Relation of Single‐Crystal Elastic Constants to Polycrystalline Isotropic Elastic Moduli of MgO: II, Temperature Dependence

Abstract

The technical adiabatic elastic moduli E [hkl] and G hkl of single crystals of magnesium oxide were measured over the temperature range 298° to about 1600°K by a Förster‐type resonance method. These data were compared with the low‐temperature values (80° to 560°K) of the principal elastic constants c ij and coefficients Sy reported by Durand. Combining Durand's data and the present data, the elastic moduli for single‐crystal magnesium oxide were evaluated for the temperature range 80° to 1600°K. Young's modulus and the shear modulus of densely formed isotropic polycrystalline magnesium oxide were measured over the temperature range 298° to 1600°K. The data on the elastic constants of the single crystals were compared with the measured elastic moduli of the isotropic polycrystalline magnesium oxide on the basis of the Voigt‐Reuss‐Hill approximation. The temperature dependence of the elastic moduli was fitted into the expression M = M c — BT exp (—T c /T) suggested by Wachtman et al. ; mean deviations were less than 0.4% for the temperature range considered. The significance of the present data is discussed with particular emphasis on the following points: (1) the temperature variation of the elastic modulus is a function of thermal expansion, (2) the temperature dependence of the elastic modulus can be well described by the foregoing expression for a wide range of temperature, (3) the expression gives a value of the elastic modulus at 0°K, and (4) it may be possible to make use of measurements on the elastic properties of a densely sintered polycrystalline material to obtain information heretofore obtainable only from the corresponding single‐crystal data.