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A novel combustion method to prepare tetrapod nano-ZnO
Materials Letters 61 (2007) 4603 – 4605 www.elsevier.com/locate/matlet
A novel combustion method to prepare tetrapod nano-ZnO Liang Chen a,⁎, Wulin Song a , Changsheng Xie a , Lufeng Lin b , Jianguo Wang b a
State Key Lab of Plastic Forming Simulation and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China b Hubei Kaei Powder Materials Co., Ltd., Ezhou 436070, PR China Received 12 January 2007; accepted 24 February 2007 Available online 3 March 2007
Abstract Tetrapod nano-ZnO was prepared based on a novel combustion method: melting–combustion method (MCM). Industrial pure zinc was melted and then ignited by the flame of acetylene and oxygen gas mixture at a proper ratio. Nano-ZnO was obtained by condensation of the combusting product, and then was examined by XRD showing that the production was pure zinc oxide without other impurities. FESEM, TEM and HRTEM were used to characterize the structure features and appearance of the product. The results showed that the zinc oxide was largely tetrapod of 20– 30 nm in diameter and 200–300 nm long in the four legs and a bottleneck phenomenon was also observed on several legs. Based on these analyses, growth mechanism was further discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Zinc oxide; Tetrapod; Microstructure
1. Introduction Recently, semiconductor nanostructures have been attracting increasing attention due to their exceptional properties, which are different from their conventional bulk counterparts. As a versatile semiconductor material, ZnO with a wide and direct fundamental band-gap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV is one of the most promising materials for the fabrication of optoelectronic devices operating in the blue and ultraviolet region and for gas sensing applications [1]. Since the novel properties of nanomaterials depend sensitively on their shape and size, it is essential to develop synthetic methods that yield nano-ZnO in a desirable shape and size [2]. Therefore, preparing several typical and complex nano-ZnO, such as tetrapod [3,4], hierarchical [5,6], mushroom [7] and dendritic [8], has recently been developed into an important branch of ZnO nanomaterials. Among them, nano-ZnO with a tetrapod shape distinguished itself by its extensive applications in shock-resistance, sound insulation, photosensitization, fluorescence, gas sensitization, and catalysis [9]. So ⁎ Corresponding author. Tel./fax: +86 27 87557453. E-mail addresses: [email protected] (L. Chen), [email protected] (W. Song), [email protected] (C. Xie), [email protected] (L. Lin), [email protected] (J. Wang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.02.059
far quite a few methods have been explored and experimented, however, most of the synthetic procedures involve long reaction time, toxic templates and exotic metal catalysts, and the outcomes are poor in both productivity and purity. Combustion method as one of the gas phase (aerosol) methods generally produce powders “ready to use”, is largely investigated and widely employed in the large-scale production of several oxides of silicon, titanium, aluminum and zirconium [10,11]; however, not all elements form volatile salt precursors (e.g. halides) easily, while this kind of precursors are prerequisites in this method. As evidenced that when zinc is burnt at high temperature in air, the produced zinc oxide particle has the peculiar shape of a tetrapod [12]. It is, however, only the tetrapod ZnO with micro-sized length
Fig. 1. The sketch of equipment applied in the preparing of nano-ZnO by MCM.
4604
L. Chen et al. / Materials Letters 61 (2007) 4603–4605
rose to about 450 °C, it went through the nozzle under the nitrogen's pressure (2 atm) shaping a liquid zinc rill. Flowed from a welding torch, oxygen (O2) and acetylene (C2H2) gas mixture was ignited to have the rill burnt. When the zinc was burned and large scale of smoke came into being, a fan was turned on at the same time to pump the smoke into the cooling collector. Powders on the plates of the collector were collected, and then the ZnO nano-tetrapods were prepared. The as-prepared products were further characterized by X-ray diffraction (XRD) (PAN B.V. X'Pert PRO with Cu Ka radiation, 40 kV), field-emission scanning electron microscopy (FESEM) (FEI, Sirion 200, 20 kV), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (FEI, Tecnai G220, 200 kV). Fig. 2. Result of X-ray diffraction for the production.
