Controlling cristobalite formation in fused silica via particle size and treatment atmosphere adjustment during ceramic processing

This study explores strategies for controlling cristobalite formation in high-purity amorphous silica by systematically adjusting particle size, heat-treatment atmosphere and surface chemistry. Silica samples with different particle size fractions were heat-treated under static gas atmospheres at precisely controlled temperatures and durations. A novel approach utilizing forming gas (95 % N 2 , 5 % H 2 ) was developed to suppress crystallization, and its effectiveness was benchmarked against treatments in oxidizing (medical air) and inert (N 2 ) environments. Unlike conventional methods - such as doping, oxygen depletion, or processing under high vacuum - the present technique employs a reducing atmosphere that not only blocks external oxygen ingress but actively removes surface hydroxyl groups from the silica. This dual action significantly inhibits crystallization. Differential scanning calorimetry (DSC) offered detailed insights into how different atmospheres affect cristobalite formation, revealing a crystallization onset near 1280 °C. Below this threshold, nucleation was substantially suppressed; above it, crystallization proceeded rapidly, even under minimal temperature, regardless of the gas environment. This research uniquely demonstrates a clear linear correlation between specific surface area and cristobalite content, underscoring the surface-controlled nature of crystallization in high-purity silica. Surface chemistry analyses using X-ray photoelectron spectroscopy (XPS) revealed distinct alterations in Si-O bonding configurations following heat treatment. Cristobalite formation was thoroughly characterized using synchrotron X-ray diffraction (XRD) and DSC, with a critical assessment of the respective strengths and limitations of these methods. Taken together, these findings provide valuable insights and practical guidance for precisely regulating cristobalite content in silica-based ceramics, thereby improving material performance in demanding industrial applications.