(533f) Metal (Ni, Cu, Ag) and Alloy (Cu/Ag, Ni/Ag) Microparticle Preparation by Spray Pyrolysis

Spray pyrolysis is used in the preparation of oxide particles, with the advantages of tunable and narrowly distributed particle sizes. However, extension to oxide free metal preparation of base metals requires high concentrations of a reducing gas, for example addition of hydrogen above the flammability limit, an impediment to spray pyrolysis for base metal powder production at industrial scales. Another challenge in spray pyrolysis of both oxides and metals is to prevent formation of hollow particles, which may be undesirable in some applications. In our approach, a co-solvent is added to the aqueous precursor solution, and it decomposes in the reactor to produce small amounts of hydrogen, less than the flammability limit in air, reducing inexpensive metal salt precursors to metallic powders. A reactor system was developed to produce several grams/hour of micron sized metallic powder, composed of a droplet generator, a tube furnace operated at temperatures up to 1200^oC and a particle filtration for collection. Nitrogen is used as the carrier gas, and particles are quenched with additional nitrogen. Various metal particles (Ag, Cu and Ni) with number average diameter on the order of one micron were made from their nitrate precursors. The carrier flow rate, co-solvent volume ratio and furnace temperature were adjusted to optimize the process. Oxide free silver particles were easily achieved at 700^oC without the addition of a co-solvent. Pure copper particle preparation required the use of a co-solvent and a furnace temperature above 900^oC. We then extended our work to multicomponent systems. Cu/Ag and Ni/Ag mixtures were investigated, and micron size bi-component particles were successfully synthesized. Different phase separation behaviors were observed. Cu/Ag formed a reasonably well-mixed structure with phase separation visible in a nanoscale. Ni/Ag mixtures exhibited phase separation on a larger scale, usually with one Ni rich phase domain and one Ag rich phase domain per particle. The results are consistent with the thermodynamics of both systems as well as kinetic considerations.