- Email: [email protected]
metal molar ratio
Applied Surface Science 280 (2013) 42–49
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of aluminum nitride nanoparticles by a facile urea glass route and influence of urea/metal molar ratio Zhifang Gao a , Yizao Wan a , Guangyao Xiong b , Ruisong Guo a , Honglin Luo a,∗ a b
School of Materials Science and Engineering, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China School of Mechanical and Electrical Engineering, East China Jiaotong University, Nanchang, Jiangxi 330013, China
a r t i c l e
i n f o
Article history: Received 16 February 2013 Received in revised form 9 April 2013 Accepted 17 April 2013 Available online 24 April 2013 Keywords: Calcination Chemical preparation Nanoparticles Aluminum nitride
a b s t r a c t Attention toward nanosized aluminum nitride (AlN) was rapidly increasing due to its physical and chemical characteristics. In this work, nanocrystalline AlN particles were prepared via a simple urea glass route. The effect of the urea/metal molar ratio on the crystal structure and morphology of nanocrystalline AlN particles was studied using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results revealed that the morphology and the crystal structure of AlN nanoparticles could be controlled by adjusting the urea/metal ratio. Furthermore, a mixture of Al2 O3 and h-AlN was detected at the urea/metal molar ratio of 4 due to the inadequate urea content. With increasing the molar ratio, the pure h-AlN was obtained. In addition, the nucleation and growth mechanisms of AlN nanocrystalline were proposed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum nitride (AlN) has attracted much attention recently. It has been applied as an important ceramic substrate material due to its high thermal conductivity, low thermal expansion coefficient, low dielectric constant, good mechanical strength, thermal stability, lack of toxicity, stable crystal structure and relatively low cost [1–3]. These unique properties have made AlN an attractive material for high-tech industrial applications such as electrical packaging and heat sinks. Therefore, much attention has been given to the preparation of AlN. In general, AlN could be fabricated through a variety of methods, such as carbothermal nitridation [4–8], direct nitridation of aluminum powders [9,10], chemical vapor deposition [11] and plasma-base process, among which carbothermal nitridation and direct nitridation of aluminum powders have been the most widely used in the industrial production. However, both of these two methods have a variety of drawbacks, i.e., high-energy consumption, large size products, and low pure products of direct nitridation of aluminum powders and complicated production processes, high production temperatures, long production process of carbothermal nitridation. Recently, it has been reported that a new approach, the urea glass route, can be used to synthesize the metal nitrides materials
∗ Corresponding author. Tel.: +86 22 8789 8601; fax: +86 22 8789 8601. E-mail address: [email protected] (H. Luo). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.075
such as TiN, VN, CrN, GaN and MoN [12–14]. In addition, compared with other methods, this route shows many advantages. Firstly, it is a simple method due to the simple and cheap raw reagent materials. Secondly, the growth temperature is lower than that of the low temperature combustion synthesis method which uses the similar starting materials [15,16]. Thirdly, no further purifications are necessary before or after the temperature treatment. Particularly, the products have nanosize. Although the preparation of nanosized AlN using the urea glass route had been reported by some researchers [17–19] but the focus was placed on metal sources, such as metal chloride and nitrate. However, no significant systematic studies have been conducted to investigate the effect of urea/metal molar ratio (R) on the synthesized AlN powders. It had been reported that the urea/metal molar ratio could be applied to tailor molecular precursors which might play an important role on the purity and crystal structure of the formed metal nitrides. Sardar et al. [18] demonstrated that the crystal structure of GaN transformed from hexagonal to cubic as increasing the urea content. Giordano et al. [12] reported that the pure Mo and W nitrides were able to be prepared by changing the urea/metal molar ratio suggesting the urea contents play an important role on the purity of the metal nitride products. Therefore, in this paper, a varying urea/metal molar ratio (R) was employed for the synthesis of the nanosized AlN via the simple urea glass route. Moreover, the effect of R on the phase composition, the morphology and microstructure of the synthesized AlN nanoparticle was investigated in detail. Furthermore, the nucleation and growth mechanisms of AlN nanoparticle were proposed.
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
43
Fig. 1. Scheme for the AlN nanoparticles samples preparation route.
2. Materials and methods 2.1. Materials preparation The aluminum chloride (AlCl3 ·6H2 O, 97%), urea (CON2 H4 , 99.0%), anhydrous ethanol (C2 H5 OH, 99.5%) and nitrogen (N2 , 99.999%) were used as raw materials to prepare AlN nanoparticle. All starting materials were purchased from commercial sources and were used without further purification. The fabrication process of the AlN nanoparticles is shown in Fig. 1. The basic procedure employed for the synthesis of AlN was to heat compounds of aluminum chloride with urea at an appropriate temperature. Firstly, aluminum chloride and urea were dissolved in anhydrous ethanol to obtain a concentrated solution, respectively. Then, the aluminum chloride solution was added slowly into the urea solution at 70 ◦ C. The urea content of the urea solution was various to obtain the different urea/metal precursor molar ratio (R). The R value was set at 4, 6 and 10 respectively and the resultant aluminum–urea chloride compound samples were named as R4, R6 and R10, respectively. The obtained aluminum–urea chloride compound was separated by filtration and dried at 80 ◦ C for 5 h. Finally, the aluminum–urea chloride compound was put into a tube furnace and heated under flowing N2 at 1000 ◦ C for 5 h to obtain AlN. The AlN nanoparticles fabricated from different aluminum–urea chloride compound samples were named as R4S, R6S and R10S, respectively.
transmission electron microscope (TEM) investigations were performed. TEM samples were prepared by dispersing a small amount of solid powder in anhydrous ethanol and then using an ultrasonic bath for dispersing without any further dispersing agent. TEM images were obtained with a JEOL JEM100CXII operating at an accelerating voltage of 300 kV. 3. Results and discussion 3.1. FTIR spectra of aluminum–urea chloride complexes Fig. 2 shows the FTIR spectra of the urea and aluminum–urea chloride complexes prepared with different R. The related peak positions and their assignments are listed in Table 1. It could be observed that the curve shapes of the R4, R6 and R10 samples were similar while they exhibited different trend in comparison with the curve of the urea spectrum. The asymmetric and symmetric stretching vibrations of NH2 groups in the urea were observed at 3442 cm−1 and 3396 cm−1 while both of two peaks in the R4, R6 and R10 samples shifted to a higher frequency (see Table 1). This revealed that an intermolecular linkage between urea molecules
2.2. Characterization Thermal gravimetric analysis (TGA) of the aluminum–urea chloride compounds and urea were performed in air at a heating rate of 10 ◦ C/min with a Swiss Mettler TGA/DSC1. Simultaneous the Fourier transform infrared spectroscopy (FTIR) of the aluminum–urea compounds and urea were recorded on a Nicolet MAGNA-560 spectrophotometer. The AlN nanoparticles were characterized by X-ray diffraction (XRD) for phase identification using Cu K␣ -radiation. XRD measurements were performed on a Bruker D8 Advanced X-ray diffractometer from 20◦ to 80◦ with a scan speed of 4◦ /min. Scanning electron microscope (SEM) examination of the AlN nanoparticle samples was carried out in FEI Nanosem 430. The samples were coated by sputtering an Au alloy prior to imaging. To provide further insight into AlN particles,
Fig. 2. The FTIR spectra results of urea and aluminum–urea chloride complexes.
