Structure and magnetic properties of nickel nanoparticles prepared by selective leaching

Structure and magnetic properties of nickel nanoparticles prepared by selective leaching

Materials Letters 137 (2014) 221–224 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet S...

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Materials Letters 137 (2014) 221–224

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Structure and magnetic properties of nickel nanoparticles prepared by selective leaching Alena Michalcová a,n, Petra Svobodová a, Radka Nováková a, Adél Len b, Oleg Heczko c, Dalibor Vojtěch a, Ivo Marek a, Pavel Novák a a

Institute of Chemical Technology, Prague, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic Budapest Nuclear Centre, Hungarian Academy of Sciences, Association of the KFKI Research Institutes Centre for Energy Research – Wigner Research Centre for Physics, P.O.B. 49, H-1525 Budapest 114, Hungary c Institute of Physics of the ASCR, v. v. i., Na Slovance 2, 182 21 Praha, Czech Republic b

art ic l e i nf o

a b s t r a c t

Article history: Received 22 August 2014 Accepted 3 September 2014 Available online 10 September 2014

In this study, we propose a novel method for preparation of nanocrystalline powder precursors for powder metallurgy processing. Nickel nanocrystalline powders were prepared by selective leaching of the Al-20Ni alloys in NaOH solution. The powders were studied by SEM, TEM, XRD, magnetometry and by small angle neutron scattering. The prepared powders had internal structure formed by grains of 5– 50 nm in size depending on the leaching temperature. It was also proven that slight grain coarsening occurred during heating of particles at 40–80 1C for one hour. Magnetometry testing revealed significantly reduced magnetization of nanocrystalline nickel which is related to the internal structure. The results indicate that nickel powders prepared by selective leaching are suitable precursor for powder metallurgical preparation of bulk products with interesting physical and possible also mechanical properties. & 2014 Elsevier B.V. All rights reserved.

Keywords: Nanoparticles Magnetic materials Metals and alloys

1. Introduction Much attention has been paid to preparation of magnetic nanoparticles in last decade [1]. Many papers describe preparation of Ni nanoparticles by reduction of aqueous solution of nickel salts alternatively with or without addition of alcohol or oil [2–7]. Some other paper describe production of nickel nanoparticles by micelles route [2,9], by utilization of plant extracts as reduction agent [3,8], by gamma irradiation [1] or laser ablation of solids in organic solutions [10]. This paper is focused on preparation of nickel nanoparticles by a selective leaching method. This process is based on preparation of binary alloy, its appropriate heat treatment to obtain supersaturated solid solution followed by possible tempering to grow nanoparticles from minor element and leaching of matrix element [11,12]. The advantage of this method is the ability to produce industrially significant amount of nanoparticles suitable for consequent processing by powder metallurgy to produce bulk nanocrystalline material. It was proven that by selective leaching of Al–20Ni alloy, Ni submicrometer nanoparticles with internal nanocrystalline structure are produced [11]. For further compaction, this is even more suitable than individual

n

Corresponding author. E-mail address: [email protected] (A. Michalcová).

http://dx.doi.org/10.1016/j.matlet.2014.09.012 0167-577X/& 2014 Elsevier B.V. All rights reserved.

nanoparticles, while it suppresses the surface oxidation of particles and improves the homogeneity of a compact product.

2. Experimental The master alloy with compositions of Al–20 wt% Ni (34 at% Ni) was prepared by melting of appropriate amount of pure metals in induction furnace followed by melt spinning process with circumferial speed of cooling wheel of 20 m/s. The rapidly solidified alloy was leached in 20% (wt.) solution of NaOH, following this reaction (1) 3.03AlNi34 þ2NaOHþ6H2O-2Na[Al(OH)4]þ 3H2 þ 1.03Ni

(1)

