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Oxidation behavior of NiAl nanoparticles prepared by hydrogen plasma–metal reaction
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Materials Chemistry and Physics 107 (2008) 381–384
Oxidation behavior of NiAl nanoparticles prepared by hydrogen plasma–metal reaction Zhong Wang a,b , Wenhuai Tian c , Xingguo Li a,∗ a
The State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China b General Research Institute for Nonferrous Metal, Beijing 100088, China c Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China Received 14 June 2007; received in revised form 29 July 2007; accepted 5 August 2007
Abstract NiAl nanoparticles were synthesized by hydrogen plasma–metal reaction method. In order to study the oxidation behavior of the intermetallic nanoparticles, the changes in morphology, particle size and crystal structure during heat treatment in air were investigated by TEM, XRD and TGA. The NiAl nanoparticles were stable in shape at 1073 K in air, the oxide film formed around the nanoparticles during heating in air was contributed to prevent the nonparticles growth and further oxidized. The oxidation of NiAl nanoparticles proceeded slowly below 973 K and rapidly above this temperature. The NiAl nanoparticles were oxidized as Al2 O3 preferentially, and then NiO formed. © 2007 Elsevier B.V. All rights reserved. Keywords: Intermetallic compounds; Nanostructures; Crystal structure; Oxidation
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]. Since metallic nanoparticles have a large specific surface area, they actively react with oxygen even at room temperature. The formation of the oxide layers might protect the particles against further oxidation at ambient temperature, but the metallic nanoparticles are oxidized easily with increasing temperature and this reduced the properties of the particles in a large scale [5,6]. This therefore prevents their applications at a little high temperature. It is well known that bulk nickel aluminides intermetallic compounds, such as NiAl and Ni3 Al, are regarded as promising candidates for the next generation of high-temperature and high-performance structural materials [7] because of their high-melting points, relatively low densities, good strength and high-temperature corrosion and oxidation resistance. As important intermetallics, they show great potential applications in automobile engines, aircraft, and electricity generation and energy conversion equipment. ∗
Corresponding author. Tel.: +86 10 62765930; fax: +86 10 62765930. E-mail address: [email protected] (X. Li).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.08.003
Recently, powders of NiAl and Ni3 Al have been prepared by heat treatment of the organometallic precursors synthesized by coprecipitation of constituent metallic salts in ammonium benzoate and hydrazinium monochloride [8], but the particle size of these powders was quite large (1–3 m). So far, however, there have been few reports on synthesis and oxidation properties of nickel aluminide intermetallic compound nanoparticles. Hydrogen plasma–metal reaction (HPMR) method developed by Ohno and Uda [9] is to produce nanoparticles by DC thermal plasma in a mixture of hydrogen and argon gas of 0.1 MPa. The production yield of nanoparticles in this method varied from tens to hundreds of grams per hour for different metals, much larger than that obtained by the ordinary vacuum evaporation method. It seems that Uda’s method is suitable to prepare metallic nanoparticles industrially at low cost. In the present paper, we intend to investigate the evolution of the oxidation of NiAl nanoparticles synthesized by HPMR. 2. Experimental The experimental equipment for producing NiAl nanoparticles was primarily composed of an arc melting chamber and a collecting system, which was described previously [10]. The bulk Al–58.3 at.% Ni ingot weighting 50 g was prepared from aluminum (purity > 99.9%) and nickel (purity > 99.7%) by arc melting in an argon gas atmosphere. Arc-melted ingots were flipped over and remelted four times to get a homogeneous composition. Then nanoparticles
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Z. Wang et al. / Materials Chemistry and Physics 107 (2008) 381–384 the oxidation experiment. The composition of nanoparticles was determined by the induction-coupled plasma (ICP) spectrophotometer. Oxidation behavior of nanoparticles was studied by heating in air at 673, 873 and 1073 K for 1 h, respectively. The crystal structures of particles were studied by X-ray diffraction (XRD) using monochromatic Cu K␣ radiation. The morphology, size distribution and shape of particles were observed using a JEOL EX transmission electron microscopy (TEM) operated at 160 kV. The thermal analysis of the as-prepared nanoparticles was observed by thermal gravitation analysis (TGA) with a heating rate of 10 K min−1 in air.
