Efficient synthesis route to quasi-aligned and high-aspect-ratio aluminum nitride micro- and nanostructures

Chemical Engineering Journal 174 (2011) 461–466

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Efficient synthesis route to quasi-aligned and high-aspect-ratio aluminum nitride micro- and nanostructures Tae-Hyuk Lee a , Hayk H. Nersisyan c , Ha-Guk Jeong d , Kap-Ho Lee a , Jae-Soo Noh e , Jong-Hyeon Lee a,b,∗ a

Graduate School of Department of Advanced Materials Engineering, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Graduate School of Green Energy Technology, Chungnam National University, 220 Gungdong, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Rapidly Solidified Materials Research Center, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea d KITECH, 994-32 Dongchun-Dong, Yeonsu-Gu, Inchen 406-130, Republic of Korea e Department of Materials Engineering, Korea University of Technology and Education, 1600 Chungjeolno, Byeongchunmyun, Cheonan, Chungnam 330-708, Republic of Korea b c

a r t i c l e

i n f o

Article history: Received 7 July 2011 Received in revised form 5 September 2011 Accepted 11 September 2011 Keywords: Aluminum nitride Combustion synthesis Ceramic–metal systems Composites Transmission electron microscopy

a b s t r a c t Quasi-aligned, high-aspect-ratio AlN micro- and nanostructures were synthesized under high nitrogen pressure by the exothermic reaction of an Al + 0.015 mol (C2 F4 )n mixture. Structurally uniform AlN microand nanofibers with hexagonal and cylindrical morphologies were obtained when the system temperature was maintained within the range of 1600–1700 ◦ C. The fibers had aspect ratios as high as 2000, diameters in the range of ∼0.05–20 ␮m, and were ∼100–1000 ␮m in length. High-resolution transmission electron microscopic and selected area diffraction analyses indicated that the as-synthesized AlN micro- and nanostructures are perfectly single crystalline with preferential growth along the [0 0 1] direction. Branching was also observed in some of the micro-fibers, giving rise to randomized, two-dimensional comb textures. Based on the results obtained in the present study, a mechanism for the formation of AlN micro- and nanostructures under combustion conditions was proposed. © 2011 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional (1D) semiconductor micro- and nanostructures, such as rods, wires, belts, and tubes, may be used as building blocks for future electronics and photonics, and consequently, have attracted significant attention in recent years due to their many unique properties. These one-dimensional micro- and nanostructures are also expected to play a key role as both interconnects and functional units in the fabrication of electronic, optoelectronic, electrochemical and electromechanical nanodevices. In this class of materials, one-dimensional AlN is particularly advantageous due to its wide band gap (6.2 eV), large exciton binding energy (0.6 eV), high thermal conductivity (320 W/m K), high electrical resistivity (1013  cm), and small dielectric constant (8.8 at 1 MHz). These physical properties make AlN especially attractive for potential application in UV LED-based white lighting, surface acoustic wave devices, chip carriers, and heat sinks [1–8]. Several methods have been applied to the synthesis of one-dimensional AlN structures, including vapor–solid–liquid (VLS) growth processes [9–12], catalyst assisted growth [13–16], direct nitridation [17–19], carbothermal reduction [20], direct

∗ Corresponding author at: Graduate School of Green Energy Technology, Chungnam National University, 220 Gungdong, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea. Tel.: +82 42 821 6596; fax: +82 42 822 5850. E-mail address: [email protected] (J.-H. Lee). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.09.047

