Studies on Preparation and Characterization of Aluminum Nitride-Coated Carbon Fibers and Thermal Conductivity of Epoxy Matrix Composites

: In this work; the effects of an aluminum nitride (AlN) ceramic coating on the thermal conductivity of carbon ﬁber-reinforced composites were studied. AlN were synthesized by a wet-thermal treatment (WTT) method in the presence of copper catalysts. The WTT method was carried out in a horizontal tube furnace at above 1500 ◦ C under an ammonia (NH 3 ) gas atmosphere balanced by a nitrogen using aluminum chloride as a precursor. Copper catalysts pre-doped enhance the interfacial bonding of the AlN with the carbon ﬁber surfaces. They also help to introduce AlN bonds by interrupting aluminum oxide (Al 2 O 3 ) formation in combination with oxygen. Scanning electron microscopy (SEM); Transmission electron microscopy (TEM); and X-ray diffraction (XRD) were used to analyze the carbon ﬁber surfaces and structures at each step (copper-coating step and AlN formation step). In conclusion; we have demonstrated a synthesis route for preparing an AlN coating on the carbon ﬁber surfaces in the presence of a metallic catalyst.


Introduction
Thermal management has always been a major concern in the design of high frequency and power electronic devices. In order to avoid the malfunction of electronic devices, composites materials used in these devices should have high thermal conductivity and electrical insulator properties. Normally, high thermal conductive performance has been obtained by adding fillers due to the thermal conductive chains or networks produced by the fillers [1][2][3][4][5]. In order to improve the thermal conductivity of composites, various inorganic ceramic fillers with high thermal conductivity have been introduced into insulation composites; notable example include silicon nitride (SiN) [6][7][8], silicon carbide (SiC) [9], boron nitride (BN) [10,11], alumina oxide (Al 2 O 3 ) [12][13][14], and alumina nitride (AlN) [15][16][17]. Among the various ceramic fillers, aluminum nitride is an important ceramic material that has attracted considerable attention due to its excellent thermal conductivity (82-170 W/mK) and electrical resistance (>10 14 Ω·cm @ R.T.), as well as its high density, wide band gap, low dielectric constant, and low thermal expansion coefficient close to that of silicon [18][19][20][21][22][23][24][25][26][27][28]. It has good chemical stability and high hardness and is used in various structure refractory composite applications. AlN also leads to an increase in critical thermal shock [29,30]. A common characteristic of various AlN composites is their utility as thermal conducting filler in electronic industries.
Commonly, a high Poisson's ratio is required for a material to be used as heat transfer filler for thermal conduction, such as fibers, whiskers, and nanowires. However, the synthesis of AlN nanostructures via methods such as molecular beam epitaxy is complicated and expensive. In this light, an industrial and low cost method is needed. To address the aforementioned issues, we report a low-cost method for synthesizing AlN nanostructures doped on a carbon fiber having extremely high Poisson's ratio. Carbon fiber is a material consisting of good thermal conductivity that does not have restrictions in terms of length and has high Poisson's ratio [31,32]. Preparation of hybrid filler, based on the ceramic and carbon fiber, can be a novel method compared with conventional AlN synthetic methods.
This method is not suitable as an AlN synthesis method for inducing high thermal conductivity when used as a filler of a composite material. This is because the shape of the precursor that can be used is limited to spherical or particulate forms due to the characteristics to be produced at a high temperature, and since the shape of the final product, AlN, is also in the form of spherical particles, it can be limited in forming a thermal conduction network in the composite system. Although AlN synthesized from aluminum powder shows low cost and high crystallinity, it has a limitation as a filler for high thermal conductive composites because of the limit of Poisson's ratio.
In our previous report [8], SiN-coated carbon fibers were successfully manufactured by simple wet-thermal method using silane coupling agent, resulting in a hybrid filler with high Poisson's ratio and thermal conductivity. Therefore, it was deemed possible to apply this method to the production of AlN-coated carbon fibers. In this paper, by a direct wet-thermal method (WTT) process, AlN nanostructures with a hexagonal crystalline structure were synthesized at above 1500 • C. The complete synthesis is achieved at temperatures up to 1500 • C under flowing nitrogen-based gases and results in agglomerated AlN due to the low melting point of AlCl 3 which is less than the temperature required for nitridation. In addition, compared with other methods, this method is simple and requires inexpensive precursors.
