Effect of calcium modification on the microstructure and oxidation property of submicron spherical palladium powders
- PDF / 256,195 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 197 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Effect of calcium modification on the microstructure and oxidation property of submicron spherical palladium powders Shenglei Che, Osamu Sakurai, Hiroshi Funakubo, Kazuo Shinozaki, and Nobuyasu Mizutani Department of Inorganic Materials, Faculty of Engineering, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152, Japan (Received 31 May 1996; accepted 29 July 1996)
Ca-modified spherical palladium particles were prepared from the mixed solution of Pd(NO3 )2 and Ca(NO3 )2 by ultrasonic spray pyrolysis. Pure palladium powder and that modified with less than 55 ppm Ca were composed of single crystal particles. However, Ca addition of more than 500 ppm resulted in polycrystalline particles. Crystallite size of the particles decreased with the increase of Ca addition and changed dramatically at the addition of some hundred ppm. Ca additive did not form solid solution with palladium but formed CaPd3 O4 on the surface and grain boundary of the particles. 50 ppm–1% of Ca addition significantly reduced the oxidation of palladium powder. More addition of Ca resulted in excess oxidation due to the reaction between palladium and calcium oxide.
I. INTRODUCTION
Palladium powder used for the internal electrodes of multilayer ceramic capacitors (MLC) oxidizes in air during heating which causes a volume change of about 68%. This phenomenon is proved to be an important reason for delamination between internal electrodes and ceramic dielectric layers during the cofiring process of MLC.1 It is evident that oxidation of palladium powder increases with decreasing crystallite size2 and increasing specific surface area.3 Most commercial palladium powders are produced by precipitating palladium from solution with a reductant.4 The crystallite sizes are generally 20 –30 nm and thus the powders usually oxidize completely on heating. Some attempts have been made recent years in synthesizing well-crystallized palladium powder with high density and large crystallite size by spray pyrolysis.5–7 Pluym et al. prepared palladium powder by spray pyrolysis,5 but the powder was reported to oxidize to the extent of 100% on heating.8 This is considered to result from the low pyrolysis temperature and short soaking time applied. In a previous work we reduced the extent of oxidation of palladium powder to about 50% by single crystallizing it in aerosol state immediately after particle formation with a 3staged spray pyrolysis system.7 Single crystal palladium powder prepared in this way was proved to cause little delamination in application to MLC manufacturing. All metals except gold oxidize in air, but the extent of oxidation is generally determined by the diffusion rate of oxygen or metal ions through the oxide scale. Thus, the reason why palladium is liable to oxidize is because the oxidation product formed on the surface of palladium particles does not retard further oxidation. For bulk metals, a heterogeneous coating layer often protects the metal from oxidation. 392
J. Mater. Res., Vol
Data Loading...
