Related Theoretical Calculations Based On Study Of Synthesis Of Cubic Boron Nitride Single Crystals

At present, the mechanical properties of cubic boron nitride (cBN) single crystal are mainly used in the industry, and research for semiconductor in high temperature and optical performance has just started, which the main reason is that large cBN crystals that research needs performance are difficult to obtain. The phase transition mechanism of cBN single crystal which has been research several decades is still inconclusive, leading the lack of theoretical guidance for synthesis of large single crystals. In this paper, by using thermodynamic theory, the first principle theory, EET theory and TFDC theory the phase transition mechanism of cBN single crystal is studied. Thermodynamics theory has been widely used to study phase transition mechanism in the field of materials research, but the main reason why it is not applied to the study of phase transition mechanism of cBN single crystal is the difficulty to obtain the thermodynamic parameters and unclear phase changing process. By the Vegard law the thermodynamic parameters of relevant phases, and the possible reaction in the process of cBN single crystal are discussed to analyze the phase transition mechanism of cBN single crystal from the thermodynamic angle. With the lattice constant of cBN single crystal under high pressure and high temperature(HPHT) changing, the VASP software based on density functional theory is used to calculate the lattice constant of the cBN single crystal of temperature and pressure in the synthesis for subsequent EET theoretical analysis. By using EET theory and TFDC theory, mutual transitional continuous conditions of heterogeneous interface between hexagonal boron nitride (hBN) and cBN single crystal are discussed for further study on the phase transition mechanism of cBN single crystal.In this paper, with lithium nitride (Li3N) as catalyst and hBN as experimental materials, cBN single crystal synthesized by static high pressure and high temperature method is studied by means of thermodynamics. Based on the first classic law of thermodynamics, considering the influence of temperature and pressure on each volume of reactant phase, the Gibbs free energy is calculated in the process of synthesis under HPHT to the analyze the possibility of each reaction. The Gibbs free energy changes of three possible reactions hBN+LiaNâ†'Li3BN2, hBNâ†'cBN and Li3BN2â†'Li3N+cBN are calculated in the lithium-based catalyst system under HPHT. It is found that free energy changes of two reaction hBNâ†'cBN, hBN+Li3Nâ†'Li3BN2 is negative, which indicates that the reactions are easy to occur, and a V shape area of pressure and temperature forms in which Li3BN2 resolves into Li3N and hBN. The area of Li3BN2 stable existence almost covers the V-shape area of pressure and temperature in which quality cBN single crystal can be obtained in the synthetic process, which means that cBN single crystal is likely transformed directly by hBN from the angle of thermodynamics.The lattice constant can be affected by temperature and pressure. In order to study the phase transition mechanism of cBN single crystal by means of the EET theory, cBN single crystal lattice constant is calculated by using USPP_LDA and PPW_GGA method, and the influence of pressure and temperature on the lattice constant is discussed. The calculation shows lattice constants cBN and hBN varies with temperature rising, along with the rapid drop in the pressure increases linearly. At the same time, the conditions of EET theory and TFDC theory analysis are chosen with temperature at 1800K and the pressure at 5.0GPa during synthesis. Under the experimental conditions, the lattice constant of cBN single crystal is 3.5883A, and the a axis lattice constant is 2.4766A, the c axis constant is 6.5106A in hBN.The EET theory and the TFDC theory are used to analyze the phase transition mechanism of cBN single crystal in this paper. By means of key distance difference method, it is found in cBN single crystal temperature and pressure (1800 K,5.0 GPa), the structures of hBN and cBN exist possible atomic configuration. In this paper, we calculate the electron density of major low index crystals in hBN and cBN, and the difference of electron density between the main crystals. The results show two groups of main crystals hBN(100)/cBN(100), hBN(110)/cBN(110) meet boundary conditions electron density between adjacent crystal need continuous present by the TFDC theory. Therefore, cBN single crystal is transformed directly by hBN from the angle of valence electron structure.Based on the above theoretical analysis, cBN single crystal is very likely be transformed directly by hBN from the angle of thermodynamics and valence electron theory. In Li3N-hBN system, Li3BN2 is the real catalyst in the synthetic process of cBN single crystal, and it can transfer electrons from hBN to cBN as a bridge,catalyzing the phase transition from hBN to cBN single crystal.