Effects Of Additive On The Synthesis Of High Quality CBN Single Crystal In The Li₃N-hBN System

Effects of Additive on the Synthesis of High Quality cBN Single Crystal in the Li₃N-hBN SystemFor the synthesis of cBN single crystals, it's a main method that crystal is synthesized under high pressure and high temperature by introduction of catalyst system. The pressure and temperature for converting hBN to cBN are about 11.5 GPa and 2000 K without use of a catalyst and they are significant reduced to 4.5-5.5 GPa, 1400-1700℃by introduction of catalyst/solvent. Compared to diamond, cBN is synthesized under higher temperature and lower pressure and its growth mechanism is still not clear though it has been applied extensively.For the moment, many kinds of catalysts were found to be effective for cBN formation. Lithium, calcium, magnesium, etc., their nitride as well as boron nitride are main catalysts for cBN growth in industry. CBN conversion and quality are very high by using these catalysts, but the cost of them is high and not easy to keep; Some metals and alloys, e.g. Sn, Ni, Ni-Al, Fe-Al, were also reported as catalysts for hBN-cBN phase transformation, and their cost is low, easy to conserve, but quality and conversion of cBN crystal is low; Though some ammonium salts and acids also can be catalysts for cBN formation, they are not fit to cBN commercial process too because of releasing gases under HPHT. It is an important technique that introduces the additives into cBN formation system for reducing the formation conditions and improving the quality of cBN before new catalysts of cBN formation becomes.In this paper, we researched the effects of different additives on the synthesis of cBN in Li₃N-hBN system based on the solvent theory and general regulation of cBN formation. Firstly, the crystallization and growth of cBN were studied by subjecting fluoride. It has been reported that the quality of cBN and synthesized conditions would markedly improved by introducing fluoride, but the details of experiments or technique process had not mentioned due to privacy of technology. We chose alkali or alkaline-earth fluorides (LiF, CaF₂, and MgF₂) to study their effects on synthesis of cBN. On cBN formation process, these fluorides can not only apply F- and do not introduce any other irrespective elements, but also no gas released under HPHT. The results show that the temperature of cBN formation is decreased obviously, and the quality of cBN improved markedly. Furthermore, the crystals and samples are examined by X-Ray diffraction and scanning electron microscopy and logical reasons of effect by introducing fluoride are found. Secondly, aluminium, titanium, and boron, which adjacent to alkaline-earth metal in the periodic table of elements, were chose to study the effect on cBN single crystal growth in Li₃N-hBN system. These materials have a high chemical activity of boron and nitrogen. That's why they have an important effect on cBN growth that cBN with different color and morphology is apt to obtain.