or diameter of legs was produced [13,14]. In this study, we conceived a novel and simple combustion process named melting– combustion method (MCM) that excluded volatile salt precursor from the fabricating process. The tetrapod nano-ZnO prepared by MCM is pure, structurally uniform, and of good decentralization and the method is simple, low cost, and controllable. 2. Experimental procedure Tetrapod nano-ZnO was synthesized by melting and combusting of 99.995% pure bulk industrial zinc without the presence of catalyst or any further disposal. As illustrated in Fig. 1, the stainless-steel container with a nozzle in its center carrying bulk metallic zinc was charged into an electric furnace. After the zinc was heated and melted as the temperature in the furnace
Fig. 3. Morphology of the nano-ZnO synthesized by MCM; inset: a single tetrapod ZnO with a bottleneck phenomenon.
Fig. 4. (a) A TEM image of tetrapod nano-ZnO; (b) An HRTEM image of the rectangular part of (a).
L. Chen et al. / Materials Letters 61 (2007) 4603–4605
3. Results and discussion XRD pattern in Fig. 2 shows peaks corresponding to wurtzite ZnO and the diffraction peaks can be indexed to a hexagonal structure of bulk ZnO with cell constants of a = b = 0.32498 nm, c = 0.52066 nm. No diffraction peaks from Zn or other impurities were detected in any of our samples. As for the reaction of oxygen and acetylene, there would be two types as follows (reactions (1) and (2)) according to the variations of the mole ratio of O2/C2H2: C2 H2 þ 5=2O2 ¼ 2CO2 þ H2 OðgÞ þ Q1
ð1Þ
C2 H2 þ O2 ¼ 2CO þ H2 þ Q2
ð2Þ
CO þ ZnO ¼ Zn þ CO2
H2 þ ZnO ¼ Zn þ H2 O
ð3Þ
When the ratio reaches to or exceeds 5:2, reaction (1) would take place, the temperature of the flame from torch could achieve 3000 °C [15]. If the quantity of oxygen is low, reaction (2) would occur, i.e. deoxidized gases CO and H2 would come into being. On this condition, at the presupposition that zinc oxide is fabricated the presence of CO and H2 could deoxidize zinc oxide into zinc (reaction (3)), and then zinc might exist in the products. However, zinc wasn't discovered in the XRD pattern of the as-prepared products, which must be profited from the adequate and proper existence of oxygen in the gas mixture. From the initial heating of metallic Zn bulk to the subsequent formation of ZnO nano-tetrapods, the process involved the following reactions: ZnðsÞ → ZnðlÞ
ð4Þ
ZnðlÞ → ZnðgÞ
ð5Þ
ZnðgÞ þ 1=2O2 →ZnOðgÞ
ð6Þ
ZnOðgÞ →ZnOðsÞ
ð7Þ
4605
catalyst is involved during our synthesis process nor droplet is observed on the tips of the legs, which proves the 1-D growth of the legs cannot be dominated by the VLS mechanism but the VS mechanism [21]. Moreover, it is also unlikely to be explained by screw dislocation evolved growth mechanism [22], because the legs are free from defects and dislocations, as evidenced by the HRTEM observation. In view of the bottleneck phenomena in Fig. 3 (inset), the 1-D growth process of the legs follows Sears' law [23]. Sears proposed a vapor–solid (VS) whisker growth model for the impingement of atoms onto a whisker side surface, temporary adsorption of impinging atoms, and diffusion of atoms along the lateral surface to a sink at the whisker tips. According to Sears' theory, the 1-D growing process happened at the tip of the legs where there were enormous atoms sinks, the observed bottleneck phenomenon can be recognized as a frozen state of dynamic growth process.
