High temperature behaviour of polycrystalline aluminosilicate fibres with mullite bulk composition .2. Kinetics of mullite formation

Kinetics and mechanisms of mullite formation in technical diphasic aluminosilicate 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 min to 32 h. The overall mullitization process can be described by an Avrami reaction law, or alternatively, by a simple exponential reaction law with a temperature-dependent induction period. Early stages of mullite formation with a low mullitization degree (alpha = 0.02) are characterized by an activation energy of similar to 650 kJ mol(-1), whereas the apparent activation energy of the overall transformation reaction is similar to 900 kJ mol(-1). Both activation energies are significantly lower than those described in the literature for other diphasic aluminosilicate gels. The relatively low activation energies of mullite formation in Nextel 440 fibres ave attributed to the small B2O3 content 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 accelerated.