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Mullite formation from gels derived from tetraethoxysilane or tetramethoxysilane with aluminium sec-butoxide
Journal of Non-Crystalline Solids 221 Ž1997. 297–301
Letter to the Editor
Mullite formation from gels derived from tetraethoxysilane or tetramethoxysilane with aluminium sec-butoxide J. Wu, P.F. James
)
Department of Engineering Materials, UniÕersity of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK Received 18 March 1997; revised 29 July 1997
Abstract Two gels corresponding to the oxide composition, 3Al 2 O 3 P 2SiO 2 were prepared using different alkoxide precursors: aluminium sec-butoxide ŽASB. mixed with either tetraethoxysilane ŽTEOS. or tetramethoxysilane ŽTMOS.. Mullite formation in the gels was compared using X-ray diffraction after heat treatment for 1 h at a series of temperatures. The crystallisation behaviour in the gels was significantly different. After 1 h heat treatment mullite was found at 7008C but not at 6008C for the gel derived from TMOS, and was observed at 10008C but not at 9008C for the gel derived from TEOS. This marked variation between the gels was attributed to the different hydrolysis and polymerisation rates of the TMOS and TEOS, resulting in a difference in the homogeneity of the two gels on a molecular scale. q 1997 Elsevier Science B.V.
1. Introduction Synthesis of mullite by various techniques and from different starting materials has been extensively investigated w1–10x. Mullite has various potential applications at high temperatures because of its excellent mechanical and functional properties, including good chemical and thermal stability and a low thermal expansion coefficient of 4.5–5.6 = 10y6 Ky1 w10x. Several mullite formation mechanisms have been suggested through different sol–gel processing methods using tetraethoxysilane ŽTEOS. as the silicon source w2,7,8x. Huling and Messing w7x have pointed out that sol–gel chemistry has a profound influence on the crystallisation of aluminosilicate
)
Corresponding author. Tel.: q44-114 222 2000; fax: q44-114 222 5943.
gels. If the gels corresponding to the mullite composition were synthesised to avoid chemical segregation or phase separation, the stable orthorhombic form of mullite Ž3Al 2 O 3 P 2SiO 2 . could form at 7008C. However, if the initial chemistry conditions led to the segregation of transitional alumina yielding species during gel synthesis, this resulted in the epitaxial nucleation of spinel at 9808C. Recently, Taylor and Holland w8x prepared mullite powders from varying homogeneity sols by a water-free sol– gel approach. They suggested that the level of homogeneity, on an atomic scale, plays a central role in the determination of the crystalline phases, which was supported by XRD and NMR results. The level of homogeneity was indicated by the amount of pentacoordinate aluminium which existed at the interface between the inhomogeneity and the surrounding amorphous silica-rich matrix. They observed crystallisation at 9808C of mullite andror spinel,
0022-3093r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 4 1 2 - 2
298
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301
Table 1 Specification of chemicals used in this study Name
Formula
Specification
Manufacturer
Tetraethoxysilane ŽTEOS. Tetramethoxysilane ŽTMOS. Aluminum sec-butoxide ŽASB. Acetyl acetone Žacac.
SiŽOC 2 H 5 .4 SiŽOCH 3 .4 AlŽOC 4 H 9 . 3 CH 3 COCH 2 COCH 3
assay ) 98 wt% ŽGC. 99 wt% assay 21 wt% Al 2 O 3 –
Fluka Aldrich Fluka Fluka
which was initiated by the transformation of the metastable pentacoordinate aluminium to octahedral and tetrahedral arrangements. In another important study Yoldas w9x prepared 3Al 2 O 3 P 2SiO 2 gels from tetramethoxysilane ŽTMOS. with either aluminium sec-butoxide ŽASB., or colloidal alumina, or aluminium nitrate as precursors. He found that crystallisation behaviour was fundamentally affected by the gel structure, as determined by NMR. Spontaneous crystallisation of mullite at 9808C, as observed by DTA, was promoted by a high degree of homogeneity and the presence of polymerical aluminium–silicon networks. In this study, a comparison is made between gels derived from different silicon bearing alkoxides, either tetramethoxysilane ŽTMOS. or tetraethoxysilane
ŽTEOS., mixed with aluminium sec-butoxide ŽASB.. Mullite formation in the gel powders was analyzed by X-ray diffraction ŽXRD. after heat treatment at different temperatures.