4. Conclusion High yield tetrapod wurtzite nano-ZnO using a novel and simple combustion approach has been successfully fabricated. The non-metal-catalyst and the proper ratio of the gas mixture guarantee the relatively high purity in the products. The volatile salt precursors which were not involved in the combustion process make the MCM simple and cost cheap. The 1-D growth of the legs follows VS mechanism and Sears' law. Further work should be done to investigate the formation mechanism and the specific application of the product. Acknowledgments
The melting of the metallic zinc (reaction (4)) and the gasification of liquid zinc (reaction (5)) can be carried out easily. The change of Gibbs free energy of reaction (6) at 1200 °C is ΔG = −192 kJ/mol as calculated in [16], which means reaction (6) is spontaneous because the temperature in this study is far above 1200 °C. The decomposition of ZnO vapor on the surface of the cooling plates (reaction (7)) was also a thermodynamically spontaneous process because of the low temperature on the plates in the collector. The general morphologies and structures of tetrapod nano-ZnO are shown in Figs. 3 and 4. The low magnification FESEM image (Fig. 3) reveals that the product consisted of densely populated nano-scaled ZnO, and has structurally uniformed tetrapod. A bottleneck phenomenon is visible at the middle parts on several legs (Fig. 3 inset), i.e. diameters of some legs are found to become abruptly narrow at the middle parts. It is seen from Fig. 4 that the diameter of the tetrapod is 20–30 nm with the length of 200–300 nm, meanwhile the legs are straight and smooth and no foreign droplet or particle was found attached to the tops of these legs. The lattice fringe from the HRTEM image (Fig. 4b) shows a clean and structurally perfect surface, indicating each ZnO leg is single crystal structure. The spacing of about 3.60 nm between adjacent lattice planes corresponds to the distance between two (1 0¯1 1)crystal planes. Various models have been proposed for the growth process of the tetrapod ZnO particles [17–20], however, only the octahedral multiple twin nucleus model proposed by Iwanaga can explain the prototype angle relation. The formation model of the tetrapod nano-ZnO prepared by MCM is also corresponding to this model. Because neither the metal
This work was funded by the National Natural Science Foundation of China (Grant No. 50471061) and the Project of Advanced Science and Technology of Ezhou Hubei China. We would like to thank Analytical and Testing Center (Huazhong University of Science and Technology) for the measurements and analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
H. Cao, J.Y. Xu, D.Z. Zhang, et al., Phys. Rev. Lett. 84 (2000) 5584. Y. Dai, Y. Zhang, Z.L. Wang, Solid State Commun. 126 (2003) 629. H. Yan, R. He, J. Pham, et al., Adv. Mater. 15 (2003) 402. V.A.L. Roy, A.B. Djurisic, W.K. Chan, et al., Appl. Phys. Lett. 83 (2003) 141. P. Gao, Z.L. Wang, J. Phys. Chem. B. 106 (2002) 12653. J.Y. Lao, J.Y. Huang, D.Z. Wang, et al., Nano Lett. 3 (2003) 235. H.X. Niu, Q. Yang, F. Yu, et al., Mater. Lett. 61 (2007) 137. H. Yan, R. He, J. Johnson, et al., J. Am. Chem. Soc. 125 (2003) 4728. M.J. Mayo, Int. mater. Rev. 3 (1996) 95. C.-H. Hung, P.F. Miquel, J.L. Katz, J. Mater. Res. 7 (1992) 1870. W. Hartmann, A.T. Liu, D. Peuckert, et al., Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 109 (1989) 243. H. Iwanaga, M. Fujii, S. Takeuchi, J. Cryst. Growth 183 (1993) 190. Y. Suyama, T. Tomokiyo, T. Manabe, et al., J. Am. Ceram. Soc. 71 (1988) 391. H. Iwanaga, M. Fujii, S. Takeuchi, J. Cryst. Growth 134 (1993) 275. C.S. Xu, Organic Chem, 2nd ed., 1993 PR China, Beijing. L.M. Raff, Principles of Phys. Chem., Prentice-Hall, New Jersey, 2001. M. Shiojiri, C. Kaito, J. Cryst. Growth 52 (1981) 173. S. Takeuchi, H. Iwanaga, M. Fujii, Philos. Mag., A 69 (1994) 1125. K. Nishio, T. Isshiki, Philos. Mag., A 76 (1997) 889. G.V. Chertihin, L. Andrews, J. Chem. Phys. 106 (1997) 3457. H.H. Michael, Y.Y. Wu, F. Henning, et al., Adv. Mater. 13 (2001) 113. C.M. Drum, J.W. Mitchell, Appl. Phys. Lett. 4 (1964) 164. G.W. Sears, Acta Met. 4 (1955) 361.
