Crystallization kinetics of mullite in alumina-silica glass fibers

The mechanism of mullite crystallization in Al₂O₃-SiO₂ glass fibers with compositions of 49 and 69 mass% Al₂O₃ was investigated using isothermal methods. The activation energies (E_a) of the nucleation and nucleation-growth of mullite were obtained from the temperature dependence of the incubation times and rate constants of mullite formation, respectively. The formation of mullite in both glasses occurred in two stages. The amount of mullite increased very steeply in the first stage within very short firing time, whereas the increase was more gradual in the second stage. The E_a values for nucleation and nucleation-growth in the first stage were 864 and 1288 kJ/mol in the 49-mass%-Al₂O₃ glass fiber and 980 and 1138 kJ/mol in the 69-mass%-Al₂O₃ glass fiber, respectively. Arrhenius plots of the second-stage data showed that the slope ratios were different at temperatures below and above 1200 °C. The E_a values at <1200 °C were 1195 and 1099 kJ/mol in the 49- and 69-mass%-Al₂O₃ glass fibers, whereas those at >1200 °C were 696 and 645 kJ/mol, respectively. Observation of the microstructures and crystallite-size data indicates that the crystallization of mullite proceeds via three different mechanisms: nucleation, nucleation-growth, and coalescence of mullite grains.