First-Principles Investigations On The Structures And Properties Of Novel Super-hard C/N Materials

Search on structure prediction and mechanical properties of super-hard carbon and carbon nitrides is a focus in the investigations of super-hard materials with light element. In this thesis, the electronic structures and the mechanical properties of elasticity, ideal strength and theoretical hardness of several novel kinds of C/N super-hard material were calculated using VASP software based on density functional theory（DFT） based on the reviews of the developments and problems existing in the researches on light element super-hard materials, and the basic principles of first-principles calculations and related calculation methods,Furthermore, It was studied that the relationship between microstructure and macroscopic properties of the materials above.First of all, the mechanical properties of the diamond-like carbon super-hard materials which proposed recently were studied systematically. Results show that P-carbon is not only more stable than others in thermodynamics, but also the most hard C-C covalent bond was found in P-carbon, its hardness value is as high as114 GPa witch significantly higher than Z-carbon, oC32-carbon, S-carbon, X-carbon,F-carbon, M585-carbon and other novel super-hard carbon phase. Its unique mechanical properties are expected to show extraordinary application prospects in the specific area.At the same time, the electronic structure, structural stability and mechanical properties such as elastic properties,ideal tensile strength, ideal shear strength and theoretical hardness of a novel carbon mononitride super-hard material — simple orthogonal structure of Pnnm-CN were studied in details by the first-principles.The results show that the ideal strength of Pnnm-CN reaches 41.0GPa, witch higher than that of P42/m-CN with 40 GPa, and the hardness value of Pnnm-CN is64 GPa higher than that of cg-CN and P42/m-CN significantly. It was shown that Pnnm-CN is a novel kind of carbon mononitride super-hard material with superior mechanical properties in potential applications.Finally, two kinds of three-dimensional covalent structures of primitive-centered cubic Pm3n-C₃N₄ and P43m-C₃N₄ were predicted, and then the electronic structures, structural stability and the mechanical properties such as elastic properties, ideal strength and theoretical hardness were studied by first-principles. The results show that Pm3n-C₃N₄ is a direct-bandgap semiconductor material with a very narrow bandgap of 0.06 eV, and the hardness value is 54 GPa, which is a kind of potential multi functional C₃N₄ material.