Research On Phase Transition Mechanism Of Cubic Boron Nitride Single Crystal/Catalyst Layer At High Temperature And High Pressure

At present,cubic boron nitride(c-BN)single crystals in industry are synthesized with the static high temperature and high pressure(HPHT)catalyst method.It is significant to clarify phase transition for synthesizing c-BN single crystals with high quality.The nucleation and growth information of c-BN single crystals at HPHT are retained to the greatest extent by means of quenching.It is found that c-BN single crystals were surrounded by white powders.The powders in the range of 30～0 μm from c-BN single crystal are defined as catalyst layer.The powders in the range of 30～20,20～10,10～0μpm are defined as the outer,middle and inner layers,respectively.In this paper,the c-BN samples were synthesized by using h-BN as the raw material,Li3N as catalyst under HPHT.The calculation is selected in Li3N-BN system.The phase structure of c-BN/catalyst interface was characterized by X-ray diffraction and high resolution transmission electron microscopy.Auger electron spectroscopy and electron energy loss spectroscopy were used to qualitatively and quantitatively analyze the variation of B and N electronic structures.Empirical electron theory of solid and molecule was used to calculate the possibility of phase transition between h-BN/c-BN and Li3BN2/c-BN at HPHT.The phase transition coexistence points of h-BN/c-BN and phase transition points of Li3BN2 were calculated by first-principles theory.The surface morphology and defects of c-BN single crystal were analyzed by scanning electron microscopy and atomic force microscopy.In this paper,the phase transformation mechanism of c-BN,the growth process of c-BN,and the catalytic effect of Li3BN2 at HPHT were analyzed with the experiment and theoretical calculation results.It was found that the main phase structures in the catalyst layer were c-BN,h-BN and Li3BN2.Meanwhile,Li3N was not found in the catalyst layer.It is found that different structures exist in catalyst layer,for example:c-BN nanoparticles,highly ordered h-BN,h-BN with various defects,disordered BN structure and c-BN wrapped in disordered BN structure.The Auger peak intensity of sp2 hybrid state of B and N atoms in the catalyst layer decreases gradually from the outer to the inner layer,while the Auger peak intensity of sp’ hybrid state increases gradually.The B-π*characteristic peak in the outer layer has a high intensity,while the intensity of the B-π*characteristic peak decreases sharply in the inner layer.The B-sp’ content from the outer to the inner layer was 10.11%,14.67%and 22.85%respectively.This indicates that the electronic structures of B,N atoms in the catalyst layer transformed from the sp2 hybridization of h-BN to the sp3 hybridization of c-BN.Using the bond distance difference method,it is found that the structure of h-BN,c-BN and Li3BN2 can exist at HPHT.The covalent electron density and bond energy of the strongest bond in Li3BN2 structure are larger than those of h-BN,which indicates that Li3BN2 is more stable than h-BN.The electron density differences of main low-index surfaces between h-BN and c-BN conform to the atomic boundary condition.Which the electron density differences is not agreed with the atomic boundary condition between Li3BN2 and c-BN.It is indicated that c-BN single crystal is directly transformed from h-BN,not the decomposition of Li3BN2.The temperature and pressure of the transition from h-BN to c-BN are lower than the transition from Li3BN2 to c-BN with the first principle.This results are consistent with calculation result with the EET theory..There are many defects on the(111)surface of c-BN single crystal,such as impurity particles,triangular holes,lamellar structure and large step structure.This indicates that there are two-dirmensional nucleation growth and dislocation growth in the growth process of c-BN single crystal.The surface energies of main low index surfaces at HPHT are calculated.and the c-BN(110)surface has the lowest surface energy.The lower the surface energy is,the higher the electron density of the crystal surface is,and the more stable the crystal surface is.The calculated results of the surface energy are consistent with those of the valence electronic structure.The Li3BN2(100)has the highest surface energy,thus it can be used as the base for c-BN nucleation and growth.Based on the experimental and theoretical results,a theoretical model of phase transition from h-BN to c-BN with the catalyst of Li3BN2 was proposed.The(BN2)3-of Li3BN2 intruded into the hexagonal phase and reduced the h-BN to a low degree of polymerization BN cluster at HPHT.The Li+ can attract an electron,and complete the electron transfer between B and N atoms.Then BN atom group of sp3 state were created and form c-BN growth units.Finally,c-BN crystal nucleus are formed by aggregation and collision.After c-BN nucleation,Li3BN2 acts as the growth base to promote the continuous growth of c-BN single crystal.