Low temperature mullite forming pre-ceramic resins of high ceramic yield for oxide matrix composites

Abstract

Monophasic mullite precursors, namely aluminosiloxanes were synthesized by a novel synthetic route, in which co-hydrolysis and condensation of aluminium tri secbutoxide and tetraethoxy silane was achieved in presence of con. HCl in a non-polar medium. Aluminosiloxanes were characterized by FT-IR, NMR, and elemental analyses. Spectral analysis confirms the presence of Si-O-Al bonds in all the samples and also validates the incorporation of more aluminium via Si-O-Al bonds with increasing Al/Si mole ratio in the precursor. These FT-IR and NMR data also attest the precursor level homogeneity in all the samples. The aluminosiloxanes are obtained as low viscous resins and are capable of giving high ceramic residue qualifying them as ceramic matrix precursors for CMCs. The effect of Al/Si ratio on the ceramic conversion was studied. All the precursors showed the formation of mullite at 1000 °C. This low temperature mullite formation is a key factor in developing oxide CMCs without fiber damage. The results obtained from the study show that the composition of the ceramic can be controlled between a silica rich mullite phase and near-stoichiometric mullite phase by suitably selecting the Al/Si monomer feed ratio of the precursors. This aspect provides a greater scope for designing application-specific ceramic matrices for space applications.