2. Experimental Two gels were prepared, the first using tetraethoxysilane ŽTEOS. and aluminium sec-butoxide ŽASB. as precursors, and the second using tetramethoxysilane ŽTMOS. instead of TEOS. The specification of these alkoxides and the other chemicals used is given in Table 1. The starting sol compositions yielding the corresponding oxide compound, 3Al 2 O 3 P 2SiO 2 are given in Table 2. Both sols were
Fig. 1. XRD of the TMOSrASB derived gels, heated at 700, 800, 900 and 10008C for 1 h respectively. M denotes crystalline orthorhombic mullite phase. Intensities are approximately 1r4 of those in Fig. 2.
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301 Table 2 Compositions of sols derived from TEOS and TMOS with ASB TEOS Žg.
TMOS Žg.
ASB Žg.
H 2 O Žml.
acac Žg.
6.77
73.9 32.84
22.5 12.8
3.14 8.36
20.84
prepared at room temperature with the aid of magnetic stirring. TEOS in ethanol or TMOS in methanol was prehydrolysed with a molar ratio of 1:1 of alkoxide to water for 12 h at 808C, before the ASB solution was introduced dropwise to achieve transparent sols. The ASB solution was composed of ASB, propan-2-ol and acetyl acetone Žacac.. Acetyl acetone was added as a chelating agent with the objective of avoiding the segregation of transitional alumina-yielding species from ASB. The sols were transformed to gels by full hydrolysis with the addition of the remaining water ŽTable 2.. The gels were dried in an oven at 908C for 24 h to yield coarse powders consisting of large transparent fragments of up to 5 mm in diameter. The fragments were gently crushed to a fairly uniform diameter of around 2–3 mm before being heated at 700, 800, 900 and 10008C for 1 h respectively in a tube furnace. The fired samples were ground into powder with a particle size of under 50 mm to reach a talc-like consistency so that when pressed into an aluminium specimen holder, the charge remained in the specimen holder
299
after rotation to the vertical position. A Philips X-ray diffractometer system using Co K a radiation and operating at 40 kV and 30 mA was used to characterise the phases in the heated gel powders, between 10 and 808 Žtwo theta. using a scanning speed of 0.58 Ž2 u .rmin. The phases present in specimens were determined by checking XRD results against ASTM standard references and card indices ŽJCPDS..
3. Results The XRD results of the TMOSrASB and TEOSrASB derived gels fired at corresponding temperatures were significantly different, as shown in Figs. 1 and 2 Žthe intensity in Fig. 2 is approximately four times higher than that in Fig. 1.. Orthorhombic mullite formation was observed in the TEOSrASB gel after heat treatment at 10008C for 1 h but after treatment at 9008C for 1 h the gel was amorphous. However, the orthorhombic mullite phase was found in the TMOSrASB gel at a temperature as low as 7008C after 1 h and the level of mullitisation was similar to that in the TEOSrASB gel heat treated at 10008C. The TMOSrASB gel was amorphous after heat treatment at 6008C for 1 h Žnot shown..
Fig. 2. XRD of the TEOSrASB derived gels, heated at 900 and 10008C for 1 h respectively. M denotes crystalline orthorhombic mullite phase.
