**2.3 Effects of the additive particle size**

In this section, CaF2 granulations with different particle sizes (<75 μm, 75~150 μm, and >150 μm) were used to evaluate the effects of the additive particle size on the nitridation rate and morphology of the final products. The nitridation pressure in the CRN process was maintained at 1 MPa. According to the particle size of CaF2 granulations changing from small to large, the obtained powders were briefly named as ACF-S, ACF-M, and ACF-L. **Table 1** summarized the AlN conversion fraction of the products synthesized at 1500 and 1800°C.

It could be clearly inferred that the AlN conversion fraction decreased with the increase of the CaF2 particle size at the low temperature of 1500°C. This is mainly because the large CaF2 particle size tended to reduce the reaction rate between CaF2 and Al2O3, leading to the slow formation rate of Ca-aluminates. This observation further demonstrated that the Ca-aluminates played a central role in the nitridation process. When the temperature increased to 1800°C, full nitridation

**69**

**Figure 5**.

**Table 1.**

**Figure 4.**

*(d) 10 wt.% [31].*

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

was achieved for all samples, and the corresponding SEM images are shown in

*SEM images of the AlN products synthesized at 1800°C with various CaF2 contents: (a) 0, (b) 3, (c) 5, and* 

**Samples CaF2 particle size (μm) AlN conversion fraction (%)**

ACF-S <75 88.03 100 ACF-M 75~150 73.01 100 ACF-L >150 57.36 100

**1500°C 1800°C**

limited, leading to the AlN products with a small particle size.

**2.4 Effects of reaction temperature**

As observed, the particle size of AlN products significantly decreased with increasing the particle size of the CaF2 additive. As mentioned, the large CaF2 particle size could lead to a low formation rate of liquid aluminates. Therefore, insufficient liquid aluminates existed in the system. As a result, small AlN particles were difficult to precipitate on the surface of large particles through the dissolutionprecipitation mechanism. The rearrangement and growth of AlN particles were

*The AlN conversion fraction of the products synthesized with different CaF2 particle sizes at 1500 and 1800°C [29].*

**Figure 6** shows the typical SEM images of the AlN granules synthesized from a typical mixture of Al2O3/C with 5 wt.% CaF2 at various reaction temperatures (1600–1900°C) under the N2 pressure of 1 MPa for 2 h. As can be seen, the temperature has a great influence on the morphology and particle size of the as-synthesized AlN granules. At 1600°C, angular AlN granules along with several tadpole-like

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

## **Figure 4.**

*Fillers - Synthesis, Characterization and Industrial Application*

established as follows [39, 40]:

**Figure 3.**

When the reaction temperature was increased to 1800°C, only the peaks of AlN were observed in **Figure 3b**, suggesting the full conversion of Al2O3 to AlN. Moreover, it is necessary to note that no diffraction peaks ascribed to the CaF2 or Ca-compounds were detected, inferring that Ca-compounds were just formed at relatively low temperature; afterwards, they were reduced and further vaporized in the atmosphere with the increase of synthesis temperature. The process was

*XRD patterns of the products synthesized with various CaF2 contents at (a) 1500°C and (b) 1800°C.*

Ca−aluminates (l) + C(s) + N2(g)→AlN(s) + CO(g) + Ca(g) (4)

**Figure 4** further shows the SEM images of the AlN products synthesized at 1800°C. As observed in **Figure 4a**, small particles accompanied with some irregular grains were obtained in the absence of the CaF2 additive. When a different amount of CaF2 ranging from 1 to 10 wt.% was added, both the sphericity and particle size of AlN granules significantly increased with the CaF2 content, as shown in **Figure 4b**–**d**. Clearly, the higher CaF2 content meant more Ca-aluminate liquids were generated, providing the liquid environment for AlN nucleation and material transport. A large amount of Ca-aluminate liquid explicitly favored for the complete wrap of AlN particles, promoting the formation of a smooth spherical morphology. In addition, the small AlN particles were more easy to dissolve in the excessive liquid phase and reprecipitate on the surface of large particles, which finally promoted the growth of

In this section, CaF2 granulations with different particle sizes (<75 μm, 75~150 μm, and >150 μm) were used to evaluate the effects of the additive particle size on the nitridation rate and morphology of the final products. The nitridation pressure in the CRN process was maintained at 1 MPa. According to the particle size of CaF2 granulations changing from small to large, the obtained powders were briefly named as ACF-S, ACF-M, and ACF-L. **Table 1** summarized the AlN conver-

It could be clearly inferred that the AlN conversion fraction decreased with the increase of the CaF2 particle size at the low temperature of 1500°C. This is mainly because the large CaF2 particle size tended to reduce the reaction rate between CaF2 and Al2O3, leading to the slow formation rate of Ca-aluminates. This observation further demonstrated that the Ca-aluminates played a central role in the nitridation process. When the temperature increased to 1800°C, full nitridation

AlN particles via the dissolution-precipitation mechanism.

sion fraction of the products synthesized at 1500 and 1800°C.

**2.3 Effects of the additive particle size**

**68**

*SEM images of the AlN products synthesized at 1800°C with various CaF2 contents: (a) 0, (b) 3, (c) 5, and (d) 10 wt.% [31].*


## **Table 1.**

*The AlN conversion fraction of the products synthesized with different CaF2 particle sizes at 1500 and 1800°C [29].*

was achieved for all samples, and the corresponding SEM images are shown in **Figure 5**.

As observed, the particle size of AlN products significantly decreased with increasing the particle size of the CaF2 additive. As mentioned, the large CaF2 particle size could lead to a low formation rate of liquid aluminates. Therefore, insufficient liquid aluminates existed in the system. As a result, small AlN particles were difficult to precipitate on the surface of large particles through the dissolutionprecipitation mechanism. The rearrangement and growth of AlN particles were limited, leading to the AlN products with a small particle size.
