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Synthesis of cordierite powder by spray-drying
]OURNA
Journal of Non-Crystalline Solids 147&148 (1992) 554-558 North-Holland
L OF
NON-CRYSTALLISNO ELIDS
Synthesis of cordierite powder by spray-drying A n d r e Douy CNRS, Centre de Recherches sur la Physique des Hautes Temperatures, 1D, Avenue de la Recherche Scientifique, 45071 Orldans cddex 02, France
Homogeneous multicomponent silicate powders may be prepared when a silicon alkoxide is hydrolyzed in an acidic aqueous solution of metallic salts and the resulting sol rapidly transformed into a powder. By spray-drying a sol of A1 and Mg nitrates and TEOS at 200°C, followed by calcination to 750°C, a cordierite precursor powder is produced which crystallizes into c¢-cordierite via ix-cordierite, but expands upon heating instead of sintering. If a water-soluble organic polymer is added to the sol before spray-drying, the chemical homogeneity of the powder is improved, the specific surface area greatly increased and the compacted powder densities before crystallizing.
1. Introduction
In the preparation of multicomponent oxide ceramics and glasses, sol-gel chemistry has enabled improved synthetic methods, control over structure and lower processing temperatures, resulting in better material properties. Silicate gels, films, fibers and powders prepared mainly from alkoxides in organic medium, by hydrolysis, condensation and polymerization have been widely studied. However, when the number of cations increases, much care has to be taken for achieving a true copolymerization because of great differences of reactivity between the alkoxides. It has been shown that, for the elaboration of multiple oxide powders, the polyacrylamide gel is a convenient auxiliary [1]. This organic hydrophilic gel may be associated with the 'amorphous citrate process' [2], since practically all aqueous solutions may be gelled at acidic or neutral pH. The aqueous gel is then directly calcined in a ventilated oven. The chemical homogeneity of the powder results mainly from the cations complexation by the citric acid, but the organic gel has also its own contribution. This may thus be considered as a simplification and an improvement of the citrate process. The polyacrylamide gel may also be applied to other aqueous chemical processes such as homogeneous by-
droxide co-precipitation by thermal decomposition of urea in solution [3]. The co-precipitation domains are then confined within the gel pores. By further calcination, ultrafine and chemically homogeneous powders are obtained. A great number of complex oxide compositions may be prepared by aqueous polyacrylamide gel-assisted processes. However, the case of silicates is different because silicon cannot be chelated in aqueous medium. However, the elaboration of mullite, 3Al203-2SiO 2, a binary oxide [4], and cordierite, 2MgO-2A1203-5SiO2, a ternary oxide [5], has been studied, by the aqueous route, using polyacrylamide gel, and also by the alcoholic route with the aid of poly-(2-hydroxyethyl)-methacrylate (pHEMA) gel. For the aqueous preparation, a silicon alkoxide, tetraethylorthosilicate (TEOS) Si(OC2H5)4, or 3-(triethoxysilyl)-propylamine (TESPA) (C2H50)3Si(CH2)3NH2, is hydrolyzed in a diluted aqueous solution of AI salt (nitrate, citrate, chloride or sulphate) for mullite, or A1 and Mg salts (nitrates or citrates) for cordierite. The resuiting clear solution (sol) is then gelled by in situ formation of the organic network and this aqueous gel is transformed into a powder by microwave heating and calcination in air to 750°C. The properties and characteristics of the powders depend on the nature of the precursor salts and
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A. Douy / Synthesis of cordieritepowder
555
noted that the quantity of secondary phase, MgA1204, is the lowest when the fresh solution (sol) made by hydrolyzing TEOS is an aqueous A1 and Mg nitrates solution is gelled by polyacrylamide at room temperature and rapidly transformed into a dry solid by microwave heating and further calcined. TEOS is hydrolyzed into silicic acid, which further condenses and polymerizes into silica gel, the kinetics of these reactions depending on concentration, temperature and pH [6]. By a rapid treatment, the transitory step in hot aqueous medium, which is favourable for accelerating the colloid growth, may be partly avoided and the resulting powder will be more chemically homogeneous. The present paper deals with spray-drying of cordierite powders, which achieves more efficiently the rapid transformation of a sol into a powder.
