- Email: [email protected]
Rapid preparation of aluminum nitride powders by using microwave plasma
Journal of Alloys and Compounds 542 (2012) 78–84
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Rapid preparation of aluminum nitride powders by using microwave plasma Chia-Hao Li a, Li-Heng Kao a, Meng-Jie Chen a, Ya-Fen Wang b, Cheng-Hsien Tsai a,⇑ a b
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan, ROC Department of Bioenvironmental Engineering, Chung Yuan Christian University, Chung-Li, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 13 April 2012 Received in revised form 13 July 2012 Accepted 16 July 2012 Available online 24 July 2012 Keywords: Synthesis Nitridation Aluminum nitride Discharge
a b s t r a c t Traditionally, syntheses of aluminum nitride (AlN) powders by thermal nitridation approaches are carried out at high temperature for several hours. In this study, high-density hexagonal AlN powders have been synthesized from aluminum powders at a relatively low temperature and short reaction time by the microwave plasma-induced nitridation approach at atmospheric-pressure. The effects of reaction time (15–45 min), temperature (660–850 °C), content of NH4Cl (0–50%) in Al powders, applied power (800–1200 W), and inlet hydrogen concentration (0–1%) on the characteristics of the synthesized AlN powders are discussed. The results show that the active N-containing species produced from the NH4Cl and N2 accelerated the nitridation process, the addition of NH4Cl inhibited the aggregation of powders, and inlet hydrogen reduced the residual oxygen, resulting in wurtzite AlN powders with fine particles and nanowires within porous microstructures after a relatively short time (15 min), at low temperature (800 °C), and with the content of NH4Cl = 50%, and H2 = 1%. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum nitride (AlN) has been acknowledged as an important ceramic material for use as packaging materials and electrical substrates [1]. AlN is attractive due to its high thermal conductivity, low dielectric constant, low dielectric loss, high electrical resistivity, good mechanical strength, non-toxicity, low thermal expansion coefficient (4 106 K1) that is similar to silicon, and high band gap (6.2 eV) [2]. There are several thermal methods for producing AlN powders, such as gas phase synthesis by reacting AlCl3 with NH3 and N2 [3], solvent thermal synthesis by reacting AlCl3 with NaN3 in solvent (xylene) [4,5], self-propagating high-temperature synthesis by igniting the Al and NH4X (X = F, Cl, Br) compact in N2 [6], direct nitridation of Al/LiOH powders [7], and carbothermal reduction of Al2O3 [8,9]. However, the traditional thermal synthesis methods for producing AlN powders usually require a long reaction time (over several hours), high temperature (>1000 °C), or postcalcination treatment because of the slow N2 diffusion rate or reaction rate. The production of AlN powders has also been proposed via various plasma processes, such as arc plasma [10–16], DC pulsed wire discharge [1,17], and radio frequency (RF) plasma [18,19]. These have some advantages, such as short reaction time, high conversion rate, and high energy efficiency for the production of AlN powders. Moreover, in the plasma environment, active and energetic ⇑ Corresponding author. Tel.: +886 7 3814526x5110; fax: +886 7 3830674. E-mail address: [email protected] (C.-H. Tsai). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.086
species, such as excited nitrogen molecules or atoms, and nitrogen ions, could rapidly react with the Al atoms that cover the powder’s surface to form AlN films, preventing the coalescence of Al powders. By using a microwave plasma, the advantage is without the problem of electrode deterioration because the microwave plasma belongs to non-electrode discharge. Moreover, the microwave plasma with a high plasma density can be operated at a relative low temperature (<1000 °C) than the other plasma approaches for producing AlN powders, such as at 1700–2100 °C by using transferred arc plasmas [10,11]. To achieve AlN with high thermal conductivity, impurities such as excess carbon and residual oxygen in the AlN structure should to be removed in all the synthesis methods. This removal is not easily controlled by the conventional thermal approaches, due to the slow reaction rate. However, they can be rapidly removed under plasma processes. Therefore, in this study, a N2 atmospheric-pressure microwave plasma was used to synthesize AlN powders from the Al/NH4Cl ingot to avoid the coalescence of Al powders at a short reaction time and relatively low temperature. In addition, H2 was added in the N2 gas to remove residual oxygen.
2. Experiment The continuous microwave plasma system (Fig. 1) was assembled with a commercially available magnetron (National Electronics YJ-1600, 2.45 GHz) with a maximum stationary power of 5 kW in continuous-wave mode. The microwave passed through a circulator and a waveguide with a three stub tuner, then reached the cavity with an arc to ignite the plasma. A quartz tube intersected the waveguide (ASTEX WR340), while the resonator was placed perpendicular to it.
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
79
plasma reactor by melting the aluminum from the surface, which then reacts with the active N-containing species to form AlN. In order to set the range of synthesis temperature in the microwave discharge zone, thermal analyses of raw Al powders in N2 and commercial AlN powders in air were then carried out (Fig. 2). The results show that there was no variation of weight below 800 °C (Fig. 2A), while the weight of powders increased significantly above 800 °C due to the formation of AlN. In addition, Fig. 2B shows that the weight of the commercial AlN powders fell a little below 850 °C, while the weight increased significantly above 850 °C due to the formation of Al2O3 [21]. Hence, in this study, the reaction temperature of the sample was set in the range of 660–850 °C and in the discharge zone. 3.2. Effects of reaction temperature on the synthesized AlN powders Fig. 1. Experimental apparatus.
The flow rate of gas was regulated by a mass flow controller. The gases supplied in the axial gas were N2 and H2 with flow rates of 3 slm (standard liter min1) and 0–120 sccm (standard cm3 min1), respectively. The flow rate of the swirl N2 gas was 9 slm. Before the reaction, the chamber was flushed by high-purity N2 to remove the residual oxygen and steam. Various contents of NH4Cl (0–50%) were added to the aluminum powders with particles sizes of 1–3.8 lm and pressed into ingots with a 13 mm of diameter to be used as the feedstock. Then the ingots were placed into the sample holder which is a stainless steel container. Without additional temperature supply was used in this study, and the Al/NH4Cl ingots in the sample holder were located at the position of 800 °C in the discharge zone. The effects of the operating parameters, including reaction temperature (660–850 °C), content of NH4Cl (0–50%), inlet hydrogen concentration (0–1%), reaction time (15–45 min), and applied power (800–1200 W) on the characteristics of synthesized aluminum nitride powders were examined, and these are discussed later in this paper. The crystal structure of the aluminum nitride powders was determined using an X-ray diffraction spectrometer (XRD, Rigaku RINT-2000) with CuKa radiation that scanned from 15° to 85° (2h). The contents of O and C elements were analyzed by elemental analyzers (HORIBA EMGA-620 W). The powders were also examined using a scanning electron microscope (SEM, Hitachi S3000N) to determine their sizes and morphologies. The thermal characteristics of the powders were determined with a thermogravimetric analyzer (TGA, SDTQ600) by heating raw Al powders (purity > 99.5%, 1–3.8 lm) or commercial AlN powders (purity > 98.5%, 8– 12 lm) from room temperature to 1200 °C at a heating rate of 10 °C/min in nitrogen gas (for Al) or air (for AlN) with a flow rate of 100 ml/min. The microstructures of the AlN nanowires were studied by high-resolution transmission electron microscopy (HRTEM, JEOL TEM-3010, 200 kV). The surface analyses and the determination of the chemical composition were performed by X-ray photoelectron spectroscopy (XPS, Fison ESCA210). The function groups of the powders mixed in a KBr disk were examined by using Fourier Transform Infrared Spectroscopy (FTIR, Nicolet iS10) in the range of 400–4000 cm1. The dielectric property was measured by an LCR meter (Agilent 4294A). The Raman spectroscopy was measured using an excitation wavelength of 488 nm (Argon ion laser). An optical emission spectrometer (OES, Ocean Optics, HR 4000CG) was also used to measure the active species involved in the formation of AlN in the discharge zone.
