The Coloration Process Study on the Synthesis of cBN by hBN-LiH/hBN-Li3N-B System

Using hBN-LiH or hBN-Li3N-B as starting materials, we synthesied black cBN crystals under high pressure and high temperature. The results of Raman spectrum show that the reason why cBN crystals become black is that the redundant B atoms get into them. The cBN crystal is synthesized in hBN-Li3N-B system, and then we observe a trigonal shadow is formed clearly. This phenomenon indicates that there are more B atoms from center to apex in cBN crystal, and the B atoms content gradually decrease from center to edge. However, the color of cBN crystal which is synthesized in hBN-LiH system was from transparent black to opaque black directly, and the trigonal shadow does not be formed. This phenomenon indicates that the B atoms as impurity diffuse equably in cBN crystals. The two circumstances illustrate that as an impurity atom, B has two kinds of distributions when it enter into cBN crystal: one is centeral symmetric distribution, the other is uniform distribution. In this paper, we analyze the reason why cBN crystals have different phenomenon above according to its growth environment and its ability of excluding impurity.