**2. Influence of carbothermal synthetic parameters**

The typical carbothermal process for synthesizing spherical AlN fillers consists of three steps: first, the raw materials (Al2O3, carbon black, and additives) were homogenized by ball milling; second, the CRN process was conducted in a graphite furnace with a high temperature and an elevated N2 gas pressure; finally, the obtained powders were transferred to a muffle oven and heated in air to remove the residual carbon. It was found that various synthetic parameters, such as N2 pressure, additives, reaction temperature, reaction time, and carbon content had an important impact on the nitridation rate, the morphology, and the particle size of the final products.
