Relationship Research Between Catalyst Microstructures And Catalytic Mechanism Of Cbn Single Crystals

Cubic boron nitride (cBN) single crystal, as a functional material, has many interesting properties, such as thermal conductivity, high hardness only second to diamond and excellent chemical stability. As a potential functional material, cBN has attracted much attention for its electrical and optical property, semiconductor property and high-frequency characteristics. At present, the static catalyst synthetic method under high pressure and high temperature (HPHT) is commonly used to synthesize cBN single crystals, and it is of great importance to carry on the investigation about growth mechanism in order to obtain large-sized cBN single crystals with high quality. The experiments show that cBN crystal is always surrounded by a thick fluffy covering, and this covering has the tendency to grow into cBN crystal. This covering is formed with the rapid cooling from HPHT state, and it is the catalyst layer composed with catalyst and hexagonal boron nitride (hBN). The nucleation and growth of cBN crystal should be finished in the catalyst layer by its catalysis and diffusion. Since there are solid structures in short-range order in the layer within the synthetic temperature and pressure, further studying about interaction of phases in the layer could help to elucidate the cBN growth mechanism under HPHT conditions.In this paper, cBN single crystals were synthesized from hBN in the presence of Li3N as a raw solvent catalyst under HPHT conditions. Scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to investigate the morphology and phase structure of catalyst layer. The changes of valance electrons of boron and nitrogen atoms in the catalyst layer were revealed clearly by Raman spectroscopy (Raman), auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). On this basis, the possibilities of different phase transition to cBN with Li3N as catalyst under HPHT were analyzed by use of the second law, and the boron and nitrogen sources were further analyzed combined with thermodynamic calculation. In addition, the relationship between nucleation and synthetic conditions were investigated in the viewpoint of growth kinetics.The XRD examinations show that there exist different phases, such as hBN, cBN, and Li3BN2, and there is no Li3N existed in any location of catalyst layer. HRTEM examinations show that there existed a lot of nanometer-sized cubic boron nitride nucleus in catalyst layer, and it is deduced that cBN crystal nuclei should be generated all over the catalyst layer under HPHT conditions. TEM examinations show that hexagonal phase was joined to cubic phase directly, and it can be explained that the hBN could be transformed to cBN directly by the catalysis of Li3BN2.For the concentration difference of boron and nitrogen in catalyst layer, the single crystals would grow up by diffusion modes after the cBN crystal nucleus was formed in the melt. The fine structures by Raman and AES show that electron structure of boron and nitrogen atoms in different regions of catalyst layer are quite different, which results from the difference of chemical environment of catalyst. It was suggested that during boron and nitrogen atoms diffuse through catalyst layer to the as-grown cBN crystal, some changes had been taken place. Such changes may be related to the catalytic effect of lithium boron nitride and closely associate with the transition from sp2Ï€ bonding of hBN to the covalent sp3 bonding of cBN. So the catalyst layer was considered that not only plays the role of a medium for delivering boron and nitrogen atoms through diffusion, but also plays the role of catalysis. The AES experiments show the fine structure of outermost catalyst layer is similar to hBN, and it becomes closer to cBN in the innermost catalyst layer.The variation of electronic structures from boron nitride of different depth in catalyst layer on the cBN single crystal had been investigated by XPS. Combining the atomic concentration analysis, it was shown that from the outside to inside in catalyst layer, the sp2 fractions are decreasing, and the sp3 fractions are increasing in the film at the same time. The Gauss/Lorenz blend function was used to XPS peak differentiation imitating analysis for B1s peak. The results showed that, from outside catalyst layer to cBN crystal, the content of sp2 fraction decrease from 61.18%to 28.24%, and the content of sp3 fraction increase from 38.82% to 71.76%. The EELS of sp3-B content is 63.47%, 67.24% and 79.53% in outer, inter and inner catalyst layer respectively. These results indicate that the catalysis was enhancing in the cBN growth process, and the electronic configurations of boron and nitrogen atoms were gradually changed from sp2Ï€state to sp3 state.Study on cBN crystals by AFM suggests that there exist some fine particles in dimension on the (111) and (100) crystal surfaces, and the size of particles on (100) surface is obvious lager than it on (111) surface. The cBN crystal growth under HPHT, in a sense, could be considered as a process of these cBN fine particles unification or boron nitride atomic clusters recombination on the growing cBN crystal surface. Successive growth interlayer steps on the (111) singular surface indicated that the cBN crystals grow layer by layer under HPHT. Some dislocations, which are generated in the cBN crystal single crystal during the growth process, may form spiral growth steps on the crystal surface. These steps let cBN crystals continue growing under lower synthesized conditions.The transformation kinetics of cBN crystals was described by a nucleation and growth process within thermodynamic stability region of cBN crystal. The theoretical description was developed based on the layer growth mechanism and heterogeneous nucleation. The critical crystal radius, r*, would increases with the temperature under constant pressure, and the change with temperature more pronounced at lower pressure. The crystal growth velocity increased with synthesized temperature, and it showed parabolic shape with temperature increasing under certain pressure. Under 5.5GPa pressure, the growth velocity reached the maximum.Based on the above experiments, considering the influences of high temperature and high pressure to different phase volumes, the source of boron and nitrogen elements was analyzed by using thermodynamic law. The results show that Gibbs free energy changes of hBNâ†'eBN reactions are both negative within the synthetic temperature and pressure range, but the pressure and temperature scope of â–³G>0 in Li3BN2â†'Li3N+cBN reaction shows V-shaped area. This area is less distinct with the cBN growing V-shaped area. It confirms that the cBN cannot be decomposed by Li3BN2, and it should be transformed from hBN directly.Combined with above evidences, the catalytic process of catalyst layer during cBN formation under HPHT could be understood as follows. Under HPHT conditions, cBN could be transformed from hBN directly by the catalysis of Li3BN2. The BN23- influences the intermolecular force of boron and nitrogen from hBN, and the long-range order of hBN is reduced to form BN cluster with low polymerization degree. At the same time, the Li+could attract the electron from N atom and transmit it to B atom under HPHT, so the electronic structures of B and N atoms are both changed to 2s22p2. The electrons which located on s-orbit of B and N atoms would be motivated to the empty p-orbit, and the B-N atoms clusters which act as the crystal nucleus with sp3 state are formed. Small fluctuations of temperature and pressure in synthetic chamber would prompt the sp3 clusters to coalesce to form the cBN crystal structure. With synthetic process going on, the content of sp3-hybridization BN particles in the catalyst layer/cBN crystal interface increases and the particles gradually grow into as-grown cBN crystal. For there exists the concentration gradient in catalyst layer, hBN diffuses towards the as-grown cBN crystals, and be transformed to the cBN clusters continually. The cBN crystals would grow up with grow step method.