3. Results and discussion 3.1. Thermal analyses of raw Al and commercial AlN powders The morphology of the raw Al powders that were used as the feedstock for synthesizing AlN powders was nearly spherical, with the particle sizes being in the range of 1–3.8 lm. Moreover, the element analysis data indicated that amounts of the impurities, including O, C, and Fe elements were 2.4%, 0.1%, and 0.06 ppm, respectively. The chemical equilibrium compositions calculated in Al–N2 system indicated a complete conversion of gaseous Al and the formation of thermodynamically stable h-AlN being favorable at about 2000–2650 K [11,20]. Such a high temperature can be achieved by using a thermal plasma, which needs more power to be applied as well as safer construction of the experimental equipment. Hence, at a moderate temperature, for example, above the melting point of bulk aluminum (660 °C), this can be easily carried out in a
The nitridation of Al to AlN is usually carried out at a high temperature of 1150–1500 °C. In this study, when AlN powders were synthesized in the microwave plasma for 45 min at various temperatures (660–800 °C) and with an applied power of 1200 W without the addition of NH4Cl, the XRD patterns show that the conversion of Al into AlN increased along with the reaction temperature from 660 to 800 °C (Fig. 3). The reaction temperature greatly promoted the AlN formation. However, even at 800 °C, it is difficult for Al to convert completely into AlN. Moreover, only a crystallite size of 35 nm, as calculated by the Debye–Scherrer equation, was found in AlN powders. This is because when the temperature is above the melting point of Al (660 °C), Al melts and the initial oxide film on the surface of the powders is broken by the significant tensile stress arising due to increased volume of the particles, then the liquid Al agglomerates to larger particles. Hence, when AlN was formed on the shell, a low diffusion rate for the N-containing species passing through the hard AlN layer and the initial oxide film would be found, resulting in slowing down the nitridation rate [22]. The morphology and shape of the Al powders (Fig. 4A) are nearly spherical, with particle sizes in the range of 1–3.8 lm. At 800 °C, agglomerated AlN/Al powders with a larger particle sizes of 3–6 lm (Fig. 4B) were produced, because Al powders were melted, resulting in the liquid Al forming larger clusters and the agglomeration of AlN particles. 3.3. Effects of NH4Cl on the characteristics of AlN powders The molten Al powders agglomerated into large clusters, resulting in the inhibition of N2 gas diffusing into the unreacted Al core during the nitridation process. Therefore, different contents of NH4Cl powders were dispersed in the Al powders and were compressed as ingots at high-pressure. Fig. 5 shows the XRD patterns of the as-synthesized AlN powders from as-prepared ingots reacting for 45 min at 800 °C and 1200 W. The results show that the conversion of Al into AlN rose significantly from 70.8% to about 100% as the NH4Cl content increased from 0 to 50 wt.%. The results show that by rapidly sublimating and decomposing NH4Cl within the ingot, numerous porous structures (Fig. 6A) with a pore size similar with the size of NH4Cl powders were produced. This structure can prevent the aggregation or coalescence of molten Al by means of rapidly dissociating NH4Cl and carrier gas (N2) to produce energetic N- and Cl-containing species that react with Al on the surface to further form a hard, thin film of nitrides. Subsequently, N-containing species can thus diffuse more easily into the Al particles and promote the nitridation rate, resulting in a greater AlN yield [22,23]. In addition, the sublimation of NH4Cl occurs around 400 °C, and this then decomposed to HCl and NH3. HCl can further react with
80
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
Fig. 2. TGA patterns of Al powders in N2 (A) and standard AlN powders in air (B).
Fig. 3. XRD patterns of AlN powders synthesized at various temperatures (A) 660 °C; (B) 700 °C; (C) 800 °C (45 min, without NH4Cl, 1200 W, H2 = 0.25%).
Al to produce intermediate gaseous AlCl3 and H2. The pores can trigger a vapor reaction between AlCl3 and N2, which causes the AlN crystals to deposit on the surface of the pores to form AlN fine particles (Fig. 6A) and AlN nanowires or nanorods (Fig. 6B) by the following reactions [24]:
NH4 ClðsÞ ¼ NH3ðgÞ þ HClðgÞ 3 Alðs;lÞ þ 3HClðgÞ ¼ AlCl3ðgÞ þ H2ðgÞ 2 1 3 AlCl3ðgÞ þ N2ðgÞ þ H2ðgÞ ¼ AlNðsÞ þ 3HClðgÞ 2 2 Furthermore, NH3 and N2 can be dissociated into active N atoms and ions and excited N2 in the plasma, and these energetic N-containing species can thermodynamically easily react with Al or AlCl3 than N2 molecules to form AlN:
Fig. 5. XRD patterns of AlN powders synthesized with various NH4Cl contents (A) 0 wt.%; (B) 25 wt.%; (C) 50 wt.% (45 min, 800 °C, 1200 W, H2 = 1%).
3þ
excited N2 ðor N;N2þ ; Nþ Þ þ Alðor Al Þ ! AlN þ In addition, the high solubility of H2 in the molten Al will increase the surface area of the vapor–solid phase reaction to enhance the yield of AlN nanowires with low coalescence [24]. From the TEM images (Fig. 7A), the synthesized AlN nanowires with a diameter of approximate 79 nm were identified. The selected area electron diffraction (SAED) pattern and the corresponding high-resolution TEM image (Fig. 7B) taken from the nanowires can be indexed based on the h-AlN cell (JCPDS No. 251133). The SAED pattern of the h-AlN nanowires was taken from the [0 0 1] zone axis and the HRTEM image of the AlN nanowire shows the interplanar spacing of the lattice planes is 0.27 nm,
Fig. 4. SEM micrographs of (A) Al powders and (B) AlN powders produced at 800 °C, reaction time = 45 min, without NH4Cl, 1200 W, and H2 = 0.25%.
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
81
Fig. 6. SEM micrograph of AlN powders with (A) porous microstructures and fine particles; (B) nanowires (15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
Fig. 7. (A) TEM image of as-synthesized AlN nanowire, (B) high-resolution TEM image and the selected area electron diffraction pattern taken from the AlN nanowire.
which matches well with the distance value of (0 1 0) lattice planes of the h-AlN. A TEM image and its corresponding SAED pattern of a single AlN nanowire show that the growth direction for the nanowires was along [0 1 0]. In order to distinguish the effects of NH4Cl on the characteristics of AlN powders, FTIR spectra of the synthesized AlN powders for the reaction times of 5 and 15 min were obtained. Fig. 8 shows that for reaction time if 5 min, Al–Cl bonds were observed at about 600 and 1350 cm1 because AlCl3 was deposited [3], and did not react completely into AlN. The broad peaks between 650 and 780 cm1 correspond to Al–N lattice vibrations [5,8,25]. Moreover, as the reaction time increases, the Al–N peak becomes stronger. However, the as-synthesized powders showed N–H bonds, and these were perhaps caused by NH4Cl or AlN reacting with hydrogen. In addition, O–H bonds were found at around 3400 cm1 at reaction time = 5 min, due to the fact that Al(OH)3 could be formed on the surface of the powder when AlN reacted with the adsorbed moisture. Furthermore, the surface compositions and functional groups of the powders synthesized at 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1% were measured by XPS. The peak regions at 73.9 and 396.5 eV correspond to Al2p (Fig. 9A) and N1s (Fig. 9B), respectively, and should be attributed to the Al–N bonds [25,26]. 3.4. Effects of inlet hydrogen concentration on the AlN powders The thermal conductivity of AlN is sensitive to its crystal structure and impurities, and residual oxygen in particular can cause point defects in the structure [27]. Achieving AlN with high thermal conductivity thus requires a reduction in residual oxygen. Therefore, hydrogen was added to the inlet gas to remove the rare oxygen in the raw Al powders or inlet N2 gas. Fig. 10 shows the
Fig. 8. FTIR spectra of synthesized AlN powders with a reaction time of 5 and 15 min ([NH4Cl] = 50%, 800 °C, 1200 W, [H2] = 1%).
