A novel process for fabricating electrical contact SnO 2 /Ag composites by reciprocating extrusion
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I. INTRODUCTION
METAL matrix composites reinforced with ceramic particles have great potential in electrical contact application because of their superior combination of strength at high elevated temperature, good thermal stability, and high electrical and thermal conductivity.[1,2,3] Examples include Al2O3/Cu, SiC/Cu, CdO/Ag, SnO2/Ag, ZnO/Ag, etc.[1,2,3] Among them, CdO/Ag composites are the most popular contact materials because of their superior electrical conductivity and arc erosion resistance. However, CdO is now being gradually replaced by other ceramic particles due to its toxicity as CdO decomposes into poisonous cadmium under arc formation.[1,2] Therefore, SnO2/Ag composites are considered as the most promising candidates for electrical contacts.[4] Traditional processing methods for oxide-reinforced electrical contact composites include internal oxidation and powder metallurgy. In the internal oxidation process, the matrix metal and oxide metal are prealloyed and then oxidized in a high oxygen atmospheric furnace at high temperature, while the powder metallurgy process uses matrix metal and oxide powders to compose the composites by pressing and sintering.[2] Since the diffusion process is necessary for oxidation, the internal oxidation method has a disadvantage on the uniformity of oxide dispersion. On the other hand, the powder metallurgy method also has a property issue of densification and uniformity. To obtain better material performance, the composite billets are usually wrought to improve the uniformity of oxide dispersion and to enhance the densification of the specimens.[1,5,6] However, these excess YU-LIANG CHEN and SHIH-WEI LEE, Postdoctoral Candidates, and JIEN-WEI YEH, Professor, are with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China. Contact e-mail: [email protected] CHIH-FU YANG, Professor, is with the Department of Materials Engineering, Tatung University, Taipei, Taiwan, Republic of China. SHUI-SHU HUNG, Chairman, is with the Catcher Technology Company, Tainan, Taiwan, R.O.C. Manuscript submitted July 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
processes would bring a raise of both processing time and prime cost. In this study, a novel reciprocating extrusion method combined with simple powder compaction was performed to produce SnO2/Ag electrical contact composites with improved properties for simplifying the process and reducing the cost. This method has been demonstrated to be effective in obtaining fine and fully-densified microstructures with improved mechanical properties for various kinds of alloys and Al matrix composites.[7–13] II. EXPERIMENTAL PROCEDURE Silver powder with a purity of 99.99 pct and SnO2 powder of reagent grade were used for the preparation of SnO2/Ag composites. Figures 1(a) and (b) show the surface morphologies of the silver and SnO2 powders, respectively. The silver particles were several tens of microns, but in skeleton form, they were composed of many small silver grains o
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