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Synthesis and structural features of Ni–Al nanoparticles by hydrogen plasma–metal reaction
Materials Letters 60 (2006) 2227 – 2231 www.elsevier.com/locate/matlet
Synthesis and structural features of Ni–Al nanoparticles by hydrogen plasma–metal reaction Zhong Wang a,b , A.L. Fan b , W.H. Tian b , Y.T. Wang a , X.G. Li a,⁎ a
The State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
b
Received 27 April 2005; accepted 29 December 2005 Available online 23 January 2006
Abstract Eight kinds of Ni–Al nanoparticles have been prepared by hydrogen plasma–metal reaction. The morphology, crystal structure and chemical composition of the nanoparticles obtained in this study were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and induction-coupled plasma (ICP) spectroscopy. The particle size was determined by TEM and BET gas adsorption. It was found that all the nanoparticles have spherical shapes, with average particle size in the range of 14∼62 nm. The crystal structures of Ni–Al nanoparticles vary with the composition of master alloys. Pure Al3Ni2 (D513), NiAl (B2) and Ni3Al (L12) structures were successfully produced with 55.0, 58.3 and 72.6 at.% Ni in bulk, respectively. The analysis result about the phase equilibrium based on the crystal structures of nanoparticles is not consistent with those based on the equilibrium phase diagram. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Intermetallic alloys and compounds; Crystal structure
1. Introduction Metallic nanoparticles are of great interest due to their unique properties and a wide range of potential applications including information storage, catalysis, hydrogen storage, permanent magnet and ferrofluid [1–4]. Recently metallic nanoparticles become very promising for optoelectronics due to existence of the surface plasmon resonance excitation [5]. Nanoparticles of pure metals and alloys have been reported to be produced by chemical reduction, sputtering, inert gas condensation and hydrogen plasma–metal reaction (HPMR) and so on [6–14]. Aluminide intermetallic compounds, such as NiAl and Ni3Al, are regarded as promising candidates for the next generation of high temperature and high-performance structural materials [15–17] because of their high melting points, relatively low densities, good strength and high temperature
⁎ Corresponding author. Tel./fax: +86 10 62765930. E-mail address: [email protected] (X.G. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.116
corrosion and oxidation resistance. As important intermetallics, they showed great potential applications in automobile engines, aircraft, and electricity generation and energy conversion equipment. There are several routes for producing nickel aluminides, such as casting, rapid solidification, mechanical alloying (MA) and self-sustained reaction and so on [18–20]. To overcome some of its shortcomings, e.g. brittle fracture and low ductility at ambient temperature, considerable efforts have been made recently in an attempt to synthesize and characterize nanocrystalline materials. The synthesis of nickel aluminides by MA was reported in a number of researches [21,22]. However, it usually takes a long time to prepare alloys and it is easy for the sample to be polluted and oxidized during the milling process. Moreover, it is difficult to obtain a homogeneous nanoparticles sample instead of nanocrystalline materials by the MA method. So far, there have been seldom reports on synthesis, oxidation and other properties of Ni–Al intermetallic compound nanoparticles; The HPMR is a method suitable for preparing nanoparticles of various metals, alloys and intermallic nanoparticles industrially at low cost. Nanoparticles of
