ELASTICITY, EQUATION OF STATE AND PHASE TRANSITIONS IN ADVANCED CERAMIC MATERIALS

Murli H. Manghnani

University of Hawaii, Hawaii Institute of Geophysics and Planetology, Honolulu, Hawaii 96822

 

The temperature and pressure dependencies of the elastic moduli and the equations of state (P-V relationships) of various advanced ceramic materials (e.g., SiC, c-BN, AlN, TiB2, B4C) are reviewed in light of the experimental results from ultrasonic, X-ray, shock wave and spectroscopic (Brillouin and Raman scattering) measurements.

Measurements of the five single-crystal elastic constants (C11, C33, C44, C12 and C13) for a-SiC (hexagonal, P63mc symmetry) to 1000oC, using Brillouin light scattering technique, are in excellent agreement with theoretical prediction of the dCij/dT values reported by Li and Bradt (1987). The isotropic moduli and their temperature dependencies, deduced from the single-crystal elastic data from Brillouin spectroscopy, are compared with the high-precision ultrasonic measurements on fully dense polycrystalline specimens of a-SiC and b-SiC (synthesized by CVD process). Except for shear moduli, the elastic moduli and their temperature derivatives for b-SiC measured to ~ 900oC, are higher than those for both single-crystal and polycrystalline specimens of a-SiC.

In contrast to SiC, the temperature dependencies of the elastic properties of polycrystalline c-BN, measured by ultrasonic interferometer to 900oC, are larger and non-linear, consistent with the reported non-linear variation of the first-order Raman shifts of TO and LO modes of c-BN (Herchen and Cappeli, 1993).

Complementary in-situ high pressure X-ray diffraction (EDXRD) and Raman scattering studies have been carried out on AlN in a diamond-anvil cell at room temperature to 30 and 20 GPa, respectively, in order to characterize the phase transition from wurtzite (hexagonal) to rock salt (cubic) structure. The pressure dependencies of all the six observed Raman modes are found to be a linear function of pressure. The average Grüneisen parameter calculated from the mode gammas is 0.9 - 1.0, in good agreement with gth. Recent high pressure Raman scattering studies on single-crystal SiC (4H, 6H) are reviewed in similar manner.