sublimation [21], gas-reduction–nitridation [22], and combustion synthesis (CS), which is also known as self-propagating high temperature synthesis [23–26]. Due to the time and energy efficient nature of the combustion synthesis procedure and its potential to facilitate the low-cost production of 1D AlN micro- and nanostructures, our research has focused primarily on this technique. The synthesis of 1D AlN structures by the combustion synthesis technique has been reported in a few studies, the majority of which utilized NH4 Cl and/or NH4 F additives to facilitate the formation of the desired structures. Shi et al. [23,24] reported the combustion synthesis of quasi-aligned AlN nanowhiskers at a low nitrogen gas pressure of 0.25 MPa. The initial reaction mixture consisted of Al, AlN diluent (40/60 mol%) as well as CaO and NH4 Cl additives (5 wt.% each). It was shown that the formation of AlN nanowhiskers through the vapor–solid (VS) mechanism was enhanced by NH4 Cl. Moya et al. [26] reported the self-propagating high-temperature synthesis (SHS) of AlN fibers elongated to lengths of up to 20 ␮m and with diameters of up to 40 ␮m from a mixture of Al, AlN, and NH4 Cl under N2 at 100 atm. These fibers were grown by a VS mechanism in the axial geode-like region of the cylindrical AlN cake obtained after the combustion. Jiang et al. [27,28] synthesized AlN whiskers by combustion of Al powder under high nitrogen pressure. Several types of whisker structures, such as wavy, crossbranched, and bead-necklace structures, and dendrite crystals were obtained by varying the growth conditions. Fu et al. [29] studied the effect of using an NH4 F additive and Fe mineralizer on the growth tendency of AlN fibers from Al powder at N2 pressures of 8 and

462

T.-H. Lee et al. / Chemical Engineering Journal 174 (2011) 461–466

12 MPa. On the basis of the clear detection of liquid droplets on the fibers, it was concluded that, in this case, the fibers were grown by a VLS mechanism. Of the additives used, the ammonium halide, NH4 Cl, was generally found to be the most versatile and most convenient for combustion synthesis of 1D micro- and nanostructures. However, the lengths of the nanofibers produced by the aforementioned techniques (including the combustion technique) are limited to lengths of less than 50 ␮m, depending on the growth conditions. Recently, Yazdi et al. [30] have successfully produced highly oriented AlN single-crystal micro- and nanowires with aspect ratios of up to 600, diameters in the range of 40–500 nm, and lengths of 100 ␮m using a VS growth technique at 1750 ◦ C and 850 Mbar (or 8.5 × 107 MPa). Despite their potential advantages, fabrication of high-quality, well-aligned AlN micro- and nanostructures remains a prime challenge, notwithstanding the large number of studies that have been carried out in this regard. Note that from a practical viewpoint long AlN fibers are desirable because longer AlN fibers afford stronger composites (in terms of tensile strength, shear strength, etc.), whereas short-fiber reinforced materials have been known to suffer from catastrophic failure. Long AlN fibers may also improve the thermal conductivity, elastic modulus, and other characteristics of their composites. In the present study, we have developed a simple and efficient combustion approach for the synthesis of 1D AlN micro- and nanofibers (single crystals) with aspect ratios of up to 2000, diameters in the range of ∼0.05–20 ␮m, and lengths of ∼100–1000 ␮m. This process is based on rapid nitridation of an Al + 0.015 mol (C2 F4 )n mixture under a nitrogen pressure of 2.5 MPa. 2. Experimental Aluminum spherical powder (AAl-10SF, particle size: ≤20 ␮m; Chang Sung Co. Ltd., Korea) with an average particle size of 10 ␮m was used as the feedstock powder. Teflon (C2 F4 )n powder (particle size: ≤10 ␮m, Aldrich) and copper spherical powder (particle size: ≤10 ␮m, Aldrich) were used as additives. Two reaction mixtures of different compositions, namely, Al + 0.015 (C2 F4 )n and 0.9Al + 0.1Cu + 0.015 (C2 F4 )n were used for nitridation purposes. Initial mixing of the powders was carried out for 15–20 min using a mortar and pestle, and the as-prepared powder mixture was loosely packed into a rectangular paper pocket with dimensions of 2.0 cm × 2.0 cm × 5.0 cm. The pocket was then placed in the center of a high-pressure reactor under a nickel–chromium wire. The air in the vessel was evacuated, and the combustion vessel was filled with nitrogen gas to a pressure of 2.5 MPa. Local ignition of the sample was initiated using a resistivity-heated nickel–chromium wire, after which a combustion wave was formed and propagated throughout the reaction mixture. The reaction mass was cooled to room temperature, and a fluffy, grayish-white mass was obtained. During the combustion process, a data acquisition system (GL200A, Graphtec Co., Japan) was used to continuously record the time histories of two tungsten–rhenium thermocouples (W/Re-5 versus W/Re-20; diameter, 100 ␮m) previously inserted into the reaction pellet for temperature measurements. Phase analysis of the final products was performed by X-ray diffraction (Siemens D5000, Germany) using CuK␣ radiation. Cross sections of the microstructures of the combusted samples were observed using scanning electron microscopy (SEM; JSM 5410, JEOL, Japan), field emission scanning electron microscopy (FESEM; JSM 6330F), and transmission electron microscopy (TEM, JEM 2010, Japan). 3. Results and discussion 3.1. Combustion process Our studies have shown that protective oxide layers on Al particles inhibit the process of aluminum nitridation under