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Z. Wang et al. / Materials Letters 60 (2006) 2227–2231
metallic alloys with the desired composition and less impurity can be easily prepared by HPMR method. Several metals, alloys, and intermatallics such as Fe, Ni, Co and their alloys, Ti–Fe intematallics, Al–Fe intermallics have been fabricated by HPMR, and the formation mechanism of nanoparticles was extensively studied [13,23–25]. In this study, a serious of Al–Ni nanoparticles such as pure intermetallic compound Al3Ni2 (D513), NiAl (B2) and Ni3Al (L12) were prepared by HPMR, and their microstructures and characteristics were investigated in terms of transmission electron microscopy (TEM), X-ray diffraction (XRD) and induction-coupled plasma (ICP) spectroscopy. 2. Experimental procedure Fig. 1 shows a schematic illustration of the experimental equipment, which was used for production of Ni–Al nanoparticles. It mainly consists of an arc melting chamber and a collecting system. The Ni–Al ingots were prepared from aluminum (purity N 99.9 wt.%) and nickel (purity N 99.7 wt.%) by arc melting in an argon gas atmosphere. Arc-melted ingots were flipped over and remelted four times to get a homogeneous composition. Then Ni–Al nanoparticles were produced by arc melting in a 50% Ar (purity N99.99%) and 50% H 2 (purity N 99.99%) mixture of 0.1 MPa atmosphere. The flow rate of the circulation gas for collection of nanoparticles is 100 l/min. Since the production rate of ultrafine particles (UFPs) is affected by master sample mass, identical master alloy buttons (Ni–Al ingot) weighing 50 g were used in this work. Arc current and voltage were selected as 180A and 25V, respectively. Pure Ni and Al and six master Ni–Al alloys containing 23.7, 48.2, 55.0, 58.3, 63.4 and 72.6 at.% of Ni were used. Before the nanoparticles were taken out from the arc-melting chamber, they were slowly passivated with a mixture of argon and air to prevent the particles from burning. The X-ray diffraction (XRD) was done using the wavelength of Cu K alpha radiation to characterize the crystal structure of the as-prepared nanoparticle samples. The powders were ultrasonicated about 10 min
Fig. 2. Relationship between Ni contents in Ni–Al nanoparticles and master alloys.
in anhydrous ethanol and then dropped on a copper grid, as the TEM sample. The morphology, size dispersion and shape of particles were observed using a JEOL 200EX transmission electron microscope (TEM) operated at 160 kV. The BET specific surface areas of nanoparticles were measured by N2 adsorption using a Counter SA 3100 volumetric gas adsorption analyzer and the average particle size was evaluated. The chemical composition of the UFPs was determined by the induction-coupled plasma (ICP) spectroscopy. 3. Results and discussion Fig. 2 shows the corresponding relationship between Ni contents in the Ni–Al nanoparticles and the master alloys. The chemical compositions of the Ni–Al nanoparticles are quit different from that of master Ni–Al alloys. For the master alloy containing 23.7, 48.2, 55.0, 58.3, 63.4, 72.6 at.% Ni, the corresponding Ni contents are 1.06, 20.5, 37.2, 46.2, 61.1 and 79.2 at.%, respectively, in the UFP samples. The dotted line in Fig. 2 shows the case of ideal solid solution. It can be considered that one of the reasons, which lead to the deviation from the ideal solid solution, is that the generation speed of pure Al is different from that of pure Ni. According to Ohno and Uda's theory [10], the generation speed is essentially determined by the evaporation speed of pure element metals, approximately proportional to the reaction parameter Rp: Rp ¼ −
Fig. 1. Schematic illustration of the equipment for production of nanoparticles.
