**3. Formation mechanism of micro-size spherical AlN fillers**

In the above section, we summarized the influence of synthetic parameters on the morphology of final AlN products. It can be concluded that high N2 pressure, suitable additives, and appropriate reaction temperature were essential to synthesize the micro-sized spherical AlN fillers. Compared with the traditional CRN method, the as-synthesized AlN fillers exhibited two main obvious morphological changes: the growth of the particle size and the sphericity of the particle shape. In this section, we will discuss the formation mechanism of the two morphological changes based on the experimental results.