300
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301
4. Discussion The difference in mullitisation behaviour of the two gels probably results from the different starting materials, TMOS and TEOS used, and in particular their different hydrolysis and polymerisation Žcondensation. rates. It is well known w11x that steric or spatial factors exert the greatest effect on the hydrolytic stability of organoxysilanes and use of a more complex alkoxy group retards the hydrolysis of alkoxysilanes. The alkyl chain length generally has a negative effect on the hydrolysis rate and the longer the chain the slower the hydrolysis rate for the alkoxide. Schmidt and co-workers have compared the hydrolysis of TEOS and TMOS under acidic and basic conditions and the much more rapid hydrolysis of TMOS than that of TEOS is evidence for the retarding effect of the bulkier ethoxide group from TEOS w12x. The difference in the hydrolysis rates of TMOS and TEOS derived sols prepared under the same experimental conditions using solid state 29 Si nuclear magnetic resonance spectroscopy was also confirmed w13x. The hydrolysis rate of aluminium alkoxides is generally much higher than that of silicon alkoxides w11x. Hence when preparing alumina–silica gels it is common to prehydrolyse the silicon alkoxides before introduction of the aluminium bearing sol to avoid any segregation of the aluminium containing species. In the present system TMOS has a higher hydrolysis rate than TEOS, which is therefore closer to that of ASB than TEOS. Consequently, it is suggested that higher homogeneity can be achieved in the TMOSrASB sol than in the TEOSrASB sol, which will facilitate the formation of a more homogeneous Si–O–Al network on the molecular scale in the TMOSrASB derived gel. Thus, a lower mullitisation temperature can be achieved in the TMOSrASB derived gel. Another factor which may influence the suggested higher degree of homogeneity of the ASBrTMOS gel compared with the ASBrTEOS gel is the faster polymerisation rate Žsubsequent to hydrolysis. of TMOS relative to TEOS w11x, since the rates of polymerisation of the TEOS or TMOS relative to the ASB would affect the size of the oligomers formed in the sols prior to gelation. Although the optical transparency of both sols of TMOSrASB and TEOSrASB was observed to be qualitatively the
same, at the molecular level the segregation of transitional alumina yielding species in the TEOSrASB derived sol might have occurred andror metastable phases might have formed during the processing, which would delay the formation of the equilibrium orthorhombic phase until higher temperatures. Further studies of the detailed chemistry of the two gels in the present work using NMR, and of the crystallisation mechanism in the gels using transmission electron microscopy ŽTEM., and further experiments without partial hydrolysis of the silicon alkoxides before ASB addition, should help clarify the roles of hydrolysis and polymerisation.
5. Summary and conclusions Ž1. Two gels with composition 3Al 2 O 3 P 2SiO 2 were prepared using aluminium sec-butoxide with tetraethoxysilane and aluminium sec-butoxide with tetramethoxysilane. Ž2. The tetramethoxysilane-derived gel crystallized to mullite on heat treatment at a markedly lower temperature than the tetraethoxysilane-derived gel. Ž3. The different crystallisation behaviour of the tw o gels is attributed to the different hydrolysisrpolymerisation rates of the silicon alkoxides used, which affected the molecular scale homogeneity of the gels. It is suggested that the tetramethoxysilaneraluminium sec-butoxide gel had the more homogeneous Si–O–Al network on the molecular scale.
Acknowledgements Thanks are due to the Chinese Government and the British Council for financial support ŽJ. Wu., and to Dr D. Ou for useful discussions.
References w1x K.S. Mazdiyasni, L.M. Brown, J. Am. Ceram. Soc. 55 Ž1972. 549. w2x B.E. Yoldas, D.P. Partlow, J. Mater. Sci. 23 Ž1988. 1895. w3x M. Sacks, H.W. Lee, J.A. Pask, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans., Mullite and Mullite Matrix
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301
w4x
w5x
w6x
Composites, vol. 6, American Ceramic Society, Westerville, OH, 1990, p. 167. K.S. Mazdiyasni, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans. vol. 6, Mullite and Mullite Matrix Composites, American Ceramic Society, Westerville, OH, 1990, p. 243. S. Mitachi, M. Matsuzawa, K, Kaneko, S. Kanzaki, H. Tabata, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans. vol. 6, Mullite and Mullite Matrix Composites, American Ceramic Society, Westerville, OH, 1990, p. 275. H. Suzuki, H. Saito, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans. vol. 6, Mullite and Mullite Matrix Composites, American Ceramic Society, Westerville, OH, 1990, p. 263.
301
w7x J.C. Huling, G.L. Messing, J. Non-Cryst. Solids 147&148 Ž1992. 213. w8x A. Taylor, D. Holland, J. Non-Cryst. Solids 152 Ž1993. 1. w9x B.E. Yoldas, J. Mater. Sci 27 Ž1992. 6667. w10x W.H. Hawkes, Trans. Br. Ceram. Soc. 61 Ž1962. p689. w11x C.J. Brinker, G.W. Scherer, Sol–Gel Science, the Physics and Chemistry of Sol–Gel Processing, Academic Press, New York, 1990, p. 121. w12x H. Schmidt, A. Kaiser, M. Rudolph, A. Lentz, in: L.L. Hench, D.R. Ulrich ŽEds.., Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 87. w13x D.L. Ou, A.B. Seddon, J. Sol-Gel Sci. Technol. 8 Ž1997. 139.
