**1. Introduction**

With the rapid progress of microelectronics technology, electronic products and devices are developing toward the direction of miniaturization and high integration. Although powerful functions are introduced, the heat dissipation has become an important bottleneck restricting the development of electronic technology. In the field of heat dissipation, thermal interface materials (TIMs) play an important role. TIMs are mainly used to fill the microvoids or uneven holes generated by the contact between heating devices and radiators, establishing an effective channel and improving the efficiency of heat dissipation [1]. Therefore, the TIMs are receiving more and more attention.

In order to achieve optimum heat dissipation, the TIMs should have good ductility to fill the air gap completely. Polymeric materials have attracted increasing interest owing to their excellent processability, high ductility, and low cost. However, most of the polymers have thermal conductivity lower than 0.5 W/m K [2], which is difficult to meet the demand for heat dissipation. To solve this problem, one effective approach is to introduce high–thermal-conductivity fillers into the polymers. Inorganic ceramic powders have been considered as ideal fillers benefiting from their high thermal conductivity, low dielectric constant, and good insulating properties. At present, oxides and nitrides are the most commonly considered fillers.

Silica (SiO2) is widely used in the field of electronic heat dissipation because of its safety, reliability, and low cost [3, 4]. However, its intrinsic thermal conductivity is only 1.5–1.6 W/m K [5], so it is difficult to obtain composite materials with higher thermal conductivity. Zinc oxide (ZnO) has a high thermal conductivity up to 60 W/m K, but the high dielectric constant greatly restricts its practical application as fillers [6]. Beryllia (BeO) has the highest thermal conductivity (~240 W/m K) in all the inorganic oxides. However, the high cost and high toxicity make it unattractive for commercial use [5]. Comparatively, alumina (Al2O3) has a much higher thermal conductivity than SiO2 and also presents remarkable electrical and mechanical properties, as well as the cheap producing cost [7–9], so it is the most widely used commercial filler at present. Kozato et al. [7] successfully prepared epoxy composites filling with 60 vol% Al2O3, achieving a high thermal conductivity of 4.3 W/m K. However, due to the relatively low intrinsic thermal conductivity of 38~42 W/m K [5], it is still difficult for Al2O3 to prepare high-performance TIMs to satisfy the increasing heat-dissipation requirements in future.

Compared with oxides, nitride powders are more attractive owing to the relatively high thermal conductivity. For example, boron nitride (BN) has a thermal conductivity as high as 280 W/m K and also shows the stable chemical property [10]. Xu et al. [11] used modified BN particles as fillers and finally prepared the composites with a high thermal conductivity of 10.3 W/m K. Nevertheless, the high price still limits its wide application. Silicon nitride (Si3N4) has a low coefficient of thermal expansion and a low dielectric constant, but it has been seldom used as fillers for high-thermal conductivity TIMs owing to its moderate thermal conductivity of 86~120 W/m K [12].

In comparison, aluminum nitride (AlN) has attracted tremendous attention in the electronic industry thanks to its outstanding properties such as high intrinsic thermal conductivity (~320 W/m K), good electrical resistivity, low dielectric constant, and low thermal expansion coefficient close to that of silicon [13, 14]. Ohashi et al. [15] filled epoxy resin with 74 vol% approximately spherical AlN particles, obtaining a composite thermal conductivity as high as 8.2 W/m K. Zhou et al. [16] prepared TIMs using angular AlN powders to replace Al2O3, and the composite thermal conductivity was increased to 2.6 times with a filling fraction of 68.5 vol%. Therefore, AlN fillers have shown prosperous application prospects for preparing high-performance TIMs.

Besides the intrinsic thermal conductivity of fillers, the thermal properties of TIMs are also affected by the filling fraction, the shape and particle size of fillers. In order to prepare the composites with higher thermal conductivity, it is important to raise the filler loading as high as possible and meanwhile retain the good fluidity of the composites for facile processability [17]. Compared with angular and plate-like particles, spherical fillers offer greater advantages in this regard owing to their better fluidity in the polymers [2]. In addition, it is generally believed that the thermal conductivity of composites increases with increasing the particle size of fillers [18]. This can be explained by the following two reasons: on the one hand, larger particles tend to result in the smaller fillers/matrix interfaces, leading to less photon scattering and lower thermal resistance; on the other hand, the fillers with a larger particle size are more easy to achieve higher filling fraction owing to the better fluidity and lower viscosity of the fillers/matrices. In general, the micro-sized fillers can give higher composite thermal conductivity than nano-sized fillers. Therefore, with the above considerations in mind, it is great significant and imperative to synthesize micro-sized spherical AlN particles as thermally conductive fillers for the next generation TIMs.

Despite the potentially high commercial importance, the large-scale synthesis of micro-sized spherical AlN fillers remains a huge challenge to date since nitrides tend to decompose at high temperature and cannot be converted to a spherical morphology just by the traditional surface tension method [19]. Up to now, limited related literatures can be retrieved. Among the existing studies, Ohashi et al. [15]

**65**

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

fillers with high sphericity and enhanced properties.

spherical AlN particles will be discussed as well.