DHr NH2 ðT Þ d Ls NH2 ð273Þ
ð1Þ
where ΔHr is the reaction enthalpy between hydrogen and metal, Ls is the vaporization heat of metal at temperature T, NH2 (T), and NH2 (273) are densities of the hydrogen molecular in metal at temperature T and 273 K, respectively. According to this equation, the reaction parameters of Ni and Al are evaluated to be 0.171 and 0.411, respectively. So Ni contents in nanoparticles should be lower than those in the master alloys. It is agree with Ohno and Uda's theory while the content of Ni in master alloy is below 65 at.%. However, with Ni contents in master alloy increasing to 65 at.%, Ni contents in nanoparticles is higher than those in master alloys instead. This cannot be explained by Ohno and Uda's theory. The curve of the relationship between Ni contents in
Z. Wang et al. / Materials Letters 60 (2006) 2227–2231
Ni–Al nanoparticles and master alloys is like a letter “S” in shape. This is different from the result of Fe–Ni, Fe–Co and Fe–Al nanoparticles produced by HPMR [23–25], which is like a bow. It may be correlative with the formation of intermetallic compound in master alloys. Actually, the generation speed is not only affected by the evaporation speed of pure element metals but also done by the element existence state in master alloys ingot. Once Ni–Al intermallics in master alloys
2229
formed, Ni and Al combined more closely, evaporation speed of Al would reduce obviously. Fig. 3 shows the bright field TEM images of Ni–Al UFP samples. The inset figure is the histogram of size dispersion. All the particles of the samples are spherical in shape and have a size dispersion ranging from several nanometers to 100 nm in diameter, with most of them in a narrow range of 20–40 nm in diameter. The specific surface area ranges
Fig. 3. TEM bright field images of samples (a)–(h), respectively, with the same scale shown in (h) (the histogram of size dispersion is shown in the inset).
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from 16.5 to 100.1 m2/g, summarized in Table 1. The mean particle diameters of all UFP samples predicted from the specific surface area are in the range of 14 to 62 nm, in well agreement with the results from TEM observation. Fig. 4 shows the X-ray diffraction patterns of the as prepared UFP samples, and the crystal structures are shown in Table 1. It can be determined from Fig. 4 that sample (a) and (b) have the crystal structure of Al (FCC), and sample (h) has the crystal structure of Ni (FCC). As to samples (d), (e) and (g), pure Al3Ni2 (D513), NiAl (B2) and Ni3Al (L12) intermetallics are successfully produced. Sample (c) has the crystal structure of Al3Ni2 (D513), Al and a small amount of unidentified phase. Both NiAl and Ni3Al intermetallic compounds were founded in sample (f). The formation of the UFPs from Ni–Al binary alloy can be explained as following: Firstly, Ni and Al vapors form under H2 and Ar plasma simultaneously. Ni and Al vapors then begin to condense and solidify. As not hydrogen absorbing metals, they do not react with hydrogen during vaporing and cooling process. During the condensation stage, Al atoms collide with Ni atoms and begin to condense in the same metal cluster and then grow to nanoparticles. There exist five intermetallic compounds in the Ni–Al binary phase diagram that are Al3Ni, Al3Ni2, Al3Ni5, NiAl and Ni3Al [26]. However, in this study, only three intermetallic compounds, Al3Ni2, NiAl and Ni3Al, were found. This result is different from that expected from the equilibrium phase diagram. For example, sample (c) containing 20.5 at.% Ni in nanoparticle should be composed of the pure Al and Al3Ni finally at room temperature. However, the actual products are Al and Al3Ni2. Al3Ni2 and liquid exist above 1135 K according to the equilibrium phase diagram. In fact, because the cooling rate is as high as 105 K/s during HPMR processing, peritectoid reaction that needs some time to diffuse cannot occur, Al3Ni did not appear. Likewise, for sample (f) with 61.1 at.% Ni, the final phase in equilibrium should be NiAl and Al3Ni5 intermetallic compounds. However, NiAl and Ni3Al were formed instead. For sample (g) with 79.2 at.% Ni, single-phase Ni3Al (L12) was obtained at room temperature, which is not the single-phase field of Ni3Al on the equilibrium phase diagram. It indicates that the phase diagram of nanoparticles is quite different from the equilibrium phase diagram at high cooling rate.
