Preparation and Properties of Fine Particles of Noble Metals by Spray-pyrolysis Technique

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Fine metal particles (Ag, Pd and Ag-Pd alloy) were prepared by spray-pyrolysis technique from metal nitrate, by spraying the aqueous solutions into the flame (H_2-O_2-N_2). There were three routes for the formation of metal particles depending on the flame temperature. When the flame temperature was lower than the melting point of the metal, the metal particles were nonspherical and hollow (route 1). When the flame temperature was above the melting point but not so high, the metal particles were formed via the melt and became perfactly spherical (route 2). When the flame temperature was so high that the evaporation of metal particles took place in large part, ultrafine metal particles were produced from the metal vapor (route 3). In routes 1 and 2, the particle sizes were determined mainly by the concentration of the metal nitrate solution. In route 3, the particle sizes were determined mainly by the feed rate of the metal nitrate.The properties of the electric conduction films prepared from the Ag powders produced by routes 2 or 3 were affected markedly by the particle size and dispersiveness of the Ag particles. Ag powders consisting of fine and highly dispersive particles could give thin films having low electrical resistances (thickness: 2.0-2.5 m, sheet resistivity: 11.6-18.1m /square, specific resistivity: 2.3-4.7 .cm).