alkoxide. For mullite, the powder made from aluminium citrate and TESPA crystallizes completely into mullite at 970°C for a heating rate of 10 K min -1, through a strong exothermic peak. For cordierite, starting from aluminium and magnesium nitrates and TEOS, a powder is obtained which crystallizes first into Fx-cordierite and then transforms into a-cordierite, the stable phase, with only a little amount of MgAlzO 4 spinel as secondary phase. This powder, when compacted and heated, densifies completely by a viscous flow before crystallizing, at low temperature (below 1000°C). The alcoholic route consists in dissolving nitrates and TEOS in 96% ethanol, gelling the solutions by pHEMA and calcining the gels. Very homogeneous powders are obtained. For the binary oxide, a nearly complete crystallization into mullite occurs at 970°C while, for the ternary oxide, the stable phase, c~-cordierite, is obtained at a particularly low temperature, at 1000°C for a heating rate of 10 K min -1, without any trace of secondary phase. The use of organic gels is thus very promising in the preparation of chemically homogeneous silicate powders, and one interesting point is the attainment of low temperature-sinterable cordierite powders by an aqueous route. It has been
2. Experimental The precursor sol (about 0.06 mol 1-1) is made by hydrolyzing TEOS in an aqueous solution of A1 and Mg nitrates, under strong stirring. It is then spray-dried at 200°C (Biichi 190 mini spray dryer) and the powder is calcined to 750°C for 5 h (cord A).
955 o
833
I
I
800
900
[
1000
I
1100
TEMPERATURE (°E) Fig. 1. DSC curve of the powder A, after annealing for 2 h at 875°C; heating rate: 10 K m i n - 1.
556
A. Douy / Synthesis of cordierite powder
98O
3. Results 3.1. Cord A
><
I
I
I
800
1000 1200 TEMPERATURE(°C) Fig. 2. DTA curve of the powder B; heating rate: 10 K rnin 1.
Cord B is obtained by the same manner but after addition of a water-soluble organic polymer (polyacrylamide, Mw 50 000) to the precursor solution (2 g of polymer for 10 g of oxide). Cord C is prepared as cord B, but after aging the precursor sol for 5 weeks at room temperature.
o<.Cord.
i
50
40
Cord A precursor powder is made of hollow spherical particles (5-15 F m diameter). The specific surface area is 5 m= g-l, suggesting an impermeable coating of the particles. This powder, after compaction, and when heated at 5 K min-1, expands considerably instead of sintering, between 860 and 970°C, and a porous and very low density (0.2-0.3 g cm -3) cordierite ceramic is obtained. This expansion results from a gas release, confirmed by thermogravimetric analysis, beyond the glass transition temperature (833°C). Figure 1 shows the differential scanning calorimetry (DSC) curve of the powder, after annealing at 875°C for 2 h. There are two crystallization peaks: tx-cordierite at 955°C and oL-cordierite at 1053°C. A trace of MgAlzO 4 spinal is noticed on the X-ray diffraction patterns. 3.2. Cord B
Cord B powder does not appear very different from cord A, except the specific surface area is
~
b
30 20 10 28CuK~( ° ) Eig. 3. XRD diagramsof the cord B powder, heated at 10 K min -1 to (a) 980°Cand (b) ll00tC.
A. Douy / Synthesis of cordieritepowder
557
976
t
CD
,,>I,
m
~
866
836 [
I
I
I
I
700
800
900
1000
1100
TEMPERATURE (°E) Fig. 4. DSC curve of the powder 12: heating rate: 10 K min -
increased to 260 m 2 g-1. Thus the particles are porous, the powder more reactive and the compacted samples densify at low temperature, as for homogeneous cordierite powders synthesized by the sol-gel method [7-9]. Final densities of 98% dth are obtained by annealing for 2 h at 950°C. Moreover the chemical homogeneity is increased, the crystallization temperature of a-cordierite is lowered and even a single crystallization exotherm may be observed by thermal analysis (fig. 2). At the end of this peak single phase a-cordierite is obtained, while for the sample just heated to 980°C at 10 K min-~, the two crystalline structures ix and e~ are present (fig. 3). No trace of spinel has been detected for this powder. Thus, by just adding an organic polymer in solution before spray-drying, a very homogeneous powder is produced with high specific surface area. This can be understood by a greater viscosity of the sol during the dehydration step in the spray dryer, preserving the chemical homogeneity, and porosities in the particle shells created by the thermal decomposition of the organic additive. 3.3. Cord C
The precursor solution, after 5 weeks aging, has changed to a slightly opalescent sol, by the
1.
