Mullite, formation

It is believed that, when steaming the gel at high temperatures, the V0+i attacks and breaks the Si-O-Al bonds promoting mullite formation and the collapse of the gel macroporous structure (3). The XRD pattern in Fig. 2B shows that mullite formation in the gel can be observed with only 1.5% V and when this occurs, there is a 81% decrease in surface area, Table 1. Mullite level increased with V-loadings, see Fig. 2. Data in the literature (20) indicates that when the steaming temperature is decreased to 730 C from 760 C (as in the present work) gel stability to V improved and only a 23% reduction in surface area was observed in a similar gel loaded with 1.5% V. Aluminosilicate gels are clearly less resistant than aluminas to V attack at hydrothermal conditions, Table 1. 

Oxycations of V (V02 or VO ) can easily penetrate the gel microspace and break Si-O-Al bonds causing the collapse of the gel microstructure and mullite formation even at low (1.5%) V levels. Excess V generates an x-ray amorphous, V205-like species which is highly hydrated and less susceptible to dnemical reduction than bulk... 

The calculated orthorhombic unit cell parameters for several mullites, crystallized with and without V, have indicated that (in a qualitative sense) V causes an expansion of the unit cell volume resulting mainly from an increase in the a dimension [22], Thus, incorporation of V into the crystal lattice of this mineral can occur during mullite formation. 

In this chapter, developments in the understanding of mullite over the last few decades are reviewed. A discussion of crystal structures and phase stability is presented to provide the reader with an overview of certain characteristics of this material. The next part of this chapter examines the effect of process chemistry on the synthesis and microstructure of mullite. The role of various synthetic methods that are used to modify mullite formation will be discussed, followed by a compilation of selected materials properties. 

Y.-M. Sung, Kinetics analysis of mullite formation reaction at high temperatures, Acta Mater. 48, 2157-2162 (2000). 

As kaolinite is heated beyond 980°C, the small fraction of mullite crystals that formed at 980°C continue to grow, albeit at a slow rate. Mullite growth is accompanied by the disappearance of the spinel phase, although the amount of mullite formed is lower than expected based on the spinel loss [33], Mullite formation does not approach completion until a second exothermic event occurs at approximately 1200°C, as recorded by differential thermal analysis [33], When formed by solid-state reaction, mullite has... 

Synthetic silica-alumina catalysts containing 25% alumina are converted to gamma-alumina and mullite when thermally treated at 700-1260°. The phase transformation is accompanied by loss of catalytic activity and by collapse of the porous structure of the catalyst. The gamma-alumina phase is formed apparently by crystallization of the amorphous alumina in the catalyst, while the mullite formation apparently results from a combined silica-alumina amorphous phase. At sufficiently high temperature all alumina is converted to mullite. Silica-alumina catalysts made from more stable silicas have a greater tendency to form gamma-alumina. Such catalysts have lower initial catalytic activity and maintain relatively high catalytic activity after steam deactivation. 

Schneider, H., L. Merwin, and A. Sebald. 1992. Mullite formation fi-om non-crystalline precursors. Journal of Materials Science 27 805-812. ... 

Nucleation of mullite occurs at the Al203-Si02interfece. At 1450°C, the viscosity of SiCh glass decreases and hence diffusion through the viscous phase is enhanced, more so because the sizes of the diffusing complexes decrease. Above 1500°C, the mullite formation occurs by the reaction between cristobalite and a- AI2O3. 

B. E. Yoldas, Mullite formation from aluminum and silicon alkoxides, Ceram. Trans., 6,255-261 (1990). 

B. 0. Hildmann, H. Schneider and M. Schmucker, High temperature behaviour of polycrystalline aluminosilicate fibres with mullite bulk composition. II. Kinetics of mullite formation, J. Europ. Ceram. Soc., 16, 287-292 (1996). 

The Si02 level is more or less constrained by that of the other components. A high silica level leads to an undesired highly viscous and seedy glass, whereas a lower content, coupled with a high alumina level, enhances the probability of mullite formation. 

Mullite formation from an all-alkoxide system was achieved by Schneider et al. (1992). In the method used by them, tetraethyl orthosilicate (TEOS) and aluminum butylate were mixed in the equivalent oxide ratio 72 wt% (60mol%) AI2O3 and 28 wt% (40mol%) SiOi as the standard conposition. 2-propanol was added to the mixture as the solvent, followed by the addition ofwater in drops to cause hydrolysis. The final product was a homogeneous white gel, which was dried at 110°C and calcined at different temperatures for 15 h ... 

Physical properties such as the compressive strength of clays and zirconia have been enhanced by the application of pressure during a chemical reaction ( ). Chemical reactions can occur at lower temperatures under an applied pressure than under vacuum. Carruthers and Wheat ( ) found mullite formation at 650°C in a china clay hot pressed during the dehydroxylation reaction with 30,000 psi. The process of dissolution can also be affected by an applied pressure. 

W.E. Lee, and Y. Iqbal, Influence of Mixing on Mullite Formation in Porcelain, Journal of the European Ceramic Society 21, 2583-2587,2001. 

---