3. Results and discussion
Fig. 1. X-ray diffraction pattern of the as-prepared NiAl nanoparticles sample. were produced by arc melting this master alloy in a mixing 50%Ar + 50%H2 gas of 0.1 MPa. The flow rate of the circulation gas for collection of nanoparticles is 100 l min−1 . Arc current and voltage was selected as 180 A and 25 V, respectively. After passivated in an Ar + 5%O2 atmosphere for 24 h, the nanoparticles were taken out from the arc-melting chamber. The nanoparticles, hereafter referred to as the “as-prepared nanoparticles”, were used as an initial sample for
NiAl nanoparticle was successfully synthesized with the master alloy containing 58.3 at.%Ni by optimizing the ratio of Ni to Al for several times in HPMR processing. Ni content in the NiAl nanoparticles sample is 46.2 at.%, determined by ICP. Fig. 1 shows the XRD patterns of the as-prepared nanoparticles sample. It is found that pure NiAl nanoparticle with B2 structure is produced by HPMR. No oxide is detectable in the XRD result. This reveals that these nanoparticles possess excellent oxidation resistance property at room temperature in air. It is observed in Fig. 2a that the as-prepared NiAl nanoparticles are nearly spherical in shape, and have a size distribution ranging from 10
Fig. 2. TEM bright field images of NiAl nanoparticles samples heated in air at (a) as-prepared, (b) 673 K, (c) 873 K and (d) 1073 K, with the same scale shown in (d).
Z. Wang et al. / Materials Chemistry and Physics 107 (2008) 381–384
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Fig. 3. X-ray diffraction patterns of NiAl nanoparticle samples heated in air at (a) as-prepared, (b) 673 K, (c) 873 K and (d) 1073 K.
to 80 nm in diameter, with the mean particle diameter of about 46 nm. It is known that features of nanoparticles of metallic alloys are easily changed when heated. To understand the thermal stability of NiAl nanoparticles, variations of NiAl nanoparticles in particle feature after heat treatment at different temperature in air, respectively, for 3.6 ks were investigated. Fig. 2 shows the morphological variations of nanoparticles heated at different temperature in air. It is found that the thin layers around the nanoparticles, which are detected to be oxide in XRD, get thicker with temperature increasing. Heated up to 873 K, the oxide film can be observed obviously by TEM and the thickness is about 5 nm. At 1073 K, the thickness increases to about 10 nm, the nanoparticles size has some grown a little. But the samples still remain sphere, so it seems that the NiAl nanoparticles are stable in shape at 1073 K in air. It demonstrates clearly that the oxide film formed around the nanoparticles during heating in air are contributed to prevent the nonparticles growth and further oxidized. That is, the oxide film has higher oxide-resistance property. The particle growth for NiAl nanoparticles occurs at a higher temperature compared with some alloy nanoparticles such as Fe–Co and Fe–Al [11,12]. Fig. 3 shows the XRD patterns of the as-prepared nanoparticles heated at different temperatures. While the particles are heated at 673 K, the sample is basically composed of NiAl (B2) phase at 673 K and aluminum oxides (␥-Al2 O3 ) is formed in very small amount, implying that NiAl nanoparticles are stable at 673 K, in agreement with the results from TEM observation. At 873 K, the product of oxidation is not only Al2 O3 , but also NiO; however, NiAl nanoparticles are still majority. Up to 1073 K, the amount of NiO increases, but the nanoparticles are not oxidized completely. Because NiAl peaks are still observed obviously from in the XRD patterns and more than half of the sample is still NiAl phase. It was previously reported by Li’s group that Fe nanoparticles heated in air changed thoroughly to oxide at 473 K [13], Fe–Co, Fe–Cr and Fe3 Al nanoparticles did at 873 K [11,12,14]. This indicates that NiAl nanoparticles have the very strong oxidation resistance. Since the features of metallic nanoparticles with large specific area are easily change when heated in air, the high-oxidation resistance property of NiAl nanoparticles is to extend their utilization. However, the
Fig. 4. TGA curves of the NiAl nanoparticle samples heated in air at a heating rate of 10 K min−1 .
