Electroless polyol deposition of FeNi-based powders and films
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Iron-nickel-based powders and thin films were synthesized by an electroless polyol method. The growth process as a function of deposition time was studied. The Fe concentration of deposited films and precipitated powders were similar and independent of deposition time. The metalorganic intermediates were present in powders and their amount decreased with time. The morphological investigations suggested that 60 min was the optimum deposition time to deposit dense films with particles of narrow size distribution. In addition, the films deposited at 60 min also possessed the highest saturation magnetization due to a more ordered atomic environment of nickel. The as-deposited Fe was apparently oxidized in the films.
I. INTRODUCTION
A nonaqueous electroless process known as the polyol method can be used to produce metal films. Powder precipitation occurs simultaneously in the solution during film deposition. This method, mitigating the problem of hydrolysis in commonly encountered aqueous electroless deposition, involves the reduction of metal precursors in refluxing ethylene glycol (EG). Powders of noble metals such as silver,1 palladium,2 copper,3 and even less easily reducible metals such as nickel4 and cobalt-copper alloy5 have been precipitated in the alcohol media. Thin films such as Ni,6 Ni–Cu complex,7 NiCo,8,9 and MoS210 have also been deposited by the polyol method. The main feature of this process is that the reduction reaction proceeds via the solution rather than in the solid state. A duplicative oxidation of acetaldehyde previously produced by dehydration of EG was found to reduce metal precursors.11 The overall reaction can be schematized as Eq. (1): −2H2O
2CH2OH−CH2OH —→ 2CH3CHO M共II兲
→ CH3−C−C−CH3 + H2O + M 冏冏 冏冏 O O
.
(1)
FeNi thin films find uses in magnetic recording heads due to their low coercivity, high permeability, and nearzero magnetostriction.12 Although the polyol process can be easily used for Ni deposition, it is not suitable for the synthesis of Fe. Since Fe has a more negative standard
a)
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J. Mater. Res., Vol. 18, No. 1, Jan 2003 Downloaded: 07 Apr 2015
reduction potential in EG than Ni [E298 (Fe2+ /Fe) ⳱ −1.240 V, E2980 (Ni2+ /Ni) ⳱ −0.926 V with respect to AgCl/Ag reference electrode],13 EG is too weak to reduce Fe2+. Therefore, it is difficult to simultaneously deposit Fe and Ni by the same reduction mechanism in EG. An alternative way to synthesize Fe, namely, the disproportionation of Fe(II), as shown in Eq. (2), was used to prepare metallic Fe in polyols with the addition of NaOH:14 4Fe共OH兲2 → Fe + Fe3O4 + 4H2O
.
(2)
To increase the yield of Fe, a large excess of NaOH with respect to Fe(II) is necessary. However, due to the low yield of Fe disproportionation, the Fe concentration in synthesized FeNi particles was still below 30 at.%. The Fe concentration could not be further increased with increasing concentration of Fe precursor. The reason for Fe deficiency has
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