A novel combustion method to prepare tetrapod nano-ZnO Liang Chen a,⁎, Wulin Song a , Changsheng Xie a , Lufeng Lin b , Jianguo Wang b a
State Key Lab of Plastic Forming Simulation and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China b Hubei Kaei Powder Materials Co., Ltd., Ezhou 436070, PR China Received 12 January 2007; accepted 24 February 2007 Available online 3 March 2007
Abstract Tetrapod nano-ZnO was prepared based on a novel combustion method: melting–combustion method (MCM). Industrial pure zinc was melted and then ignited by the flame of acetylene and oxygen gas mixture at a proper ratio. Nano-ZnO was obtained by condensation of the combusting product, and then was examined by XRD showing that the production was pure zinc oxide without other impurities. FESEM, TEM and HRTEM were used to characterize the structure features and appearance of the product. The results showed that the zinc oxide was largely tetrapod of 20– 30 nm in diameter and 200–300 nm long in the four legs and a bottleneck phenomenon was also observed on several legs. Based on these analyses, growth mechanism was further discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Zinc oxide; Tetrapod; Microstructure
1. Introduction Recently, semiconductor nanostructures have been attracting increasing attention due to their exceptional properties, which are different from their conventional bulk counterparts. As a versatile semiconductor material, ZnO with a wide and direct fundamental band-gap of 3.37 eV at room temperature and a large exciton binding energy of 60 meV is one of the most promising materials for the fabrication of optoelectronic devices operating in the blue and ultraviolet region and for gas sensing applications [1]. Since the novel properties of nanomaterials depend sensitively on their shape and size, it is essential to develop synthetic methods that yield nano-ZnO in a desirable shape and size [2]. Therefore, preparing several typical and complex nano-ZnO, such as tetrapod [3,4], hierarchical [5,6], mushroom [7] and dendritic [8], has recently been developed into an important branch of ZnO nanomaterials. Among them, nano-ZnO with a tetrapod shape distinguished itself by its extensive applications in shock-resistance, sound insulation, photosensitization, fluorescence, gas sensitization, and catalysis [9]. So ⁎ Corresponding author. Tel./fax: +86 27 87557453. E-mail addresses: [email protected] (L. Chen), [email protected] (W. Song), [email protected] (C. Xie), [email protected] (L. Lin), [email protected] (J. Wang). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.02.059
far quite a few methods have been explored and experimented, however, most of the synthetic procedures involve long reaction time, toxic templates and exotic metal catalysts, and the outcomes are poor in both productivity and purity. Combustion method as one of the gas phase (aerosol) methods generally produce powders “ready to use”, is largely investigated and widely employed in the large-scale production of several oxides of silicon, titanium, aluminum and zirconium [10,11]; however, not all elements form volatile salt precursors (e.g. halides) easily, while this kind of precursors are prerequisites in this method. As evidenced that when zinc is burnt at high temperature in air, the produced zinc oxide particle has the peculiar shape of a tetrapod [12]. It is, however, only the tetrapod ZnO with micro-sized length
Fig. 1. The sketch of equipment applied in the preparing of nano-ZnO by MCM.
4604
L. Chen et al. / Materials Letters 61 (2007) 4603–4605
rose to about 450 °C, it went through the nozzle under the nitrogen's pressure (2 atm) shaping a liquid zinc rill. Flowed from a welding torch, oxygen (O2) and acetylene (C2H2) gas mixture was ignited to have the rill burnt. When the zinc was burned and large scale of smoke came into being, a fan was turned on at the same time to pump the smoke into the cooling collector. Powders on the plates of the collector were collected, and then the ZnO nano-tetrapods were prepared. The as-prepared products were further characterized by X-ray diffraction (XRD) (PAN B.V. X'Pert PRO with Cu Ka radiation, 40 kV), field-emission scanning electron microscopy (FESEM) (FEI, Sirion 200, 20 kV), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (FEI, Tecnai G220, 200 kV). Fig. 2. Result of X-ray diffraction for the production.