The nickel nanoparticles were prepared by selective leaching at different temperatures -  20, 0, 40, 60 and 80 1C. Leaching at 0– 80 1C was performed using magnetic stirring and heating device, while leaching at  20 1C took place in cryostat cell with mechanical stirring. The leaching was performed for 3 h at high temperatures (40–80 1C), 2 days at middle temperatures (0–20 1C) and 14 days at the lowest temperature (  20 1C). Phase composition of initial materials and prepared nanoparticles was determined by X-ray diffraction (PAN analytical X'Pert PROþHigh Score Plus, Cu anode). The structure of nanoparticles was observed by scanning electron microscope TESCAN VEGA 3 LMU equipped by EDS

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detector (Oxford Instruments) and by transmission electron microscope Jeol JEM 3010. As the nickel nanoparticles are magnetic, the quality of images is not very good and it is necessary to measure them by some other method. For this purpose, neutron small angle scattering was chosen assuming that the radius of gyration corresponds to the grain size of nickel powder. SANS was performed in Budapest Neutron Centre. The samples were measured in glass cuvettes in water environment. Magnetic properties of the samples in form of dry powder were measured by vibrational magnetometer PAR 4500 at room temperature.

60

Fraction (%)

222

-20 -20+HT40 -20+HT60 -20+HT80 0 20 40 60 80

40

20

3. Results and discussion 0

The nickel nanoparticles were prepared by selective leaching at different temperatures -  20, 0, 40, 60 and 80 1C. The size of prepared nickel particles is almost the same with differing temperature (about 200 nm, as illustrated in Fig. 1(a). Fig. 1 (b) shows that Ni particles are composed of several crystalline grains separated by high angle grain boundaries. Detailed analysis of HRTEM images revealed different sizes of nickel grains forming the particles. At the lowest leaching temperature of  20 1C, smallest grains of approximately 2 nm are formed. At higher

Fig. 1. Microstructure of nickel powder prepared by selective leaching at  20 1C a) overview, b) detailed HRTEM image.

0

5

10

15

20

Grain size (nm) Fig. 2. Grain size of nickel nanoparticles (measured from HRTEM images) prepared by selective leaching at various temperatures and prepared at  20 1C with consequent heat treatment (HT) at 40, 60 and 80 1C for 1 h.

leaching temperatures, the size increases and after leaching at 80 1C it reaches 10–15 nm. The grain size of the sample prepared at 20 1C was verified by SANS measurement. The radius of gyration was 22.787 0.06 Å, which is in good agreement with TEM observation. The observed difference can be attributed to accelerated diffusion of Ni atoms during selective leaching. The differences between size of grains leached at  20 1C and 0 1C can be also partially explained by the change of experimental conditions. While the nanoparticles prepared at 0–80 1C were leached using magnetic mixing device, the experimental setup did not allow to use magnetic mixing at  20 1C (leaching in cryostat). The presence of magnetic field during nickel nanoparticle formation has crucial effect to its size and shape [1]. As stated before, the sample prepared at  20 1C exhibits the finest grain size. When the powder is heated the grains start to grow (Fig. 2), but the changes are not so significant compared to samples prepared at different temperatures. It is obvious from Fig. 2 that a slight grain coarsening from approximately 3 nm to 5 nm takes place during heating at 80 1C/1 h. The change is small because it is controlled by solid state diffusion of Ni atoms which is much slower than diffusion in the liquid state during leaching. The grain size coarsening of nanoparticles heated for 1 h was confirmed also by X-ray diffraction shown in Fig. 3. The diffraction pattern of nickel nanoparticles leached at 201 exhibit broad diffraction maxima of nickel. After heating the peaks of nickel become narrower, which indicates crystallites (grains) coarsening. The crystallite size marked in Fig. 3 is estimated from Scherrer equation. To grain size values obtained in this study are slightly different than in our previous work, which can be caused by changes in experimental set-up. In [11], the AlNi20 alloy was prepared at higher cooling rate leading to finer structure. It can explain the change in shape of forming particles a partially also the change in grain size. Moreover the leaching process [11] was performed with mechanical stirring and as it was discussed above, the presence of magnetic field during selective leaching may have influence on grain size of formed particles [1]. To characterize the change in properties with grains coarsening, magnetization curves were measured. Fig. 4 shows magnetization curves of as-prepared nanopowder leached at  20 1C and nanopowders consequently heated at 40, 60 and 80 1C for 1 h. These curves are compared to the curve of electrolytic nickel taken as standard material. All studied material had significantly lower saturated magnetization than standard material. It indicates very fine structure of particles formed by grains with size of few nanometers. The suppression of saturated magnetization is due

