Modification of the sintering kinetics of palladium by a surface oxide
- PDF / 1,374,422 Bytes
- 6 Pages / 603.28 x 783.28 pts Page_size
- 40 Downloads / 172 Views
THE presence of a surface oxide layer on metallic powders has long been recognized as contributing to the kinetics of sintering. Earlier investigations attributed the unsinterability (in reality, lack of densification) of low melting metals to the presence of stable surface oxide layers) ,2 Others, however, have suggested that the presence of surface oxides contribute to enhancement of sintering. It has been suggested that metal powders whose surface oxides are rendered unstable just prior to their sintering are more "active" and hence sinterable? In fact, suggestions have been made that the oxide layers themselves are the reason for the sintering enhancement in copper, iron, and nickel: A more recent investigation has demonstrated that the presaence of the oxide layer shifts the dominance of the sintering process from bulk to surface mass transport in the case of lead: An analytical assessment of the role of surface oxide layers in the sintering of metals has been recently provided 6 and systematically applied to a large number of metals. 7 Predictions of this model were found to be consistent with available experimental observations. Three of these observations concern the sintering of palladium powders? -~~Palladium has an oxide which is stable in air up to 1062 K." Thus sintering in air above this dissociation temperature, Ta, proceeds unhindered by a surface layer. However, unlike the case of lead: palladium oxide can be made thermodynamically unstable by heating it in an ordinary vacuum (10 -8 atom) at a much lower temperature than T d. Anderson, g using submicron Pd powders (average particle size 600 ~,), demonstrated that sintering in a 10 -8 atm vacuum was accompanied by densification even at temperatures as low as 573 K. He concluded that under these conditions palladium sinters through grain boundary diffusion. Using powders with a much larger size range (60 to 525 /tin), Anderson 8 sintered Pd at 1783 K in air, and on the basis of calculated diffusion coefficients he concluded that the dominant mass transport process in sintering is Z. A. MUNIR is Associate Dean, Graduate Studies, Division of Materials Science, Department of Mechanical Engineering, University of California, Davis, CA 95616. P. K. HIGGINS is with Optical Coating Laboratories, Inc., 2789 Giffen Avenue, Santa Rosa, CA 95401. Manuscript submitted March 30, 1981. METALLURGICAL TRANSACTIONS B
volume diffusion. In a more recent investigation by Imm and Saeger, 9 the dependence of densification on various experimental conditions was studied. The dependence of densification on temperature showed a maximum in the range 1073 to 1273 K. In the most recent of the investigations on palladium, German and Ham ~~concluded that grain boundary diffusion is the rate controlling mechanism for the sintering of ~ 4 4 # m sized sponge powder in the range 1017 to 1223 K under a flowing argon atmosphere. In this paper we describe the results of investigations of the sintering of palladium powders conducted in air at temperatures higher than the dissociation tem
Data Loading...
