Influence of MgO on the growth of cubic boron nitride using the catalyst Mg3N2

Abstract

Cubic boron nitride (cBN) may be synthesized under high pressures and high temperatures in the presence of a catalyst, provided that hexagonal boron nitride (hBN) is used as starting material. Typical catalysts are known to be the nitrides of the alkali and alkaline earth metals. Sato et al. [1] and Endo et al. [2] reported the effect the oxygen content ( B 2 0 3 ) of hBN on the growth of cBN. The present work investigated the influence of MgO content in the catalyst magnesium nitride (Mg3N2) on the growth of cBN. HBN powder and the sintered body used as starting material contained 0.6 wt % oxygen and had a graphitization index of 1.6. Mg3N2 was synthesized by heating magnesium metal under a nitrogen stream at 600°C for 4 h, and subsequently at 900 ° C for 8 h. Mg3N2 was mixed with MgO in the weight ratio Mg3Nz:MgO of 5:1. Oxygen contents were measured by neutron activation analysis and the graphitization index was determined by X-ray diffraction patterns. A belt-type high-pressure cell was used for the experiments. The catalyst Mg3N 2 was placed in the centre of the reaction cell and was surrounded by hBN. The pressure was calibrated at fixed points for the phase transitions in bismuth (2.55, 7.7 GPa), thallium (3.68GPa) and barium (5.5GPa). The temperature was determined from an electric power input against temperature plot, calibrated by thermocouples. The crystal habit and the surface structure of cBN samples were observed by optical and scanning electron microscopy. The conversion rate and the crystal growth rate of cBN were investigated in the temperature range 1600 to 2200 K at pressures from 6.5 GPa. The crystal growth rate had a maximum at Figure 2 MgO layers on the crystal surfaces.