FIGURE 9 (a), (b) (HR)TEM images of the dark grey AlN powder sample; (c) SAED spectrum
In Figure 8a, the O-enriched micro-zones are located by the EDS results in Figure 8b-e. Among them, Area 1 is rich in Al, O, and C elements, and Area 2 is composed of only Al and O elements. Focusing on Area 1 in Figure 8a, its high-magnification TEM image is shown in Figure 9a. In the HRTEM image of the circle in Figure 9a, planar spacing of 2.750 ± 0.029 Å was determined, which was very close to that of the Al2OC(100) (2.759 Å) [24] (Figure 9b). Similarly, the SAED spectrum of the same area in Figure 9c also indicates the Al2OC mesophase. By the same method, Area 2 in Figure 8a could be determined as an unreduced Al2O3 particle.
In summary, the formation of the Al2OC mesophase has been confirmed in the CRN-synthesized AlN powder when the involved solid-state reaction is incompletely performed. The Al2OC mesophase brings the C and O impurities into the AlN powder together, resulting in degraded quality of the powder. Next, the formation and transformation thermodynamics of the Al2OC mesophase of AlN powder in the CRN process can be further deduced.
Thermodynamic calculations
Under the conditions of the calcining temperature, T = 1973.15 K (1700 °C), and the atmospheric pressure in the synthesis furnace, Pt = 101.667 kPa (1 atm), the equilibrium constant Q of each reaction in the CRN process was calculated, based on the relevant thermodynamic data. Then, a function expression on the CO(g) partial pressure in the synthesis furnace,PCO , was deduced to determine the boundary regions between two different phases. Last, the isothermal section of the AlN-Al2O3-Al2OC ternary phase diagram at 1973.15 K was derived.
It is known that the total reaction for AlN powder synthesis by CRN is as follows:
Al2O3(s) + 3C(s) + N2(g) = 2AlN(s) + 3CO(g) (2)
By means of the thermochemical data in Ref. [30], an equation concerning Gibbs free energy difference of the reaction,ΔG1 , was derived (detailed in the Appendix):
ΔG1 =708.101 - 0.374T + 2.303RT logQ1<1>
where R is the gas constant, 8.314×10-3kJ/(K·mol); T is the reaction temperature, K;Q1 is the equilibrium constant of reaction 2,Q1 =(PCO /Pθ )3/(PN2 /Pθ );PCO and PN2 are the CO/N2 equilibrium partial pressures in the synthesis furnace, kPa; Pθ is a constant equal to 100 kPa.
When reaction 2 is in an equilibrium state (ΔG1 = 0), PN2 can be expressed as:
logPN2= 3logPCO - 4.765 <2>
According to the results reported by Lefort et al. [31], the carbothermal reduction reaction to form the Al2OC mesophase is described as:
Al2O3(s) + 3C(s) = Al2OC(s) + 2CO(g) (3)
Based on the relevant thermodynamic data in Ref. [30] and HSC thermodynamic software data (Al2OC) [32], an equation on Gibbs free energy difference of reaction (3), ΔG2 , was deduced:
ΔG2 = 776.802 - 0.354T + 2.303RT logQ2 <3>
where Q2 is the equilibrium constant of reaction (3),Q2 =(PCO /Pθ )2.
When reaction 3 is in equilibrium at 1973.15 K, then
logPCO =0.973 <4>
According to Ref. [31], when reaction 3 takes place, Al(g) exists and reacts with CO, resulting in the formation of the Al2OC mesophase. However, as thePCO in the atmosphere is decreased, reaction 4 goes on to allow for the Al2OC mesophase to decompose to form A1(g):
Al2OC(s) = 2A1(g) + CO(g) (4)
A1(g) then reacts with N2 in the atmosphere to form AlN:
2A1(g) + N2(g) = 2A1N(s) (5)