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)