Letter to the Editor
Mullite formation from gels derived from tetraethoxysilane or tetramethoxysilane with aluminium sec-butoxide J. Wu, P.F. James
)
Department of Engineering Materials, UniÕersity of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK Received 18 March 1997; revised 29 July 1997
Abstract Two gels corresponding to the oxide composition, 3Al 2 O 3 P 2SiO 2 were prepared using different alkoxide precursors: aluminium sec-butoxide ŽASB. mixed with either tetraethoxysilane ŽTEOS. or tetramethoxysilane ŽTMOS.. Mullite formation in the gels was compared using X-ray diffraction after heat treatment for 1 h at a series of temperatures. The crystallisation behaviour in the gels was significantly different. After 1 h heat treatment mullite was found at 7008C but not at 6008C for the gel derived from TMOS, and was observed at 10008C but not at 9008C for the gel derived from TEOS. This marked variation between the gels was attributed to the different hydrolysis and polymerisation rates of the TMOS and TEOS, resulting in a difference in the homogeneity of the two gels on a molecular scale. q 1997 Elsevier Science B.V.
1. Introduction Synthesis of mullite by various techniques and from different starting materials has been extensively investigated w1–10x. Mullite has various potential applications at high temperatures because of its excellent mechanical and functional properties, including good chemical and thermal stability and a low thermal expansion coefficient of 4.5–5.6 = 10y6 Ky1 w10x. Several mullite formation mechanisms have been suggested through different sol–gel processing methods using tetraethoxysilane ŽTEOS. as the silicon source w2,7,8x. Huling and Messing w7x have pointed out that sol–gel chemistry has a profound influence on the crystallisation of aluminosilicate
)
Corresponding author. Tel.: q44-114 222 2000; fax: q44-114 222 5943.
gels. If the gels corresponding to the mullite composition were synthesised to avoid chemical segregation or phase separation, the stable orthorhombic form of mullite Ž3Al 2 O 3 P 2SiO 2 . could form at 7008C. However, if the initial chemistry conditions led to the segregation of transitional alumina yielding species during gel synthesis, this resulted in the epitaxial nucleation of spinel at 9808C. Recently, Taylor and Holland w8x prepared mullite powders from varying homogeneity sols by a water-free sol– gel approach. They suggested that the level of homogeneity, on an atomic scale, plays a central role in the determination of the crystalline phases, which was supported by XRD and NMR results. The level of homogeneity was indicated by the amount of pentacoordinate aluminium which existed at the interface between the inhomogeneity and the surrounding amorphous silica-rich matrix. They observed crystallisation at 9808C of mullite andror spinel,
0022-3093r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 7 . 0 0 4 1 2 - 2
298
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301
Table 1 Specification of chemicals used in this study Name
Formula
Specification
Manufacturer
Tetraethoxysilane ŽTEOS. Tetramethoxysilane ŽTMOS. Aluminum sec-butoxide ŽASB. Acetyl acetone Žacac.
SiŽOC 2 H 5 .4 SiŽOCH 3 .4 AlŽOC 4 H 9 . 3 CH 3 COCH 2 COCH 3
assay ) 98 wt% ŽGC. 99 wt% assay 21 wt% Al 2 O 3 –
Fluka Aldrich Fluka Fluka
which was initiated by the transformation of the metastable pentacoordinate aluminium to octahedral and tetrahedral arrangements. In another important study Yoldas w9x prepared 3Al 2 O 3 P 2SiO 2 gels from tetramethoxysilane ŽTMOS. with either aluminium sec-butoxide ŽASB., or colloidal alumina, or aluminium nitrate as precursors. He found that crystallisation behaviour was fundamentally affected by the gel structure, as determined by NMR. Spontaneous crystallisation of mullite at 9808C, as observed by DTA, was promoted by a high degree of homogeneity and the presence of polymerical aluminium–silicon networks. In this study, a comparison is made between gels derived from different silicon bearing alkoxides, either tetramethoxysilane ŽTMOS. or tetraethoxysilane
ŽTEOS., mixed with aluminium sec-butoxide ŽASB.. Mullite formation in the gel powders was analyzed by X-ray diffraction ŽXRD. after heat treatment at different temperatures.