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Fig. 3. Diffraction patterns of nickel nanoparticles prepared by selective leaching at  20 1C: a) as prepared, (b) heated at 40 1C/1 h, (c) heated at 60 1C/1 h, (d) heated at 80 1C/1 h. Crystallite sizes are estimated by the Scherrer calculator.

magnetic moment [emu/g]

4. Conclusion

-20 -20 + HT40 -20 + HT60 -20 + HT80 electrolytic Ni

60 40 20 0 -20 -40 -60 -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

magnetic field [T]

It is concluded from results obtained in this study that the selective leaching process produces nanocrystalline nickel powder with grain size of even below 5 nm. With increasing the leaching temperature from  20 1C to 80 1C the grain size increases to 10– 15 nm. Importantly leaching at 40–60 1C produces Ni powder with similar grain size between 5 and 10 nm. These temperatures thus appear as suitable for preparation of powders in amounts sufficient to be compacted into bulk nanocrystalline products. Leaching at  20 1C is not very suitable from the technological point of view because of a low leaching kinetics and problematic cooling. On the other hand, this process results in powder with interestingly low ferromagnetical properties which can be a promising base for further fundamental exploration of magnetic phenomena in nanocrystalline structures.

Fig. 4. Magnetization curves of nickel nanoparticles prepared by selective leaching at  20 1C and heated at 40, 60 and 80 1C for 1 h.

Acknowledgment

Table 1 Saturation magnetization of Ni nanoparticles and electrolytic nickel (standard sample). Sample

Form

Ms [emu/g]

Ni: Selective leaching:  20 1C Ni: Selective leaching:  20 1C, heated 40 1C, 1 h Ni: Selective leaching:  20 1C, heated 60 1C, 1 h Ni: Selective leaching:  20 1C, heated 80 1C, 1 h Electrolytic Ni (standard sample)

Nanopowder Nanopowder Nanopowder Nanopowder Bulk

8 21 22 26 57

to so called “dead zones” [13]. These are areas of grain boundaries where the structure is deformed and, therefore they do not contribute to ferromagnetic behavior of the material. The increase of magnetization of thermally treated nanoparticles indicates most probably the grain coarsening. The values of saturated magnetization are given in Table 1. It is interesting to notice that the selective leaching process enables preparation of nanocrystalline nickel where saturation magnetization is eight fold reduced as compared to the bulk microcrystalline metal.

Alena Michalcová, Dalibor Vojtěch and Ivo Marek thank for financial support by Czech Science Foundation, Project no. P108/ 12/G043. Oleg Heczko and Pavel Novák thank for support by Czech Science Foundation under Project no. 14-03044S. Adél Len thanks for support by the European Commission under the 7th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures under Grant Agreement no. 283883-NMI3-II. References [1] Wang F, Zhang Z, Chang Z. Effects of magnetic field on the morphology of nickel nanocrystals prepared by gamma-irradiation in aqueous solutions. Mater Lett 2002;55:27–9. [2] Zhang DE, Ni XM, Zheng HG, Li TY, Zhang XJ, Yang ZP. Synthesis of needle-like nickel nanoparticles in water-in-oil microemulsion. Mater Lett 2005; 59:2011–4. [3] Jiang Ch, Zou G, Zhang W, Yu W, Qian Y. Aqueous solution route to flower-like microstructures of ferromagnetic nickel nanotips. Mater Lett 2006;60: 2319–21. [4] Bai L, Yuan F, Tang Q. Synthesis of nickel nanoparticles with uniform size via a modified hydrazine reduction route. Mater Lett 2008;62:2267–70. [5] Hu H, Sugawara K. Selective synthesis of metallic nickel particles with control of shape via wet chemical process. Mater Lett 2008;62:4339–42.

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