**2.5 Effects of reaction time**

To investigate the effects of reaction time on the morphology of AlN products, the raw materials with 5 wt.% CaF2 were heated at 1800°C and under the N2 pressure of 1 MPa for various reaction times (0.5, 1, 2, and 4 h). **Figure 7** shows the SEM images of the as-synthesized powders.

As the reaction time prolonging from 0.5 to 1 h and further to 2 h, the size and uniformity of AlN particles significantly increased. It was expected that the dissolution and reprecipitation of AlN aiding with liquid phases contributed greatly to the particle growth. In addition, as the prolongation of reaction time, the residual liquid phases in the system could also adjust themselves under the action of the interfacial energy to uniformly wrap the AlN particles, further improving the uniformity of the particle size. However, when the reaction time further increased to 4 h, the growth of AlN particles was no longer obvious. Instead, most of the AlN particles were sintered each other to form large aggregates, and the individual particles tended to change from the spherical to angular morphology, as demonstrated by the

**Figure 7.** *SEM images of the AlN products synthesized for various reaction times: (a) 0.5, (b) 1, (c) 2, and (d) 4 h [29].*

inserted images in **Figure 7c** and **d**. As the reaction proceeding, Ca-aluminates were continuously consumed until they were completely reduced to Ca vapor following Eq. (4). The residual liquid phase was too less to completely encapsulate the AlN particles. The driving force of small AlN particles migrating to large particles decreased, and the energy required for the growth of individual particles increased. Therefore, the AlN particle growth gradually stopped and was replaced by the neck sintering between particles.
