First-principles Study Of Boron-titanium Compounds

Titanium borides?Ti₂B,TiB and TiB₂?has the advantages of high melting point,high hardness,high thermal conductivity,low resistivity,low power function and excellent conductivity,etc.,which has become a research hotspot in the new generation of multi-functional materials.In addition to this potential application,titanium borides compounds can be used as interconnection materials in integrated circuits.It can also act as a diffusion barrier to prevent the interaction between the element and the substrate.This dissertation systematically studies the crystal structure,stability,electrical properties,optical properties,mechanical properties and thermodynamic properties of boron-titanium compounds based on first principles:First of all,based on the first principles,the crystal structure,band structure and density of states of titanium borides compounds were studied.The lattice constant calculated by the generalized gradient approximation agrees well with the experimental values,which ensures the reliability of the calculation results.The energy band structure shows that there are many bands overlaps at the Fermi level and there is no significant band gap,indicating the presence of metallic properties.By analyzing the density of states,the free electrons of the Ti-3d states are the main reason for the metallicity,and the pseudogap is a general feature around Fermi level in the DOS spectrum of titanium borides.Analyzing the partial density of states,we know that the Ti-3d and B-2p states hybridization is the main reason for the pseudogap.With the increase of boron component,the density of B-2s and B-2p states in the valence band top region move to the high energy region and the low energy region,respectively.The Ti-3d states in the bottom region of the conduction band moves to the low energy region and become a major contribution to the total density of states of titanium borides.Secondly,the optical properties of titanium borides were studied by first principles.Titanium borides has a large static dielectric constant?0?,indicating that they are potential dielectric materials.In the high energy region?30 eV to 40 eV?,the dielectric constant oscillates.As the boron component increases,the light absorption coefficient of titanium borides increases and shifts to a higher energy region?blue shift?,which has potential as a photoelectric material.The high reflectivity in visible indicates that it is a promising coating material to avoid solar heating.In the visible and high energy areas,the conductivity is relatively high,which is a good material for photoconductive devices.At last,the mechanical properties and thermodynamic properties of titanium borides compounds were analyzed by the first principle.For the orthorhombic?FeB-type and CrB-type?TiB,it has higher shear modulus and smaller anisotropy in the x,y,and z directions,and the FeB-type TiB has strong chemical bonds in the[100]direction and largest anisotropy in y direction.For the cubic?NaCl-type?TiB,it has the smallest hardness and high bulk modulus in the x direction due to its high atomic coordination number.For the hexagonal TiB₂,it has a strong elastic anisotropy,and its elastic modulus has a strong direction dependence.Because the Ti-B bond is weaker than the B-B bond,it may be the main reason for the elastic anisotropy of TiB₂.The orthorhombic TiB exhibits brittle and the cubic TiB exhibits ductile.TiB₂ has the largest shear modulus and hardness among titanium borides and Young's modulus has the largest difference in all directions.For the titanium borides we studied,the elastic modulus can be reduced by reducing the boron content in the crystals.TiB₂ has a small density and maximum elastic modulus,so it has the maximum elastic sound velocity and Debye temperature.Since the cubic TiB has the smallest elastic constant,it has the smallest transverse acoustic velocity,longitudinal wave velocity,average acoustic velocity,and Debye temperature.Therefore,TiB₂ has the highest thermal conductivity and cubic TiB is the weakest.