growth of silica colloids. However, after addition of organic polymer, spray-drying and calcination, a cord C powder is obtained which crystallizes into c~-cordierite via Ix-cordierite (fig. 4) but the stable form appears at higher temperature. A little amount of spinel is again revealed on the XRD patterns. This powder is thus less homogeneous but the speed of the transformation of the sol into a powder makes possible the obtainment of a cordierite powder which has still a good densification behaviour before crystallization. Similar results have been obtained for another glass-ceramic composition: 0 . 5 M g O - 0 . 5 L i 2 0 A1203-4SIO2, and with another organic polymer (polyvinyl alcohol). Without organic polymer the spray-dried powder expands; wi~k-additAon of organic polymer to the sol, the compacted samples densify before crystallizing. Kanzaki et al. [10] obtained very homogeneous mullite precursor powder by spray pyrolyzing at 350-650°C a solution made by hydrolyzing TEOS into a water-methanol solution of aluminium nitrate. This powder crystallized completely into mullite at 970°C. This illustrates again the importance of the speed in the transformation of a solution (sol) into a powder when a silicon alkoxide is completely hydrolyzed in a large excess of water.
558
A. Dowy / Synthesis of cordieritepowder
4. Conclusion Silicate p o w d e r s a r e usually s y n t h e s i z e d by t h e a l c o h o l i c r o u t e starting f r o m alkoxides, or by t h e a q u e o u s r o u t e f r o m salts a n d sols. I n o r g a n i c m e d i u m , t h e m a j o r difficulty in achieving a m o n o p h a s i c c o p o l y m e r i z a t i o n is in t h e g r e a t diff e r e n c e s in t h e reactivities o f t h e alkoxides. F o r t h e a q u e o u s p r e p a r a t i o n , w h e n s t a r t i n g with sols which a r e h o m o p o l y m e r s , t h e m e d i u m will b e m u l t i p h a s i c a n d t h e c h e m i c a l h o m o g e n e i t y will b e r e a c h e d only at h i g h e r t e m p e r a t u r e s . T h e d i r e c t hydrolysis of T E O S into an acidic s o l u t i o n o f m e t a l l i c salts (nitrates), a n d t h e r a p i d t r a n s f o r m a tion into a p o w d e r is thus a very s i m p l e p r o c e s s for a q u e o u s p r e p a r a t i o n of silicate p o w d e r s . M o r e o v e r t h e a d d i t i o n o f an o r g a n i c p o l y m e r in solution before spray-drying makes the powders m o r e c h e m i c a l l y h o m o g e n e o u s a n d m o r e reactive by i n c r e a s i n g t h e specific surface areas.
T h e a u t h o r wishes to t h a n k M r s J. C o u t u r e s , M r P. C a n a l e a n d D r s M. G e r v a i s a n d P. O d i e r for s t i m u l a t i n g discussions a n d t e c h n i c a l assistance.
References [1] A. Douy and P. Odier, Mater. Res. Bull. 24 (1989) 1119. [2] C. Marcilly, P. Courty and B. Delmon, J. Am. Ceram. Soc. 53 (1970) 56. [3] A. Kato, K. Inoue and Y. Katakae, Mater. Res. Bull., 22 (1987) 1275. [4] A. Douy, J. Europ. Cerarn. Soc. 7 (1991) 117. [5] A. Douy, J. Europ. Ceram. Soc. 7 (1991) 397. [6] R.K. Iler, The Chemistry of Silica (Wiley, New York, 1978). [7] C. Gensse and U. Chowdhry, Mater. Res. Soc. Symp. Proc. 72 (1986) 297. [8] J.C. Bernier, J.L. Rehspringer, S. Vilminot and P. Poix, Mater. Res. Soc. Symp. Proc. 73 (1986) 129. [9] H. Vesteghem, A.R. Di Giampaolo and A. Dauger, J. Mater. Sci. Lett. 6 (1987) 1187. [10] S. Kanzaki, H. Tabata, T. Kumazawa and S. Ohta, J. Am. Ceram. Soc. 68 (1985) C-6.
