First Principles Study Of The New Six-party Super-hard Materials

In this paper, the lattice constants and the elastic constants of w-BN、w-BC2N、 r3m-B2CN and r3m-BC2N are calculated and the phonon dispersion relations are of these are analyzed by first-principles method based on density functional theory and perturbation theory. Then, the detailed analysis on the mechanical, optical, thermal properties and changing rule of four hexagonal superhard materials as well as their anisotropic conditions is also performed. The purpose of the study provide reliable theory basis for synthesis of this kind of superhard materials.In the third chapter, the electronic structure of w-BN、w-BC2N、r3m-B2CN and r3m-BC2N are calculated. The calculation results show that the band structure of four material belong to direct transition type semiconductor; Some valence band enter into conduction band across Fermi level in the band structure graph of r3m-B2CN and formed some free electrons, which causes that band gap of r3m-B2CN is minimum and indicates it is for conductive properties.In the fourth chapter, the mechanical properties structure of w-BN、w-BC2N、 r3m-B2CN and r3m-BC2N are calculated and Poisson’s ratio, elastic modulus, elastic anisotropy factor of four materials are studied by using elastic constants. We found that the atomic binding forces of w-BN、w-BC2N and r3m-BC2N are non-central forces, whereas that of r3M-B2CN is a central force; the four materials are proven to have elastic anisotropy; the different shapes in the three-dimensional stereograms of Young’s modulus on all surfaces, as well as the tensile and shear stress-strain curves in different crystal orientations can further account for the anisotropies of these materials; r3m-B2CN has a large degree of anisotropy, and the three-dimensional diagram of its Young’s modulus has an’8’shape.In the fifth chapter, the optical properties of w-BN、w-BC2N、r3m-B2CN and r3m-BC2N are studied and the refractive index, absorptions, reflectivities, and loss functions of the four materials are calculated and analyzed by using results of complex refractive index. Analysis results show that four superhard materials are difficult to absorb the photon energy in high-energy area; Some valence band enter into conduction band across Fermi level in the band structure graph of r3m-B2CN, which is correspond with formed free electrons when p electronic state through the Fermi level in the dos graph. This explains optical properties of r3m-B2CN is different from other three materials, and we found that r3m-B2CN is for excellent optical selectivity. In the sixth chapter, the thermodynamic properties of w-BN、w-BC2N、 r3m-B2CN and r3m-BC2N are analyzed. We found that w-BN、w-BC2N、r3m-BC2N are for good thermal stability, whereas that of r3m-B2CN is for poor thermal stability Because the phonon dispersion curves of r3m-B2CN cross zero point and there is into negative territory. This suggests that r3m-B2CN will produce phase transition at high temperature.the minimum value of thermal conductivities of ploycrystalline systems of the four superhard materials at high temperature show the anisotropies, and r3m-B2CN has a large degree of anisotropy.Through the research of physical properties of four hexagonal new superhard materials, we found that synthesis of w-BN、w-BC22N、r3m-BC2N on the experiment is relatively easy, but synthesis of r3m2-B2CN would become a huge challenge because the degree of the anisotropic is bigger and thermal stability is poorer.