**Figure 2.**

*SEM images of the AlN powders synthesized at 1800°C but under various N2 pressures: (a) 0.1 Mpa, (b) 0.5 Mpa, and (c) 1 Mpa [31].*

**Figure 3.** *XRD patterns of the products synthesized with various CaF2 contents at (a) 1500°C and (b) 1800°C.*

When the reaction temperature was increased to 1800°C, only the peaks of AlN were observed in **Figure 3b**, suggesting the full conversion of Al2O3 to AlN. Moreover, it is necessary to note that no diffraction peaks ascribed to the CaF2 or Ca-compounds were detected, inferring that Ca-compounds were just formed at relatively low temperature; afterwards, they were reduced and further vaporized in the atmosphere with the increase of synthesis temperature. The process was established as follows [39, 40]:

$$\text{Ca-aluminates (l)} + \text{C(s)} + \text{N}\_2\text{(g)} \rightarrow \text{AlN(s)} + \text{CO(g)} + \text{Ca(g)}\tag{4}$$

**Figure 4** further shows the SEM images of the AlN products synthesized at 1800°C. As observed in **Figure 4a**, small particles accompanied with some irregular grains were obtained in the absence of the CaF2 additive. When a different amount of CaF2 ranging from 1 to 10 wt.% was added, both the sphericity and particle size of AlN granules significantly increased with the CaF2 content, as shown in **Figure 4b**–**d**. Clearly, the higher CaF2 content meant more Ca-aluminate liquids were generated, providing the liquid environment for AlN nucleation and material transport. A large amount of Ca-aluminate liquid explicitly favored for the complete wrap of AlN particles, promoting the formation of a smooth spherical morphology. In addition, the small AlN particles were more easy to dissolve in the excessive liquid phase and reprecipitate on the surface of large particles, which finally promoted the growth of AlN particles via the dissolution-precipitation mechanism.