Journal of Non-Crystalline Solids 147&148 (1992) 554-558 North-Holland
L OF
NON-CRYSTALLISNO ELIDS
Synthesis of cordierite powder by spray-drying A n d r e Douy CNRS, Centre de Recherches sur la Physique des Hautes Temperatures, 1D, Avenue de la Recherche Scientifique, 45071 Orldans cddex 02, France
Homogeneous multicomponent silicate powders may be prepared when a silicon alkoxide is hydrolyzed in an acidic aqueous solution of metallic salts and the resulting sol rapidly transformed into a powder. By spray-drying a sol of A1 and Mg nitrates and TEOS at 200°C, followed by calcination to 750°C, a cordierite precursor powder is produced which crystallizes into c¢-cordierite via ix-cordierite, but expands upon heating instead of sintering. If a water-soluble organic polymer is added to the sol before spray-drying, the chemical homogeneity of the powder is improved, the specific surface area greatly increased and the compacted powder densities before crystallizing.
1. Introduction
In the preparation of multicomponent oxide ceramics and glasses, sol-gel chemistry has enabled improved synthetic methods, control over structure and lower processing temperatures, resulting in better material properties. Silicate gels, films, fibers and powders prepared mainly from alkoxides in organic medium, by hydrolysis, condensation and polymerization have been widely studied. However, when the number of cations increases, much care has to be taken for achieving a true copolymerization because of great differences of reactivity between the alkoxides. It has been shown that, for the elaboration of multiple oxide powders, the polyacrylamide gel is a convenient auxiliary [1]. This organic hydrophilic gel may be associated with the 'amorphous citrate process' [2], since practically all aqueous solutions may be gelled at acidic or neutral pH. The aqueous gel is then directly calcined in a ventilated oven. The chemical homogeneity of the powder results mainly from the cations complexation by the citric acid, but the organic gel has also its own contribution. This may thus be considered as a simplification and an improvement of the citrate process. The polyacrylamide gel may also be applied to other aqueous chemical processes such as homogeneous by-
droxide co-precipitation by thermal decomposition of urea in solution [3]. The co-precipitation domains are then confined within the gel pores. By further calcination, ultrafine and chemically homogeneous powders are obtained. A great number of complex oxide compositions may be prepared by aqueous polyacrylamide gel-assisted processes. However, the case of silicates is different because silicon cannot be chelated in aqueous medium. However, the elaboration of mullite, 3Al203-2SiO 2, a binary oxide [4], and cordierite, 2MgO-2A1203-5SiO2, a ternary oxide [5], has been studied, by the aqueous route, using polyacrylamide gel, and also by the alcoholic route with the aid of poly-(2-hydroxyethyl)-methacrylate (pHEMA) gel. For the aqueous preparation, a silicon alkoxide, tetraethylorthosilicate (TEOS) Si(OC2H5)4, or 3-(triethoxysilyl)-propylamine (TESPA) (C2H50)3Si(CH2)3NH2, is hydrolyzed in a diluted aqueous solution of AI salt (nitrate, citrate, chloride or sulphate) for mullite, or A1 and Mg salts (nitrates or citrates) for cordierite. The resuiting clear solution (sol) is then gelled by in situ formation of the organic network and this aqueous gel is transformed into a powder by microwave heating and calcination in air to 750°C. The properties and characteristics of the powders depend on the nature of the precursor salts and
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A. Douy / Synthesis of cordieritepowder
555
noted that the quantity of secondary phase, MgA1204, is the lowest when the fresh solution (sol) made by hydrolyzing TEOS is an aqueous A1 and Mg nitrates solution is gelled by polyacrylamide at room temperature and rapidly transformed into a dry solid by microwave heating and further calcined. TEOS is hydrolyzed into silicic acid, which further condenses and polymerizes into silica gel, the kinetics of these reactions depending on concentration, temperature and pH [6]. By a rapid treatment, the transitory step in hot aqueous medium, which is favourable for accelerating the colloid growth, may be partly avoided and the resulting powder will be more chemically homogeneous. The present paper deals with spray-drying of cordierite powders, which achieves more efficiently the rapid transformation of a sol into a powder.
