The roles of ammonia and ammonium bicarbonate in the preparation of nickel particles from nickel chloride
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The roles of NH3 ⭈ H2O and NH4HCO3 in the preparation of Ni particles from NiCl2 ⭈ 6H2O aqueous solution by ultrasonic spray pyrolysis were investigated. The results showed that both ammonia and ammonium bicarbonate had a remarkable influence on the solution chemistry and the resulting particles, and could significantly modify the reaction pathway. After the addition of these additives to the precursor solution, intermediate NiO was formed initially, followed by reduction to metallic Ni in the presence of a reductive gas. H2 is a powerful reducing agent; however, metallic Ni could also be obtained in the absence of H2 in the carrier gas. In the latter case, it was shown that NH3 was primarily responsible for Ni formation. A description of the mechanisms and processes of Ni formation during spray pyrolysis is proposed.
I. INTRODUCTION
Submicron metallic nickel powders are in demand, for multilayer ceramics and batteries, as electrode materials.1,2 Nickel plays an important role because it is lower in cost, without losing good performance, when compared with noble metals such as Pt, Pd, and Ag.3,4 Spray pyrolysis is an important method used to prepare such submicron metal, oxide, and nonoxide particles. This is because spherical, solid, and nonagglomerated particles with multiple components can be readily produced and the production apparatus is relatively simple.5–8 In spray pyrolysis a solution is atomized into an aerosol reactor where the aerosol droplets undergo drying, droplet shrinkage, solute precipitation, thermolysis, and sintering to form final particles.5 To prepare metals by spray pyrolysis, a solution of metal salt(s), such as nitrate and chloride, is sprayed into an aerosol reactor in a reductive atmosphere.9–11 Using such a method, metal or alloy submicron powders such as Pd, Cu, and Ag have been prepared with higher quality than those prepared by traditional solid-state and wet-chemical methods. The preparation of Ni particles by spray pyrolysis has been studied in recent years.12 Porous and irregularshaped Ni particles were formed using Ni(NO3)2 ⭈ 6H2O precursor at temperatures below 1000 °C.12–14 The particles tend to become denser at higher temperatures. Stopic and co-workers prepared solid Ni particles using NiCl2 ⭈ 6H2O precursor.15–17 However, metallic nickel was obtained at only 900 °C or above, which is higher than in the case of the Ni(NO3)2 precursor. All these
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Address all correspondence to this author. J. Mater. Res., Vol. 15, No. 10, Oct 2000
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investigations used H2 as the reductive gas, and no additives were employed. The available experimental data was limited to product characterization. Formation mechanisms have not yet been intensively studied; thus some of the chemical and physical processes remain poorly understood. Our research was motivated by the need to provide an interpretation to the processes and the mechanism of Ni formation by spray pyrolysis. It is well known that Ni2+ can form nickel ammine complex ions {[Ni(N
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