Microstructures Of Synthesized Catalyst And Synthesis Mechanism Of Cubic Boron Nitride

Although synthesis technology of cBN has made great progress after years exploration, systematical study on mechanism has been limited. Difficulty of on-line detection on closed cavity under HP-HT at the present is the reason. But much information which can be used to research on transition and growth mechanism of cBN at HP-HT can be remained at room temperature and ambient pressure when the cBN powder cake is cooled rapidly after the synthetic process finished. Much information on transition and growth of cBN single crystal will be remained in certain range of its surface. The Characterization of morphology, structure of cBN will provide clues for clarifying the transition and growth mechanism of cBN.cBN single crystals were synthesized using hBN as raw material and powder Li3N as catalyst by static high pressure method. The morphology of fracture, cBN single crystals and material on theris surface was observaed by means of Field Emission Scanning Electron Microscopy (SEM), Atomic Force Microscope (AFM) and investigate on the environment whrer cBN single crystals transit and gorow. The layered structures which distribute near cBN crystals and the interface fine structures were characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), High resolution transmission electron microscopy (HRTEM) and analyse the growth process of cBN single crystals through the changes law of physical structures.Meanwhile inorder to study on the the change law of electronic structures, the electronic structures of layered components on the surface of cBN single crystals were characterized by X-ray Photoelectron Spectroscopy (XPS) and by the analysis of the change law of sp2-sp3in B, N atomic the transition and growth mechanism of cBN single crystals were analysed. The observation by SEM of materials on the surface of cBN single crystals find that cBN are wrapped by a layer of uneven,meteld and flake film-like structures. The warappage on the surface of cBN are constituted of loose material and flake textures which were formed when melting material under high temperature were quickly cooled. Therefore, we can conclude that the cBN single crystals are always wrapped by melting material with the procession of grpwth of cBN. Little particles(particle clusters) which approximately melt into steps and pits on the surface of naked large particle cBN by means of SEM and AFM.The layered characterization by XRD discover that each layer has the same four structures:hBN, cBN, catalyst a-Li3BN2, impurity phase Li2CO3but the absent of raw catalyst Li3N indicate all the Li3N is transformed into a-Li3BN2through co-melting reaction with hBN under HP-HT.It is found that the distribution of little cBN particles and are different, the region of maximum content of little cBN particles only contains proper catalyst a-Li3BN2by comparison of X-ray diffraction peak intensity of different structure. The rule of distribution of different structure shows that the melting hBN catalyzed by catalyst α-Li3BN2are transformed into melting cBN with short rage order which gradually turn into little cBN particles and these little cBN particles will collide, aggregation and gradually form large cBN particles.The fine Characterization of interface layer is basically identical with the result of XRD: there are only hBN, cBN and catalyst α-Li3BN2in the interface layer. Meanwhile, the structure that contacts with little cBN particles is found to be α-Li3BN2by HRTEM which imply little cBN particles are precipitated from α-Li3BN2but not directly transformed by hBN.In order to explore the rule of changes of electronic structures as changes of structures on the surface of lager cBN particles, pure hBN and layerd material on the surface of larger cBN particles were Characterized by XPS with the same samples of XRD. It is found that: valence state peak of B, N of outer layer are almostly same standard sample, so The majority material which contain B, N is hBN in outer layer. There is0.2eV chemical shift of binding energy of B (Is), N (Is) between interface layer and pure hBN, we think the reason as follow: catalyst α-Li3BN2catalysis the transformation of cBN by the way of respectively releasing and absorption of an electron to B and N atom. When the cake was quickly cooled, part of was "freezed" to the room temperature and the change of environment of B and N atom caused the binding energy shift.It is found that the relative content of hBN gradually is decreased from outer layer to interface through the observation of the intensity of π plasmon loss peak of B (Is) and N (1s) but cBN is gradually increased according to the rule of the distribution of catalyst Li3BN2. Meanwhile, the characteristic distance (D value) of differential peak of N KLL of standard sample, outer layer, interlayer and interface is19eV,18eV,15eV and13eV respectively, so the the relative content of sp3hybrid structures,that is cBN, are increased from outer layer to interface. Outer layer, interlayer and interface respectively contains α-Li3BN2:7%,18%and14%according to the semi-quantitative synthesis of the ratio of N/B in each layer. So we can infer that only in the region of enough hBN can be effectively transformed into cBN and the growth of big cBN particles are completed in the region where cBN have a high relative concentration.Above results comes to conclusion that lots of hBN catalyzed by the way that catalyst Li3BN2respectively releasing and absorption of an electron to B and N atom are transformed into little cBN particles which will aggregate to large cBN particles and then large cBN particles grow up by consumption of little cBN particles nearby. Finally, concentration gradient favorable to the grow of large cBN particles will be formed as they grow up.