To date, AlN having one-dimensional, two-dimensional and three-dimensional structure can be synthesized through various chemical routes as described above. However, these methods generally require high temperatures, substrates, catalyst and long-term production cycles [46]. Therefore, we proposed a new synthesis route for the production of AlN that can be synthesized at relatively low temperatures in a short time. Short reaction times, simple treatments and low processing costs can be achieved through the WTT method. In this study, the preparation and thermal properties of AlN synthesized by WTT method were analyzed according to the synthesis temperature and the use of catalyst.

Sample Preparation
All starting materials were used without further purification. The aluminum nitride (AlN) nanolayers were fabricated by a wet-thermal treatment method (WTT) with a nitrogen/ammonium mix atmosphere. Five weight percent of Reagent-grade aluminum chloride powder (AlCl 3 , purity 97%, DaeJung Chemical & Metals Co, Ltd., Siheung, Korea) was dissolved in a mixture of ethanol and water (a weight ratio of 95:5).
Prior to use carbon fibers (chopped-type carbon fiber, T300, Toray Co., Tokyo, Japan), each fiber samples were treated by 1.0 mol of nitric acid at room temperature for 30 min to remove surface impurities and form surface active sites such as phenolic hydroxyl groups which can help metal loading on fiber surface by enhanced physical adhesion strength such as polar-polar interaction.
Subsequently, acid-treated carbon fibers were coated with copper using the electroless process as described below. A first, pre-treatments of carbon fibers were prepared by sensitization and activation treatments. Samples were sensitized by dispersing them in a hydrochloric acid solution containing tin chloride (SnCl 2 ). The samples were then rinsed with distilled water. After rinsing, the surfaces were activated by adding the samples in another hydrochloric acid containing palladium chloride (PdCl 2 ) followed by washing thoroughly by with distilled water. These steps are necessary for the subsequent reactions during electroless plating. Details of the conditions used for each solution are shown in Table 1. Copper coated-carbon fibers were then added into an AlCl 3 solution for 60 min, filtered, and dried in an oven at 80 • C for 120 min. The synthesis of AlN nanostructures was investigated by varying the heating temperature conditions and was carried out in an alumina boat set in the center of an alumina tube mounted in a horizontal furnace. The furnace was heated to 1500 • C with a rate of 4 • C/min under a nitrogen gas (N 2 ) flow of 200 cc/min and maintained at 1500 • C for 60 min under a 1000 ppm concentration ammonia gas atmosphere. Ammonia (NH 3 ) gas flow of 100 cc/min was introduced to the tube. Finally, the sample was cooled to room temperature under the nitrogen atmosphere. Various temperatures conditions were applied using the same method as explained above, and this should explain name of various treated sample in this work ( Table 2).

Composites Preparation
For this study, a mixture of bisphenol-A-diglycidyl (YD-128, KUKDO Chem., Seoul, Korea) and 4,4 -diamino dipheyl methane (DDM, TCI, Seoul, Korea) was prepared with a 1:0.25 equivalent weight Coatings 2017, 7, 121 4 of 12 ratio in order to use the matrix of composites. First, DDM was grind to powder shape to reduce the dissolution time and added into YD-128 resin, and then the mixture was heated in an oil bath at 120 • C for 30 min. Compounding samples were prepared using as-received carbon fiber (ARCF) and AlN coated carbon fiber as a thermally conductive filler, respectively. Finally, each sample was cured using hot-press vacuum-bag-molding at 150 • C for 30 min with 7.5 MPa. The size of final composites was cut with 25 mm × 25 mm × 1 mm.
The fiber fraction (weight ratio) of all composites was fixed at 0.5 when the resin (epoxy + DDM) weight ratio was 1.

Characterization
The morphology and element composition of the samples was characterized by using field emission scanning electron microscopy (FE-SEM, SUPRA 40VP, 0.5-30 KV, Carl Zeiss Co., Oberkochen, Germany) as well as an energy X-ray dispersive spectroscopy (EDS, SUPRA 40VP, 0.5-30 KV, Carl Zeiss Co., Oberkochen, Germany) with mapping images. The samples were coated with platinum to enhance the image resolution and to prevent electrostatic charging. Additionally, high-resolution transmission electron microscopy (HR-TEM, JEM-ARM-200F, JEOL Co., Tokyo, Japan) was used to take images of synthesized products and their crystallite structure. The sheets were dispersed in ethanol by sonication for 10 min and some pieces were collected on carbon-coated 200-mesh copper grids for TEM observation. HR-TEM was equipped with selected-area electron diffraction (SAED). The crystalline phase identification of the samples was carried out by X-ray diffraction (XRD, X'pert 3 Powder, PANalytical Co., Almelo, The Netherlands). The diffraction data were collected in the range of 30-80 • with a scan speed of 2 • /s using Cu-Kα radiation (wavelength = 1.54056 Å). The thermal conductivity of the AlN-coated carbon fiber-reinforced composite was determined at room temperature by solid thermal conductivity equipment with transient thermal type measurement (TPS 500 S, Hot Disk AB Co., Göteborg, Sweden). Two identical samples were subjected to a thermal conductivity test using a 5465 sensor (including nickel wire and the polyimide insulation layer), and the end result was an average of three values.

