Crystallization Kinetics of Internally Nucleated Vitreous Silica

Abstract

It has recently been observed that cristobalite can hetero‐geneously nucleate and grow internally in vitreous silica. When the glass‐crystal composite is cooled to room temperature, β‐cristobalite is metastably retained as a result of the large tensile stresses developed during cooling. Although these crystals are not obvious, they can be readily observed with a polarizing microscope or polariscope. As long as the cristobalite does not transform when cooled to room temperature, repeated increments of crystal growth can be measured on the same growing crystal. Crystal growth was measured on electrically melted quartz glass at 1350° to 1620°C. The observed growth rates were linear with time and were the lowest that have been measured in vitreous silica. The kinetic data agreed with the general form of the crystal growth equation μ= (AΔT)/ ν . Since the growth is internal and free from surface contamination, the measured rates are considered to be very near the intrinsic rates for the material.