Active filler controlled pyrolysis of polymethylsiloxane/aluminium powder
(PMS/Al) precursor mixtures have been used to form mullite bodies. The reaction
sequence has been characterised using simultaneous thermal analysis consisting of
TGA/DSC/EGA (thermogravimetry analysis/differential scanning calorimetry
coupled with evolved gas analysis), X-ray diffraction (XRD), microstructural study
(scanning electron microscope/SEM, electron probe microanalyser/EPMA and
transmission electron microscope/TEM), and microchemical analysis using energy
dispersive spectroscopy/EDS and wavelength dispersive spectroscopy/WDS.
The influence of the characteristics of the Al powders (size and morphology)
and the PMS/Al ratio on the temperature and mechanism o f mullite formation was
investigated in this work. The Al powders of flakes (73 pm), irregular (9 pm) and
spherical (1 Opm and 56pm) were used in mixtures with PMS with a ratio o f PMS/Al
close to that required to form 3:2 mullite (81/19 vol.%). A blend using alumina
powder (sample B5) was also prepared for comparison study on mechanism o f
mullite formation in the absence o f Al metal. Samples o f composition Bl-C, which
prepared using Al flakes (PMS/Al ratio: 81/19 vol.%), were used as a somewhat
arbitrary reference with which to compare variations in PMS/Al ratio and/ or Al
powder characteristics. Samples B2, B3 and B4 used spherical 56pm, irregular 9pm
and spherical 10pm Al powders respectively. Samples prepared with different
PMS/Al ratios used Al flakes powder in the blend; the ratios were 63/37 vol.%
(samples Bl-A) and 72/28 vol.% (samples Bl-B).
The heat treatment applied produced different intermediate products
depending on the PMS/Al ratio used. In the process leading to mullite formation,
some intermediate phases are evolved. They are Al-oxycarbides (AI2 OC’, AI4 O4C),
Al-silicon carbide (AUSiC4 ) and Al-nitride (AIN), Si, SiC, y~, 1-, and CC-AI2 O3 which
were found in samples Bl-A and Bl-B. Free Si, SiC, Si0 2 (amorphous and
cristobalite), 5-, a-AfOi, and SiAlON (Si6Alio0 2 iN4) were phases found in samples
B2,B3 and B4.
The size and shape o f the Al powder is shown to have a strong influence on
the final microstructure and phase composition o f the product. The TGA/DSC
analysis shows the reaction steps leading to mullite formation. The reaction proceeds
by a decomposition o f the PMS producing amorphous Si0 2 . Al oxidation occurs both
by reaction with the atmosphere and by reduction o f the amorphous SiC>2 to produce
a-A^Os. Crystallisation o f cristobalite was also observed prior to mullitisation. It is
these components o f the microstructure that react to produce mullite after heating to
1700°C. XRD analysis identified 3:2 mullite in samples Bl-C and B2 after heating to
1400° C and at 1700° C in samples B3, B4 and B5.
From the microstructural observation and microchemical analysis, at least
two different mechanisms were suggested to explain the mechanism o f mullite
formation via AFCOP. The first mechanism is the solid-state reaction o f a-Al2C>3
(corundum) and SiC>2 (cristobalite). This mechanism is believed to occur initially in
samples B2, B3, B4 and B5 which crystallisation o f cristobalite takes place after the
completion o f Al oxidation. TEM reveals the mullite crystals at the interface o f ctAI2 O3 and amorphous Si0 2 . This is the second mechanism o f mullitisation after
melting o f cristobalite, which takes place at a higher temperature (T >1650°C)
