The reaction reached about 70% conversion in 8 min while most of the nickel particles remained under 100 nm in size. Nickel nanoparticles thus prepared were amorphous, but crystallized into nickel after heating at 300oC for 30 minutes. Though NaBH4 helped to improve the conversion, the resistivity of the thick film of these nanosized nickel particles was higher. Finally, these nanosized nickel particles, after phosphating the surface, showed improved resistance toward oxidation in air.
Nanoparticle synthesis has been widely investigated in recent years because of its many unique characteristics in physical and chemical properties. Among the various methods to synthesize these nanoparticles, chemical reduction method has the advantages of being more convenient, cheaper, and more reliable for mass production purpose. Compared to noble metals, such as silver [1, 2], gold [3], palladium [4] and platinum [5], nickel nanoparticles have been less frequently studied probably because they are relatively difficult to be reduced and to avoid aggregation, as well as their easy tendency toward oxidation.
In the literature, nickel nanoparticles had been synthesized from either aqueous solution [6, 7] or a polyol solvent such as ethylene glycol [8, 9, 10] using hydrazine as the reducing agent at elevated temperatures. Kapoor, et al. [11] on the other hand tried gamma radiolysis of nickel sulfate solution to produce nanosized nickel. However, our previous experience [7] showed that only low conversions (about 30%) of nickel ions could be obtained if one also wishes to keep nickel particles within the nanometer range under well-dispersed condition. Conversions, of course, could be made higher by going to more extreme reaction conditions or longer reaction times, but only at the expense of large particle size. Therefore, efforts on increasing the conversion while keeping particle size small are worthy for the development of useful process to produce nanosized nickel particles.
Nickel, like other base metals, was not easy to be reduced in the absence of nuclei. In order to facilitate the reduction reaction, noble metal ions such as palladium or platinum have been used to act as nuclei to aid the synthesis of nickel nanoparticles [9]. Yet, the use of noble metals may be unfavorable due to their high prices. On the other hand, sodium borohydride (NaBH4) is a very strong reducing agent often used in the electroless plating of nickel and can produce nickel nuclei within a short reaction period. In this work, a small amount of NaBH4 ([NaBH4 ] / [Ni2+] = 0.125) was added to ignite the nucleation reaction, i.e., to generate large quantities of nickel nuclei quickly. After this burst of nuclei, additional nickel ions could be reduced by a milder reagent, i.e. hydrazine, at a noticeably enhanced rate than in the absence of these Ni nuclei. As result, the conversion can be increased.
One of the potential applications of nickel colloids is as electrode material in multilayer ceramic capacitor (MLCC). Yet, here nickel oxidation must be reduced or prevented. A number of strategies for anti-oxidation were reported in the literature. For example, Czerwinski and Szpunar [12] immersed nickel into the CeO2 colloid and then calcined at 300oC for one hour to obtain a 14 nm thick CeO2 coating on the nickel surface in order to inhibit nickel oxidation. Haugsrud et al., [13] tried to coat a layer of SiO2 on the nickel surface by utilizing a PVA/SiO2 mixed colloid. On the other hand, Rebeyrat et al. [14] soaked iron particles in a phosphorus acid/acetone mixed solution and tried to enhance iron particles' tolerance toward oxidation. In this work, surface of nickel nanoparticles would be modified by the phosphating process similar to Rebeyrat et al. report to enhance its resistance to oxidation.
The objective of this work, in view of the above facts, is therefore to study the effect of using sodium borohydride to enhance the conversion of nanosized nickel precipitates. The characteristics of this process as well as its products will be analyzed. Since the potential application of this nanosized nickel is as an electrode material in MLCC, its electrical conductivity after sintering is hence an important characteristic to be measured. In addition, we also wish to study the resistance to oxidation of these nickel colloids after a surface phosphating process. - AZoNetwork
Posted January 17th, 2007 by harminka
