Ultrasonic study of the elastic and nonlinear acoustic properties of ceramic aluminum nitride

Pulse-echo-overlap measurements of ultrasonic wave velocity have been used to determine the elastic stiffness moduli and related elastic properties of aluminum nitride (AlN) ceramic samples as functions of temperature in the range 100–295 K and hydrostatic pressure up to 0.2 GPa at room temperature. Aluminum nitride is an elastically stiff but light ceramic: at 295 K, the longitudinal stiffness (CL), shear stiffness (μ), adiabatic bulk modulus (BS), Young's modulus (E) and Poisson's ratio (σ) are 373 GPa, 130 GPa, 200 GPa, 320 GPa and 0.234, respectively. The temperature dependences of CL and BS show normal behaviour and can be approximated by the conventional model for vibrational anharmonicity. The results of measurements of the effects of hydrostatic pressure on the ultrasonic wave velocity have been used to determine the hydrostatic-pressure derivatives of elastic stiffnesses and the acoustic-mode Gruneisen parameters. The values determined at 295 K for the hydrostatic-pressure derivatives (∂CL/∂P)P=0, (∂μ/∂P)P=0 and (∂BS/∂P)P=0 are 4.7 ± 0.1, 0.22 ± 0.03 and 4.4 ± 0.15, respectively. The adiabatic bulk modulus BS and its hydrostatic-pressure derivative (∂BS/∂P)P=0 are in good agreement with the results of recent high pressure X-ray diffraction measurements and theoretical calculations. The longitudinal (γL), shear (γS), and mean (γel) acoustic-mode Gruneisen parameters of AlN are positive: the zone-centre acoustic phonons stiffen under pressure. The shear γS (=0.006) is much smaller than the longitudinal γL (=1.09) accounting for the low thermal Gr¨neisen parameter γth (=0.65) obtained for this ceramic: since the acoustic Debye temperature ΘD (=980 ± 5 K) is so high, the shear modes play an important role in acoustic phonon population at room temperature. Hence knowledge of the elastic and nonlinear acoustic properties sheds light on the thermal properties of ceramic AlN.