4. Conclusion Eight kinds of Ni–Al nanoparticles with the particle size ranging from 14 to 62 nm were prepared successfully by hydrogen plasma–metal reaction (HPMR). The crystal strucTable 1 Characteristics of Al–Ni UFP samples
a b c
0 23.7 48.2
0 1.1 20.5
48.3 46.8 77.9
46 ± 2 47 ± 2 21 ± 2
d e f
55.0 58.3 63.4
37.2 46.2 61.1
100.1 26.7 16.5
14 ± 1 46 ± 2 62 ± 2
72.6 100
79.2 100
tures of Ni–Al nanoparticles obtained by HPMR are dependent on the original chemical composition of the master alloys. As to nanoparticles with 37.2, 46.1 and 79.2 at.% Ni, pure Al3Ni2, NiAl and Ni3Al intermetallic compounds are successfully produced, respectively. For Ni–Al nanoparticles, the crystal structures of nanoparticles obtained by HPMR are not consistent with those expected from the equilibrium phase diagram. Only three intermetallic compounds were observed in these studies, which are Al3Ni2, NiAl and Ni3Al. It can be considered that one of the reasons that lead to the deviation from the ideal solid solution is the formation of intermetallic compounds in Ni–Al system. Acknowledgements
Mean Phase Samples Contents of Contents Specific particle structure Ni at.% in of Ni at.% surface master alloys in UFPs area (m2/g) size (nm) in UFPs
g h
Fig. 4. X-ray diffraction patterns of samples (a)–(h).
26.2 36.0
32 ± 2 30 ± 2
Al (FCC) Al (FCC) Al (FCC) + Al3Ni2(D513) Al3Ni2 (D513) NiAl (B2) NiAl (B2) + Ni3Al (L12) Ni3Al (L12) Ni (FCC)
This work was supported by the National Natural Science Foundation of China (No. 50274002, 20221101 and 10335040). The authors acknowledge assistant Prof. X.L. Gai and Associate Prof. F.H. Liao for their kind technical help with TEM and XRD measurements. References [1] A.K. Giri, D. Chakraorty, Trans. Indian Ceram. Soc. 50 (1991) 28. [2] E. Koster, J. Magn. Magn. Mater. 120 (1993) 1. [3] J. Ding, T. Tsuzuki, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 162 (1996) 271.
Z. Wang et al. / Materials Letters 60 (2006) 2227–2231 [4] L. Zhang, A. Manthiram, J. Appl. Phys. 80 (1996) 4534. [5] I.V. Kityk, K.J. Plucinski, J. Ebothe, Journ. Applied Physics (USA), V.98, 084304, (2005). [6] A. Inoue, J. Saida, T. Masumoto, Metall. Trans. A 19A (1988) 2315. [7] H.Y. Zhang, B.X. Gu, H.R. Zhai, M. Lu, Phys. Status Solidi A 143 (1994) 399. [8] J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259. [9] H. Hahn, R.S. Averback, J. Appl. Phys. 67 (1990) 1113. [10] S. Ohno, M. Uda, Trans. Jpn. Inst. Met. 48 (1984) 640. [11] H.Y. Shao, Y.T. Wang, H.R. Xu, X.G. Li, Mater. Sci. Eng., B 110 (2004) 22. [12] H.Y. Shao, Y.T. Wang, H.R. Xu, X.G. Li, J. Solid State Chem. 178 (2005) 2211. [13] T. Liu, H. Shao, X. Li, Intermetallics 12 (2004) 97. [14] H.Y. Shao, T. Liu, X.G. Li, Nanotechnology 14 (2003) L1. [15] P. Nash, M.F Singleton, J.L. Murry, in: P. Nash (Ed.), Phase Diagrams Of Binary Nickel Alloys, vol. 1, ASM International, Metals Park, OH, 1986.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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G.W. Goward, J. Met. 22 (1970) 31. C.A. Barett, Oxid. Met. 30 (1988) 361. S. Dong, P. Hou, H. Yang, G. Zou, Intermetallics 10 (2002) 217. C. Bartuli, Ceram. Acta 8 (1996) 59. T.S. Dyer, Z. Munir, Metall. Mater. Trans. B 26 (1995) 603. S. Surinach, J. Malagelada, M.D. Baro, Mater. Sci. Eng., A 168 (1993) 161. S.K. Pabi, B.S. Murty, Mater. Sci. Eng., A 214 (1996) 146. X.G. Li, A. Chiba, S. Takahashi, J. Magn. Magn. Mater. 170 (1997) 339. X.G. Li, T. Murai, T. Aaito, S. Takahashi, J. Magn. Magn. Mater. 190 (1998) 277. T. Liu, Y.H. Leng, X.G. Li, Solid State Commun. 125 (2003) 391. B.M. Thaddeus, L.M. Joanne, H.B. Lawrence, B. Hugh, K. Linda, Binary Alloy Phase Diagrams, American Society for Metals, Ohio, 1986.
