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Materials Chemistry and Physics 124 (2010) 41–43

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Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

aterials science communication

ynthesis, thermodynamic and magnetic properties of pureexagonal close packed nickel

bhishek Lahiri ∗, Zeenath Tadisinaepartment of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa, AL 35487, United States

r t i c l e i n f o

rticle history:eceived 26 February 2010

a b s t r a c t

A novel and fast technique for the synthesis of pure hexagonal close packed (HCP) nickel is demonstrated.The HCP nickel was electrodeposited from NiCl2-1-ethyl-3-methylimidazolium chloride (NiCl2-EmimCl)

eceived in revised form 5 June 2010ccepted 11 July 2010

eywords:CP nickel

onic liquid

ionic liquid at 160 ◦C. X-ray diffraction confirmed the formation of pure phase. A phase transforma-tion from HCP nickel to face centered cubic (FCC) nickel was observed at 422.6 ◦C and the enthalpy oftransformation was found to be 16.72 J g−1. The phase transformation resulted in the release of hydro-gen which makes HCP nickel a potential hydrogen storage material. The electrodeposited nickel showedferromagnetic properties and the magnetic coercivity was found to be 43 Oe.

pecific heatagnetic properties

Nickel exists in two crystal structures namely the stable faceentered cubic (FCC) and hexagonal close packed (HCP). However,he existence of nickel in HCP phase is still elusive and is consid-red to be metastable [1,2]. The synthesis of HCP nickel phase islso complicated and involves two to three steps. Chen et al. [3] pro-uced HCP nickel nanoparticles by thermal decomposition of nickelcetylacetonate in alkylamines. However, they reported the pres-nce of some impure and unidentified phases which might renderome error in the magnetic and thermodynamic properties. Car-uran et al. synthesized HCP nickel by reacting Ni(II) complexesith K/B alloy at 200 ◦C whereas Mi et al. used NiCl2 and reactedith KBH4 and ethelenediamine at temperatures ranging from 200

o 400 ◦C [4,5]. However, the papers speculated that HCP nickelormed was due to the presence of hydride, nitride or carbide impu-ities [5]. The difference in concentration of impurities may againnstigate error in the magnetic and thermodynamic properties. Fur-hermore, all the above mentioned techniques take greater than 5 ho synthesize HCP nickel.

Here we show that pure HCP nickel can be synthesized elec-rochemically in less than 2 h and also investigate the influence ofmpurities on the magnetic property. The thermodynamic proper-

ies of HCP nickel are also determined for the first time.

HCP nickel was electrodeposited from NiCl2. 5.4 mole% of thealt was dissolved in 1-ethyl-3-methylimidazolium chloride in a0 ml Pyrex beaker fitted with a Teflon cap. The Teflon cap hadoles for the introduction of electrodes and thermometer. During

∗ Corresponding author.E-mail address: lahiri.abhishek@gmail.com (A. Lahiri).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.07.019

© 2010 Elsevier B.V. All rights reserved.

the experiment the temperature was kept constant at 160 ◦C andwas monitored continuously. It was essential to maintain a tem-perature of 160 ◦C and about 2 wt% of moisture in the initial NiCl2salt, below which FCC nickel was produced. It was clarified that theproduction of hydrogen due to the electrolysis of moisture led tothe formation of HCP nickel which has been explained elsewhere[6]. Titanium was used as both anode and cathode and the activearea over which the electrolysis was performed had a dimensionof 25 mm × 15 mm × 0.5 mm. The two electrodes were immersedinto the electrolyte before a potential of 2.5 V was applied acrossit. Experiments were carried out for 2 h which led to the forma-tion of a thick layer (2 mm) of deposit. The electrodeposited nickelwas characterized using scanning electron microscopy (SEM) andenergy dispersive X-ray (EDX) techniques. The thermal propertyof HCP nickel produced was characterized by differential scanningcalorimetry (DSC) and the magnetic property of the HCP nickel wasmeasured using vibrating sample magnetometer (VSM).

The X-ray diffraction (XRD) of the electrodeposited materi-als obtained at the cathode is compared in Fig. 1a with standardICDD pattern of Nickel in FCC and HCP (ICDD 04-0850, ICDD 1-089-7129) phases. It is evident from the XRD that the nickelobtained from NiCl2 has a hexagonal structure and no additionalpeaks are observed which confirms the absence of other crystallineimpurities.

The microstructure of the electrodeposited HCP nickel fromNiCl2-EmimCl is demonstrated in Fig. 1b. The microstructure shows

lots of pores which might be due to the nucleation and growth char-acteristics of nickel nuclei during the electrodeposition process. Theinset EDX shows dominant peaks of nickel with small peaks of chlo-rine and oxygen. The chlorine could be from the unwashed ionicliquid.

42 A. Lahiri, Z. Tadisina / Materials Chemistry and Physics 124 (2010) 41–43

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ig. 1. (a) Comparison of XRD pattern of HCP nickel synthesized from NiCl2 andlectrodeposited HCP nickel along with EDX spectra.

