Ultrasonic study of the temperature and pressure dependences of the elastic properties of beta-silicon nitride ceramic

Ultrasonic wave velocity measurements have been used to determine the elast ic stiffness moduli and related elastic properties of high-purity, dense be ta -Si3N4 ceramic samples as functions of temperature in the range 150-295 K and hydrostatic pressure up to 0.2 GPa at room temperature. Due to its co valently bonded, rigid structural framework beta -Si3N4 is an elastically s tiff material; the elastic stiffness moduli of the ceramic at 295 K are: C- L = 396 GPa, mu = 119 GPa, B-S = 238 GPa, E = 306 GPa, Poisson's ratio sigm a = 0.285. The longitudinal elastic stiffness C-L increases with decreasing temperature and shows a knee at about 235 K; the decrease in slope below t he knee indicates mode softening. The shear elastic stiffness mu shows mode softening which results in a plateau centred at about 235 K and an anomalo us decrease with further reduction in temperature. The hydrostatic-pressure derivatives of elastic stiffnesses at 295 K are (partial derivativeC(L)/pa rtial derivativeP)(P)=0 = 4.5 +/- 0.1, (partial derivativeB(S)/partial deri vativeP)(P)=0 = 4.3 +/- 0.1 and (partial derivative mu/partial derivativeP) (P)=0 = 0.17 +/- 0.02 (pressure < 0.12 GPa). An interesting feature of the nonlinear acoustic behaviour of this ceramic is that, in the pressure range above 0.12 GPa, the values obtained for (partial derivative mu/partial der ivativeP)(P)=0 and the shear mode Gruneisen parameter (gamma (S)) are small and negative, indicating acoustic-mode softening under these higher pressu res. Both the anomalous temperature and pressure dependences of the shear e lastic stiffness indicate incipient lattice shear instability. The shear ga mma (S)(=0.005) is much smaller than the longitudinal gamma (L)(=1.18) acco unting for the thermal Gruneisen parameter gamma (t)h(=1.09): since the aco ustic Debye temperature Theta (D)(=923 +/- 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 ligh t on the thermal properties of ceramic beta -Si3N4. (C) 2001 Kluwer Academi c Publishers.