oxidation resistance property of NiAl nanoparticle is lower than that of bulk NiAl intermetallics which has good oxidation resistance, forming only slow growing Al2 O3 scale and no Ni-oxides or Ni-spinels [15]. To further observe the oxidation behavior of NiAl intermetallics, thermal gravitation analysis (TGA) was carried out in air and the results are shown in Fig. 4. The cure of TGA can be divided into four parts: Part one is almost a horizontal line below the temperature 673 K; Part two is from 673 to 973 K, the cure rises slowly in this part, which indicates the samples are oxidized slightly and the gravitation of sample increase about 8%; Part three is from 973 to 1173 K, the cure rises steeply indicating that the samples are oxidized violently and the gravitation of sample increases about 18%; Part four is up the temperature to 1173 K. It can be concluded that the oxidation rate is slow below 973 K and rapid above this temperature. The NiAl nanoparticles were oxidized to undergo the following process which can be analyzed from the curve of TGA and XRD results. In the heated environment, the Al in the NiAl is oxidized selectively at first and the surface of grain gradually became a layer, which can protect the grain in the internal part to be oxidized further, and this is corresponding to the second part of TGA curve. It is the aluminum oxide layer that increases the oxidation resistance of the intermetallic nanoparticles. The subsequent oxidation of NiAl nanoparticle is controlled by oxygen diffusion. Since the concentration of oxygen is higher in the layer close to the surface, the oxide layer grows toward the interior of the nanoparticle. The oxygen atom penetrates through the Al2 O3 film to the inner part and the internal Ni is oxidized gradually to form NiO, because specific gravity of Ni is about three times to Al, so the Ni is oxidized largely, the weight of sample increases obviously, this is corresponding to the third part of steep risings on the TGA curve. 4. Conclusion The NiAl nanoparticles prepared by HPMR were spherical in shape at 1073 K in air. The high-oxidation resistance
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of intermetallic nanoparticles was attributed to the alumina layer and extremely important for their practical utilization. The morphologies and crystal structures of nanoparticles changed with increasing temperature. The oxidation of NiAl nanoparticles proceeded slowly below 973 K and rapidly above this temperature. The NiAl nanoparticles were oxidized as Al2 O3 preferentially, and then NiO formed. The oxidation resistance property of NiAl nanoparticles is lower than the bulk NiAl intermetallics. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50274002, 20221101 and 10335040). References [1] A.K. Giri, D. Chakraorty, Trans. Ind. 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. [4] L. Zhang, A. Manthiram, J. Appl. Phys. 80 (1996) 4534. [5] M.Y. Huh, S.H. Kim, J.P. Ahn, Nanostruct. Mater. 11 (1993) 211. [6] X.Q. Zhao, B.X. Liu, Y. Liang, Z.Q. Hu, J. Magn. Magn. Mater. 164 (1996) 401. [7] D. Poquillon, D. Oquab, B. Viguier, F. S´enocq, D. Monceau, Mater. Sci. Eng. A381 (2004) 237. [8] A.P. Alivisatos, A.L. Harris, N.J. Levinos, J. Chem. Phys. 90 (1988) 4001. [9] S. Ohno, M. Uda, Trans. Jpn. Inst. Met. 48 (1984) 640. [10] X.G. Li, A. Chiba, S. Takahashi, J. Magn. Magn. Mater. 170 (1997) 339. [11] X.G. Li, T. Murai, T. Aaito, S. Takahashi, J. Magn. Magn. Mater. 190 (1998) 277. [12] T. Liu, H.Y. Shao, X.G. Li, Nanotechnology 14 (2003) 542. [13] X.G. Li, A. Chiba, T. Aaito, S. Takahashi, M. Sato, J. Appl. Phys. 83 (1998) 3871. [14] X.G. Li, A. Chiba, S. Takahashi, K. Ohsaki, J. Magn. Magn. Mater. 173 (1997) 101. [15] H.J. Grabke, Intermetallics 7 (1999) 1153.