44
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
Table 1 Infrared absorption peaks and their assignments observed for urea and the Al–urea complexes. Assignments [13,20]
Urea (cm−1 )
R4 (cm−1 )
R6 (cm−1 )
R10 (cm−1 )
as NH2 as NH2 OH C O C O + ␦NH2
3442 3396 – 1676 1639 1618 1462 1151 1055 787 – 555
3475 3435 3182 – 1637 1568 1496 1176 1035 771 638 561
3477 3437 3186 – 1637 1568 1496 1182 1037 771 638 565
3473 3425 3188 – 1639 1568 1494 1180 1035 771 638 561
␦NH2 CN ␦r NH2 s CN (ONCN) ␦(NCO) ␦(NCN)
existed. Furthermore, the (C O) peak at 1676 cm−1 disappeared in the R4, R6 and R10 samples compared with the urea samples. The disappearance of the (C O) peak revealed that the CON2 H4 molecules replaced the alcoholic group of the ethoxide and then were coordinated to the Al3+ via oxygen atoms [18,20]. It was proved that the aluminum–urea chloride complexes were formed by the bonding between the (C O) of urea and Al3+ ions of AlCl3 . Additionally, for the R4, R6 and R10 samples, the (C N) stretching vibration was observed to shift to higher wave number from 1462 cm−1 to 1496 cm−1 , indicating that the bonding between C and N became strong and a new strong peak was observed at 638 cm−1 that was attributable to a characteristic absorption band of ␦(NCO). These phenomena further revealed the formation of aluminum–urea chloride complexes under these three R. 3.2. Mass loss of aluminum–urea chloride complexes TGA measurements were performed on urea and three precursor complex samples under nitrogen between 20 ◦ C and 1000 ◦ C, which are shown in Fig. 3. For the urea, the mass loss could be divided into three major stages. At the beginning, a steep mass loss of approximately 63% was observed between approximately 152 ◦ C and 220 ◦ C. This stage proceeded via two processes: successive urea vaporization and decomposition. The main products were ammonia, cyanic acid and biuret [21]. As the temperature was in the range of 220–350 ◦ C, cyanuric acid, ammelide and ammeline were produced primarily from biuret [22] and the corresponding mass loss was about 29%. When the temperature exceeded 350 ◦ C, the curve became smooth; indicating the complete consumption of urea and the mass loss in this stage was the smallest, only about 9%. The curves exhibited similar development trend for the R4, R6
and R10 samples and could be divided into two main stages. It could be observed that the mass loss of these precursor complex samples began at approximately 250 ◦ C which was higher than the decomposition temperature of the pure urea. This phenomena also certified that the aluminum–urea chloride complexes were formed, which was in agreement with the FTIR spectra. As the temperature was in the range of 250–300 ◦ C, there was a very rapid weight loss in the curves; the fraction was similar to that of the first mass loss stage in the urea sample. In this stage, the removal of moisture and the decomposition of urea during the combustion occurred and a large number of ammonia released. However, the weights of these precursor complex declined at a slow rate as the temperature increased further. The weight loss was mainly caused by the decomposition of product from the urea. In addition, it could be noted that as the temperature increased to 1000 ◦ C, the residual weight of the precursor complex was larger than that of the urea. Furthermore, the residual weight of three precursor complex samples was similar. It could be induced that the urea/metal molar ratio had little effect on the decomposition of aluminum–urea chloride complexes. 3.3. The morphology and microstructure of AlN nanoparticles The morphology of AlN nanoparticles with various R was shown in Fig. 4. It could be seen that near uniformly dispersed particles were obtained in R4S sample (see Fig. 4a). The size of the particles was in the range of 40–60 nm. Interestingly, at high molar ratios, the morphology of the synthesized AlN particles was significantly changed. The particles in R6S sample exhibited a spherical morphology with a size in the range of 800–1000 nm. These regular spherical particles were aggregated by small crystallites. The size of the small crystallites was less than 50 nm. As the urea/metal molar ratio increased to 10, the small crystallites merged and grew into larger flower-shaped particles. Therefore, it could be concluded that the morphologies of AlN nanoparticles could be controlled by adjusting the urea/metal molar ratio. As shown in Fig. 4, the increased R stimulated the growth and aggregation of the AlN crystallites. Fig. 5 shows the TEM images of the AlN samples with various urea/metal molar ratios. The three samples were composed of sphere-shaped nanocrystallites. The electron diffraction patterns of AlN particles were also given in Fig. 5. It could be observed that the R4S sample was composed of two phases, i.e., the hexagonal AlN (hAlN) and ˛-Al2 O3 while the R6S and R10S samples only contained one phase, i.e., h-AlN. This phenomenon demonstrated that as the content of the urea increased, the ˛-Al2 O3 gradually disappeared and then the pure AlN particles were obtained. 3.4. Crystal structure of AlN nanoparticles
Fig. 3. TGA curves of urea and prepared aluminum–urea chloride complex precursors at different R.
Fig. 6 demonstrates the XRD patterns of AlN particle samples combusted at 1000 ◦ C. According to XRD patterns, a mixture of Al2 O3 and AlN was formed in the R4S sample, which was in agreement with TEM electron diffraction patterns. The difference in the intensity of the diffraction peaks for Al2 O3 and AlN suggested that AlN was the major product. Furthermore, XRD patterns of samples prepared at higher R did not exhibit any detectable peaks other than those assigned to AlN, as shown in Fig. 5(b) and (c). The AlN phase of the three samples indexed as h-AlN (a = 3.110 A˚ ˚ and the three intense peaks at 2 = 33.2◦ , 37.9◦ , and c = 4.975 A) and 59.3◦ were assigned to the plane of h-AlN (1 0 0), (1 1 0) and (1 0 1), respectively, agreeing well with the calculated diffraction patterns (ICDD-PDF No. 65-3409). The broadening nature of the XRD peaks indicated that the crystallites sizes of the samples were within the nanometer scale. The average size of AlN nano crystallites for the R4S, R6S and R10S samples was 10, 12 and 15 nm,
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
45
Fig. 4. SEM images of AlN samples with different R (a) R4S, (b) R6S, and (c) R10S.
respectively, which was calculated from the half-width of diffraction lines by using the Debye–Scherrer equation. This result was smaller than the size that estimated using the SEM images. It was due to that these small particles observed in SEM images were not primary particles, i.e., they might also be aggregations of smaller ones. Therefore, it could be deduced that the urea/metal molar ratio had little effect on the crystal structure of AlN while it significantly affected the purity of the formed AlN. The pure AlN could not be obtained at a low molar ratio. 3.5. Mechanism of nucleation and growth of AlN nanoparticle and influence of R For the aim of revealing the mechanism of the AlN formation, the fabrication of the particles were carried out at various combustion temperatures of 350, 650 and 850 ◦ C, respectively. Fig. 7 depicts the variation of the aluminum–urea complexes with various R as a function of temperature. As shown in Fig. 7, the aluminum–urea
complex precursor were Al(CON2 H4 )6 Cl3 , which could be obtained in three samples. This result was in agreement with the FTIR and TGA results. As the temperature was less than 650 ◦ C, no crystalline intermediate were formed in the three samples and the obtained products showed a glassy structure. A similar result was reported by Giordano et al. [13] who investigated the vanadium–urea complexes. Kai et al. [20] also found that the amorphous state persisted until the mixture of urea and AlCl3 ·6H2 O was heated to 700 ◦ C. At a higher temperature of 850 ◦ C, the crystalline phase was detected in three cases. In addition, the compositions of the products were AlN crystalline phases, except for the product obtained at R = 4 where Al2 O3 and AlN two phases occurred. Note that the AlN synthesized at 850 ◦ C was a mixture of hexagonal and cubic phase crystal structure. The diffraction peak intensity of h-AlN phase was higher than that of the cubic AlN phase(c-AlN), which revealed that the proportion of h-AlN phase was higher. It was interesting to find that only the h-AlN remained while the c-AlN phase disappeared as the temperature increased to 1000 ◦ C (see Fig. 6). It could be ascribed
46
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
Fig. 5. TEM images of AlN samples with different R (a) R4S, (b) R6S, and (c) R10S.
