The effect of ZrO2 on the crystallization of a glass in the system BaO/SrO/ZnO/SiO2: surface versus bulk crystallization

Abstract

Recently discovered solid solutions with the composition Ba1−xSrxZn2Si2O7 (0.1 ≤ x ≤ 0.9) exhibit low or negative thermal expansion in a wide temperature range above room temperature. Materials which have low or negative thermal expansion find applications such as gyroscopes, telescope mirrors, and cook panels. In contrast to other low expansion materials described in the past two decades, these solid solutions can be crystallized from a glass. Due to the high anisotropy of the coefficient of thermal expansion, the obtained materials show a strong tendency to micro-cracking which makes them not yet suitable for industrial application. Studying the crystallization kinetics and finding suitable nucleation agents are the main keys to obtain a crack-free material. In the present article, the effect of one of the most commonly used nucleation agents, which is ZrO2, is investigated. For the characterization of the glasses and glass–ceramics, X-ray diffraction was used in order to determine the obtained crystalline phases and scanning electron microscopy was applied to characterize the microstructure. By applying different heating rates in a differential scanning calorimetry device, it was possible to calculate the activation energy using the equations of Ozawa and Kissinger, and to determine the Avrami parameters, which provide further information on the crystallization process. Using ZrO2 concentrations of up to 5 mol% resulted in sole surface crystallization, while at a ZrO2 concentration of 6 mol% also bulk nucleation occurs.