M. Nogami and M. Tomozawa, J. Am. Ceram. Sot. 69 ( 1986) 99. [ 31 M. Guglielmi and A. Maddalena, J. Mater. Sci. Letters 4 (1985) 123. [4] H. Dishch, J. Non-Cryst. Solids 57 ( 1983) 371. [ 51 M. Palladino, F. Pirini, M. Beghi, P. Chiurlo, G. Cogliati and L. Costa, J. Non-Cryst. Solids 147 & 148 ( 1992) 335. [6] S.K. Saha and P. Pramanik, J. Non-Cryst. Solids 159 ( 1993) 31. [ 71 S.W. Wang, X.X. Huang, J.K. Guo, J. Eur. Ceram. Sot., submitted for publiction.
S. W. Wang et al. /Materials
[ 81 F.F. Lange, J. Am. Ceram. Sot. 67 ( 1984) 83.
[ 91 M. Kagawa, Y. Syono, Y. Imamura and S. Usui, J. Am. Ceram. Sot. 69 ( 1986) ~50.
[ 101 G. Monk, M.C. Marti, J. Carda, M.A. Tena, P. Escribano and M. Anglada, J. Mater. Sci. 28 ( 1993) 5852.
Letters 25 (1995) 151-155
155
[ I I] VS. Nagarajan, K.J. Rao, J. Mater. Sci. 24 ( 1989) 2140. [ 121 R.C. Garvie, J. Phys. Chem. 69 ( 1965) 1238. [ 131 A.H. Heuer, N. Claussen, W.M. Kriven and M. Ruhle, J. Am. Ceram. Sot. 65 ( 1982) 642.
1995
Materials Letters 25 (1995)
ELSEVIER
151-155
Morphological evolution of ZrO,-SiO, composite gel and stability of tetragonal ZrO, SW. Wang *, J.K. Guo, X.X. Huang, B.S. Li Shanghai Institute of Ceramics, Chinese Academy of Sciences, I295 Ding-Xi Road, Shanghai 200050, China Received 20 March 1995; revised 26 June 1995; accepted 1 July 1995
Abstract Composite ZrO,-SiO, gels were prepared using fumed silica and ZrOCl, solution as starting materials. Morphological evolution of ZrO, particles formed during heat treatment, and the particle sizes of the composites were investigated by transmission electron microscopy. The morphology of the ZrO, particles experiences several changes during heat treatment, i.e. the initially loose sponge-like body changes to dense sponge-like agglomerates, then to spherical particles which are embedded in the silica matrix, and finally to larger spherical particles. The stability of tetragonal zirconia in the gels is discussed in the light of the particle-size effect. The martensitic transformation of ZrOz (to the monoclinic phase) was found to occur in situ, when irradiated by a high-energy electron beam.
1. Introduction Silica-zirconia materials are widely studied because of their superior chemical resistance to alkaline attack [ 1] and fracture toughness [ 21. The toughness of the ZrO*--SiO, based ceramics is associated with the tetragonal to monoclinic transformation of ZrOz particles. The stabilization of the tetragonal phase up to very high temperatures appears possible by the control of microstructure using appropriate processing methods. In recent years, ZrO*-SiO, glass fibres [ 31, glass coatings [ 41, monolithic glasses [ 51 and composite powders [6] have been prepared by sol-gel techniques using alkoxides as starting materials. We are interested in preparing ZrO,-SiO, composite powders by a coprecipitation approach using fumed silica and zirconyl chloride as precursors and in studying the stability and morphology of Zr02 particles in the silica matrix. In * Corresponding 0167-577x/95/$09.50
author. 0 1995 Elsevier Science B.V. All rights reserved
SSDIOl67-577x(95)00168-9
this paper, we present the morphological features of the ZrOz-SiO;? composite powders and the interpretation of the stability of tetragonal zirconia ( t-ZrOz) .