to that the c-AlN phase was an unstable phase and would transform into the stable hexagonal phase at high temperatures. As the temperature exceeded 850 ◦ C, both AlN and Al2 O3 peaks could be seen in R4S sample while only the AlN could be detected in the other samples (Fig. 6 and Fig. 7a). During the synthesis process of AlN, some of the following chemical reactions might occur synchronously or asynchronously [17,18,20,23]:
AlCl3 ·NH3 → 1/2(Cl2 AlNH2 )2 + HCl
(5)
1/2(Cl2 AlNH2 )2 → 1/n(ClAlNH)n + HCl
(6)
1/4(ClAlNH)4 → c-AlN + HCl
(7)
1/6(ClAlNH)6 → h-AlN + HCl
(8)
Al(CON2 H4 )6 Cl3 → AlCl3 + 6CO(NH2 )2
(1)
c-AlN → h-AlN
(9)
CO(NH2 )2 → HCNO + NH3
(2)
CO(NH2 )2 → H2 CN2 + H2 O
(3)
AlCl3 + NH3 → AlCI3 ·NH3
(4)
The nucleation of AlN could be proposed as follows: firstly, the Al(CON2 H4 )6 Cl3 complex decomposed into AlCl3 and CON2 H4 and then CON2 H4 generated NH3 by decomposition. AlCl3 was likely to combine with NH3 to form AlCl3 ·NH3 . As the temperature increased, AlCl3 ·NH3 gradually decomposed to gain intermediate compounds (ClAlNH)4 or (ClAlNH)6 that were the most important
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
47
Fig. 6. XRD patterns of AlN samples with different R (a) R4S, (b) R6S, and (c) R10S.
substances in this process. Subsequently, the c-AlN phase and the h-AlN phase were generated from the (ClAlNH)4 and (ClAlNH)6 by the elimination of HCl. Finally, the c-AlN gradually transformed to the h-AlN due to its instability at elevated temperature. Although the nucleation process of AlN was the same at different R, the purity and morphology of AlN nanoparticle were different with various R. At R = 4, the complexation of urea with Al3+ was not adequate. Then, Al(OH)3 was formed by the surplus Al3+ with water of AlCl3 ·6H2 O. The presence of Al2 O3 might be due to the dehydration of Al(OH)3 : Al3+ + 3H2 O Al(OH)3 + 3H+
(10)
2Al(OH)3 Al2 O3 + 3H2 O
(11)
It was obviously noted that R played an important role in controlling the morphology of the formed AlN nanoparticle (Fig. 4). He et al. [24] found that the molar ratio of In3+ to urea could be employed to modulate the morphology of In2 O3 . The above analysis revealed that the nucleation process of AlN did not depend on the R. Therefore, the growth of the AlN in the synthesis process was a determined step controlling the morphology of the resultant AlN. The proposed mechanism is illustrated in Fig. 8 showing the effect of R on the morphology of the formed AlN nanoparticle. AlN nuclei were formed from the reactions between AlCl3 and NH3 , as depicted in Eqs. (1)–(8). A lot of unstable AlN molecular clusters were formed in gas phase, which supplied building blocks for the nuclei and the subsequent growth process [23,25]. At R = 4, the urea content was too low to produce enough AlN nuclei and then the formed AlN nuclei grew independently without neighbors and finally grew into bigger nanoparticles, as shown in Fig. 4(a). At R = 6 or 10, the amount of urea was high and could be successive to produce NH3 and H2 O. The interface of solid and gas was favorable for nucleation. As a result, the high R could induce high density of AlN nuclei. In addition, the as-formed AlN nuclei had a strong tendency to self-assemble radically and then aggregate together. This was caused by that the nucleus aggregation would reduce the total energy by removing surface energy associated with unsatisfied bonds and by eliminating the surfaces where the nuclei joined together [25]. This phenomenon was quite like with the oriented attachment process that occurred in the complex sintering process of nanocrystalline materials [26]. The growth of AlN nanoparticles took place under non-equilibrium conditions at high R ratio. The high gas content would modify the growth kinetics of AlN nanocrystals. Following the equation (4), the increasing NH3 content would promote the reaction rate and growth of AlN [27]. It was reported that kinetic, temperature, reaction time and capping agents could affect the growth pattern of nanocrystals
Fig. 7. XRD patterns of aluminum–urea chloride complexes treated at different temperature (a) R4, (b) R6, and (c) R10 (O: Al2 O3 , 夽: h-AlN, : c-AlN).
under non-equilibrium kinetic growth conditions [28]. During the growth stage, the AlN nanocrystals grew and captured each other and finally formed the spherical and flower-shaped morphology. It could be induced that the different morphology of the formed AlN could be attributed to the liberation of a large amount of gases during calcining process. The ratio of urea/metal precursor played a key role in modifying the morphology of the final AlN nanoparticles products. Nevertheless, a full description of these growth mechanisms would require more evidences from future work. As analyzed above, the urea/metal molar ratio played an important role in the phase composition and the morphology of AlN nanoparticles. The influence of R on the complexes and AlN nanoparticles was summarized in Table 2. It was discernible that
48
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
Table 2 Influence of R on the prepared complexes and AlN nanoparticles. Urea/metal molar ratio (R)
4
6
10
Structure of aluminum–urea chloride complexes
Al(CON2 H4 )6 Cl3
Al(CON2 H4 )6 Cl3
Al(CON2 H4 )6 Cl3
h-AlN + Al2 O3 10 nm
h-AlN 12 nm
h-AlN 15 nm
Morphology of AlN nanoparticle
Crystal structure of AlN nano particle Size of AlN nanocrystallites
the morphology and phase composition of the AlN nanoparticles displayed a variation with increase of R. The AlN nanoparticle prepared with R = 4 was composed of the mixture of Al2 O3 and h-AlN. As the R increases beyond 6, the product exhibited the pure h-AlN. Therefore, pure AlN could be obtain at R > 6. As the R increased, the morphology of AlN nanoparticle changed from scattered particles into aggregated spherical particles and then into aggregated flower-shaped particles. Moreover, the molar ratio R could affect the size of the synthesized nanocrystallites: the diameter of nanocrystallites increased from 10 to 12 to 15 nm as R increased from 4 to 6 to 10. It could be attributed to the influence of the metal concentration on the nucleation rate in the amorphous intermediates [15]. However, the structure of complex precursor was not affected by changing the R. It could be obviously noted that the generated AlN using the urea glass route showed nanosize and high purity when synthesized at R = 6. As well known, the thermal conductivity of AlN (320 W/(mK)) was higher than that of Al2 O3 (30 W/(mK)), which was the highest value among the ceramic materials ever found, and the coefficient of thermal expansion was low, so AlN could be used to solve the problem of thermal matching between the substrate and the semiconductor, and the cooling problem of the high power electronic devices. Several researches [29–31] reported
that the AlN had considered as an attractive thermal conductivity filler candidate. Besides, the nanoscaled materials could exhibit improved performances, such as enhanced electrical, thermal and mechanical properties for further specific applications. In a word, the as-prepared AlN nanopowders could be used as a high thermal conductivity filler to fill the conventional polymer plastics to produce the polymers with high thermal conductivity.
