触媒微结构与立方氮化硼单晶催化机理的相关性研究

发布时间：2018-01-04 01:07

  本文关键词：触媒微结构与立方氮化硼单晶催化机理的相关性研究　出处：《山东大学》2015年博士论文　论文类型：学位论文

  更多相关文章： cBN单晶 触媒微结构 高温高压 催化机理 晶体生长 热力学

【摘要】：立方氮化硼(cBN)具有类金刚石结构,作为一种硬度高、稳定性好的新型晶体材料,其高温半导体特性、高频特性和压电特性不断表现出在现代科学技术发展中的重要作用。目前,采用静态高温高压触媒法合成cBN仍然是工业合成单晶的重要方法,而研究高温高压cBN单晶的催化机理对于指导工业生产优质大单晶具有重要意义。实验发现,在合成后的cBN单晶表面总是覆盖着一层类似于熔融状物质,此物质应为触媒和六方氮化硼(hBN)融合所形成的触媒层。高温高压条件下cBN单晶正是通过触媒层的催化和扩散作用进行形核和长大。研究合成后的cBN单晶触媒层中物相的相互作用对于揭示cBN高温高压催化机理提供了重要的参考依据。本文以hBN为原料,以Li3N为触媒原料进行了高温高压条件下cBN单晶的合成实验。利用扫描电子显微镜(SEM)、原子力显微镜(AFM)、透射电子显微镜(TEM)、高分辨透射电子显微镜(HRTEM)等表征手段确定了cBN单晶触媒层的表面形貌、物相结构等,并在此基础上利用拉曼光谱仪(Raman)、俄歇电子能谱仪(AES)、X射线光电子能谱仪(XPS)和电子能量损失谱仪(EELS)等对触媒层中B、N原子的电子结构变化规律进行了系统分析,探讨高温高压cBN单晶的催化机理。同时,结合热力学理论计算了cBN合成过程中各反应的自由能变化,从而进一步验证了表征实验的结果,为cBN高温高压催化机理的研究提供了理论依据。此外,从生长动力学角度对cBN单晶的形核及生长与合成条件之间的关系进行了探讨。通过对cBN触媒层的分层XRD实验结果可知,在触媒层中主要物相结构为hBN、cBN、Li3BN2,而在各层中均未发现Li3N的存在。利用HRTEM实验在cBN单晶／触媒层界面中发现了大量的cBN纳米级颗粒。同时,利用TEM在cBN单晶／触媒界面中也发现了cBN聚晶颗粒,并且在界面上发现了hBN的存在,从而提出了cBN单晶应由hBN直接发生结构转变而形成。在高温高压条件下,hBN在触媒层中通过触媒的催化作用完成形核过程,并依靠触媒层中B、N原子的浓度差异进行生长。Raman谱和AES谱的精细结构表明,B原子和N原子的电子结构在触媒层的不同区域是不同的,在B、N原子由扩散方式通过触媒层达到cBN单晶的过程中,B、N原子的性质发生了变化。AES谱显示触媒外层B、N原子的精细结构与hBN相似,而在触媒内层,即靠近cBN单晶处,AES谱图更加接近于cBN单晶,在触媒层中由外到内,具有sp2π杂化特征的俄歇峰强度逐渐减弱。利用XPS和EELS对cBN单晶触媒层中B、N原子的电子结构变化进行定量分析。采用深度刻蚀的方法对触媒层不同深度的B、N电子结构进行分析,发现随着溅射时间的延长,即越来越接近cBN单晶,在主结合能高能端sp2π杂化所呈现的携上伴峰的强度越来越低。利用Gauss/Lorenz混合型函数对深度刻蚀过程中B1s峰进行曲线拟合分峰处理,从而得到在触媒层由外到内,sp2π的含量由61.18%降低到28.24%,而sp3的含量由38.82%增加到71.76%。EELS分层实验结果表明,在cBN单晶触媒层由外到内,sp3-B的相对含量分别是63.47%、67.24%和79.53%。这些结果表明在cBN单晶的生长中,触媒的催化作用逐渐增大,B、N原子的电子构型由hBN的sp2π杂化态逐渐向cBN的sp3杂化态转化。对cBN单晶表面的AFM研究表明在(111)晶面和(100)晶面均存在大量的cBN亚颗粒,并且(100)晶面的颗粒要明显大于(111)晶面。cBN单晶在高温高压下的生长在一定程度上可以看作是这些cBN亚颗粒或原子集团在生长的cBN界面上组合与重新排列的过程。结合SEM结果可知,cBN单晶(111)奇异面存在连续生长的台阶,表明cBN在高温高压条件下以片层机制长大。同时,单晶中位错的存在可以使cBN单晶的生长界面形成连续的螺形生长台阶,这为单晶长大过程提供了大量的台阶源,从而可在较低的合成条件下完成cBN单晶的生长。利用晶体生长动力学理论讨论了临界晶核半径、临界形核功和晶体生长速度随合成压力、温度的变化关系。结果表明：在非均匀形核时,一定温度下,临界晶核半径r*和临界形核功△G*随压力的降低而增大；压力一定时,两者随温度的增加而增加。晶体生长速度与温度、压力呈抛物线规律,其中当合成压力为5.5GPa时,cBN单晶具有最快的生长速度。以相变热力学为理论依据,综合考虑温度、压力对物相体积的影响,计算了Li3N及hBN在高温高压条件下向cBN结构转变的反应自由能变化情况。结果表明,在合成cBN单晶的温度、压力范围内,hBN→cBN反应的Gibbs自由能变化均为负值。Li3BN2→cBN+Li3N反应的△G0区域呈现“V”形区,这一区域与优质cBN单晶生长的温度、压力区域近似重合,这一结果说明cBN不应由Li3BN2分解产生。从热力学角度来看,Li3BN2的形成降低了hBN向cBN转变所需要越过的势垒,cBN单晶来源应为hBN的直接结构转变,Li3BN2在cBN合成中起到了结构催化作用。结合表征实验结果和理论计算,对cBN在高温高压下的催化机理可作如下分析：hBN是高温高压条件下cBN生长的直接B、N来源,Li3BN2作为触媒可以加速这一过程的实现,为单晶生长的催化相。高温高压条件下,Li3BN2与hBN形成共熔体,Li3BN2中的BN23-离子侵入六方相中,使hBN层间的范德华力受到影响,从而发生滑移或断裂,此时hBN的远程有序结构消失,逐渐降低为低聚合度的BN团簇。与此同时,Li+能够吸引处于高温活跃状态的hBN中N原子中的一个电子,并将其传送给B原子,促使B、N原子的电子结构均变为B、N原子的s轨道上的电子被激发到空的p轨道,形成具有类似sp3杂化状态的BN原子集团。由于熔体内存在能量起伏和结构起伏,合成腔体内温度、压力的微小波动,就会促使sp3-BN原子团聚集、碰撞从而形成cBN单晶结构。同时,由于触媒层内外存在浓度梯度,六方相不断向触媒内层扩散,并不断转化为立方相。随着合成的不断进行,熔体中具有sp3杂化态的BN原子集团数目不断增加,并在已形成的cBN晶面处含量达到最高。在随后的生长过程中,cBN单晶将以片层生长的方式不断长大。
[Abstract]:Cubic boron nitride (cBN) with diamond-like structure, as a new type of crystal materials, high hardness, good stability, the high temperature characteristics of semiconductor, high frequency characteristics and piezoelectric properties continue to play an important role in the development of modern science and technology. At present, the static high temperature high-pressure catalytic synthesis of cBN method is still an important method of industrial synthesis single crystal, catalytic mechanism and research of high temperature and high pressure cBN crystal has an important significance for guiding the industrial production of high quality single crystal. The experimental results showed that, in cBN single crystal surface after synthesis is always covered with a layer similar to the molten material, this material should be as catalyst and six boron nitride (hBN) catalyst layer formed by fusion. Under high temperature and high pressure cBN crystal is through catalytic and diffusion of the catalyst layer of nucleation and growth of cBN crystal. The catalyst layer after synthesis in the phase interaction to reveal cBN at high temperature