Electrical Conductivity of Open-cell Metal Foams
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Cellular metal foams are of interest because of the ability to tailor their mechanical, thermal, acoustic, and electrical properties by varying the relative density and cell morphology. Here, a tetrakaidecahedral unit-cell approach is used to represent an open-cell aluminum foam and a simplified electrical resistor network derived to model low frequency current flow through the foam. The analysis indicates that for the range of relative densities studied (4–12%), the conductivity of tetrakaidecahedral foams has a linear dependence upon relative density. The distribution of metal in the cell ligaments was found to significantly affect the conductivity. Increasing the fraction of metal at the ends of the ligaments resulted in a decrease in electrical conductivity at a fixed relative density. Low frequency electrical conductivity measurements of an open-cell aluminum foam (ERG Duocel) confirmed the linear dependence upon density, but the slope was smaller than that predicted by the unit-cell model. The difference between the model and experiment was found to be the result of the presence of a distribution of cell sizes and types in real samples. This effect is due to the varying number of ligaments, ligament lengths, and the cross-sectional areas available for current conduction across the cellular structure.
I. INTRODUCTION
The ability to create open cellular metal foams with properties that are dependent on the relative density and cell morphology has led to interest in their use as multifunctional, light-weight, impact and energy/ absorbing structures with high heat transfer coefficients.1 Other uses as load-supporting electrochemical storage structures also appear feasible. Recent studies of the mechanical behavior of metal foams has resulted in a significantly improved understanding of the performance of closed- and open-cell foams.2 However, the electrical properties of metal foams and their dependence upon the foam’s relative density and cell morphology are less well understood. Present-day metal foams are predominantly produced by one of several liquid-phase (melt foaming) or solidphase (powder metallurgical) methods.3,4 Both open- and closed-cell metal foams with a wide range of relative densities and cell morphologies can be made. Cellular metal structures may be characterized by the porosity (relative density), the average pore size, pore shape, the pore orientation, and the degree of pore interconnectivity (open-cell versus closed-cell foams). Here we used a four-point probe method to measure the low-frequency electrical conductivity of ERG Duocel (ERG Materials and Aerospace Corporation, Oakland, CA) open-cell J. Mater. Res., