Microstructure and mullitization of aluminosilicate matrix in Nextel 720/aluminosilicate composites prepared by LPCVI at 550°C

Nextel 720/aluminosilicate composites were prepared by low pressure chemical vapor infiltration (LPCVI) at 550 °C using AlCl3–SiCl4–CO2–H2 precursor system. The microstructure and mullitization of the aluminosilicate matrix were monitored by SEM, DTA and XRD. The matrix consisted of concentric muti-layers like the annual tree rings and contained some isolated alumina particles and pores. The phase composition of the matrix was composed of an amorphous silica, γ-Al2O3, Al–Si spinel, an amorphous monophasic mixture (in which Al3+ and Si4+ were mixed at the molecular level), and an amorphous diphasic mixture (in which Al3+ and Si4+ were mixed at a nanometer scale). The mullitization of the matrix included three stages. Complex mullitization process and inhomogeneous microstructure of the matrix suggested that AlCl3 and SiCl4 were not molecularly mixed.