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Formation of AlN nano-fibers
Journal of Crystal Growth 224 (2001) 187–189
Priority communication
Formation of AlN nano-fibers Hong Chena, Yongge Caoa,b,*, Xianwei Xianga a
Key Lab of New Packaging Materials and Technology of China, National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, Hu Nan Province, People’s Republic of China b Group 401, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603-23, Beijing 100080, People’s Republic of China Received 28 November 2000; accepted 2 February 2001 Communicated by L.F. Schneemeyer
Abstract AlN nano-fibers with diameters in the range of 70 to 500 nm and the maximum length of about 2 mm are fabricated by combustion synthetic technique. Addition of (NH4F+NH4Cl) mixtures is indispensable, which can act as solid nitrogen source, diluent agent, and catalyst. The AlN fiber has a wurtzite structure with lattice parameters of a ¼ 0:311 nm, c ¼ 0:498 nm. The selected area electron spots shows that the synthesized AlN fiber is a single crystal. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea Keywords: Al. Crystal morphology; B1. Nitrides
1. Introduction AlN, a wide-band-gap semiconductor, can form continuously solid solutions with GaN and InN, exhibiting tunable band gaps which are suitable for optical devices operating at wavelengths over the red–ultraviolet region [1]. The good thermal, chemical stability, high electrical resistivity, good dielectric strength, high strength, and high-temperature stability make it an excellent material for high-temperature and high-power devices, as well as a promising microelectronic substrate [2,3]. AlN fibers can be used as reinforcement in ceramicmatrix composites having high-fracture toughness. *Corresponding author. E-mail address: [email protected] (Y. Cao).
Furthermore, the optical properties of nano-sized AlN fibers may be varied. In this communication, we report the fabrication and Raman characteristics of AlN nano-fibers synthesized by combustion synthesis with the addition of (NH4F+NH4Cl).
2. Experimental procedures The reactants used were fine aluminum powder with a diameter of 0.5–1 mm, and the additives are (NH4F+NH4Cl) with analytical purity. The reactant of Al powder and 26% mol (NH4F+NH4Cl) additives were dry-mixed and dry-milled in a ball container for 48 h. The reactants were compressed into a green compact
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 7 6 - 9
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H. Chen et al. / Journal of Crystal Growth 224 (2001) 187–189
with a diameter of 16 mm and a height of 10 mm for combustion synthesis. The combustion experiments were performed in a high-pressure stainless steel vessel under a nitrogen pressure of 15 atm. TiN powder was used as an ignition agent and a tungsten coil was used as an ignition power supply. Details of propagation are given elsewhere [4]. After being ignited by the tungsten coil, the combustion reaction was completed within 5 min. X-ray diffraction and scanning electron microscopy (SEM) were used to characterize the phase composition and the morphology of the combusted products.
3. Results and discussion The X-ray diffraction pattern (Fig. 1) of the combusted product shows that it is substantially pure AlN. The AlN products have a wurtzite structure with lattice parameters of a ¼ 0:311 nm, c ¼ 0:498 nm. The SEM image (Fig. 2) shows that the products are composed of fibers with diameters in the range of 70 to 500 nm. The maximum length of these fibers is about 2 mm. These fibers are approximately straight, and sometimes curved or flexible. The inset in Fig. 2 is the selected area diffraction spots on a single fiber, which shows a single crystal characteristic with hexagonal structure. The additives of NH4F plays a key role in the combustion synthesis of AlN nano-fibers. During
Fig. 1. X-ray powder diffraction patterns of the synthesized AlN nano-fibers.
the combustion synthesis process, the following reactions may exist 1 2NH4 FðsÞ 1 2NH3 ðgÞ 2 3HFðgÞ
¼ 12NH3 ðgÞ þ 12HFðgÞ
¼ 14N2 ðgÞ þ 34H2 ðgÞ
¼ 13H2 ðgÞ þ 23FðgÞ
ð1Þ ð2Þ ð3Þ
Alðg; lÞ þ HFðgÞ ! AlFx ðgÞ þ H2 ðgÞ
ð4Þ
AlFx ðgÞ þ ½N2 ðgÞ þ NH3 ðgÞ=H2 ðgÞ ! AlNðsÞ þ HFðgÞ
ð5Þ
AlðgÞ þ ½N2 ðgÞ þ NH3 ðgÞ ! ½Alx ðNHÞy n ðsÞ
ð6Þ
½Alx ðNHÞy n ðsÞ ! AlNðsÞ þ NH3 ðgÞ
ð7Þ
It can be seen that the additive NH4F firstly decomposes into HF and NH3 at temperatures of about 400oC, and these components will further react with Al to form the mediate products AlFx(g) and [Alx(NH)y]n(s), and finally to form AlN(s), HF and NH3. The reaction scheme is depicted as Fig. 3. Hence, NH4F provide the active N source and also it acts as the catalyst. In addition, NH4F can lower the adiabatic
Fig. 2. The SEM image of the AlN nano-fibers with inset of the selected area electron diffraction spots.