Fig. 2a and b shows the magnetic measurements of HCP nickelbtained. It is evident from both the curves that the magneticoment decreases with increase in temperature which indi-

ates the ferromagnetic property of HCP nickel. Furthermore, its observed that with zero field cooling (ZFC) there is very little

oment in the sample and with field cooling (FC) the moments high. Magnetic hysteresis loops were performed on HCP nickelt room temperature and is illustrated in Fig. 2c. The hysteresis

oop of HCP nickel confirms the ferromagnetic behavior. The rem-nant magnetization, saturation magnetization and coercivity wereound to be 1.3 emu g−1, 10.4 emu g−1 and 43 Oe respectively. The

agnetic coercivity value found by Mi et al. [5] was 94.3 Oe whichs more than double obtained by our analysis. The difference in

ig. 2. (a) Temperature dependence on magnetization of pure HCP nickel during field coCP nickel obtained from NiCl2-EmimCl.

·6H2O with standard ICDD patterns of FCC and HCP nickel, (b) microstructure of

the magnetic coercivity could be attributed to the impurity con-tent such as potassium or boron, which might have been present inthe HCP nickel during the synthesis process. In comparison, the HCPnickel synthesized by electrolysis process contained only hydrogenwhich would not affect the magnetic coercivity value. The valueobtained here is in good agreement with that obtained by Han etal., wherein HCP nickel was synthesized from nickel oleate complex[7]. Thus, from magnetic measurements of HCP nickel we can con-

clude that the presence of inorganic impurities is likely to increasethe magnetic coercivity measurement.

The thermodynamic properties of HCP nickel was evaluatedusing DSC analysis shown in Fig. 3. An exothermic peak is observedat 422.6 ◦C which can be attributed to the phase transformation

oling (FC); (b) zero field cooling (ZFC); and (c) variable field magnetization data of

A. Lahiri, Z. Tadisina / Materials Chemist

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Fig. 3. DSC curve of HCP nickel electrodeposited from NiCl2-EmimCl.

f HCP nickel to FCC structure. During the phase transformation.2 wt% of hydrogen gas was released which makes HCP nickelpotential hydrogen storage material. Since the main intention

ere was to determine thermodynamic and magnetic property ofCP nickel, no studies on hydrogen evolution kinetics or the max-

mum hydrogen storage in HCP nickel was performed. However,he electrochemical approach demonstrates a novel technique forimultaneously production and storage of hydrogen in the material.

There are also two small endothermic events at around 180nd 280 ◦C in Fig. 3 which could be due to volatilization ofrganic matter from the unwashed ionic liquid. During cooling,o transformation was observed which indicates that the phaseransformation is not reversible. The XRD of the sample after the

SC analysis showed FCC nickel. The enthalpy of transformation

rom HCP to FCC nickel at 422.6 ◦C was found to be 16.72 J g−1.arturan et al. [4] found that the HCP to FCC phase transforma-ion temperature was around 350 ◦C which differs from the presentbservation. This could be due to higher concentration of impurities

[

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ry and Physics 124 (2010) 41–43 43

present in his sample as he observed evolution of gases at 250 ◦C.However, phase transformation from HCP to FCC nickel of around390 ◦C was obtained by Datta et al in a Ni95Si5 solid solution [8]. Thedifference in their result might be due to the presence of siliconcompared to the pure HCP Ni phase obtained by electrodeposi-tion route. The specific heat (Cp) of HCP nickel at 298 K was foundto be 1.425 J g−1 K−1. The constants a, b and c for the Cp expres-sion a + bT + cT2 was found to be 1.226, 7.6 × 10−4 and 9.94 × 10−7,respectively, where T is the temperature. The Cp of FCC nickel is0.444 J g−1 K−1 at 298 K. The high Cp value obtained in case of HCPnickel could be due to the presence of hydrogen incorporated in thecrystal structure.

In conclusion, we have shown a fast and new route for the pro-duction of HCP nickel. Phase transformation from HCP nickel to FCCoccurred at 422.6 ◦C. The enthalpy of phase transformation fromHCP to FCC nickel was found to be 16.72 J g−1 and the specific heatat 298 K was found to be 1.425 J g−1 K−1. HCP nickel showed ferro-magnetic properties with remanant magnetization found to be 10%of the total magnetization and coercivity to be 43 Oe.

Acknowledgements

The authors gratefully acknowledge the financial support fromNSF and facilities provided by The University of Alabama. Theauthors would also like to thank Key Polymer Corporation for per-forming the DSC measurements

References

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J.L. Vialle, M. Broyer, Philosophical Magazine A-Physics of Condensed MatterStructure Defects and Mechanical Properties 76 (1997) 493.

3] Y. Chen, D.L. Peng, D. Lin, X. Luo, Nanotechnology 18 (2007) 505703.4] G. Carturan, G. Cocco, S. Enzo, R. Ganzerla, M. Lenarda, Materials Letters 7 (1988)

47.5] Y. Mi, D. Yuan, Y. Liu, J. Zhang, Y. Xiao, Materials Chemistry and Physics 89 (2005)

359.6] A. Lahiri, R. Das, R.G. Reddy, Journal of Power Sources 195 (6) (2010) 1688.7] M. Han, Q. Liu, J. He, Y. Song, Z. Xu, J. Zhu, Advanced Materials 19 (2007) 1096.8] M.K. Datta, S.K. Pabi, B.S. Murty, Journal of Materials Research 15 (7) (2000) 1429.