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Microstructures of Synthesized Catalyst and Thermodynamics Analysis of Cubic Boron Nitride

Author: YangHongMei
Tutor: XuBin
School: Shandong Institute of Architecture
Course: Materials Science
Keywords: cBN HP-HT Li-based catalyst morphology structure of catalyst thermodynamics Gibbs free energy growing velocity
CLC: O782
Type: Master's thesis
Year: 2013
Downloads: 28
Quote: 0
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Abstract

Although synthesis technology of cBN has been made great progress after years of exploration, systematical study on the synthesis mechanism has been rarely reported. Because the experiment was carried out in a sealed cavity at high pressure and high temperature (HP-HT), it is difficult to detect on-line at present and the formation information of cBN is not able to be obtained. But much information which can be used to research on transition and growth mechanism of cBN at HP-HT could be remained at room temperature and ambient pressure when the cBN cake was cooled rapidly after the synthetic process. We could get some clues for clarifying the transition and growth mechanisms of cBN through the characterization of morphology, structure as well as thermodynamical analysis.cBN single crystals were synthesized by static high pressure and high temperature method, with hBN and L13N as raw materials. The morphology of fracture surface, cBN singles crystals surface, and the organization of hBN before and after reaction were observed by means of scanning electron microscope(SEM), atomic force microscope (AFM) to investigate on the environment where cBN singles transited and grew. The layered structures which distribute near cBN crystals was detected by X-ray diffraction (XRD). And the possibility of several reactions at HP-HT was analyzed by the determinant method of△G<0with second law of thermodynamics in the process of synthesizing cBN. The temperature and pressure to get high-quality cBN single crystals were discussed by analysing the critical radius of cBN and growth of cBN.Many small particles were found on the surface of cBN crystals by observation of SEM. The morphology of different distances to cBN crystal face was different. There were some pots and branch-like materials formed at HP-HT which was close to the interface of cBN and catalyst, and also some plate-like materials from a distance to cBN crystal face. hBN was found in the plate-like materials by analysis of the morphology of hBN before and after reactions. Some steps were found in the surface of cBN crystals, and particle clusters which approximately melted into steps and pits on the surface of naked large particle cBN were observed by means of AFM.The layered characterization by XRD discovered that each layer had the same four structures:hBN, cBN, catalyst a-Li3BN2, Li2and impurity phase Li2CO3. The absence of raw catalyst Li3N indicated that all the Li3N had been transformed into α-Li3BN2through co-melting reaction with hBN at HP-HT. hBN was the main phase and the content did not influence the transition of cBN. The microstructural cBN was found in maldistribution, and there were most cBN in the middle layer and the least cBN in the interface layer. However, the content change tendency of Li3BN2was different from that of the cBN particles, the content of Li3BN2gradually reduced from outer layer to interface layer. The distribution of cBN and Li3BN2showed that only proper proportion of hBN and Li3BN2could produce most cBN. The Li2O may be produced in the process of experiment, and the Li2CO3was produced in sample preparation.The possibility of reactions occured at HP-HT was analyzed by the determinant method of△G<0with second law of classical thermodynamics in the process of synthesizing cubic BN.The Gibbs free energy differentials of reactions Li3N+hBN→Li3BN2、hBN→cBN and Li3BN2→Li3N+cBN in the Li3-hBN system, were calculated taking the change of volume with temperature and pressure into account, under the synthetic condition of cubic BN. From the results of calculation we could get that it was very easy for the reaction Li3N+hBN→Li3BN2to occur, Li3BN2was produced in the process of heating and pressing. It was possible for reactionas hBN→cBN and Li3BN2→Li3N+cBN to happen, however the possibility of the latter was smaller than that of the former. It was probable for hBN to transit to cBN in the process of cBN synthesising at HP-HT from the viewpoint of thermodynamics.The influence of temperature and pressure to the critical radius and critical energy were analyzed in the perspective of heterogeneous nucleation and the linear velocity of cBN growth was calculated. It could be found that the influence of temperature and pressure to critical radius was obvious:and the smaller the pressure, the greater the nucleation radius radius, the nucleation radius increased with the temperature at low pressure. Critical energy had the same tendency with the critical radius. However, the velocity was different. The velocity increased with the temperature to a certain value and then decreased at different pressures, and the velocity at5.5GPa was bigger than others. There were many small crystal nucleus at5.5GPa and it was easier to form cBN nucleus. The melt flow rate was bigger at high pressure which was beneficial to the growth of cBN nucleus. It could be found that the cBN grain size was bigger near the hBN-cBN equilibrium line, and it was suitable to choose medium pressure and high temperature conditions to get large particles of cBN.

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