Fig. 1. Time–temperature profiles for combustion of Al + 0.015 mol (C2 F4 )n (line 1) and 0.9Al + 0.1Cu + 0.015(C2 F4 )n systems (line 2).

self-sustaining combustion mode conditions. Therefore, a small amount of Teflon powder (0.015 mol) was added to the Al feedstock powder prior to the combustion process; the role of the Teflon powder is discussed in a later section. A typical temperature–time profile in the combustion wave of the Al + 0.015 mol (C2 F4 )n system is shown in Fig. 1, line 1. The rapid rise in temperature signifies the arrival of the localized reaction zone; and the subsequent temperature increase, followed by a gradual decline, represents a prolonged post-combustion stage (i.e., the afterburning period). The rise time of the temperature from ignition (T*) to the first quasi-isothermal field (T1 = 1400–1500 ◦ C) is several seconds. The system temperature is generally maintained within the range of 1400–1500 ◦ C for a duration of 15–20 s, followed by a second increase to the maximum point (Tc ). The entire combustion process takes about 40–60 s, and is followed by a cooling process. The temperature profiles clearly indicate an increase in the temperature from T* to T1 , which occurs due to nitridation of Al by the nitrogen gas enclosed inside the pores of the reaction mixture. The subsequent nitridation of Al is supported by filtration of nitrogen from the outside of the sample to its inner layers. This process results in a second increase of the temperature, from points T1 to Tc . Slow burning rates were observed for the reaction samples (0.04–0.05 cm/s) under 2.5 MPa nitrogen pressure. Addition of 0.1 mol of Cu powder to the initial mixture decreased the combustion temperature to 1580 ◦ C (Fig. 1, line 2), without significant effects on the general characteristics of the temperature–time profile. It is worth noting that the Cu powder in the combustion experiment may also lead to the formation of Al/Cu/AlN composites, which may be potentially applicable as attractive heat dissipation materials for electronic packages [31]. 3.2. SEM and XRD characterization Fig. 2(a–e) shows the typical morphological features of the product prepared from the Al + 0.015 mol (C2 F4 )n mixture. Fig. 2(a) is a high-magnification image of the transparent-white, fluffy mass obtained after the combustion process. The SEM micrograph shows that most of these fibers are straight and very long compared to those previously reported [9–30]. The average diameter of the fibers, estimated from the SEM image, is in the range of ∼0.05–20 ␮m, and the length ranges from ∼100 to 1000 ␮m. Fig. 2(b) shows three parallel AlN microfibers: two of them have perfect hexagonal structures, whereas the other has a cylindrical morphology. Fig. 2(c) shows the oriented growth of fibers from the molten reaction mass. Some of these fibers were converted to

T.-H. Lee et al. / Chemical Engineering Journal 174 (2011) 461–466

463

Fig. 2. SEM micrographs (a–c, e and f) and EDS analysis (d) of AlN fibers produced from Al + 0.015 mol (C2 F4 )n mixture.

perfect hexagonal crystals as shown in the inset (upper right) of Fig. 2(c). EDS analysis of the fibers and “substrate” indicates the presence of only Al and N as shown in Fig. 2(d). The diameter of the fibers is generally uniform and most have a smooth surface as demonstrated in Fig. 2(e). Another fragment demonstrating the oriented growth of the fibers from the molten mass is shown in Fig. 2(f). All of the fibers are very straight and smooth and no droplets are observed in the micrographs. The morphology of these nanowires suggests likely formation by the VS mechanism; a topic which will be discussed in the later sections. Some AlN fibers having pyramid-like bases were also observed in the final products (Fig. 3(a)). The pyramids are micrometer-sized and possibly formed as a result of minimization of the surface energy. However, only a small portion of the fibers have these bases. Therefore, it can be deduced that the fiber growth process is not contingent on the formation of these pyramids. In some cases, the growth of fibers shows a switch from normal to lateral