2. Experimental
procedure
range .xZrO, composition in the Gels - (lOO--x)SiO,, x= 10, 20, 30, and 40 ~01% were prepared from fumed silica and zirconyl chloride (made into a standard solution by dissolving in water) as starting materials. The fumed silica and ZrOCl, solution were mixed thoroughly to obtain a homogeneous slurry. The gels can be obtained by adding NH, aqueous solution to the slurry under vigorous stirring. The resulting gels were filtered and washed with distilled water and finally dried at 120°C overnight. The as-dried gels were then heat-treated in air for 2 h at 500, 700, 900, 1100 and 135O”C, respectively. The phase composition was determined by use of an X-ray diffractometer (Rigaku Denki RAX- 10) with Cu
152
S. W. Wmg et cd. / Materids
Ka radiation. Fine particles of the gel powders variously heat treated were held in porous carbon grids for the observations of morphology, particle size and structure by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) (JEM2OOCX, JEOL)
3. Results and discussion Fig. 1 shows the morphological evolution of the ZrO*-SiOz composite powder (20 ~01% ZrO,, 80 ~01% SiO,, denoted as 20ZR) as a function of temperature. The fumed silica dispersed in water and then dried was found to be about 20 nm in size (Fig. 1a). The particles
Letters 2.5 (IYYS) IS/-15.5
grow larger with increasing temperature. The silicapartitle size in the heat-treated composite gel is about 0.50.7 km after being heat treated at 1350°C for 2 h (Fig. If. the black phase encapsulated in the Si02 particle is zirconia, which will be discussed below). The results of IR spectroscopy [ 71 revealed that the formation of Si-0-Si bonds among different SiOz particles, deduced from the intensity of the 800 cm- ’ band, becomes stronger with the increase of temperature, and is due to the growth of silica particles with temperature. From Fig. 1, it is easily seen that the morphological evolution of ZrO, is more complicated than that of SiO,. Fig. lb is the morphology of the 20ZR powder calcined at 5OO”C, and we can see the loose spongelike agglomerates which consist of small grains and
Fig. I. Transmission electron micrographs of fumed silica (a) and 20ZR gel (20 ~01% ZrOz, 80 ~01% SiO,) after heat treatments at: (b) 5OO”C, CC) 700°C (d) 900°C. (e) 1100°C and (f) 1350°C. The black round phase in (e) or (f) are zirconia particles.
S. W. Wang et al. /Materials
(a)
(b)
(4
Fig. 2. Schematic illustrations of the morphological evolution of ZrO,: (a) sponge-like body in a loose state, (b) the dense spongelike body, (c) spherical particles (black) of tetragonal zirconia (tZrO,) embedded in silica matrix, (d) the growth of t-ZrO, spherical particles.
pores. With the increase of calcining temperature, the agglomerates become denser (from Fig. 1b to Fig. Id). Fig. Id shows the microstructure of the same powder calcined at 900°C for 2 h. The agglomerates appear clearly and are more compacted, and only pores between agglomerates remain. As mentioned above, the SiOz particles grow rapidly with increasing temperature. When the temperature rises to I lOO”C, the dense agglomerates of ZrOz shrink further and evolve into a spherical shape (Fig. le, black round particles). The spherical particles of zirconia are embedded in silica particles, which are of irregular shape (Fig. 1f) . The particle size of ZrO, is about 8-10 nm at 1100°C and finally 50 nm or so at 1350°C. The structure of the agglomerated ZrO,-SiO, fine powder in the loose state during calcining is schematically illustrated in Fig. 2. As we observed, the morphological evolution of ZrO, in the composite powder can be divided into four steps. First, the morphology of ZrO, after calcining at 500°C is the sponge-like body (Fig. 2a), which already existed in the Zr( OH), gels. The sponge-like body (agglomerates) then changes gradually from loose to dense with increasing temperature. The characteristic of the second stage is the coalescence in agglomerates. As argued by Lange [ 81, for agglomerates first sintered and densified, a tensile stress should be produced between combined agglomerates and cause them to break away from the surrounding matrix, leaving gaps between them (Fig. 2b). The third step is that the densified agglomerates of zirconia change into spherical form at a higher temperature ( 1100°C). We can see that the spherical zirconia particles (Figs. 2c and 2d, black phase) are entrapped in the silica particles, which is consistent with the suggestion of Kagawa et al. [ 91, revealed by electrophoretic mobility measurements, that the surfaces of the heated ZrO,-SiO, particles consist predominantly of SiOz. Finally, the particles of ZrO, or Si02 grow larger
Letters 25 (1995) 151-155
153
with increasing temperature. Still, some smaller grains of t-ZrO, can be seen in Fig. lf, which indicates that there are different sizes of the agglomerates in the asprepared gels. It is known that zirconia can exist in three crystalline forms: cubic (c), tetragonal (t) and monoclinic (m) Generally, it is possible to stabilize metastable zirconia at room temperature (t- or c-phase) by doping with MgO, Y,O,, and CeO, or their mixtures. For the ZrO,SiO, composite powders, however, the t-ZrO, can survive to higher temperatures without doping with stabilizers. Many papers [ 10,111 reported that the SiO, matrix has the ability to stabilize zirconia in a tetragonal phase. According to the literature, the stabilizing effects of the tetragonal phase in zirconia are of two kinds: one is the particle-size effect, reported by Garvie [ 121 and the other is the constraint effect, reported by Heuer et al. [ 131. Since the ZrO, particles are dispersed in the silica matrix, grain growth of ZrO, is difficult due to encasement in the SiOz. For example, the grain size of t-ZrO, is about 10 nm after heat treatment at 1100°C. It is reasonable to expect t-ZrO, with smaller grain size to be stable at room temperature on the basis of the particle-size effect. After calcining at 1350°C for 2 h, the grains of t-ZrO, grow larger and the average size of t-ZrO, particles is about 50 nm. Fig. 3a shows that crystallization of the as-prepared gel into the tetragonal form of ZrOz has occurred at 900°C. and no transformation from the tetragonal to the monoclinic phase is detected by XRD (Fig. 3b) when the composite gel was subsequently heat-treated for 2 h at 1350°C. The critical particle size of a t-ZrO, free particle is around 30 nm, while the critical particle size of the
Fig. 3. X-ray diffraction pattern of 20ZR gel after being calcined at (a) 9OO”C, and heated to 1350°C (b) for 2 h, respectively.
