Microwave Preparation of Mullite Powders

ABSTRACTMullite, Al6Si2O13 composition gels were made by the single phase and diphasic routes. The wet mullite gels were heated in the insulated cavity of an ordinary kitchen microwave oven (2.45 GHz; 700 watts). Crystalline mullite powders were obtained from single phase gels while the diphasic gels yielded α-Al2O3 and mullite in a matter of 20–25 minutes of microwave heating. Trans- mission electron microscopy showed that the mullite particle size varied between 50 nm to 1 μm. The results of this study suggest that fine powders of mullite can be obtained from single phase mullite gels by optimizing the microwave processing parameters.

Crystallization of ThSiO4 from structurally and/or compositionally diphasic gels

The use of structurally and/or compositionally diphasic gels has been shown earlier to have profound effects in lowering the crystallization temperature and enhancing densification in several systems. In this study of solid-state epitaxy the ThSiO4 composition has been investigated. The use of compositional diphasicity allows one to lower the huttonite (β-ThSiO4) crystallization temperature by as much as 200°C. Using crystalline thorite (α-ThSiO4) as second phase nuclei partially stabilizes this phase (hypothetically metastable), while the introduction of huttonite nuclei completely inhibits the formation of the α form, which appears in detectable quantities in the unseeded precursors. This confirms the structural (epitaxial) control of the solid-state reaction. The use of structural or compositional nanoheterogeneity clearly helps to lower the crystallization temperature and to stabilize a particular polymorph in ccramics processed by the sol-gel route, and this principle can certainly be utilized more generally.