XRD patterns of the as-synthesized AlN powders for various inlet hydrogen concentrations (0–1%) with a reaction time of 45 min, NH4Cl = 50%, 800 °C, and 1200 W. The structure clearly consists of Al and AlN, at [H2] = 0% or 0.25%, while the intensity of diffraction peaks corresponding to Al is almost not observed when the inlet H2 concentration rises to 1%. Moreover, the results also show that adding hydrogen prevented the formation of Al2O3. The elemental analysis of the AlN powders was performed using an elemental analyzer and ICP-MS. The as-prepared AlN powders
82
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
Fig. 9. XPS of AlN powders (A) Al2p and (B) N1s (15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
Fig. 10. XRD patterns of powders synthesized for various inlet hydrogen concentrations (A) 0%; (B) 0.25%; (C) 1.0% (45 min, NH4Cl = 50%, 800 °C, 1200 W).
produced from raw Al powders (O: 2.4%, C: 0.1%, Fe: 0.06 ppm) with a reaction time of 15 min, 800 °C, and 1200 W consisted of 1.3 wt.% of O atoms, 0.13 wt.% of C atoms, and 0.10 ppm of Fe atoms. This is consistent with the composition of commercial AlN powders (O: 1.0%, C: 0.13%, Fe: 0.1 ppm), and indicates that the oxygen content in AlN powders can be reduced when H2 is introduced into the process. In addition, the oxygen content of the as-synthesized powders was lower than that obtained by previous plasma processes, such as when using RF and DC arc plasmas with O atoms at 5 and 12 wt.%, respectively [18,28]. 3.5. Effects of reaction time on the characteristics of AlN powders The synthesis of AlN powders was usually carried out with a long reaction time (1–6 h) (Table 1) because the low diffusion rate of nitrogen or the low reaction rate. A large amount of active
Fig. 11. XRD patterns of AlN powders synthesized at various reaction times (A) 15 min; (B) 30 min; (C) 45 min (NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
N-containing species were produced using a microwave plasma to accelerate the nitridation of Al in a relative short reaction time (15–45 min) in this study. Fig. 11 shows the XRD pattern of the AlN powders produced from the ingots made of Al and NH4Cl powders (Al:NH4Cl = 1:1) for various reaction times (15–45 min) at 800 °C, 1200 W, and H2 = 1%. All the samples were composed of hexagonal AlN. Moreover, the results show that the conversion of Al into AlN almost reached 100% at a reaction time of 15 min (Fig. 11). This short synthesis time can be achieved because in the discharge zone active and energetic N-containing species, such as atomic N, excited N2, and N or N2 ions, were produced from nitrogen gas [29–32] or NH4Cl, and involved in the reaction with Al or AlCl3 to rapidly produce AlN [10]. Fig. 12 shows that the large amounts of intermediate species were detected by using optical emission spectrometer in the MW discharge zone occurred at a
Table 1 The operating conditions for the synthesis of AlN powders by different methods. Synthesis method
Reactants
Reaction time (h)
Temperature (°C)
Ref.
Gas-phase synthesis Carbothermal reduction
MW plasma
Al, NH4Cl, N2, H2
1 Ignition 2 1 6 Calcination 48 h 15–45 min
1000 (yield = 80%) – 1500 500–1000 280 °C (amorphous) 700 800
[3] [8]
Direct nitridation Solvent thermal synthesis
AlCl3, NH3, N2 Al(NO3)3, CO(NH2)2, NH4NO3 Al2O3, C, N2 Al, LiOH, N2 AlCl3, NaN3, xylene (solvent)
[7] [5] This study
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
83
reaction time of 45 min, and it can be seen that the intensities of the diffraction peaks corresponding to AlN were elevated by increasing the applied power, because the higher power applied was accompanied by increased plasma density, resulting in the increased concentration of active N-containing species for involving in the synthesis reaction. 3.7. Characteristics of synthesized AlN powders
Fig. 12. Optical emission spectra for nitrogen MW plasma at 760 torr, 800 and 1200 kW.
pressure of 760 torr and 800 or 1200 W. Moreover, a higher concentration of active species was found at 1200 W than at 800 W, resulting in a shorter AlN synthesis time. The main plasma processes, such as dissociative ionization, ionization, and excitation reactions, are carried out through electron-impact reaction and the Penning ionization reaction. For nitrogen plasma, the emission lines of N2 from the first positive band (B3Pg ? A3Ru+ transition, at 550–900 nm) and the second positive band (C3Pu ? B3Pg transition, at 300–400 nm) [30,31,33], and the emission lines of nitrogen molecular ions from the first negative band (B2Ru+ ? X2Rg+ transition, at 381–470 nm) [30,33] were measured. However, atomic lines and electronically excited species were difficult to be observed in the discharge environment.
3.6. Effects of applied power on AlN powders Fig. 13 shows the XRD patterns of the AlN powders synthesized at various operating powers (800–1200 W) at 800 °C with a
Fig. 13. XRD patterns of AlN powders synthesized at various operation powers (A) 800 W; (B) 1000 W; (C) 1200 W (45 min, NH4Cl = 50%, 800 °C, H2 = 1%).
Fig. 14 shows the Raman spectra of the synthesized AlN powders prepared with a reaction time of 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1%. The four distinct peaks centered at 249, 610, 655, and 670 cm1 are indexed to the first-order vibrational modes of E2(low), A1(TO), E2(high), and E1(TO), respectively. In addition, the low-intensity broad peaks around 895 and 905 cm1 are correlated to the overlap of the modes A1(LO) and E1(LO), respectively. These results agree with those found for hAlN crystallization [12–14]. AlN powders can be used as the ceramic substrate for applications that require the transmission of high frequency electrical signals without delays. Hence, the dielectric constant of the synthesized AlN powders (prepared at a reaction time of 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1%) and commercial AlN powders were measured within the range of 1 k Hz to 1 MHz at room temperature. Fig. 15 shows that the dielectric constant decreased as the frequency increased, and that the as-synthesized AlN powders had a little lower dielectric constant than the commercial ones. Specifically, the values of the dielectric constants obtained at a frequency of 1 MHz were 7.3 and 7.9 for the asprepared and commercial AlN powders, respectively. The sintering additives of CaO (1 wt.%) and Y2O3 (1 wt.%) were added to the as-prepared AlN powders produced at 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1%, and the commercial AlN powders, respectively, and then were sintered at 1600 °C for 3 h in argon gas. The results show that the thermal conductivity of the as-prepared AlN powders reached 141 W/(mK), being higher than that of the commercial AlN powders (68 W/(mK)). That is because in the plasma environment, hydrogen in the inlet gas can remove the residual oxygen in the raw Al powders or inlet N2 gas. Moreover, the sintering additives of Y2O3 and CaO can also enhance the thermal conductivity of AlN by reducing the oxygen content of AlN particles via forming the secondary phases around the grain boundaries [34].