alkoxide. For mullite, the powder made from aluminium citrate and TESPA crystallizes completely into mullite at 970°C for a heating rate of 10 K min -1, through a strong exothermic peak. For cordierite, starting from aluminium and magnesium nitrates and TEOS, a powder is obtained which crystallizes first into Fx-cordierite and then transforms into a-cordierite, the stable phase, with only a little amount of MgAlzO 4 spinel as secondary phase. This powder, when compacted and heated, densifies completely by a viscous flow before crystallizing, at low temperature (below 1000°C). The alcoholic route consists in dissolving nitrates and TEOS in 96% ethanol, gelling the solutions by pHEMA and calcining the gels. Very homogeneous powders are obtained. For the binary oxide, a nearly complete crystallization into mullite occurs at 970°C while, for the ternary oxide, the stable phase, c~-cordierite, is obtained at a particularly low temperature, at 1000°C for a heating rate of 10 K min -1, without any trace of secondary phase. The use of organic gels is thus very promising in the preparation of chemically homogeneous silicate powders, and one interesting point is the attainment of low temperature-sinterable cordierite powders by an aqueous route. It has been
2. Experimental The precursor sol (about 0.06 mol 1-1) is made by hydrolyzing TEOS in an aqueous solution of A1 and Mg nitrates, under strong stirring. It is then spray-dried at 200°C (Biichi 190 mini spray dryer) and the powder is calcined to 750°C for 5 h (cord A).
955 o
833
I
I
800
900
[
1000
I
1100
TEMPERATURE (°E) Fig. 1. DSC curve of the powder A, after annealing for 2 h at 875°C; heating rate: 10 K m i n - 1.
556
A. Douy / Synthesis of cordierite powder
98O
3. Results 3.1. Cord A
><
I
I
I
800
1000 1200 TEMPERATURE(°C) Fig. 2. DTA curve of the powder B; heating rate: 10 K rnin 1.
Cord B is obtained by the same manner but after addition of a water-soluble organic polymer (polyacrylamide, Mw 50 000) to the precursor solution (2 g of polymer for 10 g of oxide). Cord C is prepared as cord B, but after aging the precursor sol for 5 weeks at room temperature.
o<.Cord.
i
50
40
Cord A precursor powder is made of hollow spherical particles (5-15 F m diameter). The specific surface area is 5 m= g-l, suggesting an impermeable coating of the particles. This powder, after compaction, and when heated at 5 K min-1, expands considerably instead of sintering, between 860 and 970°C, and a porous and very low density (0.2-0.3 g cm -3) cordierite ceramic is obtained. This expansion results from a gas release, confirmed by thermogravimetric analysis, beyond the glass transition temperature (833°C). Figure 1 shows the differential scanning calorimetry (DSC) curve of the powder, after annealing at 875°C for 2 h. There are two crystallization peaks: tx-cordierite at 955°C and oL-cordierite at 1053°C. A trace of MgAlzO 4 spinal is noticed on the X-ray diffraction patterns. 3.2. Cord B
Cord B powder does not appear very different from cord A, except the specific surface area is
~
b
30 20 10 28CuK~( ° ) Eig. 3. XRD diagramsof the cord B powder, heated at 10 K min -1 to (a) 980°Cand (b) ll00tC.
A. Douy / Synthesis of cordieritepowder
557
976
t
CD
,,>I,
m
~
866
836 [
I
I
I
I
700
800
900
1000
1100
TEMPERATURE (°E) Fig. 4. DSC curve of the powder 12: heating rate: 10 K min -
increased to 260 m 2 g-1. Thus the particles are porous, the powder more reactive and the compacted samples densify at low temperature, as for homogeneous cordierite powders synthesized by the sol-gel method [7-9]. Final densities of 98% dth are obtained by annealing for 2 h at 950°C. Moreover the chemical homogeneity is increased, the crystallization temperature of a-cordierite is lowered and even a single crystallization exotherm may be observed by thermal analysis (fig. 2). At the end of this peak single phase a-cordierite is obtained, while for the sample just heated to 980°C at 10 K min-~, the two crystalline structures ix and e~ are present (fig. 3). No trace of spinel has been detected for this powder. Thus, by just adding an organic polymer in solution before spray-drying, a very homogeneous powder is produced with high specific surface area. This can be understood by a greater viscosity of the sol during the dehydration step in the spray dryer, preserving the chemical homogeneity, and porosities in the particle shells created by the thermal decomposition of the organic additive. 3.3. Cord C
The precursor solution, after 5 weeks aging, has changed to a slightly opalescent sol, by the
1.