Synthesis and structural features of Ni–Al nanoparticles by hydrogen plasma–metal reaction Zhong Wang a,b , A.L. Fan b , W.H. Tian b , Y.T. Wang a , X.G. Li a,⁎ a
The State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
b
Received 27 April 2005; accepted 29 December 2005 Available online 23 January 2006
Abstract Eight kinds of Ni–Al nanoparticles have been prepared by hydrogen plasma–metal reaction. The morphology, crystal structure and chemical composition of the nanoparticles obtained in this study were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and induction-coupled plasma (ICP) spectroscopy. The particle size was determined by TEM and BET gas adsorption. It was found that all the nanoparticles have spherical shapes, with average particle size in the range of 14∼62 nm. The crystal structures of Ni–Al nanoparticles vary with the composition of master alloys. Pure Al3Ni2 (D513), NiAl (B2) and Ni3Al (L12) structures were successfully produced with 55.0, 58.3 and 72.6 at.% Ni in bulk, respectively. The analysis result about the phase equilibrium based on the crystal structures of nanoparticles is not consistent with those based on the equilibrium phase diagram. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Intermetallic alloys and compounds; Crystal structure
1. Introduction Metallic nanoparticles are of great interest due to their unique properties and a wide range of potential applications including information storage, catalysis, hydrogen storage, permanent magnet and ferrofluid [1–4]. Recently metallic nanoparticles become very promising for optoelectronics due to existence of the surface plasmon resonance excitation [5]. Nanoparticles of pure metals and alloys have been reported to be produced by chemical reduction, sputtering, inert gas condensation and hydrogen plasma–metal reaction (HPMR) and so on [6–14]. Aluminide intermetallic compounds, such as NiAl and Ni3Al, are regarded as promising candidates for the next generation of high temperature and high-performance structural materials [15–17] because of their high melting points, relatively low densities, good strength and high temperature
⁎ Corresponding author. Tel./fax: +86 10 62765930. E-mail address: [email protected] (X.G. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.116
corrosion and oxidation resistance. As important intermetallics, they showed great potential applications in automobile engines, aircraft, and electricity generation and energy conversion equipment. There are several routes for producing nickel aluminides, such as casting, rapid solidification, mechanical alloying (MA) and self-sustained reaction and so on [18–20]. To overcome some of its shortcomings, e.g. brittle fracture and low ductility at ambient temperature, considerable efforts have been made recently in an attempt to synthesize and characterize nanocrystalline materials. The synthesis of nickel aluminides by MA was reported in a number of researches [21,22]. However, it usually takes a long time to prepare alloys and it is easy for the sample to be polluted and oxidized during the milling process. Moreover, it is difficult to obtain a homogeneous nanoparticles sample instead of nanocrystalline materials by the MA method. So far, there have been seldom reports on synthesis, oxidation and other properties of Ni–Al intermetallic compound nanoparticles; The HPMR is a method suitable for preparing nanoparticles of various metals, alloys and intermallic nanoparticles industrially at low cost. Nanoparticles of
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Z. Wang et al. / Materials Letters 60 (2006) 2227–2231
