Theoretical Study On The Phase Transition Mechanism Of CBN Single Crystal At High Temperature And High Pressure

Cubic boron nitride?cBN?single crystal has excellent properties and is a very valuable superhard material,which is widely used in industry,aviation,military and other fields.On the one hand,because of its high hardness,high wear resistance,excellent thermal stability and chemical inertia,it can be used as a super abrasive to manufacture tools and abrasives,on the other hand,it has excellent infrared transmission and wide band gap,and can also be used as a potential functional material to manufacture high temperature semiconductors.Industrial production of cBN single crystals usually uses high temperature and high pressure catalyst method.The initial composition of the synthesis system is generally hexagonal boron nitride?hBN?₊catalyst?alkali metal,alkali earth metal,nitride,boron-nitrogen compound,ammonium salt,etc.?.This method needs the pressure range is approximately 2.5⁷.5 Gpa and and temperature range is approximately 1200²000?.It is generally believed that the molten catalyst plays the role of boron and nitrogen fluxes in the synthesis system according to literatures.In the thermodynamic stable region of cBN,saturated cBN separation by crystallization in the meantime unsaturated hBN dissolve into catalyst.The hBN structure is directly transformed into cBN structure during the process.Others think that wurtzite boron nitride?wBN?is the intermediate phase in the phase transformation process from hBN to cBN,and the phase transformation process may be hBN?wBN?cBN.From the view of valence electron structure and phase composition of the catalytic layer,the possibility of the above two phase transformation processes was analyzed,and the possible high temperature and high pressure phase transformation mechanism in the hBN-Li₃N synthesis system was speculated.When analyzing the phase transition mechanism of cBN,the role of catalyst during the synthesis process should also be fully analyzed.When valence electron structures are constructed by the empirical electron theory of solids and molecules?EET theory?,the precondition is that the structures of molecules or crystals are known?atomic space arrangement?.Therefore,the lattice constants of crystals at high temperature and high pressure should be calculated in the first place when analyzing the phase transition mechanism of cBN single crystals at high temperature and high pressure.The lattice constants vary with temperature and pressure.It is almost impossible to measure the lattice constants at 5 GPa and 1800 K.In this paper,the lattice constants of various phases at high temperature and pressure are simulated by the computer simulation software VASP.The valence electron structures of hBN,cBN and wBN constructed show that there may be three atomic configurations in hBN,five atomic configurations in cBN and three atomic configurations in wBN at pressure is 5 GPa and temperature is 1800 K.In each BN,however,the number of covalent electrons on each bond does not change significantly regardless of which hybrid order of B and N atoms are in.According to the criterion of continuity of electron density on crystal plane of improved Thomas-fermi-drac theory?TFDC?,the possibility of phase transformation is inferred.There are two groups of crystal plane electron density differences between wBN and cBN less than10%,while the difference between hBN and wBN is more than 10%.Under the first order approximation,the electronic density of wBN/cBN crystal plane is continuous,however,hBN/wBN crystal plane is discontinuous.Therefore,the indirect phase transition process of hBN?wBN?cBN may not exist.There are two groups of crystal plane electron density difference between hBN and cBN crystals is less than 10%.According to the criterion of continuity of electron density at TFDC,it is considered that the electron density of the two groups of crystal planes remains continuous under the first order approximation,so there may be a direct phase transition from hBN to cBN.The outer electronic state of B atom in hBN crystal structure is sp²+2p_z⁰,and that of N atom is sp²+2p_z².The outer electronic state of B and N atom in cBN crystal structure both are sp³.It can be seen that the phase transition of hBN?cBN at high temperature and high pressure is essentially the change of crystal structure caused by the transfer of an electron from N atom to B atom,and direct electron transfer is very difficult to promote this electron transfer.The conditions of displacement are temperature,pressure and catalyst.The actual phase transformation process is a complex instantaneous process.Whether direct phase transformation or indirect phase transformation of mesophase?wBN?occurs needs further verification.Current equipment and technology level are difficult to detect the phase transformation process of cBN single crystal at high temperature on-line.The"quenched"synthesizer will retain some high temperature and high pressure information to a large extent.In this paper,the morphology and composition of the phase in the catalytic layer of the quenched block were characterized by SEM,XRD,FTIR and HRTEM.The phase transformation mechanism of cBN single crystal in the hBN-Li₃N synthesis system at high temperature and high pressure was analyzed to support the previous calculation results.The fracture surfaces of cBN single crystal and synthesized block were analyzed by SEM.The cBN single crystal was wrapped by melt catalyst layer and tended to grow in the direction of hBN.The catalyst layer on the outside of the single crystal of cBN is discontinuous,loose and flaky.The catalytic phase in the catalyst layer not only catalyzes the reaction,but also acts as a high temperature flux in the phase transformation reaction of cBN,which promotes the crystallization of cBN structure from the high temperature melting catalytic phase.XRD,FTIR and HRTEM characterization of the catalyst layer showed that there were cBN,hBN and Li₃BN₂phases in the catalyst layer,but no wBN phase was detected.All the characterization evidence supported the direct phase transition from hBN to cBN,but no evidence supported the indirect phase transition of hBN?wBN?cBN.The results of catalytic layer characterization are consistent with the previous calculation results,which all support the direct solid phase transition process from hBN to cBN.Li₃BN₂ produced by the co-melting reaction of Li3N and hBN acts as a real phase change catalyst.Li₃BN₂ acts as both catalyst,flux and assistant electron transfer in the phase transition process.The electron transfer process is that Li₃BN₂ releases one electron to B atom first,and then N atom releases one electron to Li₃BN₂,finally formed sp³hybrid cBN crystal structure.