FIGURE 10 Calculated partial equilibrium phase diagram at
1973.15 K
As shown in Figure 10, in the plane coordinate system withlogPCO as the x-axis andlogPN2 as the y-axis, two lines of AB and AC were
constructed, based on Eqs. 2 and 4, respectively.
The isothermal section of the
AlN-Al2O3-Al2OC ternary
phase diagram at 1973.15 K was obtained in which point A is the
three-phase equilibrium point of (AlN +
Al2O3 + Al2OC)
(PN2 = 10-1.846 kPa,PCO = 100.973 kPa). Points A, B
(PN2 = 10-5.000 kPa,PCO = 100.973 kPa) and C
(PN2 = 10-5.000 kPa,PCO = 10-0.008 kPa) show an ABC
triangular region, which is the single-phase region of the
Al2OC mesophase. With PN2 above
10-1.846 kPa (point A), as the partial pressure of CO
decreases, reaction 2 is preferential, and
Al2O3 is converted to AlN. When the
N2 pressure was below 10-1.846 kPa and
the PCO was as low as 100.973kPa, reaction 3 was promoted to form the Al2OC
mesophase. When the PN2 in the synthesis furnace
was low as 10-5 kPa, the Al2OC
mesophase was stable in the PCO range of
10-0.008-100.973 kPa. The results
above are consistent with the result that the Al2OC
mesophase is produced in the PCO range of 1-10
kPa at 2100 K and an atmospheric total pressure of 100 kPa [31].
When the PCO is further decreased to the left of
the AC line segment, reactions 4 and 5
are favored, and the
Al2OC mesophase decomposes to form A1(g), which reacts
with N2 in the atmosphere to form AlN.
In summary, to avoid the formation of the Al2OC
mesophase and/or eliminate the formed Al2OC mesophase in
the CRN process of the AlN powder, there are achievable routes i.e.,
increasing PN2 and reducingPCO in the synthesis furnace. In this situation,
reaction 2 is thermodynamically favored, meanwhile, the formed
Al2OC mesophase decomposes into AlN.