growth, resulting in microrods as shown in Fig. 3(b). Yazdi et al. [30] reported that lateral growth occurs due to a decrease of the nitrogen concentration in a quasi-closed growth cell. In the current case, lateral growth may similarly be related to a low concentration of AlF3 in the reaction zone. Interestingly, a branching phenomenon was observed for some of the AlN fibers grown within this system. Fig. 4(a and b) shows several peripheral fibers that branch from a single fiber to form a 2D, comb-like, randomized texture. These branches are uniform and grow parallel to each other at an angle of 90◦ to the “mother” fiber. The branching phenomenon observed for the AlN wires may be related to the formation of new crystallization centers during the synthesis. These crystallization centers may contain elements such as C, F and N in addition to Al. A detailed analysis of the branching phenomenon is a topic for further investigation. Worth noting is that the AlN branched structures are scientifically attractive, because they provide the opportunity to create crystalline

464

T.-H. Lee et al. / Chemical Engineering Journal 174 (2011) 461–466

Fig. 3. (a) AlN microfiber with pyramid like base; (b) AlN microrod.

2D and 3D networks by manipulation of the synthesis parameters [32,33]. The SEM micrographs of the AlN nanowires produced upon addition of the Cu diluent are shown in Fig. 5. The overall view of the sample presented in Fig. 5(a) reveals the large-scale, highdensity, formation of AlN nanowires. Higher magnification SEM images Fig. 5(b and c) revealed a decrease in the diameter of the Cu-added fibers relative to those of the fibers prepared from the Al + 0.015(C2 F4 )n mixture. The diameters of the as-produced Cuadded nanowire fibers estimated from the SEM images are in the 50–1000 nm range with lengths of 100 ␮m and greater. Under the optimized experimental conditions, the volume ratio of AlN fibers in the final combustion product when Cu is added may reach 3–5%. We postulate that this decrease in fiber production was mainly promoted by the drop in the combustion temperature from 1670 to 1580 ◦ C when 0.1 mol of Cu was added to the mixture (Fig. 1). This drop in the temperature may cause a decrease in the concentration of Al in the gas phase (in the sample pores) and consequently, in the diameter of the AlN fibers. Additionally, the diameter of the AlN fibers may be also affected by the high thermal conductivity of Cu (400 W/m K) which may increase the cooling rate of the entire sample and thus reduce the period of formation of the AlN fibers and hence decrease the diameter. Varying the nitrogen pressure produced no positive influence on the size and uniformity of the AlN fibers. At a pressure below 2.0 MPa, the combustion wave becomes unstable and an attenuation of the process occurred frequently. Above 3.0 MPa pressure, a strong increase in the combustion temperature results in more

intensive melting and evaporation of aluminum and the volatile Al compounds, resulting in short AlN fibers with non-uniform morphology. The typical XRD pattern of the as-synthesized product presented in Fig. 6 shows that the obtained fluffy product can be identified as hexagonal AlN (h-AlN). The high crystallinity of the AlN fibers is indicated by the narrowness of the peaks observed in the XRD pattern. In addition, all of the peaks can be indexed to AlN; thus, no other phases are present in the final products, which indicates a complete transformation of Al to AlN and that there are no impurities (AlF3 , Al4 C3 , Al2 O3 , etc.) in the product. 3.3. Combustion chemistry and growth mechanism of AlN micro and nanofibers Our experimental results show that Teflon plays two key roles in the combustion process: first, it activates the nitridation of Al particles, and secondly, it promotes the formation of AlN fibers. Nitridation of the Al particles is activated in the presence of Teflon due to distortion of the oxide shells surrounding the Al particles. This process is represented by Eq. (1): 2Al2 O3 + 3(C2 F4 )n → 4nAlF3 + 6CO

(1)

Due to the high combustion temperature, molten Al in the pores rapidly pours out and reacts with nitrogen gas via a direct nitridation pathway. The reaction can be expressed as Eq. (2): Al(liq.) + N2 (gas) → AlN(solid)

Fig. 4. Branching phenomenon in AlN microfiber from Al + 0.015 mol (C2 F4 )n system.