metallic alloys with the desired composition and less impurity can be easily prepared by HPMR method. Several metals, alloys, and intermatallics such as Fe, Ni, Co and their alloys, Ti–Fe intematallics, Al–Fe intermallics have been fabricated by HPMR, and the formation mechanism of nanoparticles was extensively studied [13,23–25]. In this study, a serious of Al–Ni nanoparticles such as pure intermetallic compound Al3Ni2 (D513), NiAl (B2) and Ni3Al (L12) were prepared by HPMR, and their microstructures and characteristics were investigated in terms of transmission electron microscopy (TEM), X-ray diffraction (XRD) and induction-coupled plasma (ICP) spectroscopy. 2. Experimental procedure Fig. 1 shows a schematic illustration of the experimental equipment, which was used for production of Ni–Al nanoparticles. It mainly consists of an arc melting chamber and a collecting system. The Ni–Al ingots were prepared from aluminum (purity N 99.9 wt.%) and nickel (purity N 99.7 wt.%) by arc melting in an argon gas atmosphere. Arc-melted ingots were flipped over and remelted four times to get a homogeneous composition. Then Ni–Al nanoparticles were produced by arc melting in a 50% Ar (purity N99.99%) and 50% H 2 (purity N 99.99%) mixture of 0.1 MPa atmosphere. The flow rate of the circulation gas for collection of nanoparticles is 100 l/min. Since the production rate of ultrafine particles (UFPs) is affected by master sample mass, identical master alloy buttons (Ni–Al ingot) weighing 50 g were used in this work. Arc current and voltage were selected as 180A and 25V, respectively. Pure Ni and Al and six master Ni–Al alloys containing 23.7, 48.2, 55.0, 58.3, 63.4 and 72.6 at.% of Ni were used. Before the nanoparticles were taken out from the arc-melting chamber, they were slowly passivated with a mixture of argon and air to prevent the particles from burning. The X-ray diffraction (XRD) was done using the wavelength of Cu K alpha radiation to characterize the crystal structure of the as-prepared nanoparticle samples. The powders were ultrasonicated about 10 min
Fig. 2. Relationship between Ni contents in Ni–Al nanoparticles and master alloys.
in anhydrous ethanol and then dropped on a copper grid, as the TEM sample. The morphology, size dispersion and shape of particles were observed using a JEOL 200EX transmission electron microscope (TEM) operated at 160 kV. The BET specific surface areas of nanoparticles were measured by N2 adsorption using a Counter SA 3100 volumetric gas adsorption analyzer and the average particle size was evaluated. The chemical composition of the UFPs was determined by the induction-coupled plasma (ICP) spectroscopy. 3. Results and discussion Fig. 2 shows the corresponding relationship between Ni contents in the Ni–Al nanoparticles and the master alloys. The chemical compositions of the Ni–Al nanoparticles are quit different from that of master Ni–Al alloys. For the master alloy containing 23.7, 48.2, 55.0, 58.3, 63.4, 72.6 at.% Ni, the corresponding Ni contents are 1.06, 20.5, 37.2, 46.2, 61.1 and 79.2 at.%, respectively, in the UFP samples. The dotted line in Fig. 2 shows the case of ideal solid solution. It can be considered that one of the reasons, which lead to the deviation from the ideal solid solution, is that the generation speed of pure Al is different from that of pure Ni. According to Ohno and Uda's theory [10], the generation speed is essentially determined by the evaporation speed of pure element metals, approximately proportional to the reaction parameter Rp: Rp ¼ −
Fig. 1. Schematic illustration of the equipment for production of nanoparticles.