H. Chen et al. / Journal of Crystal Growth 224 (2001) 187–189
189
4. Conclusions AlN nano-fibers with diameters in the range of 70 to 500 nm and the maximum length of about 2 mm are fabricated by combustion synthesis technique. During the reaction, (NH4F+NH4Cl) additives play very important roles such that they act as solid nitrogen source, diluent agent, and catalyst. The synthesized AlN fiber is single crystal with a wurtzite structure.
Fig. 3. The proposed reaction mechanism during the combustion synthesis of AlN nano-fibers.
temperature of the strong exothermal reactions. The last but not least important function is that NH4F can create and provide space for fiber growth by its decomposition. If the green compact is too loose, the growth space will be so large that the diameter of fibers will be larger than several hundreds micrometers. Alternatively, NH4Cl plays the same role as that of NH4F. From the above mentioned, it can be concluded that the (NH4F+NH4Cl) additives virtually act as solid nitrogen source, diluent agent, and catalyst. The growth of AlN fibers may be dominated by two mechanisms, i.e. VLS mechanism along the elongated direction, and the VS mechanism along the radical direction.
Acknowledgements This work was supported by the National Natural Sciences Fundation of China and Key Lab of New Packaging Materials and Technology of China.
References [1] S. Nakamura, Science 281 (1998) 956. [2] G.A. Slack, R.A. Tanzilli, R.O. Pohl, J.W. Vandersande, J. Phys. Chem. Solids 48 (1987) 641. [3] W.J. Meng, in: J.H. Edgar (Ed.), Properties of Group III Nitrides, The institution of Electrical Engineers, London, 1994, p. 22. [4] Osamu Yamada, Kiyoshi Hirao, Mitsue Kozumi, Yoshinari Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1735.
Priority communication
Formation of AlN nano-fibers Hong Chena, Yongge Caoa,b,*, Xianwei Xianga a
Key Lab of New Packaging Materials and Technology of China, National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, Hu Nan Province, People’s Republic of China b Group 401, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603-23, Beijing 100080, People’s Republic of China Received 28 November 2000; accepted 2 February 2001 Communicated by L.F. Schneemeyer
Abstract AlN nano-fibers with diameters in the range of 70 to 500 nm and the maximum length of about 2 mm are fabricated by combustion synthetic technique. Addition of (NH4F+NH4Cl) mixtures is indispensable, which can act as solid nitrogen source, diluent agent, and catalyst. The AlN fiber has a wurtzite structure with lattice parameters of a ¼ 0:311 nm, c ¼ 0:498 nm. The selected area electron spots shows that the synthesized AlN fiber is a single crystal. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea Keywords: Al. Crystal morphology; B1. Nitrides
1. Introduction AlN, a wide-band-gap semiconductor, can form continuously solid solutions with GaN and InN, exhibiting tunable band gaps which are suitable for optical devices operating at wavelengths over the red–ultraviolet region [1]. The good thermal, chemical stability, high electrical resistivity, good dielectric strength, high strength, and high-temperature stability make it an excellent material for high-temperature and high-power devices, as well as a promising microelectronic substrate [2,3]. AlN fibers can be used as reinforcement in ceramicmatrix composites having high-fracture toughness. *Corresponding author. E-mail address: [email protected] (Y. Cao).
Furthermore, the optical properties of nano-sized AlN fibers may be varied. In this communication, we report the fabrication and Raman characteristics of AlN nano-fibers synthesized by combustion synthesis with the addition of (NH4F+NH4Cl).
2. Experimental procedures The reactants used were fine aluminum powder with a diameter of 0.5–1 mm, and the additives are (NH4F+NH4Cl) with analytical purity. The reactant of Al powder and 26% mol (NH4F+NH4Cl) additives were dry-mixed and dry-milled in a ball container for 48 h. The reactants were compressed into a green compact
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 7 6 - 9
188
H. Chen et al. / Journal of Crystal Growth 224 (2001) 187–189
with a diameter of 16 mm and a height of 10 mm for combustion synthesis. The combustion experiments were performed in a high-pressure stainless steel vessel under a nitrogen pressure of 15 atm. TiN powder was used as an ignition agent and a tungsten coil was used as an ignition power supply. Details of propagation are given elsewhere [4]. After being ignited by the tungsten coil, the combustion reaction was completed within 5 min. X-ray diffraction and scanning electron microscopy (SEM) were used to characterize the phase composition and the morphology of the combusted products.