(2)

T.-H. Lee et al. / Chemical Engineering Journal 174 (2011) 461–466

465

Fig. 5. SEM micrographs of AlN micro- and nanowires produced from 0.9Al + 0.1Cu + 0.015 (C2 F4 )n mixture.

AlF3 has relatively low melting (1040 ◦ C) and boiling (1272 ◦ C) points, and consequently, it may be transformed to the liquid and gas phases, thereby promoting the AlN formation process. In this case, the nuclei of AlN formed by reactions (2) and (4) tend to grow by the VS mechanism, whereby epitaxial growth of the AlN fibers is promoted by the continuous supply of AlF3 and N2 gases to the AlN nuclei. Therefore, growth of the AlN fibers occurred mainly in the pores of the sample where the gaseous species were highly concentrated. Note that the spontaneous fluorination–nitridation reactions were facile for the 1D AlN nanostructures [23–25]. From a simple analysis of the obtained results, it can be concluded that the growth rate of AlN fibers during the combustion process is remarkably high. The average growth rate of the fibers can be calculated as follows: V=

Fig. 6. XRD patterns of AlN fibers.

Direct nitridation of Al (reaction (2)) generally produces crystalline AlN particles as shown in Fig. 2(c), rather than AlN fibers. It is postulated that the addition of (C2 F4 )n to the starting Al powder offers an alternative reaction pathway for AlN formation, which involves spontaneous fluorination–nitridation sequences similar to that of the Al/NH4 Cl mixture reported in [25]. The ensuing reactions can be described according to the following equations, along with Eq. (2): 4nAl(liq.) + 3(C2 F4 )n → 4nAlF3 (gas) + 6nC(sol.)

(3)

2AlF3 (gas) + N2 → 2AlN(sol.) + 3F2 (gas)

(4)

Al(liq.) + F2 (gas) → AlF3 (gas)

(5)

1 100–2000 = = 1.5–30 ␮m/s t 30–60

(6)

where V is the growth rate, l is the length of the fiber and t is the combustion time. It is to be noted that the growth rate of nanofibers during vapor–solid growth is approximately 0.22 ␮m/s [30], which is ten times slower than that in the combustion process. This marked difference can be attributed to the presence of AlF3 phases and the unique conditions of the combustion processes. The growth orientation of the AlN nanofibers was investigated using the low-magnification TEM image, SAED pattern, and HRTEM image of a single nanofiber, 50 nm in diameter. The results are presented in Fig. 7. The SAED pattern can be indexed as the (0 0 2) and (1¯ 1 0) planes of h-AlN, which indicates a single-crystalline h-AlN nanofiber. During condensation of vapor phase AlN, the atoms may pack together in the lattice with the greatest possible density in the (0 0 2) plane. The lower left inset in Fig. 7 shows a low magnification TEM image of the h-AlN nanofiber. Further analysis of the structural characteristics of the h-AlN nanofibers by HRTEM (Fig. 7, mainframe) shows that the surface of the AlN fiber has a thickness of 2 nm and is structurally amorphous, which can be attributed to

466

T.-H. Lee et al. / Chemical Engineering Journal 174 (2011) 461–466

Fig. 7. Typical TEM (lower left inset) and HRTEM image (mainframe) of a single AlN nanowire; and SAED pattern taken along [0 0 1] (upper right inset).

