Microwave-assisted combustion synthesis of Ni powder using urea
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A novel microwave-assisted combustion method was used to prepare Ni powder. The method involves the combustion reaction of nickel nitrate and urea as a fuel in the microwave field. The initiation of the exothermic peak of the combustion reaction was found to vary as a function of urea content. The microwave-prepared Ni powder was characterized using x-ray diffraction (XRD), scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, thermogravimetric (TG) analysis, differential thermal analysis (DTA), and magnetic measurement. The XRD pattern revealed that the Ni powder crystallizes with the cubic phase when the molar ratio of fuel to nitrate is varied between 5:1 and 6:1. Above or below that molar ratio, NiO phase coexists as an impurity along with the Ni phase. The magnetization value of Ni measured at room temperature is 53.5 Am2/kg, which is close to the value observed for commercial Ni powder (55.0 Am2/kg). The mechanism for the formation of the Ni and NiO phase is discussed based on the infrared, TG, and DTA data. The method shows that highly pure Ni powder can be prepared using urea as a fuel and microwaves as a source of energy via the solution combustion method.
I. INTRODUCTION
In the last few decades, different methods have been used to obtain Ni powder for various applications as catalysts, in magnetic devices, and for powder metallurgical components. The production of Ni powder by chemical routes requires special attention to counter the high reactivity of Ni metal whereas the preparation of NiO is simple. Therefore, a judicious choice of method is required for the preparation of Ni. Ni powder is most often prepared by reduction of nickel salt using sodium borohydride1 and ethylene glycol2,3 as reducing agents. During synthesis, Ni powder is contaminated with the solvent and byproducts of the reducing agents. The process also requires careful control of atmosphere and water content. For the above reasons, the reactions are typically carried out in Ar/H2 atmosphere and in a nonaqueous medium such as dimethylglyoxime, tetrahydrofuran, etc., to obtain pure Ni-metal powder.4 Moreover, only the amorphous powder is formed in the final product, which requires further heat treatment to obtain crystalline powder. The thermolysis of organometallic precursor is also reported to yield highly pure Ni-metal powder.5 Another important process for the Ni-powder preparation is the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0211 1664
J. Mater. Res., Vol. 21, No. 7, Jul 2006
spray pyrolysis of Ni-organic complex salts in reducing atmosphere.6 These processes require costly equipment or precursors. Thus, development of a new method is desired for the preparation of good-quality Ni-powder by an economical and simple chemical route with a potential for large-scale production capability. Several researchers have reported using the solution combustion method, which is economical and simple, to synthesize simple and mixed metal oxides
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