Abstract
Cristobalite is formed when initially amorphous Pyrex borosilicate glass (Coming 7740) is fired at temperatures ranging from 700 to 1000 °C. The sigmoidal devitrification kinetics of cristobalite obeys Avrami-like three-dimensional diffusion-controlled kinetics. Activation energy analysis indicates that the diffusion of Na + in the glass is the rate-limiting step during phase transformation. The above conclusion is further confirmed by calculated and measured results of linear growth rates.