**Table 3.**

*The AlN conversion fraction at different temperatures for the investigated samples [32].*

At 1400°C, all samples with additives showed a higher AlN conversion fraction than that of without additives, further indicating additives favored for the nitridation process. However, it should be noted that the sample ACF showed a lower AlN fraction than AP at 1500°C. This was mainly because most of CaF2 melts to the liquid phase due to the general melting point of 1418°C. As a result, excessive CaF2 liquid wrapped on the surface of raw materials, hindering the nitridation process. The rest of samples all present higher AlN conversion fraction than that of AP and ACF, in accordance with the order of ACFYF > AYO > ACFYO. The sample ACFYF presented the highest AlN conversion fraction among all samples. According to the studies of Qiao et al. [38], the formation of (Ca,Y)F2 intermediate and the further appearance of liquid Ca-Y-aluminates at a relatively low temperature of 1350°C were the main reasons for achieving the highest nitridation rate for the Al2O3-CaF2-YF3 system.

**Figure 10** further shows the SEM images of the AlN particles synthesized at 1800°C from various samples. As observed, micro-sized spherical AlN fillers were successfully synthesized from all samples, but their morphology and particle size were slightly different, which might be related to the different formation rate and viscosity of liquid aluminates in the process. From the whole, the sample ACF presented the highest individual sphericity and the best uniformity. Although the near-spherical morphology can also be observed in the remaining samples of AYO, ACFYO, and ACFYF, several hard agglomerates cannot be overlooked. **Figure 11** shows the typical morphology of these hard agglomerates. It can be seen that many compounds precipitated on the particle surface and the neck of sinter particles, which were identified as Y-aluminates by EDS analysis. This indicated that Y-containing compounds could not volatilize as Ca-containing compounds after the CRN reaction, but deposited in the system in the form of precipitates.

**75**

**Figure 10.**

**Figure 11.**

*1800°C for 2 h [32].*

*(b) AYO, (c) ACFYO, and (d) ACFYF [32].*

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

The appearance of these agglomerates was mainly related to the addition of Y2O3. According to other studies [47], in the CRN process, the additive Y2O3 firstly reacted with Al2O3 to form Y-aluminates, which were further reacted with carbon and N2 to

*SEM images of typical microstructures showing Y-aluminate compounds in the sample ACY5 after treatment at* 

*SEM images of the AlN particles synthesized at 1800°C from samples with various kinds of additives: (a) ACF,* 

Al2O3(s) + Y2O3(s)→Y−aluminates(l) (5)

Y−aluminates(l) + C(s) + N2(g)→AlN(s) + CO(g) + Y−compounds (6)

When the reaction temperature was high enough or the time was long enough,

form AlN based on the following equations:

the Y-aluminates could be reduced completely to Y2O3:

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

### **Figure 10.**

*Fillers - Synthesis, Characterization and Industrial Application*

*Compositions of the investigated powder mixtures [32].*

**Samples AlN conversion fraction (%)**

**Samples Formulation (in wt.%)**

*The AlN conversion fraction at different temperatures for the investigated samples [32].*

At 1400°C, all samples with additives showed a higher AlN conversion fraction than that of without additives, further indicating additives favored for the nitridation process. However, it should be noted that the sample ACF showed a lower AlN fraction than AP at 1500°C. This was mainly because most of CaF2 melts to the liquid phase due to the general melting point of 1418°C. As a result, excessive CaF2 liquid wrapped on the surface of raw materials, hindering the nitridation process. The rest of samples all present higher AlN conversion fraction than that of AP and ACF, in accordance with the order of ACFYF > AYO > ACFYO. The sample ACFYF presented the highest AlN conversion fraction among all samples. According to the studies of Qiao et al. [38], the formation of (Ca,Y)F2 intermediate and the further appearance of liquid Ca-Y-aluminates at a relatively low temperature of 1350°C were the main reasons for achieving the highest nitridation rate for the Al2O3-CaF2-YF3

AP 1.78 42.94 96.02 100.00 100.00 ACF 13.96 32.85 100.00 100.00 100.00 AYO 11.21 67.48 100.00 100.00 100.00 ACFYO 10.70 50.45 100.00 100.00 100.00 ACFYF 35.60 75.62 100.00 100.00 100.00

AP 66.7 33.3 — — — ACF 63.3 31.7 5.0 — — AYO 63.3 31.7 — 5.0 — ACFYO 63.3 31.7 3.0 2.0 — ACFYF 63.3 31.7 3.0 — 2.0

**1400°C 1500°C 1600°C 1700°C 1800°C**

**Al2O3 C CaF2 Y2O3 YF3**

**Figure 10** further shows the SEM images of the AlN particles synthesized at 1800°C from various samples. As observed, micro-sized spherical AlN fillers were successfully synthesized from all samples, but their morphology and particle size were slightly different, which might be related to the different formation rate and viscosity of liquid aluminates in the process. From the whole, the sample ACF presented the highest individual sphericity and the best uniformity. Although the near-spherical morphology can also be observed in the remaining samples of AYO, ACFYO, and ACFYF, several hard agglomerates cannot be overlooked. **Figure 11** shows the typical morphology of these hard agglomerates. It can be seen that many compounds precipitated on the particle surface and the neck of sinter particles, which were identified as Y-aluminates by EDS analysis. This indicated that Y-containing compounds could not volatilize as Ca-containing compounds after the CRN reaction, but deposited in the

**74**

system in the form of precipitates.

system.

**Table 3.**

**Table 2.**

*SEM images of the AlN particles synthesized at 1800°C from samples with various kinds of additives: (a) ACF, (b) AYO, (c) ACFYO, and (d) ACFYF [32].*

## **Figure 11.**

*SEM images of typical microstructures showing Y-aluminate compounds in the sample ACY5 after treatment at 1800°C for 2 h [32].*

The appearance of these agglomerates was mainly related to the addition of Y2O3. According to other studies [47], in the CRN process, the additive Y2O3 firstly reacted with Al2O3 to form Y-aluminates, which were further reacted with carbon and N2 to form AlN based on the following equations:

$$\text{Al}\_2\text{O}\_3\text{(s)} + \text{Y}\_2\text{O}\_3\text{(s)} \rightarrow \text{Y-aluminates(l)}\tag{5}$$

$$\text{Y-aluminates(I)} + \text{C(s)} + \text{N}\_2\text{(g)} \rightarrow \text{AlN(s)} + \text{CO(g)} + \text{Y-compounds} \quad \text{(6)}$$

When the reaction temperature was high enough or the time was long enough, the Y-aluminates could be reduced completely to Y2O3:

$$\text{Y-aluminates (l)} + \text{C(s)} + \text{N}\_2\text{(g)} \rightarrow \text{AlN}(\text{s}) + \text{CO(g)} + \text{Y}\_2\text{O}\_3(\text{s}) \tag{7}$$

According to the studies of sintering AlN ceramics, the existence of these Y-aluminates tended to hinder the connection between AlN grains, leading to the decrease of thermal conductivity of AlN products [48]. However, direct experimental evidence was still necessary for the spherical AlN fillers.