Provides important reference for the high pressure catalytic mechanism. This paper takes hBN as the raw material, the synthesis experiments under high temperature and high pressure cBN single crystal with Li3N as catalyst materials. By using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) etc. the characterization of the surface morphology of cBN single crystal catalyst layer, phase structure, and on this basis by Raman spectroscopy (Raman), Auger electron spectroscopy (AES), X ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) on the catalyst layer in B, electronic structure changes of N atoms were analyzed, to investigate the catalytic mechanism of cBN crystal under high temperature and high pressure. At the same time, the reaction in the synthesis of cBN free energy change was calculated with the theory of thermodynamics, which verifies the characterization of experimental results for cBN high temperature and high pressure. The study provides the theoretical basis for the mechanism. In addition, the relationship between nucleation and growth and synthesis conditions of cBN crystal growth from the perspective of dynamics are discussed. Through the experimental results of hierarchical XRD cBN catalyst layer shows that in the main catalyst layer in phase structure of hBN, cBN, Li3BN2, and in each layer were not found in the presence of Li3N. The HRTEM assay showed that cBN nanoparticles in a large number of single crystal cBN / catalyst layer interface. At the same time, the use of TEM cBN polycrystalline particles are also found in the cBN crystal / catalyst interface, and found the existence of hBN in the interface, and then put forward the cBN single crystal by hBN the structural change happens. Formed under high temperature, hBN in the catalyst layer by the catalytic action of the catalyst to complete the nucleation process, and rely on the catalyst layer in B, shows that the fine structure of the growth of.Raman and AES spectra of N atom concentration difference, B Different areas of electronic structure and N atoms in the catalyst layer is different, in the process of B, N atoms by diffusion through the catalyst layer to cBN crystal, B, properties of N atoms changed.AES spectrum shows that the catalyst layer B, hBN and N atomic fine structure is similar, and in the media contact the inner layer, which is near to cBN crystal, the AES spectrum is more close to the cBN crystal in a catalyst layer from outside to inside, the auger peak intensity with SP2 PI hybrid characteristics gradually weakened. For single crystal cBN catalyst layer in B by XPS and EELS, the change of electronic structure of N atoms were quantitatively analyzed. The method adopts deep etching the catalyst layer in different depth of B, analyze the electronic structure of N, with the increase of the sputtering time, which is more and more close to the cBN crystal, the main binding energy of hybrid PI SP2 end with more and more low intensity peaks. Using Gauss/Lorenz mixed function of the depth of moment The B1s peak erosion process curve fitting peak processing, resulting in a catalyst layer from outside to inside, the content of SP2 PI decreased from 61.18% to 28.24%, while the SP3 content increased from 38.82% to 71.76%.EELS layered experimental results show that cBN single crystal in the catalyst layer from outside to inside, the relative content of sp3-B was 63.47% 67.24% and 79.53%., these results indicated that the growth of cBN crystal, catalytic effect of catalyst increases, B electron configuration N atoms by SP2 PI hybrid state hBN to SP3 hybrid state cBN transformation. The research on AFM cBN single crystal surface indicated that in (111) plane and (100) crystal there were cBN large number of sub grains, and (100) crystal plane of the particles to be significantly greater than the (111) crystal face of.CBN crystal under high temperature and