3. Results and discussion The X-ray diffraction pattern (Fig. 1) of the combusted product shows that it is substantially pure AlN. The AlN products have a wurtzite structure with lattice parameters of a ¼ 0:311 nm, c ¼ 0:498 nm. The SEM image (Fig. 2) shows that the products are composed of fibers with diameters in the range of 70 to 500 nm. The maximum length of these fibers is about 2 mm. These fibers are approximately straight, and sometimes curved or flexible. The inset in Fig. 2 is the selected area diffraction spots on a single fiber, which shows a single crystal characteristic with hexagonal structure. The additives of NH4F plays a key role in the combustion synthesis of AlN nano-fibers. During
Fig. 1. X-ray powder diffraction patterns of the synthesized AlN nano-fibers.
the combustion synthesis process, the following reactions may exist 1 2NH4 FðsÞ 1 2NH3 ðgÞ 2 3HFðgÞ
¼ 12NH3 ðgÞ þ 12HFðgÞ
¼ 14N2 ðgÞ þ 34H2 ðgÞ
¼ 13H2 ðgÞ þ 23FðgÞ
ð1Þ ð2Þ ð3Þ
Alðg; lÞ þ HFðgÞ ! AlFx ðgÞ þ H2 ðgÞ
ð4Þ
AlFx ðgÞ þ ½N2 ðgÞ þ NH3 ðgÞ=H2 ðgÞ ! AlNðsÞ þ HFðgÞ
ð5Þ
AlðgÞ þ ½N2 ðgÞ þ NH3 ðgÞ ! ½Alx ðNHÞy n ðsÞ
ð6Þ
½Alx ðNHÞy n ðsÞ ! AlNðsÞ þ NH3 ðgÞ
ð7Þ
It can be seen that the additive NH4F firstly decomposes into HF and NH3 at temperatures of about 400oC, and these components will further react with Al to form the mediate products AlFx(g) and [Alx(NH)y]n(s), and finally to form AlN(s), HF and NH3. The reaction scheme is depicted as Fig. 3. Hence, NH4F provide the active N source and also it acts as the catalyst. In addition, NH4F can lower the adiabatic
Fig. 2. The SEM image of the AlN nano-fibers with inset of the selected area electron diffraction spots.
H. Chen et al. / Journal of Crystal Growth 224 (2001) 187–189
189
4. Conclusions AlN nano-fibers with diameters in the range of 70 to 500 nm and the maximum length of about 2 mm are fabricated by combustion synthesis technique. During the reaction, (NH4F+NH4Cl) additives play very important roles such that they act as solid nitrogen source, diluent agent, and catalyst. The synthesized AlN fiber is single crystal with a wurtzite structure.
Fig. 3. The proposed reaction mechanism during the combustion synthesis of AlN nano-fibers.
temperature of the strong exothermal reactions. The last but not least important function is that NH4F can create and provide space for fiber growth by its decomposition. If the green compact is too loose, the growth space will be so large that the diameter of fibers will be larger than several hundreds micrometers. Alternatively, NH4Cl plays the same role as that of NH4F. From the above mentioned, it can be concluded that the (NH4F+NH4Cl) additives virtually act as solid nitrogen source, diluent agent, and catalyst. The growth of AlN fibers may be dominated by two mechanisms, i.e. VLS mechanism along the elongated direction, and the VS mechanism along the radical direction.
Acknowledgements This work was supported by the National Natural Sciences Fundation of China and Key Lab of New Packaging Materials and Technology of China.
References [1] S. Nakamura, Science 281 (1998) 956. [2] G.A. Slack, R.A. Tanzilli, R.O. Pohl, J.W. Vandersande, J. Phys. Chem. Solids 48 (1987) 641. [3] W.J. Meng, in: J.H. Edgar (Ed.), Properties of Group III Nitrides, The institution of Electrical Engineers, London, 1994, p. 22. [4] Osamu Yamada, Kiyoshi Hirao, Mitsue Kozumi, Yoshinari Miyamoto, J. Am. Ceram. Soc. 72 (1989) 1735.