The Surface Energy Calculation And Morphology Characterization In CBN Single Crystal Synthesis System

Cubic boron nitride,as a superhard material,has many interesting properties,such as thermal conductivity,wide band gap,high hardness only second to diamond and excellent chemical stability.It has broad prospects of application in the fields of mechanics,optics,electricity and so on.However,it is still difficult to obtain cBN single crystal with large size,in addition to the influence of the synthesis equipment,the lack of theoretical guidance of transformation mechanism of cBN single crystal may be the main factor for restricting the acquisition of cBN single crystal with high quality.In this paper,based on first principle density functional theory and molecular dynamics,the surface energy of the low index facts in hBN and cBN was calculated under high pressure and high temperature,to investagate the transformation mechanism of cBN single crystal.Meanwhile,the relevant structures were characterized by scanning electron microscopy（SEM）and atomic force microscopy（AFM）.Because the lattice constants may change with the change of temperature and pressure,in to improve the accuracy of the surface energy calculation,the lattice constants of hBN and cBN were calculated under 1800 K,5.5GPa and 2000 K,6.0GPa repectively using PAW_GGA and USPP_LDA methods.Calculated results show that the lattice constants of hBN are the value of a =0.244760 nm,c=0.643217 nm and the lattice constants of cBN single crystal are the values of a=0.358328 nm under 1800 K,5.5GPa,respectively.The lattice constants of h BN are the values of a=0.244174 nm,c=0.639819 nm and the lattice constants of cBN single crystal are of a=0.358292 nm under 2000 K,6.0GPa,respectively.First,the models of the crystal facts（110）,（100）and（001）in hBN and the crystal facets（110）,（100）and（111）in cBN were built under 1800 K,5.5GPa and 2000 K,6GPa by MS software,respectively.Then,the surface energy of hBN and c BN was calculated by VASP software based on the first principle theory.As a result,the surface energy of hBN（110）,（100）,（001）facets and cBN（110）,（100）,（111）facets using the GGA is 0.200 eV,0.427 eV,0.336 eV and 0.220eV、0.430eV、0.410 eV under 1800 K and 5.5GPa,respectively.And the surface energy value of the hBN and cBN facets is 0.221eV、0.444eV、0.354 eV and 0.250 eV、0.436 eV、0.475 eV under 2000 K and 6.0GPa,respectively.The calculation results show that the surface energy value of the facet hBN（100）is similar to the value of the facet（100）in cBN under 1800 K,5.5GPa and 2000 K,6.0GPa,which relative difference is only 0.7% and 1.2%,respectively.The Hohenberg-Kohn theory indicates that ground state energy can be received after system energy minimization using ground state density as a variable,and there is a one-to-one correspondence between the energy and the electron density.Therefore,it is believed that the consecutiveness of surface energy is consistent with the consecutiveness of electron density.The TFDC theory indicates that the boundary condition between the atoms of the material must satisfy the continuity of the electron density（electron density difference <10%）,which can induce the phase transition between different phases.Because the relationship of plane h BN（100）and plane cBN（100）satisfies the TFDC theory,direct transition from hBN into cBN can be induced.This indicates that cBN is most likely to be transformed from hBN directly under high pressure and high tempareture from the perspective of covalent electronic structure.Using Li₃N（purity 99.5%）as catalyst and hBN（purity 99.9%）as raw materials,and the the weight ratio of hBN and Li3 N was set as 9:1.The cBN singlecrystals were synthesized under high pressure and high tempareture.Generally,the synthesis pressure of cBN single crystal ranges from 5.0GPa to 5.5GPa,and the synthesis tempareture of cBN single crystal ranges from 1600 K to 1800 K.In this work,the morphology of typical facets in cBN was characterized through JSM-6380 LA type scanning electron microscopy（SEM）.It is found that the crystal shape is very regular and the crystal facets with much less pits and other defects are pretty smooth.And the typical facets in cBN are the triangular（111）and hexagonal（110）facets.According to the results of the first principle calculation,it is found that the theoretical analysis is consistent with the test results.Furthermore,the above conclusion indicates that the theoretical results are reliable.Then,the morphology of cBN（111）crystal surface was characterized by AFM,it is found that growth steps are formed on the crystal planes（111）of cBN and a large number of c BN growth units gather here,which has a direct relationship with higher energy.The results show that the cBN single crystals growth along the vertical direction of（111）crystal plane.According to the analysis of the above theoretical calculation and the characterization of the relecant structure,the cBN single crystal is transformed directly from hBN,the results of this paper support the theory of direct transformation of solid phase.