Electrical conductivity in directionally solidified lead-9 and -20 wt pct copper alloys
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I.
INTRODUCTION
P R E S E N T commercial battery grids, based on Pb-Ca and Pb-Ca-Sn alloys, have been known to experience large resistance losses, even if w a t e r loss is reduced during the charging cycle, such as in the so-called maintenance-free battery. The large currents necessary in the application require a large surface area for chemical reaction, and this accounts for the size and weight o f the battery. Therefore, new alloys w h i c h improve the electrical conductivity and mechanical strength while reducing w e i g h t are o f continuing interest. I1-4~ One suggestion is to utilize a composite consisting o f a less-dense, continuous, high-conductivity phase within a protective lead matrix. Alloy systems such as P b - C u , Pb-A1, Pb-Ni, and Pb-Zn, w h i c h exhibit immiscibility in the liquid phase, have been proposed as possible candidates. Unfortunately, the density difference between the respective phases promotes segregation during conventional casting. Rapid solidification techniques can promote a uniform distribution o f a conducting phase in the l e a d , but connectivity o f this phase is not ensured, and upon testing, only a slight increase in electrical conductivity is seen.~31 Alternatively, controlled directional solidification would promote growth o f a composite composed o f an aligned and connective phase (in this study, copper dendrites) within the lead matrix, a process w h i c h would ensure increased conductivity and strength. Consequently, considerable effort has been directed to correlating the resulting microstructure with the solidification parameters o f growth velocity, composition, and growth orientation with respect to the body force vector (gravity) acting on the alloy, tS-w] W.F. FLANAGAN, Professor of Materials Science and Engineering, B.D. LICHTER, Professor of Materials Science and Engineering, and R.N. GRUGEL, Research Associate Professor, Center for Microgravity Research and Applications, are with the Department of Materials Science and Engineering, Vanderbilt University,Nashville, T N 37235. SHINWOO KIM, formerly Doctoral Student, Department of Materials Science and Engineering, Vanderbilt University, is presently in Seoul Korea. This paper is based on work leading t o the successful completion of his Ph.D. degree at Vanderbilt University. Manuscript submitted September 3 , 1992. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL P R O C E D U R E
Two alloy compositions, Pb-20 and -9 wt pet Cu, were selected for investigation. Figure 1Ell1 shows that relatively pure copper, long appreciated f o r its high conductivity, initially solidifies upon cooling from the bulk liquid. The overall low volume fraction o f copper ensures that it will be surrounded by the lead phase and thus protected from a potential electrolyte. Alloys were prepared by melting high-purity constituents in alumina crucibles under an argon atmosphere in a pot furnace. The homogeneous liquid was sucked directly into 5 mm ID quartz tubes, a f t e rw h i c h the alloy rods were cleaned, cut into lengths
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