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.