Elastic Properties of Mullite Single Crystals up to 1400°C

Elastic constants and their temperature derivatives of orthorhombic 2/1‐mullite single crystals were determined using resonant ultrasound spectroscopy between ambient temperature and 1400°C in air. The values at room temperature given in GPa were c11=279.5(4), c22=234.9(2), c33=360.6(3), c44=109.49(7), c55=74.94(4), c66=79.89(4), c12=103.1(2), c13=96.1(3), and c23=135.6(1). The high temperature ultrasound data sets were corrected for thermal expansion effects using the coefficients, which were measured in the range −170° to 1400°C by means of dilatometry. Up to about 1000°C, the temperature evolution of the elastic stiffnesses was linear and the corresponding thermoelastic constants Tij=d log cij/dT given in 10−6°C−1 were T11=−100(2), T22=−126(2), T33=−78.2(6), T44=−75.3(7), T55=−81.3(9), T66=−113(1), T12=−153(3), T13=−94(4), and T23=−47(2). While the temperature derivatives of the longitudinal stiffnesses and the transverse interaction coefficients remained linear, the shear resistances c44, c55, and c66 displayed an increased softening above about 1000°C, which was accompanied by a rapidly increasing ultrasound dissipation. In the same temperature region, small, but clearly recognizable discontinuities in the longitudinal and in the volume thermal expansion coefficients occurred. Taking into account the behavior of the heat capacity, the observed anomalies may indicate a glass‐like transition of mullite above about 1000°C. An unexpected peak in the ultrasound dissipation at about 70°C was most likely caused by anelastic point defect relaxations.