First-principles Study Of Si_3-xC_xN₄ Hardness Materials

Superhard materials, which are defined by a micro-hardness exceeding 40 GPa, have attracted extensive interest for decades. They are composed of group III, IV and/or V (carbides and nitrides) and elemental crystal (diamond). Except for diamond, all these materials can only be synthesized by artificial methods. Other interesting properties of these materials include their wide band gap, stability under high temperature and chemical inertness. Current research on these materials concentrates on diamond, cubic boron nitride (c-BN), carbon nitride (C₃N₄, CNx), boron carbonitride (BCN) and diamond-like carbon (DCL).In this thesis, we investigate the ground state electronic structure and elastic properties of spinel-Si_3-xC_xN₄ system using pseudopotential plane-wave method based on the density functional theory. The main content is as follows:Firstly, using pseudopotential plane-wave method with the local density approximation and general gradient approximation, we have optimized the unit of the system of spinel- Si_3-xC_xN₄ to obtain the most stable state with the lowest energy and the lattice parameters.Secondly, on the base of the optimation of geometric structures, we have calculated the total energy, elastic constants, bulk modulus, band structure and density of states. Then the stability and electronic structure have been analyzed.Finally, we calculated the hardness of the three compounds using different method.Our calculation results implicate a very important fact that three conditions should be met for a superhard material: higher bond density or electronic density, shorter bond length, and greater degree of covalent bonding. The link between hardness and the theoretical calculation is farther confirmed, and thus can play an important role in the design of new superhard materials.