Results and Discussion
The FE-SEM images of the samples with different surfaces treatments are shown in Figure 1. Elemental mapping analysis by EDS images were performed to confirm the elemental changes of the fiber surface according to each aluminum chloride (AlCl 3 ) and copper (Cu) surface treatment. The image of acid-treated carbon fiber is shown in Figure 1a. The shapes of the fiber surfaces treated with AlCl 3 (AlClF), Cu plating (CuF) and hybrid coating (AlCuF) were compared and shown. The hybrid coating was coated by a simple method of immersing Cu-plated carbon fibers in an AlCl 3 solution. The prepared hybrid coating sample of Figure 1d showed a spotty-shape on the surfaces of the carbon fiber, and the EDS mapping results show that the Al (red) and Cu (yellow) source on the carbon fiber surfaces and is a mixed pseudocolor representation of both images (Figure 1e). Figure 1f shows a schematic representation of the formation of the AlCuF. Figure 2 shows XRD and EDS analysis results of sample synthesized from AlCl 3 treated carbon fibers (AlClF) without the copper coating step, which was synthesized form AlClF under flowing N 2 and NH 3 at 1500 • C for 1 h. As presented in Figure 2b, the EDS image taken of the surface of the T15AOF can mean the Al 2 O 3 phase due to the presence of Al and oxygen. This result is correlated well with the XRD results of Al 2 O 3 , which is T15AOF shown in Figure 2a. No characteristic peak associated with other crystalline forms was detected in the XRD results, indicating that the sample is predominantly Al 2 O 3 . In comparison, Figure 3 shows the XRD results of samples synthesized at various temperatures with copper coating, acting like metallic catalyst, from AlCF. As observed in Figure 2a, a corresponding crystalline peak of typical AlN was not founded in T15AOF. Meanwhile, T15ANF showed some crystalline peaks of AlN at almost same preparation conditions with T15AOF except the presence of copper layer on the fiber surface.  It is well-known fact that the surface of carbon fibers has 10 to 15 percent of oxygen atoms [47]. When oxygen is present during AlN synthesis, Al-O bonds are formed more easily than Al-N bonds. For this reason, oxygen-free conditions are essential. Al-O is mainly formed by the oxygen released by the decomposition of the surface functional group of the carbon fiber during the high-temperature heat treatment, which is already confirmed in Figure 2. However, when the copper layer is present between the carbon fiber and the aluminum chloride, oxygen-free conditions can be formed because it acts as a trap to absorb oxygen molecules (formation of Cux-Oy) generated from the carbon fiber during the heat treatment. This condition can accelerate the formation of Al-N on carbon surfaces. On the other hand, it was confirmed that the crystallization of Al-N was increased and the Al-O was scarcely observed as the heat treatment temperature became closer to 1700 °C.
Furthermore, in order to investigate the effect of reaction temperature on the production of AlN nano-layers, the precursor from AlCuF was thermally treated at temperature of 1500 °C-1700 °C in a nitrogen-based gas. The increased temperature stimulated the growth and aggregation of the AlN   It is well-known fact that the surface of carbon fibers has 10 to 15 percent of oxygen atoms [47]. When oxygen is present during AlN synthesis, Al-O bonds are formed more easily than Al-N bonds. For this reason, oxygen-free conditions are essential. Al-O is mainly formed by the oxygen released by the decomposition of the surface functional group of the carbon fiber during the high-temperature heat treatment, which is already confirmed in Figure 2. However, when the copper layer is present between the carbon fiber and the aluminum chloride, oxygen-free conditions can be formed because it acts as a trap to absorb oxygen molecules (formation of Cux-Oy) generated from the carbon fiber during the heat treatment. This condition can accelerate the formation of Al-N on carbon surfaces. On the other hand, it was confirmed that the crystallization of Al-N was increased and the Al-O was scarcely observed as the heat treatment temperature became closer to 1700 °C.
