**2.4 Effects of reaction temperature**

**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

**Figure 5.**

*SEM images of the products synthesized with different CaF2 particle sizes at 1800°C: (a) ACF-S, (b) ACF-M, and (c) ACF-L [29].*

**Figure 6.**

*SEM images of the AlN products synthesized at various temperatures: (a) 1600 [31], (b) 1700 [31], (c) 1800, and (d) 1900°C.*

particles were obtained. As the temperature increased to 1700 and 1800°C, the tadpole-like morphology gradually disappeared, while the shape of AlN particles was changed from angular to spherical. However, when the temperature was further raised to 1900°C, the morphology was abnormal again, changing from spherical to angular.

This interesting observation can be understood from the formation and distribution of liquid aluminates. In general, two main reaction processes existed in the system: one is the formation process of Ca-aluminates through the reaction between Al2O3 and CaF2, and the other one is the nitridation process of Ca-aluminates, promoting the formation of AlN. The low formation rate and the high nitridation rate of Ca-aluminates could both result in the reduced content of liquid phases in the system, which would obviously affect the morphology of AlN particles. At a low temperature of 1600°C, the Ca-aluminates appeared in a small amount and tended to distribute unevenly in the system due to the slow reaction rate between CaF2 and Al2O3. As a consequence, AlN had a higher growth rate in the liquid concentration area, leading to the appearance of "tadpole tail." In addition, the small amount of liquid phases also resulted in a relatively slow material migration rate, thus the

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**Figure 7.**

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

with its own structure.

**2.5 Effects of reaction time**

images of the as-synthesized powders.

AlN granules preferred growing into the angular morphology according to the crystal structure [41]. When the temperature was increased to 1700 and 1800°C, the amount of liquid Ca-aluminates remarkably increased. The rapid material migration aiding with the liquid phases eventually promoted the formation of the spherical morphology with the lowest surface free energy. However, when the temperature was further increased to 1900°C, the nitridation rate of liquid Ca-aluminates increased significantly, even larger than that of the formation rate. In other words, the Ca-aluminates were nitrided immediately as soon as they were formed. There were not enough liquid phases in the system to modify the morphology of AlN. As a result, AlN presented the angular morphology again in accordance

To investigate the effects of reaction time on the morphology of AlN products, the raw materials with 5 wt.% CaF2 were heated at 1800°C and under the N2 pressure of 1 MPa for various reaction times (0.5, 1, 2, and 4 h). **Figure 7** shows the SEM

As the reaction time prolonging from 0.5 to 1 h and further to 2 h, the size and uniformity of AlN particles significantly increased. It was expected that the dissolution and reprecipitation of AlN aiding with liquid phases contributed greatly to the particle growth. In addition, as the prolongation of reaction time, the residual liquid phases in the system could also adjust themselves under the action of the interfacial energy to uniformly wrap the AlN particles, further improving the uniformity of the particle size. However, when the reaction time further increased to 4 h, the growth of AlN particles was no longer obvious. Instead, most of the AlN particles were sintered each other to form large aggregates, and the individual particles tended to change from the spherical to angular morphology, as demonstrated by the

*SEM images of the AlN products synthesized for various reaction times: (a) 0.5, (b) 1, (c) 2, and (d) 4 h [29].*

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

*Fillers - Synthesis, Characterization and Industrial Application*

particles were obtained. As the temperature increased to 1700 and 1800°C, the tadpole-like morphology gradually disappeared, while the shape of AlN particles was changed from angular to spherical. However, when the temperature was further raised to 1900°C, the morphology was abnormal again, changing from spherical to

*SEM images of the AlN products synthesized at various temperatures: (a) 1600 [31], (b) 1700 [31], (c) 1800,* 

*SEM images of the products synthesized with different CaF2 particle sizes at 1800°C: (a) ACF-S, (b) ACF-M,* 

This interesting observation can be understood from the formation and distribution of liquid aluminates. In general, two main reaction processes existed in the system: one is the formation process of Ca-aluminates through the reaction between Al2O3 and CaF2, and the other one is the nitridation process of Ca-aluminates, promoting the formation of AlN. The low formation rate and the high nitridation rate of Ca-aluminates could both result in the reduced content of liquid phases in the system, which would obviously affect the morphology of AlN particles. At a low temperature of 1600°C, the Ca-aluminates appeared in a small amount and tended to distribute unevenly in the system due to the slow reaction rate between CaF2 and Al2O3. As a consequence, AlN had a higher growth rate in the liquid concentration area, leading to the appearance of "tadpole tail." In addition, the small amount of liquid phases also resulted in a relatively slow material migration rate, thus the

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angular.

**Figure 6.**

*and (d) 1900°C.*

**Figure 5.**

*and (c) ACF-L [29].*

AlN granules preferred growing into the angular morphology according to the crystal structure [41]. When the temperature was increased to 1700 and 1800°C, the amount of liquid Ca-aluminates remarkably increased. The rapid material migration aiding with the liquid phases eventually promoted the formation of the spherical morphology with the lowest surface free energy. However, when the temperature was further increased to 1900°C, the nitridation rate of liquid Ca-aluminates increased significantly, even larger than that of the formation rate. In other words, the Ca-aluminates were nitrided immediately as soon as they were formed. There were not enough liquid phases in the system to modify the morphology of AlN. As a result, AlN presented the angular morphology again in accordance with its own structure.
