组装方式对立方氮化硼合成的影响

Effect of assembly mode on CBN synthesis

组装方式不仅可以影响高压腔内温度压力的分布，最终影响合成晶体的质量，而且也直接影响到对原料hBN的选择。本实验用国产六面顶压机，在4～5GPa，1500～1700％范围内研究了组装方式对立方氮化硼（CBN）合成的影响。通过光学显微镜观察了晶体生长的特性及形貌。结果表明，在实验中，用粉末状触媒和六角氮化硼采取分片式组装，CBN的成核会聚集在片与片的交界面处，且生长的CBN晶体多为厚板状八面体。这是由于在粉压成型的过程中，受力不均造成了上下表面的触媒与六角氮化硼的密度高于内部的，且高密度的粉体也有利于温度和压力的传导，所以导致在此处的CBN成核数量高于内部。可见，预压的片越薄，片内各处的粉体密度就越相近。因此，在以粉体为初始原料合成CBN晶体的实验中采用薄片式组装方式，选取hBN粉体堆积密度大的为原料对于提高CBN晶体单位产量更为有利。

Assembly mode influences not only the temperature and pressure distribution in the sample chamber, which ultimately affects the quality of synthesized CBN crystals, but also makes some demand on the selection of starting hBN. In this paper the effect of assembly mode on the CBN synthesis was studied under 4 -5GPa and 1500 - 1700℃. The synthesized products were observed through optic microscope （OM）. The results indicated that CBN nucleation mostly located at the interface between the two flakes and the dominant crystal shape of CBN is octahedral thick flake when the flake assembly mode was used with the mixed powder of catalyst and hBN. The reason was as follows ：the mixture powder of catalyst and hBN would be pre-pressured into flakes for synthesis. In this process, because the flake thickness resulted in bad transmission of pressure, the density of the mixed powder at the surface is higher than that at the inside. The high density of the mixed powder was also in favor of the transmission of the temperature and the pressure. These all made CBN grow at the interface easily. It was concluded that the thinner the flake was, the more symmetrical and higher the density of the flake was. So it was preferred to select high density hBN with thin flake assembly mode to improve the CBN yield.