Crystallization kinetics of high-cordierite glass

Crystallization kinetics of high-cordierite (2MgO · 2Al 2O 3 · 5SiO 2) powdered glass containing P 2O 5 and B 2O 3 were studied using X-ray diffraction (XRD) and differential thermal analysis (DTA). Metastable high-quartz, which has a structure similar to high-cordierite, appeared first. In addition to the main phase of high-cordierite, two other phases (forsterite and protoenstatite) appear with an increase in temperature. The overall activation energy of crystallization, 303.5 kJ/mol, of high-cordierite was computed by using Marseglia's equation based on DTA traces taken at different heating rates. This result is in fair agreement with that (301.9 kJ/mol) calculated using the Johnson-Mehl-Avrami equation from the temperature dependence of the fraction of cordierite crystallized in glass as measured by XRD. The rate-determining step of crystallization in glass of this system is concluded to be a volume diffusion process governed by the thin boundary layer produced at a glass-crystal interface following surface nucleation.