Thermal Equation of State of α Silicon Nitride

The high pressure–temperature (P–T) synchrotron X-ray diffraction (XRD) technique was used to detect the changes in the α phase of silicon nitride (α-Si3N4) under varied conditions up to 8.1 GPa and 1100 K. The lattice parameters, a and c, as well as the unit-cell volume, V, of the sample at a series of P–T points were obtained from the refinement of the XRD patterns. Then, a modified high-T Birch–Murnaghan equation of state (EOS) was adopted to fit the P–V–T dataset, yielding values of the thermoelastic properties of α-Si3N4. Upon the pressure derivative of the bulk modulus, K0′, fixed at 4.0, the fitting results are the ambient condition bulk modulus K0 = 210(11) GPa, temperature derivative of the bulk modulus at constant pressure (∂K/∂T)P = −0.016(7) GPa·K–1, and volumetric thermal expansivity αT(K–1) = a + bT with a = 6.24(10) × 10–7 K–1 and b = 1.09(10) × 10–8 K–2. Meanwhile, the temperature-dependent linear compressibilities of the sampling crystal alone lattice axes a and c, respectively, were calculated. Furthermore, the thermal pressure approach was applied to derive the temperature derivative, (∂K/∂T)V, of the bulk modulus at a constant volume, leading to a value of −0.0095(11) GPa·K–1. The measured ambient bulk modulus in the present study is comparable to those reported in the literature. In addition, the thermal elastic properties of α-Si3N4 were compared with those of another similar ceramic material, silicon carbide (SiC), to further explore its unique mechanical characteristics.