4. Conclusions The AlN nanoparticle was synthesized by a simple urea glass route. As the urea/metal molar ratio was higher than 6, the pure hAlN nanoparticles were obtained. Otherwise, a mixture of Al2 O3 and AlN was obtained due to the surplus AlCl3 ·6H2 O content. Furthermore, AlN nuclei were formed from the reactions between AlCl3 and NH3 at different R. However, the variable R was a crucial experimental parameter in mediating the growth of AlN nanoparticle. The formed AlN nanocrystals were apt to aggregate at high molar ratio due to the high amount gas in the growth stage and the morphology of AlN nanoparticles could be controlled by adjusting the urea/metal molar ratio. Under the conditions with different values of R, scattered AlN nanoparticles (R = 4), aggregated AlN spherical particles (R = 6) and aggregated AlN flower-shape particles (R = 10) were obtained, respectively. In addition, the synthesized AlN nanoparticles could be applied as reinforcement to produce the polymers with high thermal conductivity.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grants No. 51172158 and 81200663) and the Science and Technology Support Program of Tianjin (Grant No. 11ZCKFSY01700).
References
Fig. 8. Schematic illustration of the effect of R on the AlN structure growth.
[1] Y. Guojun, C. Guangde, L. Huiming, Solid-state metasynthesis and characterization of AlN nanocrystals, International Journal of Refractory Metals and Hard Materials 26 (2008) 5–8. [2] J. Wang, W. Wang, P. Ding, Y. Yang, L. Fang, J. Esteve, Synthesis of cubic aluminum nitride by carbothermal nitridation reaction, Diamond and Related Materials 8 (1999) 1342–1344. [3] H.L. Wang, H.M. Lv, G.D. Chen, H.G. Ye, Synthesis of hexagonal AlN microbelts at low temperature, Journal of Alloys and Compounds 477 (2009) 580–582. [4] W.S. Jung, S.K. Ahn, Monitoring the conversion of ␣-alumina to AlN under a flow of nitrogen by Al-27 MAS NMR spectroscopy, Journal of Materials Science Letters 16 (1997) 1573–1575. [5] J. Kuang, C. Zhang, X. Zhou, S. Wang, Influence of processing parameters on synthesis of nano-sized AlN powders, Journal of Crystal Growth 263 (2004) 12–20.
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49 [6] M. Qin, X. Du, J. Wang, I.S. Humail, X. Qu, Influence of carbon on the synthesis of AlN powder from combustion synthesis precursors, Journal of the European Ceramic Society 29 (2009) 795–799. [7] M. Qin, X. Du, Z. Li, I.S. Humail, X. Qu, Synthesis of aluminum nitride powder by carbothermal reduction of a combustion synthesis precursor, Materials Research Bulletin 43 (2008) 2954–2960. [8] W.S. Jung, Synthesis of aluminum nitride powder from ı-alumina nanopowders under a mixed gas flow of nitrogen and hydrogen, Ceramics International 38 (2012) 871–874. [9] V. VasanthiPillay, K. Vijayalakshmi, Microstructural evolution and dielectric properties of 1D AlN powders synthesized by microwave technique, Superlattices and Microstructures 51 (2012) 860–867. [10] C. Li, L. Kao, M. Chen, Y. Wang, C. Tsai, Rapid preparation of aluminum nitride powders by using microwave plasma, Journal of Alloys and Compounds 542 (2012) 78–84. [11] N.C. Wu, M.S. Tsai, M.C. Wang, H.S. Liu, The morphology and formation mechanism of aluminum nitride nanocrystals synthesized by chemical vapor deposition, Journal of Crystal Growth 208 (2000) 189–196. [12] C. Giordano, C. Erpen, W. Yao, M. Antonietti, Synthesis of Mo and W carbide and nitride nanoparticles via a simple “urea glass” route, Nano Letters 8 (2008) 4659–4663. [13] C. Giordano, C. Erpen, W. Yao, B. Milke, M. Antonietti, Metal nitride and metal carbide nanoparticles by a soft urea pathway, Chemistry of Materials 21 (2009) 5136–5144. [14] Y. Qiu, L. Gao, Novel synthesis of nanocrystalline gallium nitride powder from gallium(III)–urea complex, Chemistry Letters 32 (2003) 774–775. [15] A. Chu, M. Qin, D. Rafi ud, B. Jia, H. Lu, X. Qu, Effect of urea on the size and morphology of AlN nanoparticles synthesized from combustion synthesis precursors, Journal of Alloys and Compounds 530 (2012) 144–151. [16] A. Chu, M. Qin, d. Rafi ud, B. Jia, H. Lu, X. He, Effect of aluminum source on the synthesis of AlN powders from combustion synthesis precursors, Materials Research Bulletin 47 (2012) 2475–2479. [17] Y. Qiu, L. Gao, Metal–urea complex – a precursor to netal nitrides, Journal of the American Ceramic Society 87 (2004) 352–357. [18] K. Sardar, M. Dan, B. Schwenzer, C.N.R. Rao, A simple single-source precursor route to the nanostructures of AlN, GaN and InN, Journal of Materials Chemistry 15 (2005) 2175.
49
[19] K. Sardar, C.N.R. Rao, AlN nanocrystals by new chemical routes, Solid State Sciences 7 (2005) 217–220. [20] A. Kai, Y. Kamita, T. Miki, Formation process of AlN through precursors and its application to joining AlN ceramics, Journal of the American Ceramic Society 95 (2012) 3788–3796. [21] P.M. Schaber, J. Colson, S. Higgins, D. Thielen, B. Anspach, J. Brauer, Thermal decomposition (pyrolysis) of urea in an open reaction vessel, Thermochimica Acta 424 (2004) 131–142. [22] O. CARP, Considerations on the thermal decomposition of urea, Revue Roumaine de Chimie 46 (2001) 735–740. [23] A.Y. Timoshkin, H.F. Bettinger, H.F. Schaefer III, The chemical vapor deposition of aluminum nitride: unusual cluster formation in the gas phase, Journal of the American Ceramic Society 119 (1997) 5668–5678. [24] Z. He, Z. Chen, Y. Li, Q. Zhang, H. Wang, Molar ratio of In to urea directed formation of In2 O3 hierarchical structures: cubes and nanorod-flowers, CrystEngComm 13 (2011) 2557–2565. [25] X. Ji, H. Li, Z. Wu, S. Cheng, H. Hu, D. Yan, Growth of AlN hexagonal oriented complex nanostructures induced by nucleus arrangement, CrystEngComm 13 (2011) 5198–5203. [26] K. Lu, Sintering of nanoceramics, International Materials Reviews 53 (2008) 21–38. [27] S. Dhamodaran, D.S. Chander, J. Ramkumarto, Anti-reflective and hydrophobic surface of self-organized GaN nano-flowers, Applied Surface Science 257 (2011) 9612–9615. [28] M. Chen, Y. Wang, L. Song, P. Gunawan, Z. Zhong, X. She, Urchin-like ZnO microspheres synthesized by thermal decomposition of hydrozincite as a copper catalyst promoter for the Rochow reaction, RSC Advances 2 (2012) 4164–4168. [29] C. Seran, I. Hyungu, K. Jooheon, The thermal conductivity of embedded nano-aluminum nitride-doped multi-walled carbon nanotubes in epoxy composites containing micro-aluminum nitride particles, Nanotechnology 23 (2012) 065303. [30] E. Lee, S. Lee, D.J. Shanefield, W.R. Cannon, Enhanced thermal conductivity of polymer matrix composite via high solids loading of aluminum nitride in epoxy resin, Journal of the American Ceramic Society 91 (2008) 1169–1174. [31] C.Y. Hsieh, S.L. Chung, High thermal conductivity epoxy molding compound filled with a combustion synthesized AlN powder, Journal of Applied Polymer Science 102 (2006) 4734–4740.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of aluminum nitride nanoparticles by a facile urea glass route and influence of urea/metal molar ratio Zhifang Gao a , Yizao Wan a , Guangyao Xiong b , Ruisong Guo a , Honglin Luo a,∗ a b
School of Materials Science and Engineering, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China School of Mechanical and Electrical Engineering, East China Jiaotong University, Nanchang, Jiangxi 330013, China
a r t i c l e
i n f o
Article history: Received 16 February 2013 Received in revised form 9 April 2013 Accepted 17 April 2013 Available online 24 April 2013 Keywords: Calcination Chemical preparation Nanoparticles Aluminum nitride
