Mechanical,Electronic And Thermodynamic Properties Of Boron-carbon Compounds

The various applications of superhard materials in many industrial areas, such as abrasives, polishing, cutting tools, wear-resistant, protective coatings, etc, have stimulated great interest in design and preparation study of those materials. Traditionally, diamond is a good choice for the above operations due to its high Vickers hardness beyond 100 GPa. However, its oxidizable property leads to the formation of iron carbides at high temperature for high-speed cutting of ferrous materials including steel, and the high cost and difficulty for synthesizing diamond artificially further limit the industrial applications about it. Those restrictions spur researchers to quest for the more stable compounds than diamond. As every two carbon atoms in diamond are substituted with one boron atom and one nitrogen atom, the constructed compound, cubic boron nitride, possesses excellent mechanical and thermal stability despite showing a relatively lower hardness (i.e.,48 GPa) than that of diamond. Another typical intrinsic superhard compound is diamond-like c-BC2N, whose Vickers hardness is about 76 GPa. Those facts suggest that the superhard materials with high electron density and strong covalent bond might be formed by the light and covalent-bond-forming elements (B, C, N, and 0). The B-C compounds have many advantages over diamond, performing special ability in oxidation resistance and exhibiting interesting electrical properties rather than insulation, which prompt us to pay considerable attention to the study of B-C compounds. Experimental attempts have been made intensively to synthesize boron-carbon compounds, including B4C, BC3, BC5, and BC7.The boron-carbon compounds are synthesized under high temperature and high pressure conditions due to the positive value of the formation enthalpy. However, as a result of the limitation of experimental conditions, the corresponding study can not be performed frequently under extreme conditions. In addition, the exact site occupancies by carbon and boron atoms are still debated, which is due to the similarity in both electronic and nuclear scattering cross-sections for boron and carbon, the crystal structures of boron-carbon compounds are hard to deduce from XRD measurement. Therefore, it is necessary to perform the structure prediction only requiring chemical compositions for a boron-carbon compound. In this thesis, we have predicted nine kinds of boron-carbon compounds by using CALYPSO package. In terms of their elastic constants, bulk and shear moduli, and hardness, five boron-carbon compounds, BC, BC2, BC3, BC4, and BC5 are chosen as the potential superhard materials. We have systematically calculated anisotropy and ideal strength of those five boron-carbon compounds. The results indicate that all of boron-carbon compounds possess high degree of anisotropy, and the lowest critical stresses of the boron-carbon compounds are less than 40 GPa under shear deformation.In the first chapter, we introduce the research status of superhard materials and the recent development of the boron-carbon compounds.In chapter 2, the first-principle calculation methods are introduced including the major approximation methods and the Density Functional Theory (DFT) method. Generally, the hardness should be estimated in order to decide whether a material is superhard. Therefore, we summarized three frequently-used methods for estimating the hardness of a material. In addition, the method for calculating elastic constants, the theory for studying the anisotropy, the method for deducing ideal strength, the theory of electron localization function, and the quasi-harmonic Debye model for calculating the thermodynamic properties are also presented.In chapter 3, we predicted the crystal structure of boron-carbon compounds by using CALYPSO package. Nine kinds of boron-carbon compounds were determined by consulting the previous research results carefully.In chapter 4, the mechanical properties, such as elastic constants, hardness, anisotropy, ideal strength were investigated systematically. Five carbon-rich B-C compounds, BC, BC2, BC3, BC4, and BC5 were chosen to the potential superhard materials according to the calculations of elastic constants, bulk and shear moduli, and hardness. In terms of the ideal strength calculations, we conclude that BC4 is not a potential superhard material despite possessing high bulk modulus and hardness, which is because the lowest ideal strength is only 5 GPa. The crystal structure is transformed from tetragonal structure to Cmmm space group over 5 GPa.In chapter 5, the electron localization functions (ELF) are calculated under the lowest shear ideal strength for carbon-rich B-C compounds. The isovalues of ELF show the fracture of the B-C bonds can be accounted for the shear plastic deformation.In chapter 6, the Debye temperature and the thermodynamic properties under high temperature and high pressure conditions are discussed according to the quasi-harmonic Debye model. The main investigation includes the dependences of the heat capacity at constant volume, the heat capacity at constant pressure, and the thermal expansion coefficient on temperature and pressure.In chapter 7, we summarized all of the main results and proposed the prospects for future research on the boron-carbon compounds.