DHr NH2 ðT Þ d Ls NH2 ð273Þ
ð1Þ
where ΔHr is the reaction enthalpy between hydrogen and metal, Ls is the vaporization heat of metal at temperature T, NH2 (T), and NH2 (273) are densities of the hydrogen molecular in metal at temperature T and 273 K, respectively. According to this equation, the reaction parameters of Ni and Al are evaluated to be 0.171 and 0.411, respectively. So Ni contents in nanoparticles should be lower than those in the master alloys. It is agree with Ohno and Uda's theory while the content of Ni in master alloy is below 65 at.%. However, with Ni contents in master alloy increasing to 65 at.%, Ni contents in nanoparticles is higher than those in master alloys instead. This cannot be explained by Ohno and Uda's theory. The curve of the relationship between Ni contents in
Z. Wang et al. / Materials Letters 60 (2006) 2227–2231
Ni–Al nanoparticles and master alloys is like a letter “S” in shape. This is different from the result of Fe–Ni, Fe–Co and Fe–Al nanoparticles produced by HPMR [23–25], which is like a bow. It may be correlative with the formation of intermetallic compound in master alloys. Actually, the generation speed is not only affected by the evaporation speed of pure element metals but also done by the element existence state in master alloys ingot. Once Ni–Al intermallics in master alloys
2229
formed, Ni and Al combined more closely, evaporation speed of Al would reduce obviously. Fig. 3 shows the bright field TEM images of Ni–Al UFP samples. The inset figure is the histogram of size dispersion. All the particles of the samples are spherical in shape and have a size dispersion ranging from several nanometers to 100 nm in diameter, with most of them in a narrow range of 20–40 nm in diameter. The specific surface area ranges
Fig. 3. TEM bright field images of samples (a)–(h), respectively, with the same scale shown in (h) (the histogram of size dispersion is shown in the inset).
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Z. Wang et al. / Materials Letters 60 (2006) 2227–2231
from 16.5 to 100.1 m2/g, summarized in Table 1. The mean particle diameters of all UFP samples predicted from the specific surface area are in the range of 14 to 62 nm, in well agreement with the results from TEM observation. Fig. 4 shows the X-ray diffraction patterns of the as prepared UFP samples, and the crystal structures are shown in Table 1. It can be determined from Fig. 4 that sample (a) and (b) have the crystal structure of Al (FCC), and sample (h) has the crystal structure of Ni (FCC). As to samples (d), (e) and (g), pure Al3Ni2 (D513), NiAl (B2) and Ni3Al (L12) intermetallics are successfully produced. Sample (c) has the crystal structure of Al3Ni2 (D513), Al and a small amount of unidentified phase. Both NiAl and Ni3Al intermetallic compounds were founded in sample (f). The formation of the UFPs from Ni–Al binary alloy can be explained as following: Firstly, Ni and Al vapors form under H2 and Ar plasma simultaneously. Ni and Al vapors then begin to condense and solidify. As not hydrogen absorbing metals, they do not react with hydrogen during vaporing and cooling process. During the condensation stage, Al atoms collide with Ni atoms and begin to condense in the same metal cluster and then grow to nanoparticles. There exist five intermetallic compounds in the Ni–Al binary phase diagram that are Al3Ni, Al3Ni2, Al3Ni5, NiAl and Ni3Al [26]. However, in this study, only three intermetallic compounds, Al3Ni2, NiAl and Ni3Al, were found. This result is different from that expected from the equilibrium phase diagram. For example, sample (c) containing 20.5 at.% Ni in nanoparticle should be composed of the pure Al and Al3Ni finally at room temperature. However, the actual products are Al and Al3Ni2. Al3Ni2 and liquid exist above 1135 K according to the equilibrium phase diagram. In fact, because the cooling rate is as high as 105 K/s during HPMR processing, peritectoid reaction that needs some time to diffuse cannot occur, Al3Ni did not appear. Likewise, for sample (f) with 61.1 at.% Ni, the final phase in equilibrium should be NiAl and Al3Ni5 intermetallic compounds. However, NiAl and Ni3Al were formed instead. For sample (g) with 79.2 at.% Ni, single-phase Ni3Al (L12) was obtained at room temperature, which is not the single-phase field of Ni3Al on the equilibrium phase diagram. It indicates that the phase diagram of nanoparticles is quite different from the equilibrium phase diagram at high cooling rate.
