TY - JOUR

T1 - Synthesis and structural characterization of non-crystalline mullite precursors

AU - Schneider, H.

AU - Voll, D.

AU - Saruhan, B.

AU - Sanz, J.

AU - Schrader, G.

AU - Rüscher, C.

AU - Mosset, A.

N1 - Funding Information: The authors wish to express their gratitude to Dr T. Rymon-Lipinski for carrying out the DTA studies. Thanks are also due to Dr S.H. Risbud for providing roller-quenched melt glasses. H.S. acknowledges the financial support of the Deutsche Forschungsgemeinschaft (DFG, Bonn, Germany).

PY - 1994/11/3

Y1 - 1994/11/3

N2 - Two different types of non-crystalline mullite precursor with identical bulk composition (72 wt% Al2O3, 28 wt% SiO2) were prepared from tetraethoxysilane and silicon chloride, respectively, and aluminium sec-butozide, by using different methods of hydrolysis. The precursors, designated as type I and III, display different crystallization processes above ≈ 900°C: type I precursors directly form mullite, while type III precursors yield crystallization of transient γ-alumina. Infrared (IR) spectroscopy, large angle X-ray scattering (LAXS) and 27Al nuclear magnetic resonance spectroscopic studies, and 29Si nuclear magnetic resonance (NMR) literature data give evidence for a high degree of structural mixing in type I precursors and for a beginning of segregation into Al2O3-rich domains in type III precursors prior to crystallization (≤ 900°C). Both precursors are composed of (SiO) tetrahedra and of (AlO) octahedra, tetrahedra and pentahedra although pentahedra are dominant in type I while octahedra occur more frequently in type III precursors. The driving force for mullitization (type I) and γ-alumina formation (type III) taking place at the same temperature is believed to be the instability of pentahedrally coordinated Al above ≈ 900°C. The sudden disappearance of Al pentahedra probably depends on the formation of reactive network centers during dehydroxylation. This hypothesis is derived from the observation that dehydroxylation and condensation strongly take place in a similar temperature range prior to crystallization.

AB - Two different types of non-crystalline mullite precursor with identical bulk composition (72 wt% Al2O3, 28 wt% SiO2) were prepared from tetraethoxysilane and silicon chloride, respectively, and aluminium sec-butozide, by using different methods of hydrolysis. The precursors, designated as type I and III, display different crystallization processes above ≈ 900°C: type I precursors directly form mullite, while type III precursors yield crystallization of transient γ-alumina. Infrared (IR) spectroscopy, large angle X-ray scattering (LAXS) and 27Al nuclear magnetic resonance spectroscopic studies, and 29Si nuclear magnetic resonance (NMR) literature data give evidence for a high degree of structural mixing in type I precursors and for a beginning of segregation into Al2O3-rich domains in type III precursors prior to crystallization (≤ 900°C). Both precursors are composed of (SiO) tetrahedra and of (AlO) octahedra, tetrahedra and pentahedra although pentahedra are dominant in type I while octahedra occur more frequently in type III precursors. The driving force for mullitization (type I) and γ-alumina formation (type III) taking place at the same temperature is believed to be the instability of pentahedrally coordinated Al above ≈ 900°C. The sudden disappearance of Al pentahedra probably depends on the formation of reactive network centers during dehydroxylation. This hypothesis is derived from the observation that dehydroxylation and condensation strongly take place in a similar temperature range prior to crystallization.

UR - http://www.scopus.com/inward/record.url?scp=0028763345&partnerID=8YFLogxK

U2 - 10.1016/0022-3093(94)90295-X

DO - 10.1016/0022-3093(94)90295-X

M3 - Article

AN - SCOPUS:0028763345

VL - 178

SP - 262

EP - 271

JO - Journal of non-crystalline solids

JF - Journal of non-crystalline solids

SN - 0022-3093

IS - C

ER -