or diameter of legs was produced [13,14]. In this study, we conceived a novel and simple combustion process named melting– combustion method (MCM) that excluded volatile salt precursor from the fabricating process. The tetrapod nano-ZnO prepared by MCM is pure, structurally uniform, and of good decentralization and the method is simple, low cost, and controllable. 2. Experimental procedure Tetrapod nano-ZnO was synthesized by melting and combusting of 99.995% pure bulk industrial zinc without the presence of catalyst or any further disposal. As illustrated in Fig. 1, the stainless-steel container with a nozzle in its center carrying bulk metallic zinc was charged into an electric furnace. After the zinc was heated and melted as the temperature in the furnace
Fig. 3. Morphology of the nano-ZnO synthesized by MCM; inset: a single tetrapod ZnO with a bottleneck phenomenon.
Fig. 4. (a) A TEM image of tetrapod nano-ZnO; (b) An HRTEM image of the rectangular part of (a).
L. Chen et al. / Materials Letters 61 (2007) 4603–4605
3. Results and discussion XRD pattern in Fig. 2 shows peaks corresponding to wurtzite ZnO and the diffraction peaks can be indexed to a hexagonal structure of bulk ZnO with cell constants of a = b = 0.32498 nm, c = 0.52066 nm. No diffraction peaks from Zn or other impurities were detected in any of our samples. As for the reaction of oxygen and acetylene, there would be two types as follows (reactions (1) and (2)) according to the variations of the mole ratio of O2/C2H2: C2 H2 þ 5=2O2 ¼ 2CO2 þ H2 OðgÞ þ Q1
ð1Þ
C2 H2 þ O2 ¼ 2CO þ H2 þ Q2
ð2Þ
CO þ ZnO ¼ Zn þ CO2
H2 þ ZnO ¼ Zn þ H2 O
ð3Þ
When the ratio reaches to or exceeds 5:2, reaction (1) would take place, the temperature of the flame from torch could achieve 3000 °C [15]. If the quantity of oxygen is low, reaction (2) would occur, i.e. deoxidized gases CO and H2 would come into being. On this condition, at the presupposition that zinc oxide is fabricated the presence of CO and H2 could deoxidize zinc oxide into zinc (reaction (3)), and then zinc might exist in the products. However, zinc wasn't discovered in the XRD pattern of the as-prepared products, which must be profited from the adequate and proper existence of oxygen in the gas mixture. From the initial heating of metallic Zn bulk to the subsequent formation of ZnO nano-tetrapods, the process involved the following reactions: ZnðsÞ → ZnðlÞ
ð4Þ
ZnðlÞ → ZnðgÞ
ð5Þ
ZnðgÞ þ 1=2O2 →ZnOðgÞ
ð6Þ
ZnOðgÞ →ZnOðsÞ
ð7Þ
4605
catalyst is involved during our synthesis process nor droplet is observed on the tips of the legs, which proves the 1-D growth of the legs cannot be dominated by the VLS mechanism but the VS mechanism [21]. Moreover, it is also unlikely to be explained by screw dislocation evolved growth mechanism [22], because the legs are free from defects and dislocations, as evidenced by the HRTEM observation. In view of the bottleneck phenomena in Fig. 3 (inset), the 1-D growth process of the legs follows Sears' law [23]. Sears proposed a vapor–solid (VS) whisker growth model for the impingement of atoms onto a whisker side surface, temporary adsorption of impinging atoms, and diffusion of atoms along the lateral surface to a sink at the whisker tips. According to Sears' theory, the 1-D growing process happened at the tip of the legs where there were enormous atoms sinks, the observed bottleneck phenomenon can be recognized as a frozen state of dynamic growth process.
