First-principles Study On The Structural And Physical Properties Of Typical Transition Metal Borides Under High Pressure

Superhard materials are functional materials with more advantages,such as high hardness,high melting point,better heat resistance and thermal stability,which make them widely used in industry,national defense,microelectronics,aerospace and other fields,and have become the object of focus for many researchers.With the rapid development of science and technology and the progress of society,the requirements for superhard materials is getting higher and higher,so the search for new,high hardness multifunctional materials has become a hot area of scientific research and a pressing problem to be solved at present.In recent years,scientists have found that the combination of transition metal elements and light elements(B,C,N,O)can form new superhard materials that circumvent the limitations of traditional superhard materials,especially transition metal borides are not only high hardness,but also corrosion resistance and high temperature resistance,which are powerful candidates to replace traditional superhard materials.Therefore,in this thesis,the structure and physical properties of several typical transition metal borides under high pressure are investigated by first-principles calculations based on density generalized theory,hoping to provide some theoretical references for the application and research of superhard materials.The main research contents and innovations are as follows:(1)The structural,mechanical,and electronic properties of TMB₂(TM=Y,Sc,Ti)from 0 to 100 GPa were calculated and analyzed.The results show that TMB₂(TM=Y,Sc,Ti)has structural stability,mechanical stability,brittleness and metallicity in the studied pressure range.In addition,it was found that both Ti B₂and Sc B₂ are superhard material,while YB₂ is a hard material.The order of hardness of TMB₂(TM=Y,Sc,Ti)at zero pressure from high to low is Ti B₂>Sc B₂>YB₂,which indicates that Ti B₂ has superior mechanical properties than Sc B₂ and YB₂ at 0 GPa.With the increase of pressure,the Sc B₂ material gradually becomes softer,and when the pressure increases to 80 GPa,the Sc B₂ changes from superhard material to hard material.Bond calculations show that the bond length of TMB₂(TM=Y,Sc,Ti)decreases monotonically with increasing pressure,and the TM-B bond is more sensitive to pressure.Under the same pressure,the relationship between the bond lengths of TM-B and B-B is YB₂>Sc B₂>Ti B₂,indicating that the shorter the bond length,the stronger the bond interactions,which may be the reason for the strongest compression resistance of Ti B₂.(2)The structural,mechanical,and electronic properties of VB_x(x=1,2,3,4)from 0 to 40 GPa were calculated and analyzed.The results show that VB_x(x=1,2,3,4)is structurally stable,mechanically stable,brittle,and metallic in the studied pressure range.Based on the hardness calculations,it is concluded that VB has better mechanical properties than VB₂,VB₃ and VB₄,and the order of Vickers hardness of VB_x(x=1,2,3,4)is from high to low:VB>VB₂>VB₃>VB₄,i.e.,Vickers hardness decreases with increasing boron content.In addition,VB belongs superhard materials,VB₃ and VB₄ belong to hard materials.Interestingly,when the pressure increases to 15 GPa,VB₂ changes from hard material to superhard material.The calculated B/G increases in the order of VB234,and VB₄ is more ductile than VB,VB₂ and VB₃ under pressure.The Poisson's ratio values for all the VB_x(x=1,2,3,4)compounds are lower,indicating that VB_x(x=1,2,3,4)has a strong bond orientation bonding ability,which contributes to the hardness of the material.