Fig. 14. Raman spectrum of the synthesized AlN powders (15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
84
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
electron density with a higher concentration of N-containing compounds. Acknowledgments We would like to thank the National Science Council of Taiwan for its financial support under Grants NSC 100-2221-E-151-004MY2 and NSC 100-2622-E-151-007-CC3. References
Fig. 15. Dielectric constant for synthesized AlN powders (at 15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%) and commercial AlN powders.
4. Conclusion In this study, high purity AlN powders were synthesized using an atmospheric-pressure microwave plasma at a short reaction time of 15 min and a relatively low temperature of 800 °C, with conventional thermal methods usually requiring a long reaction time (over several hours) and high temperature (>1000 °C) due to the slow diffusion rate or reaction rate between Al metal and N2 gas. The results show that while operating at a higher reaction temperature raises the conversion of Al, the Al powders agglomerate due to the molten Al, making the diffusion of N-containing species into the core of the aggregated particles more difficult, even a reaction time of 45 min. When NH4Cl powders were mixed with Al powders and pressed into a compact ingot, the yield of AlN rose significantly with increasing NH4Cl content, and reached almost 100% at the NH4Cl content of 50%, 800 °C, 1200 W, and an inlet hydrogen concentration of 1%, even for only reacting 15 min. This is because the sublimation and decomposition of NH4Cl rapidly produces AlN film to form porous microstructures, avoid the coalescence of molten Al, and synthesize fine particles and nanowires. In addition, NH4Cl and inlet H2 in the discharge zone generate more active N-containing species, not only enhancing the nitridation rate and shortening the synthesis time (15 min), but also removing and reducing the residual O atoms in the structure, especially at a higher applied power (1200 W) by generating a higher
[1] C. Sangural, Y. Kinemuchi, T. Suzuki, W. Jiang, K. Yatsui, Jpn. J. Appl. Phys. 40 (2001) 1070–1072. [2] G. Selvaduray, L. Sheet, Mater. Sci. Technol. Ser. 9 (1993) 463–473. [3] J.R. Park, S.W. Rhee, K.H. Lee, J. Mater. Sci. 28 (1993) 57–64. [4] X.P. Hao, M.Y. Yu, D.L. Cui, X.G. Xu, Y.J. Bai, Q.L. Wang, M.H. Jiang, J. Cryst. Growth 242 (2002) 229–232. [5] L. Li, X. Hao, N. Yu, D. Cui, X. Xu, M. Jiang, J. Cryst. Growth 258 (2003) 268–271. [6] S.L. Chung, W.L. Yu, C.N. Liu, J. Mater. Res. 14 (1999) 1928–1933. [7] Y. Kameshima, M. Irie, A. Yasumori, K. Okada, Solid State Ionics 172 (2004) 182–190. [8] J.C. Kuang, C.R. Zang, X.G. Zhou, S.Q. Wang, J. Cryst. Growth 256 (2003) 288– 291. [9] M. Qin, X. Du, Z. Li, X. Qu, I.S. Humail, Mater. Res. Bull. 42 (2008) 2954–2960. [10] H. Ageorges, S. Megy, K. Chang, J.M. Baronnet, J.K. Williams, C. Chapman, Plasma Chem. Plasma Process. 13 (1992) 613–632. [11] M. Iwata, K. Adachi, S. Furukawa, T. Amakawa, J. Phys. D: Appl. Phys. 37 (2004) 1041–1047. [12] L. Shen, T. Cheng, L. Wu, X. Li, Q. Cui, J. Alloys Compd. 465 (2008) 562–566. [13] Y. Tian, Y. Jia, Y. Bao, Y. Chen, Diam. Relat. Mater. 16 (2007) 302–305. [14] L.H. Shen, X.F. Li, J. Zgang, Y.M. Ma, F. Wang, G. Peng, Q.L. Cui, G.T. Zou, Appl. Phys. A 84 (2006) 73–75. [15] K. Kim, J. Cryst. Growth 283 (2005) 540–546. [16] H.D. Li, G.T. Zou, H. Wang, H.B. Yang, D.M. Li, M.H. Li, S. Yu, Y. Wu, Z.F. Meng, J. Phys. Chem. B 102 (1998) 8692–8695. [17] K.A. Khor, K.H. Cheng, L.G. Yu, F. Boey, Mater. Sci. Eng., A 346 (2003) 300–305. [18] K. Baba, N. Shohata, M. Yonezawa, Appl. Phys. Lett. 54 (1989) 2309–2311. [19] M. Yamada, T. Yasui, M. Fukumoto, K. Takahashi, Thin Solid Films 515 (2007) 4166–4171. [20] S.M. Oh, D.K. Park, J. Ind. Eng. Chem. 6 (2000) 1–7. [21] F. Ansat, H. Ganda, R. Saporte, J.P. Traverse, Thin Solid Films 260 (1995) 38–46. [22] V. Rosenband, A. Gany, J. Mater. Process Technol. 147 (2004) 197–203. [23] Y. Qiu, L. Gao, J. Eur. Ceram. Soc. 23 (2003) 2015–2022. [24] R.K. Paul, H.Y. Song, K.H. Lee, B.T. Lee, Mater. Chem. Phys. 112 (2008) 562–565. [25] C. Wu, Q. Yang, C. Hung, D. Wang, P. Yin, J. Solid State Chem. 177 (2004) 3522– 3528. [26] G. Yan, G. Chen, H. Lu, L. Guo, J. Wuhan Univ. Technol. 23 (2008) 334–337. [27] S. Fukumoto, T. Hookabe, H. Tsubakino, J. Mater. Sci. 35 (2000) 2743–2748. [28] F.J. Moura, R.J. Munz, J. Am. Ceram. Soc. 80 (1997) 2425–2428. [29] E.A.H. Timmermans, J. Jonkers, A. Rodero, M.C. Quintero, A. Sola, A. Gamero, D.C. Schram, J.A.M. Mullen van der, Spectrochim. Acta B 53 (1998) 1553–1566. [30] E.A.H. Timmermans, J. Jonkers, A. Rodero, M.C. Quintero, A. Sola, A. Gamero, D.C. Schram, J.A.M. Mullen van der, Spectrochim. Acta B 54 (1999) 1085–1098. [31] H.P. Hsueh, R.T. McGrath, B. Ji, B.S. Felker, J.G. Langan, E.J. Karwacki, J. Vac. Sci. Technol., B 19 (2001) 1347–1357. [32] Q. Sun, A.M. Zhu, X.F. Yang, J.H. Niu, Y. Xu, Z.M. Song, J. Liu, Chin. Chem. Lett. 16 (2005) 839–842. [33] B.Y. Jeong, M.H. Kim, Surf. Coat. Technol. 141 (2001) 182–189. [34] J.H. Pee, J.C. Park, K.T. Hwang, S. Kim, W.S. Cho, Res. Chem. Intermediat. 36 (2010) 801–809.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Rapid preparation of aluminum nitride powders by using microwave plasma Chia-Hao Li a, Li-Heng Kao a, Meng-Jie Chen a, Ya-Fen Wang b, Cheng-Hsien Tsai a,⇑ a b
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan, ROC Department of Bioenvironmental Engineering, Chung Yuan Christian University, Chung-Li, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 13 April 2012 Received in revised form 13 July 2012 Accepted 16 July 2012 Available online 24 July 2012 Keywords: Synthesis Nitridation Aluminum nitride Discharge
a b s t r a c t Traditionally, syntheses of aluminum nitride (AlN) powders by thermal nitridation approaches are carried out at high temperature for several hours. In this study, high-density hexagonal AlN powders have been synthesized from aluminum powders at a relatively low temperature and short reaction time by the microwave plasma-induced nitridation approach at atmospheric-pressure. The effects of reaction time (15–45 min), temperature (660–850 °C), content of NH4Cl (0–50%) in Al powders, applied power (800–1200 W), and inlet hydrogen concentration (0–1%) on the characteristics of the synthesized AlN powders are discussed. The results show that the active N-containing species produced from the NH4Cl and N2 accelerated the nitridation process, the addition of NH4Cl inhibited the aggregation of powders, and inlet hydrogen reduced the residual oxygen, resulting in wurtzite AlN powders with fine particles and nanowires within porous microstructures after a relatively short time (15 min), at low temperature (800 °C), and with the content of NH4Cl = 50%, and H2 = 1%. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum nitride (AlN) has been acknowledged as an important ceramic material for use as packaging materials and electrical substrates [1]. AlN is attractive due to its high thermal conductivity, low dielectric constant, low dielectric loss, high electrical resistivity, good mechanical strength, non-toxicity, low thermal expansion coefficient (4 106 K1) that is similar to silicon, and high band gap (6.2 eV) [2]. There are several thermal methods for producing AlN powders, such as gas phase synthesis by reacting AlCl3 with NH3 and N2 [3], solvent thermal synthesis by reacting AlCl3 with NaN3 in solvent (xylene) [4,5], self-propagating high-temperature synthesis by igniting the Al and NH4X (X = F, Cl, Br) compact in N2 [6], direct nitridation of Al/LiOH powders [7], and carbothermal reduction of Al2O3 [8,9]. However, the traditional thermal synthesis methods for producing AlN powders usually require a long reaction time (over several hours), high temperature (>1000 °C), or postcalcination treatment because of the slow N2 diffusion rate or reaction rate. The production of AlN powders has also been proposed via various plasma processes, such as arc plasma [10–16], DC pulsed wire discharge [1,17], and radio frequency (RF) plasma [18,19]. These have some advantages, such as short reaction time, high conversion rate, and high energy efficiency for the production of AlN powders. Moreover, in the plasma environment, active and energetic ⇑ Corresponding author. Tel.: +886 7 3814526x5110; fax: +886 7 3830674. E-mail address: [email protected] (C.-H. Tsai). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.086
species, such as excited nitrogen molecules or atoms, and nitrogen ions, could rapidly react with the Al atoms that cover the powder’s surface to form AlN films, preventing the coalescence of Al powders. By using a microwave plasma, the advantage is without the problem of electrode deterioration because the microwave plasma belongs to non-electrode discharge. Moreover, the microwave plasma with a high plasma density can be operated at a relative low temperature (<1000 °C) than the other plasma approaches for producing AlN powders, such as at 1700–2100 °C by using transferred arc plasmas [10,11]. To achieve AlN with high thermal conductivity, impurities such as excess carbon and residual oxygen in the AlN structure should to be removed in all the synthesis methods. This removal is not easily controlled by the conventional thermal approaches, due to the slow reaction rate. However, they can be rapidly removed under plasma processes. Therefore, in this study, a N2 atmospheric-pressure microwave plasma was used to synthesize AlN powders from the Al/NH4Cl ingot to avoid the coalescence of Al powders at a short reaction time and relatively low temperature. In addition, H2 was added in the N2 gas to remove residual oxygen.
2. Experiment The continuous microwave plasma system (Fig. 1) was assembled with a commercially available magnetron (National Electronics YJ-1600, 2.45 GHz) with a maximum stationary power of 5 kW in continuous-wave mode. The microwave passed through a circulator and a waveguide with a three stub tuner, then reached the cavity with an arc to ignite the plasma. A quartz tube intersected the waveguide (ASTEX WR340), while the resonator was placed perpendicular to it.
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
79
plasma reactor by melting the aluminum from the surface, which then reacts with the active N-containing species to form AlN. In order to set the range of synthesis temperature in the microwave discharge zone, thermal analyses of raw Al powders in N2 and commercial AlN powders in air were then carried out (Fig. 2). The results show that there was no variation of weight below 800 °C (Fig. 2A), while the weight of powders increased significantly above 800 °C due to the formation of AlN. In addition, Fig. 2B shows that the weight of the commercial AlN powders fell a little below 850 °C, while the weight increased significantly above 850 °C due to the formation of Al2O3 [21]. Hence, in this study, the reaction temperature of the sample was set in the range of 660–850 °C and in the discharge zone. 3.2. Effects of reaction temperature on the synthesized AlN powders Fig. 1. Experimental apparatus.
The flow rate of gas was regulated by a mass flow controller. The gases supplied in the axial gas were N2 and H2 with flow rates of 3 slm (standard liter min1) and 0–120 sccm (standard cm3 min1), respectively. The flow rate of the swirl N2 gas was 9 slm. Before the reaction, the chamber was flushed by high-purity N2 to remove the residual oxygen and steam. Various contents of NH4Cl (0–50%) were added to the aluminum powders with particles sizes of 1–3.8 lm and pressed into ingots with a 13 mm of diameter to be used as the feedstock. Then the ingots were placed into the sample holder which is a stainless steel container. Without additional temperature supply was used in this study, and the Al/NH4Cl ingots in the sample holder were located at the position of 800 °C in the discharge zone. The effects of the operating parameters, including reaction temperature (660–850 °C), content of NH4Cl (0–50%), inlet hydrogen concentration (0–1%), reaction time (15–45 min), and applied power (800–1200 W) on the characteristics of synthesized aluminum nitride powders were examined, and these are discussed later in this paper. The crystal structure of the aluminum nitride powders was determined using an X-ray diffraction spectrometer (XRD, Rigaku RINT-2000) with CuKa radiation that scanned from 15° to 85° (2h). The contents of O and C elements were analyzed by elemental analyzers (HORIBA EMGA-620 W). The powders were also examined using a scanning electron microscope (SEM, Hitachi S3000N) to determine their sizes and morphologies. The thermal characteristics of the powders were determined with a thermogravimetric analyzer (TGA, SDTQ600) by heating raw Al powders (purity > 99.5%, 1–3.8 lm) or commercial AlN powders (purity > 98.5%, 8– 12 lm) from room temperature to 1200 °C at a heating rate of 10 °C/min in nitrogen gas (for Al) or air (for AlN) with a flow rate of 100 ml/min. The microstructures of the AlN nanowires were studied by high-resolution transmission electron microscopy (HRTEM, JEOL TEM-3010, 200 kV). The surface analyses and the determination of the chemical composition were performed by X-ray photoelectron spectroscopy (XPS, Fison ESCA210). The function groups of the powders mixed in a KBr disk were examined by using Fourier Transform Infrared Spectroscopy (FTIR, Nicolet iS10) in the range of 400–4000 cm1. The dielectric property was measured by an LCR meter (Agilent 4294A). The Raman spectroscopy was measured using an excitation wavelength of 488 nm (Argon ion laser). An optical emission spectrometer (OES, Ocean Optics, HR 4000CG) was also used to measure the active species involved in the formation of AlN in the discharge zone.