Materials Chemistry and Physics 107 (2008) 381–384
Oxidation behavior of NiAl nanoparticles prepared by hydrogen plasma–metal reaction Zhong Wang a,b , Wenhuai Tian c , Xingguo Li a,∗ a
The State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China b General Research Institute for Nonferrous Metal, Beijing 100088, China c Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China Received 14 June 2007; received in revised form 29 July 2007; accepted 5 August 2007
Abstract NiAl nanoparticles were synthesized by hydrogen plasma–metal reaction method. In order to study the oxidation behavior of the intermetallic nanoparticles, the changes in morphology, particle size and crystal structure during heat treatment in air were investigated by TEM, XRD and TGA. The NiAl nanoparticles were stable in shape at 1073 K in air, the oxide film formed around the nanoparticles during heating in air was contributed to prevent the nonparticles growth and further oxidized. The oxidation of NiAl nanoparticles proceeded slowly below 973 K and rapidly above this temperature. The NiAl nanoparticles were oxidized as Al2 O3 preferentially, and then NiO formed. © 2007 Elsevier B.V. All rights reserved. Keywords: Intermetallic compounds; Nanostructures; Crystal structure; Oxidation
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]. Since metallic nanoparticles have a large specific surface area, they actively react with oxygen even at room temperature. The formation of the oxide layers might protect the particles against further oxidation at ambient temperature, but the metallic nanoparticles are oxidized easily with increasing temperature and this reduced the properties of the particles in a large scale [5,6]. This therefore prevents their applications at a little high temperature. It is well known that bulk nickel aluminides intermetallic compounds, such as NiAl and Ni3 Al, are regarded as promising candidates for the next generation of high-temperature and high-performance structural materials [7] because of their high-melting points, relatively low densities, good strength and high-temperature corrosion and oxidation resistance. As important intermetallics, they show great potential applications in automobile engines, aircraft, and electricity generation and energy conversion equipment. ∗
Corresponding author. Tel.: +86 10 62765930; fax: +86 10 62765930. E-mail address: [email protected] (X. Li).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.08.003
Recently, powders of NiAl and Ni3 Al have been prepared by heat treatment of the organometallic precursors synthesized by coprecipitation of constituent metallic salts in ammonium benzoate and hydrazinium monochloride [8], but the particle size of these powders was quite large (1–3 m). So far, however, there have been few reports on synthesis and oxidation properties of nickel aluminide intermetallic compound nanoparticles. Hydrogen plasma–metal reaction (HPMR) method developed by Ohno and Uda [9] is to produce nanoparticles by DC thermal plasma in a mixture of hydrogen and argon gas of 0.1 MPa. The production yield of nanoparticles in this method varied from tens to hundreds of grams per hour for different metals, much larger than that obtained by the ordinary vacuum evaporation method. It seems that Uda’s method is suitable to prepare metallic nanoparticles industrially at low cost. In the present paper, we intend to investigate the evolution of the oxidation of NiAl nanoparticles synthesized by HPMR. 2. Experimental The experimental equipment for producing NiAl nanoparticles was primarily composed of an arc melting chamber and a collecting system, which was described previously [10]. The bulk Al–58.3 at.% Ni ingot weighting 50 g was prepared from aluminum (purity > 99.9%) and nickel (purity > 99.7%) by arc melting in an argon gas atmosphere. Arc-melted ingots were flipped over and remelted four times to get a homogeneous composition. Then nanoparticles
382
Z. Wang et al. / Materials Chemistry and Physics 107 (2008) 381–384 the oxidation experiment. The composition of nanoparticles was determined by the induction-coupled plasma (ICP) spectrophotometer. Oxidation behavior of nanoparticles was studied by heating in air at 673, 873 and 1073 K for 1 h, respectively. The crystal structures of particles were studied by X-ray diffraction (XRD) using monochromatic Cu K␣ radiation. The morphology, size distribution and shape of particles were observed using a JEOL EX transmission electron microscopy (TEM) operated at 160 kV. The thermal analysis of the as-prepared nanoparticles was observed by thermal gravitation analysis (TGA) with a heating rate of 10 K min−1 in air.