high pressure growth can to a certain extent as these cBN particles or sub atomic group combination in the growth of the cBN interface and the combination of re arranged. SEM results show that cBN single crystal (111) is the continuous growth of the steps of the singular surface, showed that cBN grew up with lamellar mechanism at high temperature and high pressure. At the same time, the existence of dislocations in single crystal growth interface can make the cBN crystal formed spiral continuous growth step, this is a single crystal grow up process provides a great source of steps thus, the completion of cBN crystal growth in the condition of low synthesis. Using the crystal growth kinetics theory to discuss the critical nucleus radius, the critical nucleation energy and crystal growth velocity with synthetic pressure and temperature dependence. The results show that in heterogeneous nucleation, under certain temperature, the critical nucleus radius r* and the critical nucleation energy G* decreases with increasing pressure; pressure, which increases with the increase of the temperature. The crystal growth rate and temperature, pressure of a parabolic law and the synthesis when the pressure is 5.5GPa, cBN single crystal has the most Fast growth. In thermodynamics theory, considering the effects of temperature, pressure on the phase volume, reaction of free Li3N and hBN transition at high temperature and high pressure to cBN structure can change the calculation. The results show that the synthesis of cBN crystal temperature, pressure range, hBN, cBN the Gibbs free energy delta G0 regional changes are negative for.Li3BN2 and cBN+Li3N reaction showed "V" shape, the area with high quality cBN crystal growth temperature, pressure area is approximately coincide, this result suggests that cBN should not Li3BN2 decomposition. From the view of thermodynamics, the formation of Li3BN2 hBN decreased to cBN to cross the barrier, cBN crystal source should direct the structure of the hBN transformation, Li3BN2 has played a catalytic role in the synthesis of cBN structure. According to the experimental results and theoretical calculation of characterization, the catalytic mechanism of cBN under high temperature and high pressure can be made Analysis of the following: hBN is directly B, the growth of cBN under high temperature and high pressure N source, Li3BN2 as catalyst can accelerate the realization of this process is catalyzed by crystal growth phase. Under high temperature, Li3BN2 and the formation of hBN eutectic, BN23- ions in Li3BN2 intrusion of six party phase, hBN layer between the fan Edward affected to slip or fracture, disappear when hBN long-range order, gradually reduced to BN clusters with low degree of polymerization. At the same time, Li+ is able to attract a N atom in the active state of the electronic temperature hBN, and its transfer to the B atom, to B, the electronic structure of N atoms both are B, s orbit electrons of N atoms are excited to an empty P orbit, the formation of BN atomic group with similar SP3 hybridization state. Due to the existence of energy fluctuation and fluctuation of the melt structure, synthesis temperature cavity, small fluctuations in pressure, will promote The sp3-BN cluster aggregation, collision to form the cBN crystal structure. At the same time, the catalyst layer exists inside and outside the concentration gradient, the six phase to the catalyst layer and diffusion, and continue into the cubic phase. As the synthesis continues, the number of BN group with SP3 atomic hybridization state in the melt is increasing, and reached the highest at cBN the content of crystal surface has been formed. In the subsequent growth process, the growth of cBN single crystal by a layer the way to grow up.

【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ128.1

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