**3.1 Growth mechanism of the AlN particle size**

In the process of carbothermal synthesis of spherical AlN fillers, two typical processes occurred: the reaction between Al2O3 and additives to form the liquid aluminates and the further reduction and nitridation of aluminates to produce AlN. After formation, the process of AlN particles enlargement could be divided into three stages: nucleation, growth, and coarsening [49]. First, AlN was nucleated in the liquid aluminates, and the number of AlN grains was continuously increased. After that, the newly generated AlN grew on the basis of crystal nucleus, and the AlN particle size was continuously improved. Finally, as the reaction entered the later stage, the small AlN particles were constantly swallowed by the large particles, leading to the rearrangement and coarsening of AlN particles. Therefore, the AlN particle size was determined jointly by the three stages. The low nucleation rate, high growth rate, and high coarsening rate were all advantageous to obtain the AlN fillers with a large particle size.

As demonstrated, the elevated N2 pressure favored the formation of AlN granules with a large particle size. Under a high N2 pressure, the release of CO vapor was inhibited, becoming a barrier to limit the contact between Al2O3 and N2. As a result, the AlN nucleation rate was significantly decreased, resulting in a large particle size.

The AlN growth rate was mainly influenced by the formation and nitridation rate of liquid aluminates. In general, when the nucleation rate and holding time were constant, the fast growth rate meant that AlN particles stayed longer in the coarsening stage, which was conducive to the particle size growth. For example, both the small CaF2 particle size and high reaction temperature could accelerate the formation rate of aluminates, enhancing the AlN growth rate and further promoting the appearance of large AlN particles.

In the later stage of the reaction, the coarsening rate played a decisive role in the final particle size. Since small particles had higher interface energy, they were more easy to be swallowed by the large particles in order to reduce the overall energy of the system, leading to the growth of the AlN particle size. Therefore, the coarsening stage could be considered as the process of interface migration, and the coarsening rate was affected by the reaction time, temperature, and the difficulty of interface migration.

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smooth spherical morphology.

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

demonstrated in Section 2.5.

AlN particles as well.

It could be easily understood that longer reaction time meant longer coarsening stage, hence larger particles were more easy to be obtained. However, it should be noticed that too long reaction time could lead to the complete disappearance of small particles, and the interface migration required much more energy than the system could provide. Thus, the growth of the particle size was not that obvious, as

High reaction time tended to accelerate the rate of interface migration, promoting the growth of the particle size as well. In addition, when more liquid aluminates existed in the system, it got much easier for small particles to migrate to large particles, so the coarsening rate was significantly increased. This can well explain

Moreover, the final particle size was also influenced by the dispersion of second-

In summary, the particle size of the as-synthesized AlN fillers was influenced by a variety of synthetic parameters. In the preparation process of the raw materials, decreasing the additive particle size, increasing the additive content, and decreasing the amount of the carbon black all could lead to the increase of the AlN particle size. Additionally, using a high N2 gas pressure, increasing the reaction temperature and prolonging the holding time in the CRN reaction process could result in larger

According to the comprehensive investigations in Section 2, it can be concluded that the amount and distribution of liquid aluminates were important for the formation of the spherical AlN morphology. Based on the crystal growth theory [41], crystals preferred to grow into the lowest energy state. When there were no liquids in the system, the AlN morphology was mainly determined by the internal intrinsic structure. Thus, AlN particles tended to grow in the angular morphology to achieve the most stable state of energy. However, when additives were used in the CRN process, AlN was nucleated in the liquid aluminates, and thus its morphology was consequentially influenced by the external liquid phase environment. The solid AlN and liquid phase constituted a common system, and hence the lowest solid-liquid interface energy became the main driving force for the growth of AlN. Compared with the angular morphology, spherical particles showed the least specific surface area and the corresponding lowest solid-liquid interface energy. Therefore, AlN tended to grow into the spherical morphology with the aid of liquid Ca-aluminates. Based on the above inference, only when the AlN particles were completely wrapped in the liquids during the growth stage, the spherical morphology could be obtained. Thus, both the low liquid content and the fast AlN growth rate could lead to the disappearance of the solid-liquid interface and the difficulty to form the spherical shape. For example, when a low N2 pressure was used, the AlN growth rate was significantly increased so that the AlN particles could not be completely wrapped by the liquids, resulting in the poor sphericity. In addition, the low reaction temperature and the little additive content both led to the small liquid content, thus the spherical AlN particles could not be obtained as well. It can be concluded that the elevated N2 pressure, suitable additives, and relatively high reaction temperature greatly favored for carbothermally synthesizing AlN particles with a

why more CaF2 content resulted in a larger particle size in Section 2.2.

of AlN particles were limited, leading to the small AlN particle size.

