HIGH-TEMPERATURE BEHAVIOR OF POLYCRYSTALLINE ALUMINOSILICATE FIBERS WITH MULLITE BULK COMPOSITION .2. KINETICS OF MULLITE FORMATION

Kinetics and mechanisms of mullite formation in technical diphasic alu minosilicate fibres (Nextel(TM) 440), consisting of transition alumina and a noncrystalline silica-rich phase, were studied by quantitative X-ray phase analysis. The as-received fibres were heat-treated in the temperature and time ranges from 1130 to 1215 degrees C and from 7.5 m in to 32 h. The overall mullitization process can be described by an A vrami reaction law, or alternatively, by a simple exponential reaction law with a temperature-dependent induction period. Early stages of mu llite formation with a low mullitization degree (alpha = 0.02) are cha racterized by an activation energy of similar to 650 kJ mol(-1), where as the apparent activation energy of the overall transformation reacti on is similar to 900 kJ mol(-1). Both activation energies are signific antly lower than those described in the literature for other diphasic aluminosilicate gels. The relatively low activation energies of mullit e formation in Nextel 440 fibres ave attributed to the small B2O3 cont ent of the as-received fibres. The presence of B2O3 may be responsible for the decrease of viscosity of the coexisting non-crystalline SiO2- rich phase, by which diffusion, and mullite nucleation and growth is a ccelerated.