4. Conclusion High yield tetrapod wurtzite nano-ZnO using a novel and simple combustion approach has been successfully fabricated. The non-metal-catalyst and the proper ratio of the gas mixture guarantee the relatively high purity in the products. The volatile salt precursors which were not involved in the combustion process make the MCM simple and cost cheap. The 1-D growth of the legs follows VS mechanism and Sears' law. Further work should be done to investigate the formation mechanism and the specific application of the product. Acknowledgments
The melting of the metallic zinc (reaction (4)) and the gasification of liquid zinc (reaction (5)) can be carried out easily. The change of Gibbs free energy of reaction (6) at 1200 °C is ΔG = −192 kJ/mol as calculated in [16], which means reaction (6) is spontaneous because the temperature in this study is far above 1200 °C. The decomposition of ZnO vapor on the surface of the cooling plates (reaction (7)) was also a thermodynamically spontaneous process because of the low temperature on the plates in the collector. The general morphologies and structures of tetrapod nano-ZnO are shown in Figs. 3 and 4. The low magnification FESEM image (Fig. 3) reveals that the product consisted of densely populated nano-scaled ZnO, and has structurally uniformed tetrapod. A bottleneck phenomenon is visible at the middle parts on several legs (Fig. 3 inset), i.e. diameters of some legs are found to become abruptly narrow at the middle parts. It is seen from Fig. 4 that the diameter of the tetrapod is 20–30 nm with the length of 200–300 nm, meanwhile the legs are straight and smooth and no foreign droplet or particle was found attached to the tops of these legs. The lattice fringe from the HRTEM image (Fig. 4b) shows a clean and structurally perfect surface, indicating each ZnO leg is single crystal structure. The spacing of about 3.60 nm between adjacent lattice planes corresponds to the distance between two (1 0¯1 1)crystal planes. Various models have been proposed for the growth process of the tetrapod ZnO particles [17–20], however, only the octahedral multiple twin nucleus model proposed by Iwanaga can explain the prototype angle relation. The formation model of the tetrapod nano-ZnO prepared by MCM is also corresponding to this model. Because neither the metal
This work was funded by the National Natural Science Foundation of China (Grant No. 50471061) and the Project of Advanced Science and Technology of Ezhou Hubei China. We would like to thank Analytical and Testing Center (Huazhong University of Science and Technology) for the measurements and analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
H. Cao, J.Y. Xu, D.Z. Zhang, et al., Phys. Rev. Lett. 84 (2000) 5584. Y. Dai, Y. Zhang, Z.L. Wang, Solid State Commun. 126 (2003) 629. H. Yan, R. He, J. Pham, et al., Adv. Mater. 15 (2003) 402. V.A.L. Roy, A.B. Djurisic, W.K. Chan, et al., Appl. Phys. Lett. 83 (2003) 141. P. Gao, Z.L. Wang, J. Phys. Chem. B. 106 (2002) 12653. J.Y. Lao, J.Y. Huang, D.Z. Wang, et al., Nano Lett. 3 (2003) 235. H.X. Niu, Q. Yang, F. Yu, et al., Mater. Lett. 61 (2007) 137. H. Yan, R. He, J. Johnson, et al., J. Am. Chem. Soc. 125 (2003) 4728. M.J. Mayo, Int. mater. Rev. 3 (1996) 95. C.-H. Hung, P.F. Miquel, J.L. Katz, J. Mater. Res. 7 (1992) 1870. W. Hartmann, A.T. Liu, D. Peuckert, et al., Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 109 (1989) 243. H. Iwanaga, M. Fujii, S. Takeuchi, J. Cryst. Growth 183 (1993) 190. Y. Suyama, T. Tomokiyo, T. Manabe, et al., J. Am. Ceram. Soc. 71 (1988) 391. H. Iwanaga, M. Fujii, S. Takeuchi, J. Cryst. Growth 134 (1993) 275. C.S. Xu, Organic Chem, 2nd ed., 1993 PR China, Beijing. L.M. Raff, Principles of Phys. Chem., Prentice-Hall, New Jersey, 2001. M. Shiojiri, C. Kaito, J. Cryst. Growth 52 (1981) 173. S. Takeuchi, H. Iwanaga, M. Fujii, Philos. Mag., A 69 (1994) 1125. K. Nishio, T. Isshiki, Philos. Mag., A 76 (1997) 889. G.V. Chertihin, L. Andrews, J. Chem. Phys. 106 (1997) 3457. H.H. Michael, Y.Y. Wu, F. Henning, et al., Adv. Mater. 13 (2001) 113. C.M. Drum, J.W. Mitchell, Appl. Phys. Lett. 4 (1964) 164. G.W. Sears, Acta Met. 4 (1955) 361.