2. Experimental Two gels were prepared, the first using tetraethoxysilane ŽTEOS. and aluminium sec-butoxide ŽASB. as precursors, and the second using tetramethoxysilane ŽTMOS. instead of TEOS. The specification of these alkoxides and the other chemicals used is given in Table 1. The starting sol compositions yielding the corresponding oxide compound, 3Al 2 O 3 P 2SiO 2 are given in Table 2. Both sols were
Fig. 1. XRD of the TMOSrASB derived gels, heated at 700, 800, 900 and 10008C for 1 h respectively. M denotes crystalline orthorhombic mullite phase. Intensities are approximately 1r4 of those in Fig. 2.
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301 Table 2 Compositions of sols derived from TEOS and TMOS with ASB TEOS Žg.
TMOS Žg.
ASB Žg.
H 2 O Žml.
acac Žg.
6.77
73.9 32.84
22.5 12.8
3.14 8.36
20.84
prepared at room temperature with the aid of magnetic stirring. TEOS in ethanol or TMOS in methanol was prehydrolysed with a molar ratio of 1:1 of alkoxide to water for 12 h at 808C, before the ASB solution was introduced dropwise to achieve transparent sols. The ASB solution was composed of ASB, propan-2-ol and acetyl acetone Žacac.. Acetyl acetone was added as a chelating agent with the objective of avoiding the segregation of transitional alumina-yielding species from ASB. The sols were transformed to gels by full hydrolysis with the addition of the remaining water ŽTable 2.. The gels were dried in an oven at 908C for 24 h to yield coarse powders consisting of large transparent fragments of up to 5 mm in diameter. The fragments were gently crushed to a fairly uniform diameter of around 2–3 mm before being heated at 700, 800, 900 and 10008C for 1 h respectively in a tube furnace. The fired samples were ground into powder with a particle size of under 50 mm to reach a talc-like consistency so that when pressed into an aluminium specimen holder, the charge remained in the specimen holder
299
after rotation to the vertical position. A Philips X-ray diffractometer system using Co K a radiation and operating at 40 kV and 30 mA was used to characterise the phases in the heated gel powders, between 10 and 808 Žtwo theta. using a scanning speed of 0.58 Ž2 u .rmin. The phases present in specimens were determined by checking XRD results against ASTM standard references and card indices ŽJCPDS..
3. Results The XRD results of the TMOSrASB and TEOSrASB derived gels fired at corresponding temperatures were significantly different, as shown in Figs. 1 and 2 Žthe intensity in Fig. 2 is approximately four times higher than that in Fig. 1.. Orthorhombic mullite formation was observed in the TEOSrASB gel after heat treatment at 10008C for 1 h but after treatment at 9008C for 1 h the gel was amorphous. However, the orthorhombic mullite phase was found in the TMOSrASB gel at a temperature as low as 7008C after 1 h and the level of mullitisation was similar to that in the TEOSrASB gel heat treated at 10008C. The TMOSrASB gel was amorphous after heat treatment at 6008C for 1 h Žnot shown..
Fig. 2. XRD of the TEOSrASB derived gels, heated at 900 and 10008C for 1 h respectively. M denotes crystalline orthorhombic mullite phase.
