**2.2 Effects of additive content**

*Fillers - Synthesis, Characterization and Industrial Application*

In order to evaluate the effects of N2 gas pressure on the nitridation rate and the morphology of AlN particles, the raw mixtures containing 5 wt.% CaF2 additive were heated at a temperature ranging from 1400 to 1800°C and under various N2 pressure (0.1, 0.1, and 1 MPa), respectively. The AlN conversion fraction was determined based on XRD peak intensities of the plane (100) of AlN and (104) of Al2O3. **Figure 1** shows the relationship between AlN conversion fraction and N2 pressure at

As observed, AlN conversion fraction was significantly decreased with increasing the N2 pressure, indicating an elevated N2 pressure hampered the reductionnitridation process. Based on the studies of Forslund et al. [34, 35], under a high N2 pressure, the removal of produced CO vapor became more difficult. Thus, a barrier was set up on the surface of solid raw materials by the increased CO level, which limited the contact between Al2O3 and N2, and eventually decreased the reductionnitridation rate. In addition, it can also be observed in **Figure 1** that the nitridation rate increased with the reaction temperature. When the reaction temperature was higher than 1600°C, the nitridation rate was significantly improved due to the high reaction activity. As a result, Al2O3 was completely converted into AlN irrespective

**Figure 2** further presents the SEM images of the AlN powders synthesized at 1800°C but under various N2 pressures. Under the N2 pressure of 0.1 Mpa, as shown in **Figure 2a**, irregular AlN particles were obtained. When the N2 pressure was increased to 0.5 Mpa, approximately spherical AlN particles could be observed in **Figure 2b**. With the N2 pressure further increasing to 1 Mpa, micro-sized spherical AlN fillers were successfully prepared, as shown in **Figure 2c**. It should be noted that the both particle size and the sphericity of the as-synthesized AlN particles were improved with increasing the N2 pressure. As established, the elevated N2 pressure tended to improve the CO level in the system. The AlN nucleation rate was slowed down, resulting in the large particle size. Additionally, CaF2 additive was expected to react with Al2O3 to form low-melting Ca-aluminates, providing the liquid catalyst for AlN nucleation [36]. The slow reaction rate under the elevated N2 pressure was also beneficial for the migration and uniform distribution of Ca-aluminate liquid phases in the system. Therefore, the uniform liquid-assisted

*Relationship between AlN conversion fraction and N2 gas pressure at various reaction temperatures [31].*

**2.1 Effects of N2 gas pressure**

various reaction temperatures.

of the tested N2 pressure values.

**66**

**Figure 1.**

As mentioned above, the aluminates formed from the reaction between additives and Al2O3 played an important role to determine the CRN reaction rate and the morphology of AlN particles. Therefore, the investigation of the additive was helpful to verify the previous inference and better understand the formation mechanism of spherical AlN particles. In this section, the raw materials with various CaF2 contents (0, 3, 5, and 10 wt.%) were used to proceed the CRN reaction under the high N2 pressure of 1 MPa and at different temperatures of 1500 and 1800°C. The XRD patterns of the as-synthesized products were shown in **Figure 3**.

As observed from **Figure 3a**, Al2O3 was identified in all samples, indicating the incomplete nitridation at 1500°C. In the absence of CaF2, the peaks of Al2O3 were detected strongly. When 3 wt.% CaF2 was introduced, the relative intensity of the Al2O3 diffraction peaks decreased obviously, which indicated that a small amount of CaF2 could effectively accelerate the nitridation rate. However, with the CaF2 content further increased to 5 and 10 wt.%, the peaks of Al2O3 increased instead, suggesting the conversion fraction of AlN was decreased. This is mainly because the reaction between CaF2 and Al2O3 was relatively slow at the low temperature of 1500°C; excessive and unreacted CaF2 existed in the system, hindering the contact between reactants and further retarding the nitridation process [37]. In addition, the secondary phase of CaAl12O19 was detected in all samples with CaF2. The Ca-aluminates were mainly formed from the reaction between Al2O3 and CaF2. The process can be described by the following equations [38]:

$$\rm Al\_2O\_3(s) + \rm 3CaF\_2(s) \rightarrow 2AlF\_3(g) + \rm 3CaO(s) \tag{1}$$

$$\text{CaO}(\text{s}) + \text{Al}\_2\text{O}\_3(\text{s}) \rightarrow \text{Ca-aluminates (l)}\tag{2}$$

As the reaction proceeds, the low-melting Ca-aluminates tended to be reduced and further transformed into AlN, providing some Ca-compounds [39]:

$$\text{Ca-aluminates (l)} + \text{C(s)} + \text{N}\_2\text{(g)} \rightarrow \text{AlN(s)} + \text{CO(g)} + \text{Ca-compounds} \quad \text{(3)}$$

The reduction and nitridation of intermediate Ca-aluminate liquid undergo an easier nitridation process, promoting the conversion rate from Al2O3 to AlN.