154
Fig. 4. (a) High resolution electron micrograph of the spherical t-ZrO? particle (a quarter of a particle) dispersed in the amorphous silica matrix, (b) typical twinning is observed in the encapsulated spherical t-ZrO, particles after the tetragonal to monoclinic transformation is induced by localized electron beam irradiation.
constrained particle, such as a ZrO, particle in an A1203 matrix is larger, typically = 600 nm [ 121. Even though t-ZrO, particles were precipitated in a silica matrix, the critical particle size was closer to that of the free particle [ 21. It is not strange that there is a difference between the critical size of t-ZrO, in the present study and that in the literature [ 21, because the critical size of t-ZrO, depends upon the composition and/or temperature in the same matrix. It is suggested that the critical size of the t-ZrO, grain has different values when the ZrO, grains are dispersed in different matrices or in the same matrix with different composition and/or different heat treatment. Therefore, the stability of 50 nm t-ZrO, grains is ascribed to their smaller particle sizes than the critical size of t-ZrO, in the present system. The high resolution electron micrograph (Fig. 4a) shows the lattice image of the spherical t-ZrO, particle (a quarter of a particle) dispersed in the amorphous silica matrix for the 20ZR sample after being calcined at 1350°C for 2 h. Additionally, an interesting phenomenon has been found by HREM, namely, that some of the tetragonal (t) spherical grains become monoclinic (m) when irradiated with an intense electron beam (which results in heating) in HREM. The typical twinning that occurs during this transformation of the encapsulated tetragonal particles is clearly revealed in Fig. 4b.
4. Conclusion According to the TEM observations, some conclusions may be drawn: ( 1) homogeneously dispersed ZrO*-SiOz composite powders have been obtained by
a coprecipitation approach using fumed silica and zircony1 chloride as precursors, (2) the silica matrix is simply growing with increasing temperature, and gradually encases the ZrO, grains in it, (3) the morphological evolution of ZrO, is complicated, i.e. the loose sponge-like body + the dense sponge-like one (agglomerates) -+ the dense agglomerates change into spherical ZrO, particles (embedded in the silica matrix) + the growth of spherical grains, (4) the smaller size of the t-ZrO, grains contributes to the stability of the tetragonal zirconia, and (5) the transformation from the tetragonal to the monoclinic phase has been induced by irradiation with a high-intensity electron beam.
Acknowledgements The authors would like to thank MS Meiling Ruan for her help in the TEM and HREM experiments.
References [ 11 A. Paul, Chemistry of glasses (Chapman and Hall, London, 1982) p. 139. [ 2 I M. Nogami and M. Tomozawa, J. Am. Ceram. Sot. 69 ( 1986) 99. [ 31 M. Guglielmi and A. Maddalena, J. Mater. Sci. Letters 4 (1985) 123. [4] H. Dishch, J. Non-Cryst. Solids 57 ( 1983) 371. [ 51 M. Palladino, F. Pirini, M. Beghi, P. Chiurlo, G. Cogliati and L. Costa, J. Non-Cryst. Solids 147 & 148 ( 1992) 335. [6] S.K. Saha and P. Pramanik, J. Non-Cryst. Solids 159 ( 1993) 31. [ 71 S.W. Wang, X.X. Huang, J.K. Guo, J. Eur. Ceram. Sot., submitted for publiction.
S. W. Wang et al. /Materials
[ 81 F.F. Lange, J. Am. Ceram. Sot. 67 ( 1984) 83.
[ 91 M. Kagawa, Y. Syono, Y. Imamura and S. Usui, J. Am. Ceram. Sot. 69 ( 1986) ~50.
[ 101 G. Monk, M.C. Marti, J. Carda, M.A. Tena, P. Escribano and M. Anglada, J. Mater. Sci. 28 ( 1993) 5852.
Letters 25 (1995) 151-155
155
[ I I] VS. Nagarajan, K.J. Rao, J. Mater. Sci. 24 ( 1989) 2140. [ 121 R.C. Garvie, J. Phys. Chem. 69 ( 1965) 1238. [ 131 A.H. Heuer, N. Claussen, W.M. Kriven and M. Ruhle, J. Am. Ceram. Sot. 65 ( 1982) 642.