3. Results and discussion 3.1. Thermal analyses of raw Al and commercial AlN powders The morphology of the raw Al powders that were used as the feedstock for synthesizing AlN powders was nearly spherical, with the particle sizes being in the range of 1–3.8 lm. Moreover, the element analysis data indicated that amounts of the impurities, including O, C, and Fe elements were 2.4%, 0.1%, and 0.06 ppm, respectively. The chemical equilibrium compositions calculated in Al–N2 system indicated a complete conversion of gaseous Al and the formation of thermodynamically stable h-AlN being favorable at about 2000–2650 K [11,20]. Such a high temperature can be achieved by using a thermal plasma, which needs more power to be applied as well as safer construction of the experimental equipment. Hence, at a moderate temperature, for example, above the melting point of bulk aluminum (660 °C), this can be easily carried out in a
The nitridation of Al to AlN is usually carried out at a high temperature of 1150–1500 °C. In this study, when AlN powders were synthesized in the microwave plasma for 45 min at various temperatures (660–800 °C) and with an applied power of 1200 W without the addition of NH4Cl, the XRD patterns show that the conversion of Al into AlN increased along with the reaction temperature from 660 to 800 °C (Fig. 3). The reaction temperature greatly promoted the AlN formation. However, even at 800 °C, it is difficult for Al to convert completely into AlN. Moreover, only a crystallite size of 35 nm, as calculated by the Debye–Scherrer equation, was found in AlN powders. This is because when the temperature is above the melting point of Al (660 °C), Al melts and the initial oxide film on the surface of the powders is broken by the significant tensile stress arising due to increased volume of the particles, then the liquid Al agglomerates to larger particles. Hence, when AlN was formed on the shell, a low diffusion rate for the N-containing species passing through the hard AlN layer and the initial oxide film would be found, resulting in slowing down the nitridation rate [22]. The morphology and shape of the Al powders (Fig. 4A) are nearly spherical, with particle sizes in the range of 1–3.8 lm. At 800 °C, agglomerated AlN/Al powders with a larger particle sizes of 3–6 lm (Fig. 4B) were produced, because Al powders were melted, resulting in the liquid Al forming larger clusters and the agglomeration of AlN particles. 3.3. Effects of NH4Cl on the characteristics of AlN powders The molten Al powders agglomerated into large clusters, resulting in the inhibition of N2 gas diffusing into the unreacted Al core during the nitridation process. Therefore, different contents of NH4Cl powders were dispersed in the Al powders and were compressed as ingots at high-pressure. Fig. 5 shows the XRD patterns of the as-synthesized AlN powders from as-prepared ingots reacting for 45 min at 800 °C and 1200 W. The results show that the conversion of Al into AlN rose significantly from 70.8% to about 100% as the NH4Cl content increased from 0 to 50 wt.%. The results show that by rapidly sublimating and decomposing NH4Cl within the ingot, numerous porous structures (Fig. 6A) with a pore size similar with the size of NH4Cl powders were produced. This structure can prevent the aggregation or coalescence of molten Al by means of rapidly dissociating NH4Cl and carrier gas (N2) to produce energetic N- and Cl-containing species that react with Al on the surface to further form a hard, thin film of nitrides. Subsequently, N-containing species can thus diffuse more easily into the Al particles and promote the nitridation rate, resulting in a greater AlN yield [22,23]. In addition, the sublimation of NH4Cl occurs around 400 °C, and this then decomposed to HCl and NH3. HCl can further react with
80
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
Fig. 2. TGA patterns of Al powders in N2 (A) and standard AlN powders in air (B).
Fig. 3. XRD patterns of AlN powders synthesized at various temperatures (A) 660 °C; (B) 700 °C; (C) 800 °C (45 min, without NH4Cl, 1200 W, H2 = 0.25%).
Al to produce intermediate gaseous AlCl3 and H2. The pores can trigger a vapor reaction between AlCl3 and N2, which causes the AlN crystals to deposit on the surface of the pores to form AlN fine particles (Fig. 6A) and AlN nanowires or nanorods (Fig. 6B) by the following reactions [24]:
NH4 ClðsÞ ¼ NH3ðgÞ þ HClðgÞ 3 Alðs;lÞ þ 3HClðgÞ ¼ AlCl3ðgÞ þ H2ðgÞ 2 1 3 AlCl3ðgÞ þ N2ðgÞ þ H2ðgÞ ¼ AlNðsÞ þ 3HClðgÞ 2 2 Furthermore, NH3 and N2 can be dissociated into active N atoms and ions and excited N2 in the plasma, and these energetic N-containing species can thermodynamically easily react with Al or AlCl3 than N2 molecules to form AlN:
Fig. 5. XRD patterns of AlN powders synthesized with various NH4Cl contents (A) 0 wt.%; (B) 25 wt.%; (C) 50 wt.% (45 min, 800 °C, 1200 W, H2 = 1%).
3þ
excited N2 ðor N;N2þ ; Nþ Þ þ Alðor Al Þ ! AlN þ In addition, the high solubility of H2 in the molten Al will increase the surface area of the vapor–solid phase reaction to enhance the yield of AlN nanowires with low coalescence [24]. From the TEM images (Fig. 7A), the synthesized AlN nanowires with a diameter of approximate 79 nm were identified. The selected area electron diffraction (SAED) pattern and the corresponding high-resolution TEM image (Fig. 7B) taken from the nanowires can be indexed based on the h-AlN cell (JCPDS No. 251133). The SAED pattern of the h-AlN nanowires was taken from the [0 0 1] zone axis and the HRTEM image of the AlN nanowire shows the interplanar spacing of the lattice planes is 0.27 nm,
Fig. 4. SEM micrographs of (A) Al powders and (B) AlN powders produced at 800 °C, reaction time = 45 min, without NH4Cl, 1200 W, and H2 = 0.25%.
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
81
Fig. 6. SEM micrograph of AlN powders with (A) porous microstructures and fine particles; (B) nanowires (15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
Fig. 7. (A) TEM image of as-synthesized AlN nanowire, (B) high-resolution TEM image and the selected area electron diffraction pattern taken from the AlN nanowire.
which matches well with the distance value of (0 1 0) lattice planes of the h-AlN. A TEM image and its corresponding SAED pattern of a single AlN nanowire show that the growth direction for the nanowires was along [0 1 0]. In order to distinguish the effects of NH4Cl on the characteristics of AlN powders, FTIR spectra of the synthesized AlN powders for the reaction times of 5 and 15 min were obtained. Fig. 8 shows that for reaction time if 5 min, Al–Cl bonds were observed at about 600 and 1350 cm1 because AlCl3 was deposited [3], and did not react completely into AlN. The broad peaks between 650 and 780 cm1 correspond to Al–N lattice vibrations [5,8,25]. Moreover, as the reaction time increases, the Al–N peak becomes stronger. However, the as-synthesized powders showed N–H bonds, and these were perhaps caused by NH4Cl or AlN reacting with hydrogen. In addition, O–H bonds were found at around 3400 cm1 at reaction time = 5 min, due to the fact that Al(OH)3 could be formed on the surface of the powder when AlN reacted with the adsorbed moisture. Furthermore, the surface compositions and functional groups of the powders synthesized at 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1% were measured by XPS. The peak regions at 73.9 and 396.5 eV correspond to Al2p (Fig. 9A) and N1s (Fig. 9B), respectively, and should be attributed to the Al–N bonds [25,26]. 3.4. Effects of inlet hydrogen concentration on the AlN powders The thermal conductivity of AlN is sensitive to its crystal structure and impurities, and residual oxygen in particular can cause point defects in the structure [27]. Achieving AlN with high thermal conductivity thus requires a reduction in residual oxygen. Therefore, hydrogen was added to the inlet gas to remove the rare oxygen in the raw Al powders or inlet N2 gas. Fig. 10 shows the
Fig. 8. FTIR spectra of synthesized AlN powders with a reaction time of 5 and 15 min ([NH4Cl] = 50%, 800 °C, 1200 W, [H2] = 1%).