3. Results and discussion
Fig. 1. X-ray diffraction pattern of the as-prepared NiAl nanoparticles sample. were produced by arc melting this master alloy in a mixing 50%Ar + 50%H2 gas of 0.1 MPa. The flow rate of the circulation gas for collection of nanoparticles is 100 l min−1 . Arc current and voltage was selected as 180 A and 25 V, respectively. After passivated in an Ar + 5%O2 atmosphere for 24 h, the nanoparticles were taken out from the arc-melting chamber. The nanoparticles, hereafter referred to as the “as-prepared nanoparticles”, were used as an initial sample for
NiAl nanoparticle was successfully synthesized with the master alloy containing 58.3 at.%Ni by optimizing the ratio of Ni to Al for several times in HPMR processing. Ni content in the NiAl nanoparticles sample is 46.2 at.%, determined by ICP. Fig. 1 shows the XRD patterns of the as-prepared nanoparticles sample. It is found that pure NiAl nanoparticle with B2 structure is produced by HPMR. No oxide is detectable in the XRD result. This reveals that these nanoparticles possess excellent oxidation resistance property at room temperature in air. It is observed in Fig. 2a that the as-prepared NiAl nanoparticles are nearly spherical in shape, and have a size distribution ranging from 10
Fig. 2. TEM bright field images of NiAl nanoparticles samples heated in air at (a) as-prepared, (b) 673 K, (c) 873 K and (d) 1073 K, with the same scale shown in (d).
Z. Wang et al. / Materials Chemistry and Physics 107 (2008) 381–384
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Fig. 3. X-ray diffraction patterns of NiAl nanoparticle samples heated in air at (a) as-prepared, (b) 673 K, (c) 873 K and (d) 1073 K.
to 80 nm in diameter, with the mean particle diameter of about 46 nm. It is known that features of nanoparticles of metallic alloys are easily changed when heated. To understand the thermal stability of NiAl nanoparticles, variations of NiAl nanoparticles in particle feature after heat treatment at different temperature in air, respectively, for 3.6 ks were investigated. Fig. 2 shows the morphological variations of nanoparticles heated at different temperature in air. It is found that the thin layers around the nanoparticles, which are detected to be oxide in XRD, get thicker with temperature increasing. Heated up to 873 K, the oxide film can be observed obviously by TEM and the thickness is about 5 nm. At 1073 K, the thickness increases to about 10 nm, the nanoparticles size has some grown a little. But the samples still remain sphere, so it seems that the NiAl nanoparticles are stable in shape at 1073 K in air. It demonstrates clearly that the oxide film formed around the nanoparticles during heating in air are contributed to prevent the nonparticles growth and further oxidized. That is, the oxide film has higher oxide-resistance property. The particle growth for NiAl nanoparticles occurs at a higher temperature compared with some alloy nanoparticles such as Fe–Co and Fe–Al [11,12]. Fig. 3 shows the XRD patterns of the as-prepared nanoparticles heated at different temperatures. While the particles are heated at 673 K, the sample is basically composed of NiAl (B2) phase at 673 K and aluminum oxides (␥-Al2 O3 ) is formed in very small amount, implying that NiAl nanoparticles are stable at 673 K, in agreement with the results from TEM observation. At 873 K, the product of oxidation is not only Al2 O3 , but also NiO; however, NiAl nanoparticles are still majority. Up to 1073 K, the amount of NiO increases, but the nanoparticles are not oxidized completely. Because NiAl peaks are still observed obviously from in the XRD patterns and more than half of the sample is still NiAl phase. It was previously reported by Li’s group that Fe nanoparticles heated in air changed thoroughly to oxide at 473 K [13], Fe–Co, Fe–Cr and Fe3 Al nanoparticles did at 873 K [11,12,14]. This indicates that NiAl nanoparticles have the very strong oxidation resistance. Since the features of metallic nanoparticles with large specific area are easily change when heated in air, the high-oxidation resistance property of NiAl nanoparticles is to extend their utilization. However, the
Fig. 4. TGA curves of the NiAl nanoparticle samples heated in air at a heating rate of 10 K min−1 .