a b s t r a c t Attention toward nanosized aluminum nitride (AlN) was rapidly increasing due to its physical and chemical characteristics. In this work, nanocrystalline AlN particles were prepared via a simple urea glass route. The effect of the urea/metal molar ratio on the crystal structure and morphology of nanocrystalline AlN particles was studied using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results revealed that the morphology and the crystal structure of AlN nanoparticles could be controlled by adjusting the urea/metal ratio. Furthermore, a mixture of Al2 O3 and h-AlN was detected at the urea/metal molar ratio of 4 due to the inadequate urea content. With increasing the molar ratio, the pure h-AlN was obtained. In addition, the nucleation and growth mechanisms of AlN nanocrystalline were proposed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum nitride (AlN) has attracted much attention recently. It has been applied as an important ceramic substrate material due to its high thermal conductivity, low thermal expansion coefficient, low dielectric constant, good mechanical strength, thermal stability, lack of toxicity, stable crystal structure and relatively low cost [1–3]. These unique properties have made AlN an attractive material for high-tech industrial applications such as electrical packaging and heat sinks. Therefore, much attention has been given to the preparation of AlN. In general, AlN could be fabricated through a variety of methods, such as carbothermal nitridation [4–8], direct nitridation of aluminum powders [9,10], chemical vapor deposition [11] and plasma-base process, among which carbothermal nitridation and direct nitridation of aluminum powders have been the most widely used in the industrial production. However, both of these two methods have a variety of drawbacks, i.e., high-energy consumption, large size products, and low pure products of direct nitridation of aluminum powders and complicated production processes, high production temperatures, long production process of carbothermal nitridation. Recently, it has been reported that a new approach, the urea glass route, can be used to synthesize the metal nitrides materials
∗ Corresponding author. Tel.: +86 22 8789 8601; fax: +86 22 8789 8601. E-mail address: [email protected] (H. Luo). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.075
such as TiN, VN, CrN, GaN and MoN [12–14]. In addition, compared with other methods, this route shows many advantages. Firstly, it is a simple method due to the simple and cheap raw reagent materials. Secondly, the growth temperature is lower than that of the low temperature combustion synthesis method which uses the similar starting materials [15,16]. Thirdly, no further purifications are necessary before or after the temperature treatment. Particularly, the products have nanosize. Although the preparation of nanosized AlN using the urea glass route had been reported by some researchers [17–19] but the focus was placed on metal sources, such as metal chloride and nitrate. However, no significant systematic studies have been conducted to investigate the effect of urea/metal molar ratio (R) on the synthesized AlN powders. It had been reported that the urea/metal molar ratio could be applied to tailor molecular precursors which might play an important role on the purity and crystal structure of the formed metal nitrides. Sardar et al. [18] demonstrated that the crystal structure of GaN transformed from hexagonal to cubic as increasing the urea content. Giordano et al. [12] reported that the pure Mo and W nitrides were able to be prepared by changing the urea/metal molar ratio suggesting the urea contents play an important role on the purity of the metal nitride products. Therefore, in this paper, a varying urea/metal molar ratio (R) was employed for the synthesis of the nanosized AlN via the simple urea glass route. Moreover, the effect of R on the phase composition, the morphology and microstructure of the synthesized AlN nanoparticle was investigated in detail. Furthermore, the nucleation and growth mechanisms of AlN nanoparticle were proposed.
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
43
Fig. 1. Scheme for the AlN nanoparticles samples preparation route.
2. Materials and methods 2.1. Materials preparation The aluminum chloride (AlCl3 ·6H2 O, 97%), urea (CON2 H4 , 99.0%), anhydrous ethanol (C2 H5 OH, 99.5%) and nitrogen (N2 , 99.999%) were used as raw materials to prepare AlN nanoparticle. All starting materials were purchased from commercial sources and were used without further purification. The fabrication process of the AlN nanoparticles is shown in Fig. 1. The basic procedure employed for the synthesis of AlN was to heat compounds of aluminum chloride with urea at an appropriate temperature. Firstly, aluminum chloride and urea were dissolved in anhydrous ethanol to obtain a concentrated solution, respectively. Then, the aluminum chloride solution was added slowly into the urea solution at 70 ◦ C. The urea content of the urea solution was various to obtain the different urea/metal precursor molar ratio (R). The R value was set at 4, 6 and 10 respectively and the resultant aluminum–urea chloride compound samples were named as R4, R6 and R10, respectively. The obtained aluminum–urea chloride compound was separated by filtration and dried at 80 ◦ C for 5 h. Finally, the aluminum–urea chloride compound was put into a tube furnace and heated under flowing N2 at 1000 ◦ C for 5 h to obtain AlN. The AlN nanoparticles fabricated from different aluminum–urea chloride compound samples were named as R4S, R6S and R10S, respectively.
transmission electron microscope (TEM) investigations were performed. TEM samples were prepared by dispersing a small amount of solid powder in anhydrous ethanol and then using an ultrasonic bath for dispersing without any further dispersing agent. TEM images were obtained with a JEOL JEM100CXII operating at an accelerating voltage of 300 kV. 3. Results and discussion 3.1. FTIR spectra of aluminum–urea chloride complexes Fig. 2 shows the FTIR spectra of the urea and aluminum–urea chloride complexes prepared with different R. The related peak positions and their assignments are listed in Table 1. It could be observed that the curve shapes of the R4, R6 and R10 samples were similar while they exhibited different trend in comparison with the curve of the urea spectrum. The asymmetric and symmetric stretching vibrations of NH2 groups in the urea were observed at 3442 cm−1 and 3396 cm−1 while both of two peaks in the R4, R6 and R10 samples shifted to a higher frequency (see Table 1). This revealed that an intermolecular linkage between urea molecules
2.2. Characterization Thermal gravimetric analysis (TGA) of the aluminum–urea chloride compounds and urea were performed in air at a heating rate of 10 ◦ C/min with a Swiss Mettler TGA/DSC1. Simultaneous the Fourier transform infrared spectroscopy (FTIR) of the aluminum–urea compounds and urea were recorded on a Nicolet MAGNA-560 spectrophotometer. The AlN nanoparticles were characterized by X-ray diffraction (XRD) for phase identification using Cu K␣ -radiation. XRD measurements were performed on a Bruker D8 Advanced X-ray diffractometer from 20◦ to 80◦ with a scan speed of 4◦ /min. Scanning electron microscope (SEM) examination of the AlN nanoparticle samples was carried out in FEI Nanosem 430. The samples were coated by sputtering an Au alloy prior to imaging. To provide further insight into AlN particles,
Fig. 2. The FTIR spectra results of urea and aluminum–urea chloride complexes.
