| Superhard materials, with its extreme hardness, high elastic modulus, excellent wear resistance, and incompressibility, are extensively used in machining, cutting, aerospace, construction, wear-resistant coatings, and in other fields. The first and second extensively used superhard materials are diamond (Vickers hardness Hv of 96 GPa) and cubic boron nitride (c-BN; Hv of 63 GPa). However, diamond possesses a low thermal stability and poor chemical stability, which restrain its application fields. By contrast, c-BN has better thermal stability but lower hardness compared with diamond. Notably, researchers have discovered ternary B-C-N compounds and observed that these compounds have the merits of diamond and c-BN, i.e., the hardness of diamond and the thermal stability of c-BN. These compounds have evoked long-standing and extensive attention among researchers.This paper designed and constructed bc-BCN and bc-BC4N based on the bc-structure superhard BC2N that researched by Luo, and investigated the properties (the elastic constants, elastic modulus, hardness, ideal strength, minimum thermal conductivity at high temperature limit, energy band structure, optical properties, and anisotropy.) of these new superhard materials of bc-BCxN (including bc-BCN, bc-BC2N and bc-BC4N, in this paper referred to as BCxN or BCN, BC2N and BC4N), with diamond and c-BN as reference materials. Systematically studied the physical properties of the three orthorhombic BCxN, hope to provide some theory reference for the experimental research and industrial application.The three crystal structures of BCN, BC2N and BC4N were given in chapter three, besides, the electron density and electron density difference were also plotted and discussed. It was observed that the highest electron density is around the N atom and the lowest electron density is around the B atom. Electron density difference shows the electron transfer from N to the neighboring C-C bonds. The B-N bonds slant toward the N atom.The mechanical properties of BCxN show that BCN, BC2N and BC4N can be classified as brittle materials, in which BCN possesses the best ductility and yield strength among BCxN, BC2N ranks second. However, BC4N has the highest elastic modulus as well as the highest hardness (68 GPa in LDA method, the same below), BC2N with a lower hardness but higher than that of BCN. Furthermore, the research on ideal strength shows that the lowest stress for BC4N is 63 GPa in the (110)[001] direction, which is close to that of c-BN with the same ideal strength. The elastic anisotropy has been studied, the result indicates that BCN, BC2N and BC4N are all anisotropic, and show a slightly decreasing trend of the degree of anisotropy in the sequence of BCNâ†'BC2Nâ†'BC4N. Hence the conclusion goes that BC4N possesses the best mechanical property.Investigation on minimum thermal conductivity at high temperatures shows that a high hardness indicates a high minimum thermal conductivity Kmin. BCN has the lowest Kmin among BCxN, BC4N possesses the highest minimum thermal conductivity. Besides, the values of kmin in different crystal orientations are considerably different, which reflects the anisotropic thermal conductivity for each compound.In chapter 6, the electronic structure and optical properties of BCxN have been investigated, the band gaps for BCN, BC2N and BC4N are 0.819eV,0.595eV and 0.340eV, respectively. Besides, the bottom of conduction band and the top of valence band for BCxN are not the same point in Brillouin Zone, indicating they are indirect bandgap semiconductors. The optical properties of BCN, BC2N and BC4N are very similar with each other, and have very small absorption in the visible region, so does the reflectivity. Their refractive index across 2.0 to 2.5, in which BC4N has a slightly higher value, and BC2N ranks second. |
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