Pressure-induced phase transition in silicon nitride material

Abstract

The equilibrium crystal structures, lattice parameters, elastic constants, and elastic moduli of the polymorphs α-, β -, and γ-Si 3 N 4 , have been calculated by first-principles method. β -Si 3 N 4 is ductile in nature and has an ionic bonding. γ-Si 3 N 4 is found to be a brittle material and has covalent chemical bonds, especially at high pressures. The phase boundary of the β → γ transition is obtained and a positive slope is found. This indicates that at higher temperatures it requires higher pressures to synthesize γ-Si 3 N 4 . On the other hand, the α → γ phase boundary can be described as P = 14.37198 + 3.27 × 10 −3 T -7.83911 × 10 −7 T 2 -3.13552 × 10 −10 T 3 . The phase transition from α-to γ-Si 3 N 4 occurs at 16.1 GPa and 1700 K. Then, the dependencies of bulk modulus, heat capacity, and thermal expansion on the pressure P are obtained in the ranges of 0 GPa-30 GPa and 0 K-2000 K. Significant features in these properties are observed at high temperatures. It turns out that the thermal expansion of γ-Si 3 N 4 is larger than that of α-Si 3 N 4 over wide pressure and temperature ranges. The evolutions of the heat capacity with temperature for the Si 3 N 4 polymorphs are close to each other, which are important for possible applications of Si 3 N 4 .

High Temperature Stability of Zirconium Tungstate

“…Analysis of Negative Expansion Mechanism of Zirconium Tungstate. The occurrence of negative expansion is mainly due to the transverse thermal movement of oxygen atoms, which is also described as rigid element model(the low energy RuMs) [12,13]. The crystal structure of cubic ZrW2O8 is shown in Fig.…”

Thermal Equation of State of α Silicon Nitride

“…Because of its importance to condensed matter physics and engineering application, Si 3 N 4 has been investigated extensively by theoretical approaches and experimental methods. For example, the phase transition among the three phases was studied thoroughly under high P – T conditions through computational simulation and experimental measurement. − The mechanical properties, including hardness, bulk modulus, and shear modulus, were also characterized either by the theoretical calculation or experimental method. − However, almost all of those characterizations were conducted on Si 3 N 4 at room temperatures. Despite the yielding strength of α-Si 3 N 4 was measured under high P – T conditions, the thermal elastic properties, such as how the bulk modulus of this material changes against temperature and how this material thermally expands at a certain pressure, are lacking.…”