**3.2 Sphericity mechanism of the AlN particle shape**

phase particles. As demonstrated in Section 2.6, excessive carbon black could become the barriers for small particles to large particles. The external growth space *Carbothermal Synthesis of Spherical AlN Fillers DOI: http://dx.doi.org/10.5772/intechopen.81708*

*Fillers - Synthesis, Characterization and Industrial Application*

tal evidence was still necessary for the spherical AlN fillers.

changes based on the experimental results.

fillers with a large particle size.

ing the appearance of large AlN particles.

**3.1 Growth mechanism of the AlN particle size**

**3. Formation mechanism of micro-size spherical AlN fillers**

Y−aluminates (l) + C(s) + N2(g)→AlN(s) + CO(g) + Y2O3(s) (7)

In the above section, we summarized the influence of synthetic parameters on the morphology of final AlN products. It can be concluded that high N2 pressure, suitable additives, and appropriate reaction temperature were essential to synthesize the micro-sized spherical AlN fillers. Compared with the traditional CRN method, the as-synthesized AlN fillers exhibited two main obvious morphological changes: the growth of the particle size and the sphericity of the particle shape. In this section, we will discuss the formation mechanism of the two morphological

In the process of carbothermal synthesis of spherical AlN fillers, two typical processes occurred: the reaction between Al2O3 and additives to form the liquid aluminates and the further reduction and nitridation of aluminates to produce AlN. After formation, the process of AlN particles enlargement could be divided into three stages: nucleation, growth, and coarsening [49]. First, AlN was nucleated in the liquid aluminates, and the number of AlN grains was continuously increased. After that, the newly generated AlN grew on the basis of crystal nucleus, and the AlN particle size was continuously improved. Finally, as the reaction entered the later stage, the small AlN particles were constantly swallowed by the large particles, leading to the rearrangement and coarsening of AlN particles. Therefore, the AlN particle size was determined jointly by the three stages. The low nucleation rate, high growth rate, and high coarsening rate were all advantageous to obtain the AlN

As demonstrated, the elevated N2 pressure favored the formation of AlN granules with a large particle size. Under a high N2 pressure, the release of CO vapor was inhibited, becoming a barrier to limit the contact between Al2O3 and N2. As a result, the AlN nucleation rate was significantly decreased, resulting in a large particle size. The AlN growth rate was mainly influenced by the formation and nitridation rate of liquid aluminates. In general, when the nucleation rate and holding time were constant, the fast growth rate meant that AlN particles stayed longer in the coarsening stage, which was conducive to the particle size growth. For example, both the small CaF2 particle size and high reaction temperature could accelerate the formation rate of aluminates, enhancing the AlN growth rate and further promot-

In the later stage of the reaction, the coarsening rate played a decisive role in the final particle size. Since small particles had higher interface energy, they were more easy to be swallowed by the large particles in order to reduce the overall energy of the system, leading to the growth of the AlN particle size. Therefore, the coarsening stage could be considered as the process of interface migration, and the coarsening rate was affected by the reaction time, temperature, and the difficulty of interface migration.

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 experimen-

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It could be easily understood that longer reaction time meant longer coarsening stage, hence larger particles were more easy to be obtained. However, it should be noticed that too long reaction time could lead to the complete disappearance of small particles, and the interface migration required much more energy than the system could provide. Thus, the growth of the particle size was not that obvious, as demonstrated in Section 2.5.

High reaction time tended to accelerate the rate of interface migration, promoting the growth of the particle size as well. In addition, when more liquid aluminates existed in the system, it got much easier for small particles to migrate to large particles, so the coarsening rate was significantly increased. This can well explain why more CaF2 content resulted in a larger particle size in Section 2.2.

Moreover, the final particle size was also influenced by the dispersion of secondphase particles. As demonstrated in Section 2.6, excessive carbon black could become the barriers for small particles to large particles. The external growth space of AlN particles were limited, leading to the small AlN particle size.

In summary, the particle size of the as-synthesized AlN fillers was influenced by a variety of synthetic parameters. In the preparation process of the raw materials, decreasing the additive particle size, increasing the additive content, and decreasing the amount of the carbon black all could lead to the increase of the AlN particle size. Additionally, using a high N2 gas pressure, increasing the reaction temperature and prolonging the holding time in the CRN reaction process could result in larger AlN particles as well.
