**2.6 Effects of the carbon content**

It is generally believed that the ratio of carbon and Al2O3 has a great effect on the CRN process. Traditionally, excessive carbon was used to guarantee the full conversion of Al2O3 to AlN [42]. In order to evaluate the effects of carbon content on the synthesis of spherical AlN particles, the raw powder mixtures with 5 wt.% CaF2 and various C/Al2O3 mole ratios (2.5, 3.0, 4.0, and 5.0) were used, and the CRN process was conducted under the N2 pressure of 1 MPa and at different temperatures for 2 h.

**Figure 8** shows the relationship between the AlN conversion fraction and the C/Al2O3 mole ratio at 1500 and 1800°C, respectively. At 1500°C, no samples achieved full nitridation. When the mole ratio of C to Al2O3 was 3.0, namely, the theoretical value of the CRN reaction, a highest AlN conversion fraction of ~79% was obtained, while a lower or higher C/Al2O3 mole ratio tended to reduce the AlN conversion fraction. As discussed, the formation and nitridation process of liquid Ca-aluminates played a crucial role in the CRN process. When the carbon content was insufficient, the contact interface between carbon and Ca-aluminates decreased. As a consequence, the nitridation rate of Ca-aluminates was reduced, leading to the decrease of the AlN conversion fraction. On the other hand, when the ratio of C to Al2O3 exceeded the theoretical value, the excessive carbon tended to hinder the contact between CaF2 and Al2O3. The formation rate of Ca-aluminates was decreased correspondingly, resulting in the low AlN conversion fraction as well.

In addition, it can also be observed from **Figure 8** that the full AlN conversion occurred for the samples with the C/Al2O3 mole ratios of 3.0, 4.0, and 5.0 at 1800°C. As for the sample with the C/Al2O3 mole ratio of 2.5, a small amount of

### **Figure 8.**

*Relationship between the AlN conversion fraction and the mole ratio of the C/Al2O3 mole ratio at 1500 and 1800°C [29].*

**73**

**Figure 9.**

*(b) 3.0, (c) 4.0, and (d) 5.0 [29].*

*Carbothermal Synthesis of Spherical AlN Fillers DOI: http://dx.doi.org/10.5772/intechopen.81708*

**2.7 Effects of additive types**

all samples synthesized at 1800°C were shown in **Figure 9**.

a result, AlN powders with smaller particle size were obtained.

types on the AlN conversion fraction was summarized in **Table 3**.

Al2O3 still existed in the system due to the lack of carbon black. The SEM images of

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

*SEM images of the AlN particles synthesized at 1800°C from samples with various C/Al2O3 mole ratios: (a) 2.5,* 

When the carbon content was less than the theoretical value, many large sintering aggregates could be observed (**Figure 9a**). As increasing the carbon content, the large aggregates gradually disappeared, while the particle size significantly decreased. As known, the growth of particles was not only affected by their growth rate but also restricted by external growth space. When insufficient carbon black was used, the residual carbon black in the late stage was scare. Thus, the AlN particles were likely to contact each other, promoting the formation of large particles and hard aggregates. When the carbon content was excessive, a large amount of unreacted carbon dispersed between AlN particles, which enlarged the migration distance of small particles to AlN nucleus, and limited the growth space of AlN. As