**2. Influence of carbothermal synthetic parameters**

against humidity.

successfully synthesized spherical AlN particles via solution-reprecipitation treatment of angular AlN powders in a low-melting Ca-Al-O flux. Nevertheless, the particle size of the products was limited by the raw AlN powders, and the necessary hydrochloric acid treatment for removing residual Ca-Al-O was harmful to the product purity and the environment as well. Chowdhury et al. [20] first prepared core-shell structured C@Al2O3 composited particles, following the nitridation process in the flowing nitrogen at high temperature to ultimately obtain spherical AlN particles. However, the sphericity of the final product is very low due to the limitation of the heterogeneous mixing process. Suehiro et al. [21] synthesized spherical AlN particles by gas nitridation of spherical Al2O3, using a NH3-C3H8 gas mixture as the reduction-nitridation agent, but the undesired impurities were still presented owing to the incomplete conversion. In addition, an effective two-step method, involving the freezing granulation and subsequent sintering process, was also developed to prepare spherical AlN granules with the high sphericity and particle size more than tens of microns [22, 23]. Nevertheless, the AlN was used as raw materials, resulting in the relatively high production cost. Therefore, it is still highly desirable to explore suitable methods to directly synthesize spherical AlN

In general, commercial AlN powders are mainly synthesized by two methods. One is the direct nitridation of aluminum powders with N2 or NH3 (2Al + N2 → 2AlN); the other one is carbothermal reduction nitridation (CRN) of alumina powders in the presence of N2 (Al2O3 + 3C + N2 → 2AlN + 3CO) [24]. Comparatively, the CRN method is a better choice for industry production since the resultant AlN powders exhibit more attractive properties such as high purity, facile sinterability, and resistance

To date, many efforts have been devoted to ameliorating the quality of AlN powders synthesized by the CRN method [25–28]. Unfortunately, most of them just aimed at fabricating fine or ultrafine AlN powders via low-temperature synthesis to improve the sintering ability and reduce the fabricating cost. The obtained nano or submicron particles are too small to meet the basic requirements as promising fillers. Until recently, increasing attention has been paid on the carbothermal synthesis of coarser AlN granules, especially micro-sized spherical AlN fillers. Based on a series of studies [29–33], the authors have successfully synthesized micro-sized spherical AlN fillers by using appropriate additives and high-pressure N2 in the CRN process. The as-synthesized AlN granules presented high sphericity, uniform size distribution, and good dispersing behavior, which exhibited great potential as high-performance thermally conductive fillers. Based on this, this chapter will focus on the research progress in the carbothermal synthesis of spherical AlN fillers. The influence of various synthetic parameters on the morphology and particle size of final products will be summarized, and the growth mechanism of micro-sized

The typical carbothermal process for synthesizing spherical AlN fillers consists of three steps: first, the raw materials (Al2O3, carbon black, and additives) were homogenized by ball milling; second, the CRN process was conducted in a graphite furnace with a high temperature and an elevated N2 gas pressure; finally, the obtained powders were transferred to a muffle oven and heated in air to remove the residual carbon. It was found that various synthetic parameters, such as N2 pressure, additives, reaction temperature, reaction time, and carbon content had an important impact on the nitridation rate, the morphology, and the particle size of the final products.

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

*Fillers - Synthesis, Characterization and Industrial Application*

satisfy the increasing heat-dissipation requirements in future.

of 86~120 W/m K [12].

high-performance TIMs.

for the next generation TIMs.

thermal conductivity. Zinc oxide (ZnO) has a high thermal conductivity up to 60 W/m K, but the high dielectric constant greatly restricts its practical application as fillers [6]. Beryllia (BeO) has the highest thermal conductivity (~240 W/m K) in all the inorganic oxides. However, the high cost and high toxicity make it unattractive for commercial use [5]. Comparatively, alumina (Al2O3) has a much higher thermal conductivity than SiO2 and also presents remarkable electrical and mechanical properties, as well as the cheap producing cost [7–9], so it is the most widely used commercial filler at present. Kozato et al. [7] successfully prepared epoxy composites filling with 60 vol% Al2O3, achieving a high thermal conductivity of 4.3 W/m K. However, due to the relatively low intrinsic thermal conductivity of 38~42 W/m K [5], it is still difficult for Al2O3 to prepare high-performance TIMs to

Compared with oxides, nitride powders are more attractive owing to the relatively high thermal conductivity. For example, boron nitride (BN) has a thermal conductivity as high as 280 W/m K and also shows the stable chemical property [10]. Xu et al. [11] used modified BN particles as fillers and finally prepared the composites with a high thermal conductivity of 10.3 W/m K. Nevertheless, the high price still limits its wide application. Silicon nitride (Si3N4) has a low coefficient of thermal expansion and a low dielectric constant, but it has been seldom used as fillers for high-thermal conductivity TIMs owing to its moderate thermal conductivity