oxidation resistance property of NiAl nanoparticle is lower than that of bulk NiAl intermetallics which has good oxidation resistance, forming only slow growing Al2 O3 scale and no Ni-oxides or Ni-spinels [15]. To further observe the oxidation behavior of NiAl intermetallics, thermal gravitation analysis (TGA) was carried out in air and the results are shown in Fig. 4. The cure of TGA can be divided into four parts: Part one is almost a horizontal line below the temperature 673 K; Part two is from 673 to 973 K, the cure rises slowly in this part, which indicates the samples are oxidized slightly and the gravitation of sample increase about 8%; Part three is from 973 to 1173 K, the cure rises steeply indicating that the samples are oxidized violently and the gravitation of sample increases about 18%; Part four is up the temperature to 1173 K. It can be concluded that the oxidation rate is slow below 973 K and rapid above this temperature. The NiAl nanoparticles were oxidized to undergo the following process which can be analyzed from the curve of TGA and XRD results. In the heated environment, the Al in the NiAl is oxidized selectively at first and the surface of grain gradually became a layer, which can protect the grain in the internal part to be oxidized further, and this is corresponding to the second part of TGA curve. It is the aluminum oxide layer that increases the oxidation resistance of the intermetallic nanoparticles. The subsequent oxidation of NiAl nanoparticle is controlled by oxygen diffusion. Since the concentration of oxygen is higher in the layer close to the surface, the oxide layer grows toward the interior of the nanoparticle. The oxygen atom penetrates through the Al2 O3 film to the inner part and the internal Ni is oxidized gradually to form NiO, because specific gravity of Ni is about three times to Al, so the Ni is oxidized largely, the weight of sample increases obviously, this is corresponding to the third part of steep risings on the TGA curve. 4. Conclusion The NiAl nanoparticles prepared by HPMR were spherical in shape at 1073 K in air. The high-oxidation resistance
384
Z. Wang et al. / Materials Chemistry and Physics 107 (2008) 381–384
of intermetallic nanoparticles was attributed to the alumina layer and extremely important for their practical utilization. The morphologies and crystal structures of nanoparticles changed with increasing temperature. The oxidation of NiAl nanoparticles proceeded slowly below 973 K and rapidly above this temperature. The NiAl nanoparticles were oxidized as Al2 O3 preferentially, and then NiO formed. The oxidation resistance property of NiAl nanoparticles is lower than the bulk NiAl intermetallics. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50274002, 20221101 and 10335040). References [1] A.K. Giri, D. Chakraorty, Trans. Ind. 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. [4] L. Zhang, A. Manthiram, J. Appl. Phys. 80 (1996) 4534. [5] M.Y. Huh, S.H. Kim, J.P. Ahn, Nanostruct. Mater. 11 (1993) 211. [6] X.Q. Zhao, B.X. Liu, Y. Liang, Z.Q. Hu, J. Magn. Magn. Mater. 164 (1996) 401. [7] D. Poquillon, D. Oquab, B. Viguier, F. S´enocq, D. Monceau, Mater. Sci. Eng. A381 (2004) 237. [8] A.P. Alivisatos, A.L. Harris, N.J. Levinos, J. Chem. Phys. 90 (1988) 4001. [9] S. Ohno, M. Uda, Trans. Jpn. Inst. Met. 48 (1984) 640. [10] X.G. Li, A. Chiba, S. Takahashi, J. Magn. Magn. Mater. 170 (1997) 339. [11] X.G. Li, T. Murai, T. Aaito, S. Takahashi, J. Magn. Magn. Mater. 190 (1998) 277. [12] T. Liu, H.Y. Shao, X.G. Li, Nanotechnology 14 (2003) 542. [13] X.G. Li, A. Chiba, T. Aaito, S. Takahashi, M. Sato, J. Appl. Phys. 83 (1998) 3871. [14] X.G. Li, A. Chiba, S. Takahashi, K. Ohsaki, J. Magn. Magn. Mater. 173 (1997) 101. [15] H.J. Grabke, Intermetallics 7 (1999) 1153.