Conversion of Al2O3-SiO2 powder mixtures to 3 : 2 mullite following the stable or metastable phase diagram

A model system, composed of powder blends of amorphous isomorphic silica sp heres, being 500 nm in diameter, and also monosized crystalline alpha -Al2O 3 powder, was investigated. Two different particle sizes of the correspondi ng alumina powder were employed: 300 nm and 2 mum. This particular assembly enabled a distinction between amorphous silica and crystalline alumina mer ely by their difference in particle morphology. The powder blends were sint ered at temperatures between 1400 and 1700 degreesC and the microstructure evolution was characterized by scanning (SEM) and transmission electron mic roscopy (TEM). It is worth noting that upon annealing at 1700 degreesC, bot h microstructures were indistinguishable. However, depending on the Al2O3 p article size, different conversion mechanisms were monitored. When using th e 300 nm Al2O3 powder, fast dissolution of alumina into the coalesced silic a glass occurred, followed by homogeneous nucleation and growth of mullite within the glass. Utilizing 2 mum Al2O3 particles, however, resulted in the formation of two At-containing glasses (phase separation into a Si- and Al -rich glass). In this case, the transformation to mullite can be rationaliz ed by the conversion of the metastable At-rich transient glass into mullite , which forms an epitaxial. single crystalline coating on the host Al2O3 pa rticle. Therefore, depending on the initial Al2O3 particle size, mullite fo rmation follows either the stable or metastable phase diagram. (C) 2001 Pub lished by Elsevier Science Ltd. All rights reserved.