Density Of Functional Theory Study Of Structural Stability And High-pressure Elasticity Of Superhard Materials BC₂N And C₈B₂N₂

Superhard materials which Vickers hardness H_v > 40 GPa have a wide range of uses in military and industrial field,and it has high pressure resistance,abrasion resistance,stable chemical properties and other properties.It has been used as a raw material for cutting,grinding,polishing,and other machining tools and as an anti-wear coating material on mechanical tools.At the same time,superhard materials often have good thermal conductivity,electrical properties,and optical properties,and can be applied to special work environments.The synthesis of superhard materials with excellent properties has always been one of the most important areas of high-pressure science and technology.Diamond is the hardest material found in nature at present,but it is brittleness and easily reacts with iron-containing alloys at high temperatures,which limits its application.Cubic boron nitride?c-BN?is second only to diamond in hardness and has better stability than diamond,but its hardness is much smaller than diamond and it is difficult to synthesize large blocks.Therefore,in order to meet the industrial needs,it has become a research hotspot to explore new superhard materials that can replace c-BN and diamond.Considering that c-BN and diamond have many similar properties,such as high melting point,large bulk modulus,high specific heat,and similar thermal expansion coefficient,B-C-N series materials have become one of the research hotspots for superhard materials.In this academic dissertation,one superhard compound BC₂N with space group I4/mmm and two superhard compounds C₈B₂N₂ with space group P3m1 are designed.And their structural stability,electronic and elastic properties were analyzed.At first,based on the body-centered-tetragonal carbon structure model which have the excellent mechanical properties,a potential superhard compound of BC₂N with tetragonal symmetric structure and I4/mmm space group is constructed.The first-principles calculations based on density functional theory is used to systematically study the structural stability,elasticity,and electronic properties of the tetragonal phase BC₂N.The calculated results show that I4/mmm-BC₂N is mechanically stable at least in the pressure range of 0-100 GPa.The energy of this structure is higher than the most stable wurtzite structure of BC₂N compounds,lower than chalcopyrite structure,indicating that the new constructed tetragonal structure is a metastable structure of BC₂N compounds.For this structure,its incompressibility is greater than that of the similar materials B₂ CN and BC4 N.The elastic moduli,Poisson's ratio,Debye temperature,minimum thermal conductivity,and elastic wave velocity of the I4/mmm-BC₂N compound all appear different increase tend with increasing pressure.It is brittle,and its elastic anisotropy becomes more pronounced under high pressure.At the same time,the large bulk modulus,shear modulus,and theoretically calculated Vickers hardness indicate that I4/mmm-BC₂N is a potential superhard material.Secondly,based on hexagonal phase superhard material supercell of P3m1-BCN,two hexagonal phase structures of C₈B₂N₂ were predicted,named them C₈B₂N₂-1 and C₈B₂N₂-2,and they were proved to be superhard materials through a first-principles calculation method based on density functional theory.At the same time,the structural,electronic,elastic and mechanical properties of C₈B₂N₂-1 and C₈B₂N₂-2 were analyzed and tested.The results show that C₈B₂N₂-1 and C₈B₂N₂-2 are mechanically stable at least under pressure of 100 GPa;the band structure shows that C₈B₂N₂-1 and C₈B₂N₂-2 are direct wide bandgap semiconductors;C₈B₂N₂-1 and C₈B₂N₂-2 are both relatively large The modulus of elasticity increases with increasing pressure.The bulk modulus B tends to increase linearly and the shear modulus G grows at a slower rate.Both C₈B₂N₂-1 and C₈B₂N₂-2 show elastic anisotropy and brittleness.The Debye temperatures of both materials increase with the increase of pressure.The greater the pressure,the smaller the increase rate is.The elastic wave velocity increases with the pressure and the shear wave velocity and the average wave velocity are not very obvious.The calculation results in C₈B₂N₂.The Vickers hardness values of C₈B₂N₂-1 and C₈B₂N₂-2 ranged from 82 to 83 GPa,which was greater than that of t-C₈B₂N₂,P3m1-BCN and c-BN.The above demonstrates that C₈B₂N₂ compounds of these two structures are potential superhard materials.This research work can provide a wider range of alternative structural information for the synthesis and application of B-C-N superhard materials.