oxidation. The inter-planar spacing is 0.498 nm (marked between the two arrows), which matches the spacing of the (0 0 1) planes of h-AlN, indicating that growth of the h-AlN nanowires is in the [0 0 1] direction. Both the HRTEM image and the SAED pattern (Fig. 7, upper right inset) show that the as-synthesized h-AlN nanofibers are perfectly single crystalline with a growth direction along [0 0 1]. 4. Conclusions AlN micro- and nanofibers were produced from an Al + 0.015 mol (C2 F4 )n mixture under a nitrogen pressure of 2.5 MPa by a newly developed, simple and efficient combustion synthesis approach. Large scale and structure-uniform AlN fibers with hexagonal and cylindrical morphologies and with aspect ratios of up to 2000, diameters in the range of ∼0.05–20 ␮m and lengths of ∼100–1000 ␮m were obtained at temperatures of 1600–1700 ◦ C. It was shown that AlN micro- and nanostructures were mainly grown by the VS mechanism, in which the continuous supply of AlF3 and N2 gases to the AlN nuclei promotes epitaxial growth of AlN fibers. XRD, SAED, and HRTEM studies suggested that the preferred growth of h-AlN nanowires occurred in the [0 0 1] direction along the axis. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials (10037206, Multi-functional Metal Matrix Composites by Controlled Reaction) funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] H. Morkoc, Handbook of nitride semiconductors and devices, in: Materials Properties, Physics and Growth, vol. 1, Wiley VCH, Verlag GmbH & Co., KGaA, WeinHeim, 2008. [2] Y. Sun, J.Y. Li, Y. Tan, L. Zhang, Fabrication of aluminum nitride (AlN) hollow fibers by carbothermal reduction and nitridation of electrospun precursor fibers, J. Alloys Compd. 471 (1–2) (2009) 400–403. [3] J.C. Kuang, C.R. Zhang, X.G. Zhou, Q.C. Liu, C. Ye, Formation and characterization of cubic AlN crystalline in a carbothermal reduction reaction, Mater. Lett. 59 (16) (2005) 2006–2010.

