Effect of Thermal History on the Properties of a Willemite Glass‐Ceramic

Abstract

The effect of time and temperature on the properties and structure of a willemite glass‐ceramic were measured. ‐0 polymorphs of willemite were found to exist in most of the glass‐ceramics. Those properties of the glass‐ceramic which depend primarily on the composition of the glassy phase (helium permeation and diffusion, and chemical durability) were found to be strong functions of the thermal history of the samples, whereas those properties which depend on both the compositions and the relative amounts of both the glassy and crystalline phases (thermal expansion coefficient, density, and refractive index) were found to be relatively insensitive to thermal history. The properties of the glass‐ceramics could be described by simple mixing models using the properties measured for willemite and for glasses having theoretical residual glass compositions corresponding to varying degrees of crystallinity in the glass‐ceramic.