Furthermore, in order to investigate the effect of reaction temperature on the production of AlN nano-layers, the precursor from AlCuF was thermally treated at temperature of 1500 °C-1700 °C in a nitrogen-based gas. The increased temperature stimulated the growth and aggregation of the AlN It is well-known fact that the surface of carbon fibers has 10 to 15 percent of oxygen atoms [47]. When oxygen is present during AlN synthesis, Al-O bonds are formed more easily than Al-N bonds. For this reason, oxygen-free conditions are essential. Al-O is mainly formed by the oxygen released by the decomposition of the surface functional group of the carbon fiber during the high-temperature heat treatment, which is already confirmed in Figure 2. However, when the copper layer is present between the carbon fiber and the aluminum chloride, oxygen-free conditions can be formed because it acts as a trap to absorb oxygen molecules (formation of Cu x -O y ) generated from the carbon fiber during the heat treatment. This condition can accelerate the formation of Al-N on carbon surfaces. On the other hand, it was confirmed that the crystallization of Al-N was increased and the Al-O was scarcely observed as the heat treatment temperature became closer to 1700 • C.
Furthermore, in order to investigate the effect of reaction temperature on the production of AlN nano-layers, the precursor from AlCuF was thermally treated at temperature of 1500 • C-1700 • C in a nitrogen-based gas. The increased temperature stimulated the growth and aggregation of the AlN crystallites. Figure 3 shows that as the reaction temperature increases, the (100) peak of XRD and AlN products formation is completed. As the synthesis temperature increased, the 110 peak at 33.3 • became narrower in shape and the intensity became stronger. The Al 2 O 3 peak was decreased and the formation of AlN was increased in the product as the reaction temperature was increased. It was observed that the T15ANF was composed of two phases; hexagonal AlN and Al 2 O 3 , and Synthesis of AlN coated carbon fibers from AlCl 3 and copper hybrid treated carbon fibers at 1600 • C (T16ANF) was implying the transformation of Al 2 O 3 . A few weak peaks were also found in the XRD patterns, and they originate from the remaining Al 2 O 3 . However, the T17ANF only contained one phase; hexagonal AlN. XRD patterns of T17ANF prepared at high temperature did not exhibit any detectable peaks other than those assigned to AlN (Synthesis of AlN coated carbon fibers from AlCl 3 and copper hybrid treated carbon fibers at 1700 • C), as shown in Figure 3a. This demonstrated that as the temperature increased, the Al 2 O 3 gradually disappeared and then pure AlN was obtained. The AlN phase of the three samples indexed as hexagonal AlN and the three intense peaks at 2θ = 33.3 • , 38.1 • , 59.5 • were assigned to the plane of hexagonal AlN (100), (101), and (110), respectively, agreeing well with the calculated diffraction patterns. The overwhelming (002) peak implies that the AlN nanostructures grow preferentially along the c-axis. The corresponding structural model is presented in Figure 3. As shown in Figure 3, the growth of AlN crystals is caused by the combination of continuous Al and N atoms at the edge region [45,48].
Coatings 2017, 7,121 crystallites. Figure 3 shows that as the reaction temperature increases, the (100) peak of XRD and AlN products formation is completed. As the synthesis temperature increased, the 110 peak at 33.3° became narrower in shape and the intensity became stronger. The Al2O3 peak was decreased and the formation of AlN was increased in the product as the reaction temperature was increased. It was observed that the T15ANF was composed of two phases; hexagonal AlN and Al2O3, and Synthesis of AlN coated carbon fibers from AlCl3 and copper hybrid treated carbon fibers at 1600 °C (T16ANF) was implying the transformation of Al2O3. A few weak peaks were also found in the XRD patterns, and they originate from the remaining Al2O3. However, the T17ANF only contained one phase; hexagonal AlN. XRD patterns of T17ANF prepared at high temperature did not exhibit any detectable peaks other than those assigned to AlN (Synthesis of AlN coated carbon fibers from AlCl3 and copper hybrid treated carbon fibers at 1700 °C), as shown in Figure 3a. This demonstrated that as the temperature increased, the Al2O3 gradually disappeared and then pure AlN was obtained. The AlN phase of the three samples indexed as hexagonal AlN and the three intense peaks at 2θ = 33.3°, 38.1°, 59.5° were assigned to the plane of hexagonal AlN (100), (101), and (110), respectively, agreeing well with the calculated diffraction patterns. The overwhelming (002) peak implies that the AlN nanostructures grow preferentially along the c-axis. The corresponding structural model is presented in Figure 3. As shown in Figure 3, the growth of AlN crystals is caused by the combination of continuous Al and N atoms at the edge region [45,48].    Table 3 shows the results of analysis of the L a and L c values of the samples synthesized at different temperatures. In the case of T15ANF, it was confirmed that L a and L c values were the highest. On the other hand, in the case of T16ANF, L a and L c values were significantly reduced as compared with those of T15ANF, and T17ANF was slightly increased compared with T16ANF. The XRD results in Figure 3 show that T15ANF contains a high content of Al 2 O 3 . In T16ANF, AlN is a major component, and some Al 2 O 3 was observed, and Al 2 O 3 was not observed at all in T17ANF, indicating that it is a crystal structure composed of AlN. The reported lattice constants [49] of Al 2 O 3 are L a = 4.785 (Å) and L c = 12.99 (Å). On the other hand, the lattice constant of AlN has L a = 3.11 (Å) and L c = 4.98 (Å), which are smaller than Al 2 O 3 . Based on the above report, it can be seen that T15ANF shows the highest L a and L c values because it has a mixed crystal phase of Al 2 O 3 and AlN but Al 2 O 3 rich composition.
