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