4. Conclusion Eight kinds of Ni–Al nanoparticles with the particle size ranging from 14 to 62 nm were prepared successfully by hydrogen plasma–metal reaction (HPMR). The crystal strucTable 1 Characteristics of Al–Ni UFP samples
a b c
0 23.7 48.2
0 1.1 20.5
48.3 46.8 77.9
46 ± 2 47 ± 2 21 ± 2
d e f
55.0 58.3 63.4
37.2 46.2 61.1
100.1 26.7 16.5
14 ± 1 46 ± 2 62 ± 2
72.6 100
79.2 100
tures of Ni–Al nanoparticles obtained by HPMR are dependent on the original chemical composition of the master alloys. As to nanoparticles with 37.2, 46.1 and 79.2 at.% Ni, pure Al3Ni2, NiAl and Ni3Al intermetallic compounds are successfully produced, respectively. For Ni–Al nanoparticles, the crystal structures of nanoparticles obtained by HPMR are not consistent with those expected from the equilibrium phase diagram. Only three intermetallic compounds were observed in these studies, which are Al3Ni2, NiAl and Ni3Al. It can be considered that one of the reasons that lead to the deviation from the ideal solid solution is the formation of intermetallic compounds in Ni–Al system. Acknowledgements
Mean Phase Samples Contents of Contents Specific particle structure Ni at.% in of Ni at.% surface master alloys in UFPs area (m2/g) size (nm) in UFPs
g h
Fig. 4. X-ray diffraction patterns of samples (a)–(h).
26.2 36.0
32 ± 2 30 ± 2
Al (FCC) Al (FCC) Al (FCC) + Al3Ni2(D513) Al3Ni2 (D513) NiAl (B2) NiAl (B2) + Ni3Al (L12) Ni3Al (L12) Ni (FCC)
This work was supported by the National Natural Science Foundation of China (No. 50274002, 20221101 and 10335040). The authors acknowledge assistant Prof. X.L. Gai and Associate Prof. F.H. Liao for their kind technical help with TEM and XRD measurements. References [1] A.K. Giri, D. Chakraorty, Trans. Indian Ceram. Soc. 50 (1991) 28. [2] E. Koster, J. Magn. Magn. Mater. 120 (1993) 1. [3] J. Ding, T. Tsuzuki, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 162 (1996) 271.
Z. Wang et al. / Materials Letters 60 (2006) 2227–2231 [4] L. Zhang, A. Manthiram, J. Appl. Phys. 80 (1996) 4534. [5] I.V. Kityk, K.J. Plucinski, J. Ebothe, Journ. Applied Physics (USA), V.98, 084304, (2005). [6] A. Inoue, J. Saida, T. Masumoto, Metall. Trans. A 19A (1988) 2315. [7] H.Y. Zhang, B.X. Gu, H.R. Zhai, M. Lu, Phys. Status Solidi A 143 (1994) 399. [8] J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 18 (1988) 259. [9] H. Hahn, R.S. Averback, J. Appl. Phys. 67 (1990) 1113. [10] S. Ohno, M. Uda, Trans. Jpn. Inst. Met. 48 (1984) 640. [11] H.Y. Shao, Y.T. Wang, H.R. Xu, X.G. Li, Mater. Sci. Eng., B 110 (2004) 22. [12] H.Y. Shao, Y.T. Wang, H.R. Xu, X.G. Li, J. Solid State Chem. 178 (2005) 2211. [13] T. Liu, H. Shao, X. Li, Intermetallics 12 (2004) 97. [14] H.Y. Shao, T. Liu, X.G. Li, Nanotechnology 14 (2003) L1. [15] P. Nash, M.F Singleton, J.L. Murry, in: P. Nash (Ed.), Phase Diagrams Of Binary Nickel Alloys, vol. 1, ASM International, Metals Park, OH, 1986.
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