44
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
Table 1 Infrared absorption peaks and their assignments observed for urea and the Al–urea complexes. Assignments [13,20]
Urea (cm−1 )
R4 (cm−1 )
R6 (cm−1 )
R10 (cm−1 )
as NH2 as NH2 OH C O C O + ␦NH2
3442 3396 – 1676 1639 1618 1462 1151 1055 787 – 555
3475 3435 3182 – 1637 1568 1496 1176 1035 771 638 561
3477 3437 3186 – 1637 1568 1496 1182 1037 771 638 565
3473 3425 3188 – 1639 1568 1494 1180 1035 771 638 561
␦NH2 CN ␦r NH2 s CN (ONCN) ␦(NCO) ␦(NCN)
existed. Furthermore, the (C O) peak at 1676 cm−1 disappeared in the R4, R6 and R10 samples compared with the urea samples. The disappearance of the (C O) peak revealed that the CON2 H4 molecules replaced the alcoholic group of the ethoxide and then were coordinated to the Al3+ via oxygen atoms [18,20]. It was proved that the aluminum–urea chloride complexes were formed by the bonding between the (C O) of urea and Al3+ ions of AlCl3 . Additionally, for the R4, R6 and R10 samples, the (C N) stretching vibration was observed to shift to higher wave number from 1462 cm−1 to 1496 cm−1 , indicating that the bonding between C and N became strong and a new strong peak was observed at 638 cm−1 that was attributable to a characteristic absorption band of ␦(NCO). These phenomena further revealed the formation of aluminum–urea chloride complexes under these three R. 3.2. Mass loss of aluminum–urea chloride complexes TGA measurements were performed on urea and three precursor complex samples under nitrogen between 20 ◦ C and 1000 ◦ C, which are shown in Fig. 3. For the urea, the mass loss could be divided into three major stages. At the beginning, a steep mass loss of approximately 63% was observed between approximately 152 ◦ C and 220 ◦ C. This stage proceeded via two processes: successive urea vaporization and decomposition. The main products were ammonia, cyanic acid and biuret [21]. As the temperature was in the range of 220–350 ◦ C, cyanuric acid, ammelide and ammeline were produced primarily from biuret [22] and the corresponding mass loss was about 29%. When the temperature exceeded 350 ◦ C, the curve became smooth; indicating the complete consumption of urea and the mass loss in this stage was the smallest, only about 9%. The curves exhibited similar development trend for the R4, R6
and R10 samples and could be divided into two main stages. It could be observed that the mass loss of these precursor complex samples began at approximately 250 ◦ C which was higher than the decomposition temperature of the pure urea. This phenomena also certified that the aluminum–urea chloride complexes were formed, which was in agreement with the FTIR spectra. As the temperature was in the range of 250–300 ◦ C, there was a very rapid weight loss in the curves; the fraction was similar to that of the first mass loss stage in the urea sample. In this stage, the removal of moisture and the decomposition of urea during the combustion occurred and a large number of ammonia released. However, the weights of these precursor complex declined at a slow rate as the temperature increased further. The weight loss was mainly caused by the decomposition of product from the urea. In addition, it could be noted that as the temperature increased to 1000 ◦ C, the residual weight of the precursor complex was larger than that of the urea. Furthermore, the residual weight of three precursor complex samples was similar. It could be induced that the urea/metal molar ratio had little effect on the decomposition of aluminum–urea chloride complexes. 3.3. The morphology and microstructure of AlN nanoparticles The morphology of AlN nanoparticles with various R was shown in Fig. 4. It could be seen that near uniformly dispersed particles were obtained in R4S sample (see Fig. 4a). The size of the particles was in the range of 40–60 nm. Interestingly, at high molar ratios, the morphology of the synthesized AlN particles was significantly changed. The particles in R6S sample exhibited a spherical morphology with a size in the range of 800–1000 nm. These regular spherical particles were aggregated by small crystallites. The size of the small crystallites was less than 50 nm. As the urea/metal molar ratio increased to 10, the small crystallites merged and grew into larger flower-shaped particles. Therefore, it could be concluded that the morphologies of AlN nanoparticles could be controlled by adjusting the urea/metal molar ratio. As shown in Fig. 4, the increased R stimulated the growth and aggregation of the AlN crystallites. Fig. 5 shows the TEM images of the AlN samples with various urea/metal molar ratios. The three samples were composed of sphere-shaped nanocrystallites. The electron diffraction patterns of AlN particles were also given in Fig. 5. It could be observed that the R4S sample was composed of two phases, i.e., the hexagonal AlN (hAlN) and ˛-Al2 O3 while the R6S and R10S samples only contained one phase, i.e., h-AlN. This phenomenon demonstrated that as the content of the urea increased, the ˛-Al2 O3 gradually disappeared and then the pure AlN particles were obtained. 3.4. Crystal structure of AlN nanoparticles
Fig. 3. TGA curves of urea and prepared aluminum–urea chloride complex precursors at different R.
Fig. 6 demonstrates the XRD patterns of AlN particle samples combusted at 1000 ◦ C. According to XRD patterns, a mixture of Al2 O3 and AlN was formed in the R4S sample, which was in agreement with TEM electron diffraction patterns. The difference in the intensity of the diffraction peaks for Al2 O3 and AlN suggested that AlN was the major product. Furthermore, XRD patterns of samples prepared at higher R did not exhibit any detectable peaks other than those assigned to AlN, as shown in Fig. 5(b) and (c). The AlN phase of the three samples indexed as h-AlN (a = 3.110 A˚ ˚ and the three intense peaks at 2 = 33.2◦ , 37.9◦ , and c = 4.975 A) and 59.3◦ were assigned to the plane of h-AlN (1 0 0), (1 1 0) and (1 0 1), respectively, agreeing well with the calculated diffraction patterns (ICDD-PDF No. 65-3409). The broadening nature of the XRD peaks indicated that the crystallites sizes of the samples were within the nanometer scale. The average size of AlN nano crystallites for the R4S, R6S and R10S samples was 10, 12 and 15 nm,
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
45
Fig. 4. SEM images of AlN samples with different R (a) R4S, (b) R6S, and (c) R10S.
respectively, which was calculated from the half-width of diffraction lines by using the Debye–Scherrer equation. This result was smaller than the size that estimated using the SEM images. It was due to that these small particles observed in SEM images were not primary particles, i.e., they might also be aggregations of smaller ones. Therefore, it could be deduced that the urea/metal molar ratio had little effect on the crystal structure of AlN while it significantly affected the purity of the formed AlN. The pure AlN could not be obtained at a low molar ratio. 3.5. Mechanism of nucleation and growth of AlN nanoparticle and influence of R For the aim of revealing the mechanism of the AlN formation, the fabrication of the particles were carried out at various combustion temperatures of 350, 650 and 850 ◦ C, respectively. Fig. 7 depicts the variation of the aluminum–urea complexes with various R as a function of temperature. As shown in Fig. 7, the aluminum–urea
complex precursor were Al(CON2 H4 )6 Cl3 , which could be obtained in three samples. This result was in agreement with the FTIR and TGA results. As the temperature was less than 650 ◦ C, no crystalline intermediate were formed in the three samples and the obtained products showed a glassy structure. A similar result was reported by Giordano et al. [13] who investigated the vanadium–urea complexes. Kai et al. [20] also found that the amorphous state persisted until the mixture of urea and AlCl3 ·6H2 O was heated to 700 ◦ C. At a higher temperature of 850 ◦ C, the crystalline phase was detected in three cases. In addition, the compositions of the products were AlN crystalline phases, except for the product obtained at R = 4 where Al2 O3 and AlN two phases occurred. Note that the AlN synthesized at 850 ◦ C was a mixture of hexagonal and cubic phase crystal structure. The diffraction peak intensity of h-AlN phase was higher than that of the cubic AlN phase(c-AlN), which revealed that the proportion of h-AlN phase was higher. It was interesting to find that only the h-AlN remained while the c-AlN phase disappeared as the temperature increased to 1000 ◦ C (see Fig. 6). It could be ascribed
46
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
Fig. 5. TEM images of AlN samples with different R (a) R4S, (b) R6S, and (c) R10S.
