**2.7 Effects of additive types**

In Sections 2.1–2.6, CaF2 was selected as the typical additive to synthesize spherical AlN fillers by the CRN method. Actually, many single or composite compounds can be used as additives to promote the enhancement of the nitridation rate and the formation of the spherical morphology of AlN particles [37, 43–46]. These compounds have the same characteristic that they can react with Al2O3 to produce low-melting liquid aluminates at a relatively low temperature. However, different kinds of additives showed a slight different effect on the nitridation process and the product morphology. **Table 2** shows the powder mixtures with different additive types that the authors previously investigated. The CRN process was carried out at 1400–1800°C and under the N2 pressure of 1 MPa for 2 h. The influence of additive types on the AlN conversion fraction was summarized in **Table 3**.