XRD patterns of the as-synthesized AlN powders for various inlet hydrogen concentrations (0–1%) with a reaction time of 45 min, NH4Cl = 50%, 800 °C, and 1200 W. The structure clearly consists of Al and AlN, at [H2] = 0% or 0.25%, while the intensity of diffraction peaks corresponding to Al is almost not observed when the inlet H2 concentration rises to 1%. Moreover, the results also show that adding hydrogen prevented the formation of Al2O3. The elemental analysis of the AlN powders was performed using an elemental analyzer and ICP-MS. The as-prepared AlN powders
82
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
Fig. 9. XPS of AlN powders (A) Al2p and (B) N1s (15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
Fig. 10. XRD patterns of powders synthesized for various inlet hydrogen concentrations (A) 0%; (B) 0.25%; (C) 1.0% (45 min, NH4Cl = 50%, 800 °C, 1200 W).
produced from raw Al powders (O: 2.4%, C: 0.1%, Fe: 0.06 ppm) with a reaction time of 15 min, 800 °C, and 1200 W consisted of 1.3 wt.% of O atoms, 0.13 wt.% of C atoms, and 0.10 ppm of Fe atoms. This is consistent with the composition of commercial AlN powders (O: 1.0%, C: 0.13%, Fe: 0.1 ppm), and indicates that the oxygen content in AlN powders can be reduced when H2 is introduced into the process. In addition, the oxygen content of the as-synthesized powders was lower than that obtained by previous plasma processes, such as when using RF and DC arc plasmas with O atoms at 5 and 12 wt.%, respectively [18,28]. 3.5. Effects of reaction time on the characteristics of AlN powders The synthesis of AlN powders was usually carried out with a long reaction time (1–6 h) (Table 1) because the low diffusion rate of nitrogen or the low reaction rate. A large amount of active
Fig. 11. XRD patterns of AlN powders synthesized at various reaction times (A) 15 min; (B) 30 min; (C) 45 min (NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
N-containing species were produced using a microwave plasma to accelerate the nitridation of Al in a relative short reaction time (15–45 min) in this study. Fig. 11 shows the XRD pattern of the AlN powders produced from the ingots made of Al and NH4Cl powders (Al:NH4Cl = 1:1) for various reaction times (15–45 min) at 800 °C, 1200 W, and H2 = 1%. All the samples were composed of hexagonal AlN. Moreover, the results show that the conversion of Al into AlN almost reached 100% at a reaction time of 15 min (Fig. 11). This short synthesis time can be achieved because in the discharge zone active and energetic N-containing species, such as atomic N, excited N2, and N or N2 ions, were produced from nitrogen gas [29–32] or NH4Cl, and involved in the reaction with Al or AlCl3 to rapidly produce AlN [10]. Fig. 12 shows that the large amounts of intermediate species were detected by using optical emission spectrometer in the MW discharge zone occurred at a
Table 1 The operating conditions for the synthesis of AlN powders by different methods. Synthesis method
Reactants
Reaction time (h)
Temperature (°C)
Ref.
Gas-phase synthesis Carbothermal reduction
MW plasma
Al, NH4Cl, N2, H2
1 Ignition 2 1 6 Calcination 48 h 15–45 min
1000 (yield = 80%) – 1500 500–1000 280 °C (amorphous) 700 800
[3] [8]
Direct nitridation Solvent thermal synthesis
AlCl3, NH3, N2 Al(NO3)3, CO(NH2)2, NH4NO3 Al2O3, C, N2 Al, LiOH, N2 AlCl3, NaN3, xylene (solvent)
[7] [5] This study
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
83
reaction time of 45 min, and it can be seen that the intensities of the diffraction peaks corresponding to AlN were elevated by increasing the applied power, because the higher power applied was accompanied by increased plasma density, resulting in the increased concentration of active N-containing species for involving in the synthesis reaction. 3.7. Characteristics of synthesized AlN powders
Fig. 12. Optical emission spectra for nitrogen MW plasma at 760 torr, 800 and 1200 kW.
pressure of 760 torr and 800 or 1200 W. Moreover, a higher concentration of active species was found at 1200 W than at 800 W, resulting in a shorter AlN synthesis time. The main plasma processes, such as dissociative ionization, ionization, and excitation reactions, are carried out through electron-impact reaction and the Penning ionization reaction. For nitrogen plasma, the emission lines of N2 from the first positive band (B3Pg ? A3Ru+ transition, at 550–900 nm) and the second positive band (C3Pu ? B3Pg transition, at 300–400 nm) [30,31,33], and the emission lines of nitrogen molecular ions from the first negative band (B2Ru+ ? X2Rg+ transition, at 381–470 nm) [30,33] were measured. However, atomic lines and electronically excited species were difficult to be observed in the discharge environment.
3.6. Effects of applied power on AlN powders Fig. 13 shows the XRD patterns of the AlN powders synthesized at various operating powers (800–1200 W) at 800 °C with a
Fig. 13. XRD patterns of AlN powders synthesized at various operation powers (A) 800 W; (B) 1000 W; (C) 1200 W (45 min, NH4Cl = 50%, 800 °C, H2 = 1%).
Fig. 14 shows the Raman spectra of the synthesized AlN powders prepared with a reaction time of 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1%. The four distinct peaks centered at 249, 610, 655, and 670 cm1 are indexed to the first-order vibrational modes of E2(low), A1(TO), E2(high), and E1(TO), respectively. In addition, the low-intensity broad peaks around 895 and 905 cm1 are correlated to the overlap of the modes A1(LO) and E1(LO), respectively. These results agree with those found for hAlN crystallization [12–14]. AlN powders can be used as the ceramic substrate for applications that require the transmission of high frequency electrical signals without delays. Hence, the dielectric constant of the synthesized AlN powders (prepared at a reaction time of 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1%) and commercial AlN powders were measured within the range of 1 k Hz to 1 MHz at room temperature. Fig. 15 shows that the dielectric constant decreased as the frequency increased, and that the as-synthesized AlN powders had a little lower dielectric constant than the commercial ones. Specifically, the values of the dielectric constants obtained at a frequency of 1 MHz were 7.3 and 7.9 for the asprepared and commercial AlN powders, respectively. The sintering additives of CaO (1 wt.%) and Y2O3 (1 wt.%) were added to the as-prepared AlN powders produced at 15 min, NH4Cl = 50%, 800 °C, 1200 W, and H2 = 1%, and the commercial AlN powders, respectively, and then were sintered at 1600 °C for 3 h in argon gas. The results show that the thermal conductivity of the as-prepared AlN powders reached 141 W/(mK), being higher than that of the commercial AlN powders (68 W/(mK)). That is because in the plasma environment, hydrogen in the inlet gas can remove the residual oxygen in the raw Al powders or inlet N2 gas. Moreover, the sintering additives of Y2O3 and CaO can also enhance the thermal conductivity of AlN by reducing the oxygen content of AlN particles via forming the secondary phases around the grain boundaries [34].