to that the c-AlN phase was an unstable phase and would transform into the stable hexagonal phase at high temperatures. As the temperature exceeded 850 ◦ C, both AlN and Al2 O3 peaks could be seen in R4S sample while only the AlN could be detected in the other samples (Fig. 6 and Fig. 7a). During the synthesis process of AlN, some of the following chemical reactions might occur synchronously or asynchronously [17,18,20,23]:
AlCl3 ·NH3 → 1/2(Cl2 AlNH2 )2 + HCl
(5)
1/2(Cl2 AlNH2 )2 → 1/n(ClAlNH)n + HCl
(6)
1/4(ClAlNH)4 → c-AlN + HCl
(7)
1/6(ClAlNH)6 → h-AlN + HCl
(8)
Al(CON2 H4 )6 Cl3 → AlCl3 + 6CO(NH2 )2
(1)
c-AlN → h-AlN
(9)
CO(NH2 )2 → HCNO + NH3
(2)
CO(NH2 )2 → H2 CN2 + H2 O
(3)
AlCl3 + NH3 → AlCI3 ·NH3
(4)
The nucleation of AlN could be proposed as follows: firstly, the Al(CON2 H4 )6 Cl3 complex decomposed into AlCl3 and CON2 H4 and then CON2 H4 generated NH3 by decomposition. AlCl3 was likely to combine with NH3 to form AlCl3 ·NH3 . As the temperature increased, AlCl3 ·NH3 gradually decomposed to gain intermediate compounds (ClAlNH)4 or (ClAlNH)6 that were the most important
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
47
Fig. 6. XRD patterns of AlN samples with different R (a) R4S, (b) R6S, and (c) R10S.
substances in this process. Subsequently, the c-AlN phase and the h-AlN phase were generated from the (ClAlNH)4 and (ClAlNH)6 by the elimination of HCl. Finally, the c-AlN gradually transformed to the h-AlN due to its instability at elevated temperature. Although the nucleation process of AlN was the same at different R, the purity and morphology of AlN nanoparticle were different with various R. At R = 4, the complexation of urea with Al3+ was not adequate. Then, Al(OH)3 was formed by the surplus Al3+ with water of AlCl3 ·6H2 O. The presence of Al2 O3 might be due to the dehydration of Al(OH)3 : Al3+ + 3H2 O Al(OH)3 + 3H+
(10)
2Al(OH)3 Al2 O3 + 3H2 O
(11)
It was obviously noted that R played an important role in controlling the morphology of the formed AlN nanoparticle (Fig. 4). He et al. [24] found that the molar ratio of In3+ to urea could be employed to modulate the morphology of In2 O3 . The above analysis revealed that the nucleation process of AlN did not depend on the R. Therefore, the growth of the AlN in the synthesis process was a determined step controlling the morphology of the resultant AlN. The proposed mechanism is illustrated in Fig. 8 showing the effect of R on the morphology of the formed AlN nanoparticle. AlN nuclei were formed from the reactions between AlCl3 and NH3 , as depicted in Eqs. (1)–(8). A lot of unstable AlN molecular clusters were formed in gas phase, which supplied building blocks for the nuclei and the subsequent growth process [23,25]. At R = 4, the urea content was too low to produce enough AlN nuclei and then the formed AlN nuclei grew independently without neighbors and finally grew into bigger nanoparticles, as shown in Fig. 4(a). At R = 6 or 10, the amount of urea was high and could be successive to produce NH3 and H2 O. The interface of solid and gas was favorable for nucleation. As a result, the high R could induce high density of AlN nuclei. In addition, the as-formed AlN nuclei had a strong tendency to self-assemble radically and then aggregate together. This was caused by that the nucleus aggregation would reduce the total energy by removing surface energy associated with unsatisfied bonds and by eliminating the surfaces where the nuclei joined together [25]. This phenomenon was quite like with the oriented attachment process that occurred in the complex sintering process of nanocrystalline materials [26]. The growth of AlN nanoparticles took place under non-equilibrium conditions at high R ratio. The high gas content would modify the growth kinetics of AlN nanocrystals. Following the equation (4), the increasing NH3 content would promote the reaction rate and growth of AlN [27]. It was reported that kinetic, temperature, reaction time and capping agents could affect the growth pattern of nanocrystals
Fig. 7. XRD patterns of aluminum–urea chloride complexes treated at different temperature (a) R4, (b) R6, and (c) R10 (O: Al2 O3 , 夽: h-AlN, : c-AlN).
under non-equilibrium kinetic growth conditions [28]. During the growth stage, the AlN nanocrystals grew and captured each other and finally formed the spherical and flower-shaped morphology. It could be induced that the different morphology of the formed AlN could be attributed to the liberation of a large amount of gases during calcining process. The ratio of urea/metal precursor played a key role in modifying the morphology of the final AlN nanoparticles products. Nevertheless, a full description of these growth mechanisms would require more evidences from future work. As analyzed above, the urea/metal molar ratio played an important role in the phase composition and the morphology of AlN nanoparticles. The influence of R on the complexes and AlN nanoparticles was summarized in Table 2. It was discernible that
48
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49
Table 2 Influence of R on the prepared complexes and AlN nanoparticles. Urea/metal molar ratio (R)
4
6
10
Structure of aluminum–urea chloride complexes
Al(CON2 H4 )6 Cl3
Al(CON2 H4 )6 Cl3
Al(CON2 H4 )6 Cl3
h-AlN + Al2 O3 10 nm
h-AlN 12 nm
h-AlN 15 nm
Morphology of AlN nanoparticle
Crystal structure of AlN nano particle Size of AlN nanocrystallites
the morphology and phase composition of the AlN nanoparticles displayed a variation with increase of R. The AlN nanoparticle prepared with R = 4 was composed of the mixture of Al2 O3 and h-AlN. As the R increases beyond 6, the product exhibited the pure h-AlN. Therefore, pure AlN could be obtain at R > 6. As the R increased, the morphology of AlN nanoparticle changed from scattered particles into aggregated spherical particles and then into aggregated flower-shaped particles. Moreover, the molar ratio R could affect the size of the synthesized nanocrystallites: the diameter of nanocrystallites increased from 10 to 12 to 15 nm as R increased from 4 to 6 to 10. It could be attributed to the influence of the metal concentration on the nucleation rate in the amorphous intermediates [15]. However, the structure of complex precursor was not affected by changing the R. It could be obviously noted that the generated AlN using the urea glass route showed nanosize and high purity when synthesized at R = 6. As well known, the thermal conductivity of AlN (320 W/(mK)) was higher than that of Al2 O3 (30 W/(mK)), which was the highest value among the ceramic materials ever found, and the coefficient of thermal expansion was low, so AlN could be used to solve the problem of thermal matching between the substrate and the semiconductor, and the cooling problem of the high power electronic devices. Several researches [29–31] reported
that the AlN had considered as an attractive thermal conductivity filler candidate. Besides, the nanoscaled materials could exhibit improved performances, such as enhanced electrical, thermal and mechanical properties for further specific applications. In a word, the as-prepared AlN nanopowders could be used as a high thermal conductivity filler to fill the conventional polymer plastics to produce the polymers with high thermal conductivity.
