材料期刊网

Theoretical study of the elastic properties of titanium nitride

Dong CHEN , Jingdong CHEN , Yinglu ZHAO , Benhai YU , Chunlei WANG , Deheng SHI

金属学报(英文版) doi:10.1016/S1006-7191(08)60082-4

The equilibrium lattice parameter, relative volume V/V₀, elastic constants C_ij, and bulk modulus of titanium nitride are successfully obtained using the ab~initio plane-wave pseudopotential (PW-PP) method within the framework of density functional theory. The quasi-harmonic Debye model, using a set of total energy vs molar volume obtained with the PW-PP method, is applied to the study of the elastic properties and vibrational effects. We analyze the relationship between the bulk modulus and temperature up to 2000 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonously with increasing pressure and decreases with increasing temperature. Moreover, the Debye temperature is determined from the non-equilibrium Gibbs functions.

关键词: Elastic constants , null , null , null

First-principles calculations of LaNi_5-xSn_xH_y  intermetallics and intermediate phase

Dong CHEN , Jingdong CHEN , Yinglu ZHAO , Hailiang HUO , Benhai YU , Deheng SHI

金属学报(英文版) doi:10.1016/S1006-7191(08)60106-4

The crystal and electronic structures of LaNi_4.75Sn_0.25 intermetallics and LaNi_4.5Sn_0.5H_y (y=2.0, 2.5) intermediate phase have been investigated by the full-potential linearized augmented plane wave (FP-LAPW) method. Hydrogen occupation sites in LaNi_4.5Sn_0.5H_y have been determined based on Westlake$'$s criterions: (1) the minimum hole radius is 0.04~nm; (2) the minimum H-H distance is 0.21~nm; as well as geometry optimizations and internal coordinates optimizations. We find that hydrogen atoms prefer to occupy the 12n^*, 6m, 12o, 6m^* sites in LaNi_4.5Sn_0.5H_2.0 and the 6m^*, 4h, 6m, 12o, 12n^* sites in LaNi_4.5Sn_0.5H_2.5. The specific coordinates of hydrogen atoms in LaNi_4.5Sn_0.5H_y are also determined. The results show that hydrogen atoms tend to keep away from tin atoms. The maximum hydrogen content decreases compared with LaNi₅. The interactions between Sn and Ni with H play a dominate role in the stability of LaNi_4.5Sn_0.5-H system. Lattice expansion and increment of Fermi energy E _F show that both Sn and H atoms decrease structural stability of these alloys.

关键词: Rare-earth intermetallics , null , null , null

Predictions of high-pressure structural, electronic and thermodynamic properties of α-Si₃N₄

Benhai YU

金属学报(英文版)

The plane-wave pseudo-potential method within the framework of first-principles technique is used to investigate the fundamental structural properties of Si₃N₄. The calculated ground-state parameters agree quite well with the experimental data. Our calculation reveals that α-Si₃N₄ can retain its stability to at least 45 GPa when compressed below 300 K. No phase transition can be seen in the pressure range of 0--45 GPa and the temperature range of 0--300 K. Actually, the α→β transition occurs at 1600 K and 7.98 GPa. Many thermodynamic properties, such as bulk modulus, heat capacity, thermal expansion, Gruneisen parameter and Debye temperature of α-Si₃N₄ were determined at various temperatures and pressures. Significant differences in these properties were observed at high temperature and high pressure. The calculated results are in good agreement with the available experimental data and previous theoretical values. Therefore, our results may provide useful information for theoretical and experimental investigations of the N-based hard materials like α-Si₃N₄.

关键词: First-principle

Phase Transition Characters and Thermodynamics Modeling of the Newly-Discovered wⅡ-and Post-Spinel Si3N4 Polymorphs:A First-Principles Investigation

Benhai YU , Dong CHEN

金属学报(英文版) doi:10.1007/s40195-012-0133-1

关键词:

First-principle study on the physical properties of ultra-incompressible ReB₂

Chunlei WANG , Qiuju SUN , Xuanyu SONG , Benhai YU

金属学报(英文版) doi:10.1016/S1006-7191(08)60100-3

The elastic and physical characteristics of ReB₂ crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Five independent elastic constants are C₁₁=662 GPa, C₁₂=150 GPa, C₁₃=146 GPa, C₃₃=1090 GPa and C₄₄=263 GPa. The bulk modulus (B), shear modulus (G), Young's modulus (E), Poisson's ratio (γ) and the ratio of linear compressibility coefficient along the a- and c-axis crystal direction (Ka/Kc) are 356 GPa, 305 GPa, 711 GPa, 0.167 and 1.758, respectively. In addition, the dependence of bulk modulus (B) on temperature (T) and pressure (p) as well as the coefficient of thermal expansion (α_L) at various temperatures are evaluated and discussed. The coefficient of thermal expansion is consistent with the famous Gruneisen's law when the temperature is less than 1500K. Our results agree well with the other experimental results.

关键词: Rhenium diboride , null , null , null