富硼体系中立方氮化硼晶体的生长

本文在hBN-Li3N体系中添加不同含量的单质硼(B),研究了cBN晶体在富B条件下的生长特性.结果表明,B进入晶体的位置具有明显的区域选择性.B以占据N(111)面内N空位的方式进入晶体,并与原有的N原子一起形成一个B原子和N原子的(111)面.随着B进入量的增加,越来越多的B原子取代N空位,B和N原子的混合面与邻近的B(111)面叠加,在晶体的中心部位形成颜色较深的三角形阴影并逐步扩展,最后,使晶体完全变成黑色.由于B占据N空位造成原来N的(111)面上有大量硼原子存在,使得晶体沿＜111＞方向生长困难而有利于沿＜100＞方向生长, 从而形成了八面体或类球形晶体.同时,由于加入的B与部分Li3N发生化学反应也可生成cBN,因此,体系中cBN晶体的形成受两种机制控制:一种为溶解析出过程,另一种为化学反应过程.cBN的产率随着B添加量的增加而降低的实验结果表明,溶解析出过程占主导地位.

Growth of cBN Crystal in the Boron-rich System

In this paper,growth characters of cBN crystal in hBN-Li_3N system with different contents of B additive were investigated under high pressure and high temperature.The results show that boron atoms entering cBN crystal had distinct location character.B atoms occupy N vacancies in the(111) plane and therefore leads to forming a new plane made of B and N atoms.With B additive increasing,more N vacancies are substituted.The new B-N(111)plane and its neighboring B (111) plane overlapping with each other resulted in forming dark triangle shadow in(111) plane of cBN crystal.The triangle extended through to the whole cBN crystal,which changed the color of cBN crystal into black.Owing to excessive B locating at N(111) planes,the growth of cBN crystals along <111> direction became difficult while the growth along <100> direction was favorable.So the morphology of the obtained cBN crystals changed from thick plank into near perfect octahedron or sphere-like morphology.At the same time,cBN could be also obtained by the chemical reaction between Li_3N and B,so the formation of cBN crystal in this B-rich system could be controlled by two mechanisms: dissolve-recrystallization and chemical reaction process.The experimental results show cBN nucleation decreases with B additive increases.This indicates that the dissolve-recrystallization is dominant process in the B-rich system,if Li_3N is excessive.