In the case of T16ANF, it is considered that the Al 2 O 3 peak is decreased and AlN peak is enhanced as the temperature is increased. It is thinkable that the value of L c and L a is remarkably decreased because Al 2 O 3 having a relatively large crystal size is predominantly decreased. T17ANF has a slightly increased lattice constant value compared to T16ANF, which can be resulted by the influence of the crystal peaks of AlN grown perfectly as compared with the unstable AlN peak of T16ANF as the temperature increases. Figure 4 shows the result of elemental analyzing the surface composition of the synthesized sample using EDS in more detail. EDS results show that the content of N element in T15ANF can be considered to be almost insignificant. T15ANF is composed of Al 2 O 3 with a small amount of AlN, but it is considered that relatively weak N element was not detected as compared with Al and O elements. (N element is not detected maybe due to the limit of the instrumental resolution.) As the reaction temperature was increased to 1700 • C (T17ANF), while, the O element decreased and the N element increased. From the surface composition analysis of the samples via EDS, it was confirmed that the amount of N element increased as the reaction temperature was raised. Therefore, it could be concluded that the synthesis of AlN could be controlled by the use of the copper layer and adjusting the reaction temperature. Figure 5 shows the TEM images of the synthesized T15ANF and T17ANF samples. The T15ANF samples were composed of bough (or twig)-shaped nanocrystallites. In the case of T15ANF showing crystal peaks in which Al 2 O 3 and AlN are mixed (XRD in Figure 3), a TEM image of a typical twig shape can be identified by mixing Al 2 O 3 in powder or particulate form [50] and AlN form showing plate or linear shape [51,52] (Figure 5a). T17ANF shows a typical AlN crystal peak, and the corresponding TEM image also shows a linear shape. The selected area electron diffraction (SAED) of the samples is inserted in Figure 5 and the crystal indices are calibrated in the photograph. According to the calibration of the diffraction lattice, it is determined to be hexagonal AlN (Figure 5b insert images), which is consistent with the results of the Figure 3b in XRD pattern. In the EDS analysis, the T17ANF showed that the chemical composition mainly consisted of Al and N element (Figure 4c). The SAED pattern of the AlN (Figure 5b)   (N element is not detected maybe due to the limit of the instrumental resolution.) As the reaction temperature was increased to 1700 °C (T17ANF), while, the O element decreased and the N element increased. From the surface composition analysis of the samples via EDS, it was confirmed that the amount of N element increased as the reaction temperature was raised. Therefore, it could be concluded that the synthesis of AlN could be controlled by the use of the copper layer and adjusting the reaction temperature.
