Heat Capacity of Mullite ‐ New Data and Evidence for a High‐Temperature Phase Transformation

Abstract

The heat capacity at constant pressure (Cp) of mullite (single crystalline and dense and powdered polycrystalline) was determined between −125° and 1400°C. Measurements were performed in three laboratories using two different disk‐type differential scanning calorimeters (DLR and Netzsch) and one cylinder‐type (Setaram). On the basis of these experimental data a new master curve Cp (T) is presented. Up to about 1100°C the Cp curve agrees well with those from previously published results: The Cp master curve displays a parabolic shape with Cp values of about 0.415 J/(g·K) at −125°C, 0.78 J/(g·K) at 25°C, and 1.25 J/(g·K) at 1000°C. At temperatures above 1100°C a yet unknown, steplike, weak Cp increase is observed with ΔC°p step values ranging between 0.03 and 0.10 J/(g·K). This Cp anomaly of mullite is reproducible under various heating conditions and is reversible both on heating and on cooling. The extrapolated onset temperature of the anomaly (Ton.) is controlled by the crystalline state of the samples (single crystal, Ton. above 1100°C; polycrystal, Ton. above 1250°C) but does not depend on the composition of mullite. Sillimanite, which is structurally closely related to mullite, displays no Cp anomaly. The absence of latent heat (ΔH= 0) and the shape of the Cp anomaly of mullite are comparable to the behavior of glass‐forming materials at the glass transition. The observations can best be interpreted in terms of an onset of ion‐jumping between adjacent structural sites T and T* and O(C) and O(C)*. Other interpretations, i.e., clockwise and counterclockwise rotations of structural units, or minor decomposition of mullite to α‐Al2O3 plus SiO2 glass observed at the surface of the single crystals, are less probable.