[4] S.M. Bradshaw, J.L. Spicer, Combustion synthesis of aluminum nitride particles and whiskers, J. Am. Ceram. Soc. 82 (9) (1999) 2293–2300. [5] G.A. Slack, R.A. Tanzilli, R.O. Pohl, J.W. Vandersande, The intrinsic thermal conductivity of AlN, J. Phys. Chem. Solids 48 (7) (1987) 641–647. [6] J. Liu, X. Zhang, Y.J. Zhang, R.R. He, J. Zhu, Novel synthesis of AlN nanowires with controlled diameters, J. Mater. Res. 16 (11) (2001) 3133–3138. [7] L.M. Sheppard, Aluminum nitride: a versatile but challenging material, Am. Ceram. Soc. Bull. 69 (11) (1990) 1801–1812. [8] A. Zangil, R. Ruh, Phase relationships in the silicon carbide–aluminum nitride system, J. Am. Ceram. Soc. 71 (10) (1988) 884–890. [9] Q. Wu, Z. Hu, X.Z. Zhang, Y.N. Lu, K.F. Huo, S.F. Deng, et al., Extended vapor–liquid–solid growth and field emission properties of aluminum nitride nanowires, J. Mater. Chem. 13 (8) (2003) 2024–2027. [10] Q. Wu, Z. Hu, X.Z. Wang, Y. Chen, Synthesis and optical characterization of aluminum nitride nanobelts, J. Phys. Chem. B 107 (36) (2003) 9726–9729. [11] Q. Zhao, H.Z. Zhang, X.Y. Xu, Z. Wang, J. Xu, D.P. Yu, Optical properties of high ordered AlN nanowire arrays grown on sapphire substrate, Appl. Phys. Lett. 86 (2005) 193101–193103. [12] J.H. He, R.S. Yang, Y.L. Chueh, L.J. Chou, L.J. Chen, Z.L. Wang, Aligned AlN nanorods with multi-tipped surfaces-growth, field-emissionand cathodoluminescence properties, Adv. Mater. 18 (5) (2006) 650–654. [13] V. Cimalla, C. Foerster, D. Cengher, K. Tonisch, O. Ambacher, Growth of AlN nanowires by metal organic chemical vapor deposition, Phys. Status Solidi B 243 (7) (2006) 1476–1480. [14] H. Wang, G. Liu, W. Yang, L. Lin, Z. Xie, J.Y. Fang, et al., Bi-crystal AlN zigzag nanowires, J. Phys. Chem. C 111 (46) (2007) 17169–17172. [15] Z. Chen, C.B. Cao, H.S. Zhu, Controlled growth of aluminum nitridenanostructures: aligned tips, brushes, and complex structures, J. Phys. Chem. C 111 (5) (2007) 1895–1899. [16] J.H. Duan, S.G. Yang, H.W. Liu, J.F. Gong, H.B. Huang, X.N. Zhao, AlN nanorings, J. Cryst. Growth 283 (2005) 291–296. [17] Y.J. Tian, Y.P. Jia, Y.J. Bao, Y.F. Chen, Macro-quantity synthesisof AlN nanowires via combined technique of arc plasma jet and thermal treatment, Diamond Relat. Mater. 16 (4–7) (2007) 302–305. [18] C.K. Xu, L. Xue, C.R. Yin, G.H. Wang, Formation and photoluminescence properties of AlN nanowires, Phys. Status Solidi A 243 (2003) 329–335. [19] M. Lei, H. Yang, Y.F. Guo, B. Song, P.G. Li, W.H. Tang, Synthesis and optical property of high purity AlN nanowires, Mater. Sci. Eng. B 143 (1–3) (2007) 85–89. [20] Y.J. Zhang, J. Liu, R.R. He, Q. Zhang, X.Z. Zhang, J.S. Zhu, Synthesis of aluminum nitride nanowires from carbon nanotubes, Chem. Mater. 13 (11) (2001) 3899–3905. [21] M. Lei, B. Song, X. Guo, Y.F. Guo, P.G. Li, W.H. Tang, Large-scale AlN nanowires synthesized by direct sublimation method, J. Eur. Ceram. Soc. 29 (11) (2009) 195–200. [22] T. Suehiro, J. Tatami, T. Meguro, K. Komeya, Aluminum nitride fibers synthesized from alumina fibers using gas-reduction–nitridation method, J. Am. Ceram. Soc. 85 (3) (2002) 715–717. [23] Z. Shi, M. Radwan, S. Kirihara, Y. Miyamoto, Z. Jin, Morphology-controlled synthesis of quasi-aligned AlN nanowhiskers by combustion method: effect of NH4 Cl additive, Ceram. Int. 35 (7) (2009) 2727–2733. [24] Z. Shi, M. Radwan, S. Kirihara, Y. Miyamoto, Z. Jin, Formation and evolution of quasi-aligned AlN nanowhiskers by combustion synthesis, J. Alloys Compd. 476 (1–2) (2009) 360–365. [25] M. Radwan, Y. Miyamoto, Growth of quasi-aligned AlN nanofibers by nitriding combustion synthesis method, J. Am. Ceram. Soc. 90 (8) (2007) 2347–2351. ˜ N.S. Makhonin, M.A. Rodríguez, Sin[26] J.S. Moya, J.E. Iglesias, J. Limpo, J.A. Escrina, gle crystal AlN fibers obtained by self-propagating high temperature synthesis (SHS), Acta Mater. 45 (8) (1997) 3089–3094. [27] G.J. Jiang, H.R. Zhuang, J. Zhang, M.L. Ruan, W.L. Li, F.Y. Wu, B.L. Zhang, Morphologies and growth mechanisms of aluminum nitride whiskers by SHS method—Part 1, J. Mater. Sci. 35 (1) (2000) 57–62. [28] G.J. Jiang, H.R. Zhuang, J. Zhang, M.L. Ruan, W.L. Li, F.Y. Wu, B.L. Zhang, Morphologies and growth mechanisms of aluminum nitride whiskers by SHS method—Part 2, J. Mater. Sci. 35 (1) (2000) 63–69. [29] R. Fu, K. Chen, S. Agathopoulos, J.M.F. Ferreira, Factors which affect themorphology of AlN particles made by self-propagating high-temperature synthesis (SHS), J. Cryst. Growth 296 (1) (2006) 97–103. [30] G.R. Yazdi, P.O.A. Person, D. Gogova, R. Fornari, L. Hultman, M. Syvajarvi, R. Yakomova, Aligned AlN nanowires by self-organized vapor–solid growth, Nanotechnology 20 (1) (2009) 1–7. [31] J. Tian, K. Shobu, Hot-pressed AlN–Cu metal matrix composites and their thermal properties, J. Mater. Sci. 39 (4) (2009) 1309–1313. [32] W. Lei, D. Liu, J. Zhang, P. Zhu, Q. Cui, G. Zou, Direct synthesis, growth mechanism, and optical properties of 3D AlN nanostructures with urchin shapes, Cryst. Growth Des. 9 (3) (2009) 1489–1493. [33] Z. Shi, M. Radwan, S. Kirihara, Y. Miyamoto, Z. Jin, Enhanced thermal conductivity of polymer composites filled with three-dimensional brush like AlN nanowhiskers, Appl. Phys. Lett. 95 (2009) 224104.