4. Conclusions The AlN nanoparticle was synthesized by a simple urea glass route. As the urea/metal molar ratio was higher than 6, the pure hAlN nanoparticles were obtained. Otherwise, a mixture of Al2 O3 and AlN was obtained due to the surplus AlCl3 ·6H2 O content. Furthermore, AlN nuclei were formed from the reactions between AlCl3 and NH3 at different R. However, the variable R was a crucial experimental parameter in mediating the growth of AlN nanoparticle. The formed AlN nanocrystals were apt to aggregate at high molar ratio due to the high amount gas in the growth stage and the morphology of AlN nanoparticles could be controlled by adjusting the urea/metal molar ratio. Under the conditions with different values of R, scattered AlN nanoparticles (R = 4), aggregated AlN spherical particles (R = 6) and aggregated AlN flower-shape particles (R = 10) were obtained, respectively. In addition, the synthesized AlN nanoparticles could be applied as reinforcement to produce the polymers with high thermal conductivity.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grants No. 51172158 and 81200663) and the Science and Technology Support Program of Tianjin (Grant No. 11ZCKFSY01700).
References
Fig. 8. Schematic illustration of the effect of R on the AlN structure growth.
[1] Y. Guojun, C. Guangde, L. Huiming, Solid-state metasynthesis and characterization of AlN nanocrystals, International Journal of Refractory Metals and Hard Materials 26 (2008) 5–8. [2] J. Wang, W. Wang, P. Ding, Y. Yang, L. Fang, J. Esteve, Synthesis of cubic aluminum nitride by carbothermal nitridation reaction, Diamond and Related Materials 8 (1999) 1342–1344. [3] H.L. Wang, H.M. Lv, G.D. Chen, H.G. Ye, Synthesis of hexagonal AlN microbelts at low temperature, Journal of Alloys and Compounds 477 (2009) 580–582. [4] W.S. Jung, S.K. Ahn, Monitoring the conversion of ␣-alumina to AlN under a flow of nitrogen by Al-27 MAS NMR spectroscopy, Journal of Materials Science Letters 16 (1997) 1573–1575. [5] J. Kuang, C. Zhang, X. Zhou, S. Wang, Influence of processing parameters on synthesis of nano-sized AlN powders, Journal of Crystal Growth 263 (2004) 12–20.
Z. Gao et al. / Applied Surface Science 280 (2013) 42–49 [6] M. Qin, X. Du, J. Wang, I.S. Humail, X. Qu, Influence of carbon on the synthesis of AlN powder from combustion synthesis precursors, Journal of the European Ceramic Society 29 (2009) 795–799. [7] M. Qin, X. Du, Z. Li, I.S. Humail, X. Qu, Synthesis of aluminum nitride powder by carbothermal reduction of a combustion synthesis precursor, Materials Research Bulletin 43 (2008) 2954–2960. [8] W.S. Jung, Synthesis of aluminum nitride powder from ı-alumina nanopowders under a mixed gas flow of nitrogen and hydrogen, Ceramics International 38 (2012) 871–874. [9] V. VasanthiPillay, K. Vijayalakshmi, Microstructural evolution and dielectric properties of 1D AlN powders synthesized by microwave technique, Superlattices and Microstructures 51 (2012) 860–867. [10] C. Li, L. Kao, M. Chen, Y. Wang, C. Tsai, Rapid preparation of aluminum nitride powders by using microwave plasma, Journal of Alloys and Compounds 542 (2012) 78–84. [11] N.C. Wu, M.S. Tsai, M.C. Wang, H.S. Liu, The morphology and formation mechanism of aluminum nitride nanocrystals synthesized by chemical vapor deposition, Journal of Crystal Growth 208 (2000) 189–196. [12] C. Giordano, C. Erpen, W. Yao, M. Antonietti, Synthesis of Mo and W carbide and nitride nanoparticles via a simple “urea glass” route, Nano Letters 8 (2008) 4659–4663. [13] C. Giordano, C. Erpen, W. Yao, B. Milke, M. Antonietti, Metal nitride and metal carbide nanoparticles by a soft urea pathway, Chemistry of Materials 21 (2009) 5136–5144. [14] Y. Qiu, L. Gao, Novel synthesis of nanocrystalline gallium nitride powder from gallium(III)–urea complex, Chemistry Letters 32 (2003) 774–775. [15] A. Chu, M. Qin, D. Rafi ud, B. Jia, H. Lu, X. Qu, Effect of urea on the size and morphology of AlN nanoparticles synthesized from combustion synthesis precursors, Journal of Alloys and Compounds 530 (2012) 144–151. [16] A. Chu, M. Qin, d. Rafi ud, B. Jia, H. Lu, X. He, Effect of aluminum source on the synthesis of AlN powders from combustion synthesis precursors, Materials Research Bulletin 47 (2012) 2475–2479. [17] Y. Qiu, L. Gao, Metal–urea complex – a precursor to netal nitrides, Journal of the American Ceramic Society 87 (2004) 352–357. [18] K. Sardar, M. Dan, B. Schwenzer, C.N.R. Rao, A simple single-source precursor route to the nanostructures of AlN, GaN and InN, Journal of Materials Chemistry 15 (2005) 2175.
49
[19] K. Sardar, C.N.R. Rao, AlN nanocrystals by new chemical routes, Solid State Sciences 7 (2005) 217–220. [20] A. Kai, Y. Kamita, T. Miki, Formation process of AlN through precursors and its application to joining AlN ceramics, Journal of the American Ceramic Society 95 (2012) 3788–3796. [21] P.M. Schaber, J. Colson, S. Higgins, D. Thielen, B. Anspach, J. Brauer, Thermal decomposition (pyrolysis) of urea in an open reaction vessel, Thermochimica Acta 424 (2004) 131–142. [22] O. CARP, Considerations on the thermal decomposition of urea, Revue Roumaine de Chimie 46 (2001) 735–740. [23] A.Y. Timoshkin, H.F. Bettinger, H.F. Schaefer III, The chemical vapor deposition of aluminum nitride: unusual cluster formation in the gas phase, Journal of the American Ceramic Society 119 (1997) 5668–5678. [24] Z. He, Z. Chen, Y. Li, Q. Zhang, H. Wang, Molar ratio of In to urea directed formation of In2 O3 hierarchical structures: cubes and nanorod-flowers, CrystEngComm 13 (2011) 2557–2565. [25] X. Ji, H. Li, Z. Wu, S. Cheng, H. Hu, D. Yan, Growth of AlN hexagonal oriented complex nanostructures induced by nucleus arrangement, CrystEngComm 13 (2011) 5198–5203. [26] K. Lu, Sintering of nanoceramics, International Materials Reviews 53 (2008) 21–38. [27] S. Dhamodaran, D.S. Chander, J. Ramkumarto, Anti-reflective and hydrophobic surface of self-organized GaN nano-flowers, Applied Surface Science 257 (2011) 9612–9615. [28] M. Chen, Y. Wang, L. Song, P. Gunawan, Z. Zhong, X. She, Urchin-like ZnO microspheres synthesized by thermal decomposition of hydrozincite as a copper catalyst promoter for the Rochow reaction, RSC Advances 2 (2012) 4164–4168. [29] C. Seran, I. Hyungu, K. Jooheon, The thermal conductivity of embedded nano-aluminum nitride-doped multi-walled carbon nanotubes in epoxy composites containing micro-aluminum nitride particles, Nanotechnology 23 (2012) 065303. [30] E. Lee, S. Lee, D.J. Shanefield, W.R. Cannon, Enhanced thermal conductivity of polymer matrix composite via high solids loading of aluminum nitride in epoxy resin, Journal of the American Ceramic Society 91 (2008) 1169–1174. [31] C.Y. Hsieh, S.L. Chung, High thermal conductivity epoxy molding compound filled with a combustion synthesized AlN powder, Journal of Applied Polymer Science 102 (2006) 4734–4740.