| Cubic boron nitride (cBN) is man-made superhard material only next to diamond in hardness and natural cBN is not discovered up to now. Cubic BN does not react with the ferrous metals and has higher anti-oxidized temperature than diamond, therefore Cubic BN is an excellent material for machining ferrous metals and alloy in the field of industry. Besides, cBN has good electricity insulation,heat conductivity and the widest energy gap (～6.4eV). Cubic BN can be made easily into both p- and n-type semiconductor with suitable impurity addition. As a result, cBN has the obvious superiority in making the high power semiconductor and optoelectronic devices with the excellent properties. So, the perfect cBN crystal is prerequisite no matter where it is applied in the super hard material or semiconductor material. Therefore, the synthesis of the perfect cBN crystal is always the aim of a lot of researchers.In general, cBN can be synthesized from hexagonal boron nitride (hBN) or amorphous boron nitride (aBN) using catalysts under high temperature and high pressure [HPHT]. The method is called phase transition method. In the method, cBN was directly formed by phase transition of hBN or crystallized out indirectly from hBN-catalyst solvent system, and high pressure and high temperature is requisite in the course of synthesizing cBN crystal. Therefore, many researcher did masses of work in reducing the synthesizing condition.In this paper, the effects of two different additives on synthesizing cBN crystal in the Li3N-hBN system were studied. Though two different additives can reduce the synthesizing condition, the different conclusions were made by analyzing experiments. Namely, some additives will consume the catalysts and some additives will transform into new catalyst.In my work, firstly, cBN was synthesized in the Li3N-hBN system by technologe of once boosting pressure and once boosting temperature. Secondly, the optimal synthesizing region was found by gradually adding NH4F into the Li3N-hBN system. Therefore, the conclusion was found that the synthesizing pressure reduced about 20% after adding additive NH4F. Finally, the HPHT experiments was done as the mixed powder NH4F and Li3N was the raw material. After analyzing the X-ray diffraction spectra of the experimental products, the result showed that NH4F would react with Li3N, namely: 3NH4F+Li3N=3LiF+4NH3。We also studied cBN was synthesized in the Li3N-hBN system by technologe of twice boosting pressure and once boosting temperature. Then, after adding additive LiH into the Li3N- hBN system, the optimal synthesizing region was found by doing many repeated experiments. After LiH was added, the color of the crystal was dark and the crystal grains became like spherical. In addition, the octahedral black cBN crystal was synthesized in the LiH-hBN system adopting the same experimental process. On the basis of the experiment we guessed : LiH would react with hBN under HTHP :3LiH+2BN=Li3N+NH3+2B. By analyzing Raman patterns it was proved that dissociative B atoms existed in the black cBN crystals synthesized in the system of LiH-hBN. Finally, the testand verify experiments were made in hBN+ B system, and the results accorded with our conclusion.
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