3.4 Elimination of the Al2OC mesophase
According to the thermodynamic calculations in section 3.3, thePN2 in the synthesis furnace can be increased and the PCO in the synthesis furnace can be decreased by increasing the N2 flow in the synthesis furnace. Therefore, reactions 2, 4, and 5 take place in the positive direction. The conversion of Al2O3 to AlN and the decomposition of the formed Al2OC mesophase are promoted. To verify these assumptions, experiments synthesizing AlN powder in the CRN process at different N2 flow rates were carried out. Table 5 shows the O, N and C contents of the AlN powder synthesized in the CRN process at different N2flow rates. As the N2 flow rate of 250 L/min was employed previously, the O and C contents of the AlN powder were the highest, and the N content of the AlN powder was the lowest, after calcinating at 1700 °C for 12 h and then decarburizing and pulverizing, resulting from the formation of the Al2OC mesophase and/or inadequate reduction of Al2O3during the CRN reaction. As the N2 flow rate increased to 320 and 400 L/min, the O and C contents of the AlN powder gradually decreased, while the N content of the AlN powder increased significantly. That is, by adjusting the N2 flow rate during AlN powder synthesis in the CRN process, the concentrations of the O and C impurities of the AlN powder fabricated in batch quantities could be controlled at a very low level, which were slightly higher than those of grade H-AlN powder synthesized by the same CRN process from Tokuyama, Japan (O content: 0.8 wt%, C content: 280 ppm) [33].
TABLE 5 O, N and C contents of the AlN powder synthesized at different N2 flow rates