(a)   Figure 5 shows the TEM images of the synthesized T15ANF and T17ANF samples. The T15ANF samples were composed of bough (or twig)-shaped nanocrystallites. In the case of T15ANF showing crystal peaks in which Al2O3 and AlN are mixed (XRD in Figure 3), a TEM image of a typical twig shape can be identified by mixing Al2O3 in powder or particulate form [50] and AlN form showing plate or linear shape [51,52] (Figure 5a). T17ANF shows a typical AlN crystal peak, and the corresponding TEM image also shows a linear shape. The selected area electron diffraction (SAED) of the samples is inserted in Figure 5 and the crystal indices are calibrated in the photograph. According to the calibration of the diffraction lattice, it is determined to be hexagonal AlN (Figure 5b insert images), which is consistent with the results of the Figure 3b in XRD pattern. In the EDS analysis, the T17ANF showed that the chemical composition mainly consisted of Al and N element The thermal conductivities of the composites were obtained by a hot disk method and the results of the experiment are exhibited in Figure 6. For ARCF reinforced polymer composites (ARCFP) without the AlN coating, the room-temperature thermal conductivity was 5.8 W/mK. With the addition of T15ANP, the value was similar to the thermal conductivity of ARCFP (5.6 W/mK) duo to the low Coatings 2017, 7, 121 9 of 12 crystallinity of Al 2 O 3 -rich/AlN layer. However, with the addition of T17ANP, the thermal conductivity of the composites was significantly enhanced to 7.8 W/mK which is about 1.4 times larger than that of ARCFP without AlN. This result can be explained that the high content and crystallinity of AlN-rich layer on T17ANF possibly led to the high thermal conductivity of the T17ANP sample.

Conclusions
In this paper, AlN nanostructure has been synthesized on carbon fiber surfaces by the reactions between AlCl3 and NH3 gas via a direct wet-thermal method with a metallic catalyst under synthesis temperature of 1500 °C to 1700 °C. The effect of the copper coating on the AIN synthesis was confirmed by comparing the samples synthesized according to the presence or absence of the copper coating from the AlCl3 treated carbon fibers. The elevated reaction temperature is beneficial for the growth of AlN, it was found that the temperature was a crucial experimental parameter in mediating the synthesis of AlN. The experimental results indicate that the obtained crystalline hexagonal AlN structure has high purity and consists of nanolayers. EDS and XRD profiles reveal that the yield of AlN nanostructures increased with the synthesis temperature up to 1700 °C in an ammonia (NH3) gas atmosphere balanced by a nitrogen atmosphere. Higher temperature not only increases the density of AlN but also have good crystallinity, which together induces a fast synthesis rate by increasing surface diffusion. In addition, the synthesized AlN nanolayers could be applied as a reinforcement to produce polymers with high thermal conductivity.
Author Contributions: Hyeon-Hye Kim is the main contributor to the article who performed the experiments. Analysis of the data was a joint effort by Youn-Sik Lee and Dong Chul Chung. Byung-Joo Kim conceived and designed the experiments as a corresponding author.

Conflicts of Interest:
The authors declare no conflicts of interest.

Conclusions
In this paper, AlN nanostructure has been synthesized on carbon fiber surfaces by the reactions between AlCl3 and NH3 gas via a direct wet-thermal method with a metallic catalyst under synthesis temperature of 1500 °C to 1700 °C. The effect of the copper coating on the AIN synthesis was confirmed by comparing the samples synthesized according to the presence or absence of the copper coating from the AlCl3 treated carbon fibers. The elevated reaction temperature is beneficial for the growth of AlN, it was found that the temperature was a crucial experimental parameter in mediating the synthesis of AlN. The experimental results indicate that the obtained crystalline hexagonal AlN structure has high purity and consists of nanolayers. EDS and XRD profiles reveal that the yield of AlN nanostructures increased with the synthesis temperature up to 1700 °C in an ammonia (NH3) gas atmosphere balanced by a nitrogen atmosphere. Higher temperature not only increases the density of AlN but also have good crystallinity, which together induces a fast synthesis rate by increasing surface diffusion. In addition, the synthesized AlN nanolayers could be applied as a reinforcement to produce polymers with high thermal conductivity.

Conclusions
In this paper, AlN nanostructure has been synthesized on carbon fiber surfaces by the reactions between AlCl 3 and NH 3 gas via a direct wet-thermal method with a metallic catalyst under synthesis temperature of 1500 • C to 1700 • C. The effect of the copper coating on the AIN synthesis was confirmed by comparing the samples synthesized according to the presence or absence of the copper coating from the AlCl 3 treated carbon fibers. The elevated reaction temperature is beneficial for the growth of AlN, it was found that the temperature was a crucial experimental parameter in mediating the synthesis of AlN. The experimental results indicate that the obtained crystalline hexagonal AlN structure has high purity and consists of nanolayers. EDS and XRD profiles reveal that the yield of AlN nanostructures increased with the synthesis temperature up to 1700 • C in an ammonia (NH 3 ) gas atmosphere balanced by a nitrogen atmosphere. Higher temperature not only increases the density of AlN but also have good crystallinity, which together induces a fast synthesis rate by increasing surface diffusion. In addition, the synthesized AlN nanolayers could be applied as a reinforcement to produce polymers with high thermal conductivity.