Mullite formation from the pyrolysis of aluminium-loaded polymethylsiloxanes: The influence of aluminium powder characteristics

Polymethylsiloxane (PMS) filled with a range of aluminium powders of different size and morphology have been used to produce precursor mixtures to form mullite bodies. The size and shape of the Al powder is shown to have a strong influence on the temperature and mechanism of mullite formation, on the final microstructure and phase composition of the product. The reaction proceeds by decomposition of the PMS producing amorphous SiO2. Al oxidation occurs both by reaction with the atmosphere and by reduction of the amorphous SiO2 to produce α-Al2O3. Crystallisation of cristobalite was also observed prior to mullitisation. It is these components of the microstructure that react to produce mullite. The onset of mullite formation occurs at different temperatures, depending on the initial Al powder size and morphology. Large, flake morphology Al powders produced the greatest quantity of mullite and showed the lowest temperatures for mullite formation. XRD analysis identified 3:2 mullite in samples using large Al particles after heating to 1400 °C and at 1700 °C in samples using small Al powders.