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.