Fig. 14. Raman spectrum of the synthesized AlN powders (15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%).
84
C.-H. Li et al. / Journal of Alloys and Compounds 542 (2012) 78–84
electron density with a higher concentration of N-containing compounds. Acknowledgments We would like to thank the National Science Council of Taiwan for its financial support under Grants NSC 100-2221-E-151-004MY2 and NSC 100-2622-E-151-007-CC3. References
Fig. 15. Dielectric constant for synthesized AlN powders (at 15 min, NH4Cl = 50%, 800 °C, 1200 W, H2 = 1%) and commercial AlN powders.
4. Conclusion In this study, high purity AlN powders were synthesized using an atmospheric-pressure microwave plasma at a short reaction time of 15 min and a relatively low temperature of 800 °C, with conventional thermal methods usually requiring a long reaction time (over several hours) and high temperature (>1000 °C) due to the slow diffusion rate or reaction rate between Al metal and N2 gas. The results show that while operating at a higher reaction temperature raises the conversion of Al, the Al powders agglomerate due to the molten Al, making the diffusion of N-containing species into the core of the aggregated particles more difficult, even a reaction time of 45 min. When NH4Cl powders were mixed with Al powders and pressed into a compact ingot, the yield of AlN rose significantly with increasing NH4Cl content, and reached almost 100% at the NH4Cl content of 50%, 800 °C, 1200 W, and an inlet hydrogen concentration of 1%, even for only reacting 15 min. This is because the sublimation and decomposition of NH4Cl rapidly produces AlN film to form porous microstructures, avoid the coalescence of molten Al, and synthesize fine particles and nanowires. In addition, NH4Cl and inlet H2 in the discharge zone generate more active N-containing species, not only enhancing the nitridation rate and shortening the synthesis time (15 min), but also removing and reducing the residual O atoms in the structure, especially at a higher applied power (1200 W) by generating a higher
[1] C. Sangural, Y. Kinemuchi, T. Suzuki, W. Jiang, K. Yatsui, Jpn. J. Appl. Phys. 40 (2001) 1070–1072. [2] G. Selvaduray, L. Sheet, Mater. Sci. Technol. Ser. 9 (1993) 463–473. [3] J.R. Park, S.W. Rhee, K.H. Lee, J. Mater. Sci. 28 (1993) 57–64. [4] X.P. Hao, M.Y. Yu, D.L. Cui, X.G. Xu, Y.J. Bai, Q.L. Wang, M.H. Jiang, J. Cryst. Growth 242 (2002) 229–232. [5] L. Li, X. Hao, N. Yu, D. Cui, X. Xu, M. Jiang, J. Cryst. Growth 258 (2003) 268–271. [6] S.L. Chung, W.L. Yu, C.N. Liu, J. Mater. Res. 14 (1999) 1928–1933. [7] Y. Kameshima, M. Irie, A. Yasumori, K. Okada, Solid State Ionics 172 (2004) 182–190. [8] J.C. Kuang, C.R. Zang, X.G. Zhou, S.Q. Wang, J. Cryst. Growth 256 (2003) 288– 291. [9] M. Qin, X. Du, Z. Li, X. Qu, I.S. Humail, Mater. Res. Bull. 42 (2008) 2954–2960. [10] H. Ageorges, S. Megy, K. Chang, J.M. Baronnet, J.K. Williams, C. Chapman, Plasma Chem. Plasma Process. 13 (1992) 613–632. [11] M. Iwata, K. Adachi, S. Furukawa, T. Amakawa, J. Phys. D: Appl. Phys. 37 (2004) 1041–1047. [12] L. Shen, T. Cheng, L. Wu, X. Li, Q. Cui, J. Alloys Compd. 465 (2008) 562–566. [13] Y. Tian, Y. Jia, Y. Bao, Y. Chen, Diam. Relat. Mater. 16 (2007) 302–305. [14] L.H. Shen, X.F. Li, J. Zgang, Y.M. Ma, F. Wang, G. Peng, Q.L. Cui, G.T. Zou, Appl. Phys. A 84 (2006) 73–75. [15] K. Kim, J. Cryst. Growth 283 (2005) 540–546. [16] H.D. Li, G.T. Zou, H. Wang, H.B. Yang, D.M. Li, M.H. Li, S. Yu, Y. Wu, Z.F. Meng, J. Phys. Chem. B 102 (1998) 8692–8695. [17] K.A. Khor, K.H. Cheng, L.G. Yu, F. Boey, Mater. Sci. Eng., A 346 (2003) 300–305. [18] K. Baba, N. Shohata, M. Yonezawa, Appl. Phys. Lett. 54 (1989) 2309–2311. [19] M. Yamada, T. Yasui, M. Fukumoto, K. Takahashi, Thin Solid Films 515 (2007) 4166–4171. [20] S.M. Oh, D.K. Park, J. Ind. Eng. Chem. 6 (2000) 1–7. [21] F. Ansat, H. Ganda, R. Saporte, J.P. Traverse, Thin Solid Films 260 (1995) 38–46. [22] V. Rosenband, A. Gany, J. Mater. Process Technol. 147 (2004) 197–203. [23] Y. Qiu, L. Gao, J. Eur. Ceram. Soc. 23 (2003) 2015–2022. [24] R.K. Paul, H.Y. Song, K.H. Lee, B.T. Lee, Mater. Chem. Phys. 112 (2008) 562–565. [25] C. Wu, Q. Yang, C. Hung, D. Wang, P. Yin, J. Solid State Chem. 177 (2004) 3522– 3528. [26] G. Yan, G. Chen, H. Lu, L. Guo, J. Wuhan Univ. Technol. 23 (2008) 334–337. [27] S. Fukumoto, T. Hookabe, H. Tsubakino, J. Mater. Sci. 35 (2000) 2743–2748. [28] F.J. Moura, R.J. Munz, J. Am. Ceram. Soc. 80 (1997) 2425–2428. [29] E.A.H. Timmermans, J. Jonkers, A. Rodero, M.C. Quintero, A. Sola, A. Gamero, D.C. Schram, J.A.M. Mullen van der, Spectrochim. Acta B 53 (1998) 1553–1566. [30] E.A.H. Timmermans, J. Jonkers, A. Rodero, M.C. Quintero, A. Sola, A. Gamero, D.C. Schram, J.A.M. Mullen van der, Spectrochim. Acta B 54 (1999) 1085–1098. [31] H.P. Hsueh, R.T. McGrath, B. Ji, B.S. Felker, J.G. Langan, E.J. Karwacki, J. Vac. Sci. Technol., B 19 (2001) 1347–1357. [32] Q. Sun, A.M. Zhu, X.F. Yang, J.H. Niu, Y. Xu, Z.M. Song, J. Liu, Chin. Chem. Lett. 16 (2005) 839–842. [33] B.Y. Jeong, M.H. Kim, Surf. Coat. Technol. 141 (2001) 182–189. [34] J.H. Pee, J.C. Park, K.T. Hwang, S. Kim, W.S. Cho, Res. Chem. Intermediat. 36 (2010) 801–809.