Preparation of silver particles by spray pyrolysis of silver-diammine complex solutions
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Preparation of silver particles by spray pyrolysis of silver-diammine complex solutions N. Kiedaa) and G. L. Messing Particulate Materials Center and Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 9 May 1997; accepted 10 July 1997)
A novel precursor solution system, containing NH3 as a complexing agent, was used for the production of Ag powders by spray pyrolysis. Solutions of Ag2 CO3 , Ag2 O, and AgNO3 with NH4 HCO3 were used in this study. Ag powders were obtained at unexpectedly low temperatures, i.e., 400 ±C or less. The Ag powders with a shell-like morphology were obtained from Ag2 CO3 and Ag2 O solutions, whereas dense Ag particles of about 1 mm diameter were obtained from AgNO3 –NH4 HCO3 solution. These morphologies are explained in terms of NH3 release during spray pyrolysis.
I. INTRODUCTION
The production of ceramics and metal powders by spray pyrolysis (SP) has been extensively studied in the past twenty years.1,2 SP is classified as a solution process, in which micrometer-sized droplets of precursor solution are heated to form particles. The process involves several stages, i.e., evaporation of solvents and condensation of solutes, decomposition and/or reactions of the precipitates, and sintering of the particles. This means that most processes required for producing powders via solution routes are performed continuously within each droplet during SP. In theory, powders with unique characteristics, e.g., dense and spherical, narrow size distribution, agglomerate free and high purity, can be produced by SP. In addition, the intentional preparation of particles with a unique morphology (e.g., hollow spheres or short fibers) or composite particles has been reported.3–5 The primary ingredient for successful powder production by SP is undoubtedly the choice of precursor, because the condensation (i.e., gelation or precipitation) and decomposition characteristics strongly influence the properties of the powder produced. The number of available precursors for a material is usually limited by the solubility requirement. Most nitrates and chlorides have relatively high solubility in water; thus they have been frequently used as precursors of SP. The general problems of these precursors are melting before decomposition and the relatively high decomposition temperature of chlorides. Alcoholic solutions of metal alkoxides are also common precursors for oxide powders, but condensation a)
Address all correspondence to this author. Present address: Shonan Institute of Technology, 1-1-25 Tsujido-nishikaigan, Fujisawa, Kanagawa 251, Japan.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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occurs through the hydrolysis and the polymerization of the alkoxide. In this case, residual carbon is sometimes difficult to remove completely and is undesirable for most powder applications. Addition of a chelating agent to form complexes has been reported to avoid the
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