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 spheres, being 500 nm in diameter, and also monosized crystalline α-Al2O3 powder, was investigated. Two different particle sizes of the corresponding alumina powder were employed: 300 nm and 2 μm. This particular assembly enabled a distinction between amorphous silica and crystalline alumina merely by their difference in particle morphology. The powder blends were sintered at temperatures between 1400 and 1700°C and the microstructure evolution was characterized by scanning (SEM) and transmission electron microscopy (TEM). It is worth noting that upon annealing at 1700°C, both microstructures were indistinguishable. However, depending on the Al2O3 particle size, different conversion mechanisms were monitored. When using the 300 nm Al2O3 powder, fast dissolution of alumina into the coalesced silica glass occurred, followed by homogeneous nucleation and growth of mullite within the glass. Utilizing 2 μm Al2O3 particles, however, resulted in the formation of two Al-containing glasses (phase separation into a Si- and Al-rich glass). In this case, the transformation to mullite can be rationalized by the conversion of the metastable Al-rich transient glass into mullite, which forms an epitaxial, single crystalline coating on the host Al2O3 particle. Therefore, depending on the initial Al2O3 particle size, mullite formation follows either the stable or metastable phase diagram.