In comparison, aluminum nitride (AlN) has attracted tremendous attention in the electronic industry thanks to its outstanding properties such as high intrinsic thermal conductivity (~320 W/m K), good electrical resistivity, low dielectric constant, and low thermal expansion coefficient close to that of silicon [13, 14]. Ohashi et al. [15] filled epoxy resin with 74 vol% approximately spherical AlN particles, obtaining a composite thermal conductivity as high as 8.2 W/m K. Zhou et al. [16] prepared TIMs using angular AlN powders to replace Al2O3, and the composite thermal conductivity was increased to 2.6 times with a filling fraction of 68.5 vol%. Therefore, AlN fillers have shown prosperous application prospects for preparing

Besides the intrinsic thermal conductivity of fillers, the thermal properties of TIMs are also affected by the filling fraction, the shape and particle size of fillers. In order to prepare the composites with higher thermal conductivity, it is important to raise the filler loading as high as possible and meanwhile retain the good fluidity of the composites for facile processability [17]. Compared with angular and plate-like particles, spherical fillers offer greater advantages in this regard owing to their better fluidity in the polymers [2]. In addition, it is generally believed that the thermal conductivity of composites increases with increasing the particle size of fillers [18]. This can be explained by the following two reasons: on the one hand, larger particles tend to result in the smaller fillers/matrix interfaces, leading to less photon scattering and lower thermal resistance; on the other hand, the fillers with a larger particle size are more easy to achieve higher filling fraction owing to the better fluidity and lower viscosity of the fillers/matrices. In general, the micro-sized fillers can give higher composite thermal conductivity than nano-sized fillers. Therefore, with the above considerations in mind, it is great significant and imperative to synthesize micro-sized spherical AlN particles as thermally conductive fillers

Despite the potentially high commercial importance, the large-scale synthesis of micro-sized spherical AlN fillers remains a huge challenge to date since nitrides tend to decompose at high temperature and cannot be converted to a spherical morphology just by the traditional surface tension method [19]. Up to now, limited related literatures can be retrieved. Among the existing studies, Ohashi et al. [15]

**64**

successfully synthesized spherical AlN particles via solution-reprecipitation treatment of angular AlN powders in a low-melting Ca-Al-O flux. Nevertheless, the particle size of the products was limited by the raw AlN powders, and the necessary hydrochloric acid treatment for removing residual Ca-Al-O was harmful to the product purity and the environment as well. Chowdhury et al. [20] first prepared core-shell structured C@Al2O3 composited particles, following the nitridation process in the flowing nitrogen at high temperature to ultimately obtain spherical AlN particles. However, the sphericity of the final product is very low due to the limitation of the heterogeneous mixing process. Suehiro et al. [21] synthesized spherical AlN particles by gas nitridation of spherical Al2O3, using a NH3-C3H8 gas mixture as the reduction-nitridation agent, but the undesired impurities were still presented owing to the incomplete conversion. In addition, an effective two-step method, involving the freezing granulation and subsequent sintering process, was also developed to prepare spherical AlN granules with the high sphericity and particle size more than tens of microns [22, 23]. Nevertheless, the AlN was used as raw materials, resulting in the relatively high production cost. Therefore, it is still highly desirable to explore suitable methods to directly synthesize spherical AlN fillers with high sphericity and enhanced properties.

In general, commercial AlN powders are mainly synthesized by two methods. One is the direct nitridation of aluminum powders with N2 or NH3 (2Al + N2 → 2AlN); the other one is carbothermal reduction nitridation (CRN) of alumina powders in the presence of N2 (Al2O3 + 3C + N2 → 2AlN + 3CO) [24]. Comparatively, the CRN method is a better choice for industry production since the resultant AlN powders exhibit more attractive properties such as high purity, facile sinterability, and resistance against humidity.

To date, many efforts have been devoted to ameliorating the quality of AlN powders synthesized by the CRN method [25–28]. Unfortunately, most of them just aimed at fabricating fine or ultrafine AlN powders via low-temperature synthesis to improve the sintering ability and reduce the fabricating cost. The obtained nano or submicron particles are too small to meet the basic requirements as promising fillers. Until recently, increasing attention has been paid on the carbothermal synthesis of coarser AlN granules, especially micro-sized spherical AlN fillers. Based on a series of studies [29–33], the authors have successfully synthesized micro-sized spherical AlN fillers by using appropriate additives and high-pressure N2 in the CRN process. The as-synthesized AlN granules presented high sphericity, uniform size distribution, and good dispersing behavior, which exhibited great potential as high-performance thermally conductive fillers. Based on this, this chapter will focus on the research progress in the carbothermal synthesis of spherical AlN fillers. The influence of various synthetic parameters on the morphology and particle size of final products will be summarized, and the growth mechanism of micro-sized spherical AlN particles will be discussed as well.