300
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301
4. Discussion The difference in mullitisation behaviour of the two gels probably results from the different starting materials, TMOS and TEOS used, and in particular their different hydrolysis and polymerisation Žcondensation. rates. It is well known w11x that steric or spatial factors exert the greatest effect on the hydrolytic stability of organoxysilanes and use of a more complex alkoxy group retards the hydrolysis of alkoxysilanes. The alkyl chain length generally has a negative effect on the hydrolysis rate and the longer the chain the slower the hydrolysis rate for the alkoxide. Schmidt and co-workers have compared the hydrolysis of TEOS and TMOS under acidic and basic conditions and the much more rapid hydrolysis of TMOS than that of TEOS is evidence for the retarding effect of the bulkier ethoxide group from TEOS w12x. The difference in the hydrolysis rates of TMOS and TEOS derived sols prepared under the same experimental conditions using solid state 29 Si nuclear magnetic resonance spectroscopy was also confirmed w13x. The hydrolysis rate of aluminium alkoxides is generally much higher than that of silicon alkoxides w11x. Hence when preparing alumina–silica gels it is common to prehydrolyse the silicon alkoxides before introduction of the aluminium bearing sol to avoid any segregation of the aluminium containing species. In the present system TMOS has a higher hydrolysis rate than TEOS, which is therefore closer to that of ASB than TEOS. Consequently, it is suggested that higher homogeneity can be achieved in the TMOSrASB sol than in the TEOSrASB sol, which will facilitate the formation of a more homogeneous Si–O–Al network on the molecular scale in the TMOSrASB derived gel. Thus, a lower mullitisation temperature can be achieved in the TMOSrASB derived gel. Another factor which may influence the suggested higher degree of homogeneity of the ASBrTMOS gel compared with the ASBrTEOS gel is the faster polymerisation rate Žsubsequent to hydrolysis. of TMOS relative to TEOS w11x, since the rates of polymerisation of the TEOS or TMOS relative to the ASB would affect the size of the oligomers formed in the sols prior to gelation. Although the optical transparency of both sols of TMOSrASB and TEOSrASB was observed to be qualitatively the
same, at the molecular level the segregation of transitional alumina yielding species in the TEOSrASB derived sol might have occurred andror metastable phases might have formed during the processing, which would delay the formation of the equilibrium orthorhombic phase until higher temperatures. Further studies of the detailed chemistry of the two gels in the present work using NMR, and of the crystallisation mechanism in the gels using transmission electron microscopy ŽTEM., and further experiments without partial hydrolysis of the silicon alkoxides before ASB addition, should help clarify the roles of hydrolysis and polymerisation.
5. Summary and conclusions Ž1. Two gels with composition 3Al 2 O 3 P 2SiO 2 were prepared using aluminium sec-butoxide with tetraethoxysilane and aluminium sec-butoxide with tetramethoxysilane. Ž2. The tetramethoxysilane-derived gel crystallized to mullite on heat treatment at a markedly lower temperature than the tetraethoxysilane-derived gel. Ž3. The different crystallisation behaviour of the tw o gels is attributed to the different hydrolysisrpolymerisation rates of the silicon alkoxides used, which affected the molecular scale homogeneity of the gels. It is suggested that the tetramethoxysilaneraluminium sec-butoxide gel had the more homogeneous Si–O–Al network on the molecular scale.
Acknowledgements Thanks are due to the Chinese Government and the British Council for financial support ŽJ. Wu., and to Dr D. Ou for useful discussions.
References w1x K.S. Mazdiyasni, L.M. Brown, J. Am. Ceram. Soc. 55 Ž1972. 549. w2x B.E. Yoldas, D.P. Partlow, J. Mater. Sci. 23 Ž1988. 1895. w3x M. Sacks, H.W. Lee, J.A. Pask, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans., Mullite and Mullite Matrix
J. Wu, P.F. James r Journal of Non-Crystalline Solids 221 (1997) 297–301
w4x
w5x
w6x
Composites, vol. 6, American Ceramic Society, Westerville, OH, 1990, p. 167. K.S. Mazdiyasni, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans. vol. 6, Mullite and Mullite Matrix Composites, American Ceramic Society, Westerville, OH, 1990, p. 243. S. Mitachi, M. Matsuzawa, K, Kaneko, S. Kanzaki, H. Tabata, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans. vol. 6, Mullite and Mullite Matrix Composites, American Ceramic Society, Westerville, OH, 1990, p. 275. H. Suzuki, H. Saito, in: S. Somiya, R.F. Davis, J.A. Pask ŽEds.., Ceram. Trans. vol. 6, Mullite and Mullite Matrix Composites, American Ceramic Society, Westerville, OH, 1990, p. 263.
301
w7x J.C. Huling, G.L. Messing, J. Non-Cryst. Solids 147&148 Ž1992. 213. w8x A. Taylor, D. Holland, J. Non-Cryst. Solids 152 Ž1993. 1. w9x B.E. Yoldas, J. Mater. Sci 27 Ž1992. 6667. w10x W.H. Hawkes, Trans. Br. Ceram. Soc. 61 Ž1962. p689. w11x C.J. Brinker, G.W. Scherer, Sol–Gel Science, the Physics and Chemistry of Sol–Gel Processing, Academic Press, New York, 1990, p. 121. w12x H. Schmidt, A. Kaiser, M. Rudolph, A. Lentz, in: L.L. Hench, D.R. Ulrich ŽEds.., Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 87. w13x D.L. Ou, A.B. Seddon, J. Sol-Gel Sci. Technol. 8 Ž1997. 139.