growth of silica colloids. However, after addition of organic polymer, spray-drying and calcination, a cord C powder is obtained which crystallizes into c~-cordierite via Ix-cordierite (fig. 4) but the stable form appears at higher temperature. A little amount of spinel is again revealed on the XRD patterns. This powder is thus less homogeneous but the speed of the transformation of the sol into a powder makes possible the obtainment of a cordierite powder which has still a good densification behaviour before crystallization. Similar results have been obtained for another glass-ceramic composition: 0 . 5 M g O - 0 . 5 L i 2 0 A1203-4SIO2, and with another organic polymer (polyvinyl alcohol). Without organic polymer the spray-dried powder expands; wi~k-additAon of organic polymer to the sol, the compacted samples densify before crystallizing. Kanzaki et al. [10] obtained very homogeneous mullite precursor powder by spray pyrolyzing at 350-650°C a solution made by hydrolyzing TEOS into a water-methanol solution of aluminium nitrate. This powder crystallized completely into mullite at 970°C. This illustrates again the importance of the speed in the transformation of a solution (sol) into a powder when a silicon alkoxide is completely hydrolyzed in a large excess of water.
558
A. Dowy / Synthesis of cordieritepowder
4. Conclusion Silicate p o w d e r s a r e usually s y n t h e s i z e d by t h e a l c o h o l i c r o u t e starting f r o m alkoxides, or by t h e a q u e o u s r o u t e f r o m salts a n d sols. I n o r g a n i c m e d i u m , t h e m a j o r difficulty in achieving a m o n o p h a s i c c o p o l y m e r i z a t i o n is in t h e g r e a t diff e r e n c e s in t h e reactivities o f t h e alkoxides. F o r t h e a q u e o u s p r e p a r a t i o n , w h e n s t a r t i n g with sols which a r e h o m o p o l y m e r s , t h e m e d i u m will b e m u l t i p h a s i c a n d t h e c h e m i c a l h o m o g e n e i t y will b e r e a c h e d only at h i g h e r t e m p e r a t u r e s . T h e d i r e c t hydrolysis of T E O S into an acidic s o l u t i o n o f m e t a l l i c salts (nitrates), a n d t h e r a p i d t r a n s f o r m a tion into a p o w d e r is thus a very s i m p l e p r o c e s s for a q u e o u s p r e p a r a t i o n of silicate p o w d e r s . M o r e o v e r t h e a d d i t i o n o f an o r g a n i c p o l y m e r in solution before spray-drying makes the powders m o r e c h e m i c a l l y h o m o g e n e o u s a n d m o r e reactive by i n c r e a s i n g t h e specific surface areas.
T h e a u t h o r wishes to t h a n k M r s J. C o u t u r e s , M r P. C a n a l e a n d D r s M. G e r v a i s a n d P. O d i e r for s t i m u l a t i n g discussions a n d t e c h n i c a l assistance.
References [1] A. Douy and P. Odier, Mater. Res. Bull. 24 (1989) 1119. [2] C. Marcilly, P. Courty and B. Delmon, J. Am. Ceram. Soc. 53 (1970) 56. [3] A. Kato, K. Inoue and Y. Katakae, Mater. Res. Bull., 22 (1987) 1275. [4] A. Douy, J. Europ. Cerarn. Soc. 7 (1991) 117. [5] A. Douy, J. Europ. Ceram. Soc. 7 (1991) 397. [6] R.K. Iler, The Chemistry of Silica (Wiley, New York, 1978). [7] C. Gensse and U. Chowdhry, Mater. Res. Soc. Symp. Proc. 72 (1986) 297. [8] J.C. Bernier, J.L. Rehspringer, S. Vilminot and P. Poix, Mater. Res. Soc. Symp. Proc. 73 (1986) 129. [9] H. Vesteghem, A.R. Di Giampaolo and A. Dauger, J. Mater. Sci. Lett. 6 (1987) 1187. [10] S. Kanzaki, H. Tabata, T. Kumazawa and S. Ohta, J. Am. Ceram. Soc. 68 (1985) C-6.