Hysteresis upon Repeated Cycling through the Beta-Alpha Cristobalite Transformation

摘要

Beta-to-alpha transformation in cristobalite was examined by means of differential scanning calorimetry (DSC) to address how enthalpy, transformation hysteresis, and transformation onset temperature change upon repeated thermal cycling. Cristobalite powder was repeatedly cycled from 190-280 °C and back, cycling through the first-order displacive transformation between high-temperature, cubic beta-cristobalite and low-temperature, tetragonal alpha-cristobalite. The enthalpy of the transformation did not change with cycling, but the exothermic beta-to-alpha enthalpy at 1560 + 27 J/mol was larger than endothermic alpha-to-beta enthalpy at 1260 ± 8 J/mol. Transformation onset temperatures and hysteresis varied systematically with repeated cycling of the transformation. The onset temperature of the beta-to-alpha transformation increased logarithmically with cycling, resulting in reduced undercooling and hysteresis. The onset temperature of the alpha-to-beta transformation decreased logarithmically with cycling, resulting in reduced super-heating and hysteresis. The reduced hysteresis indicates a lowered barrier to transformation. We propose this reduced hysteresis indicates particle refinement through microfracture caused by the -4.9 % volume change on the beta-to-alpha transformation. This is supported by the observation of powder size dependence. Powder with particles finer than 38 um had no change on cycling, suggesting 38 um is below the critical minimum size.