FIGURE 4 Al2OC crystal structure schematic in which the black circles are the C, O atoms, and the others are the Al atoms [19]
In the Al-O-C ternary system, there are two ternary compounds, i.e., Al2OC and Al4O4C [20]. Between them, Al2OC has a similar wurtzite-type crystal structure to AlN, and the formation of Al2OC can be achieved through N atom replacement in the AlN crystal lattices with C and O atoms (Figure 4) [19, 21, 22]. As reported by Qiu et al. [23], a solid solution between Al2OC and AlN should be formed in the Al4C3-AlN-Al2O3ternary system. The defect reaction equation for this process can be described as follows:
(1)
When 2 mol AlN dissolves into 1 mol Al2OC, half of the N atoms may occupy 1 mol O atom positions in Al2OC, and the other N atoms may occupy 1 mol C atom position therein. No additional vacancies are generated, so, the lattice distortion is relatively small. Furthermore, the solid solubility of AlN in Al2OC can be as high as 50 mol.%. When the AlN content exceeds 15 mol%, the stability of the Al2OC-AlN solid solution increases, and, as a result, Al2OC is not easily decomposed into Al4O4C [19, 24-26]. According to the results mentioned above, it may be concluded that Al2OC reasonably acts as the mesophase during AlN powder synthesis in the CRN process because of their extremely similar crystal structures.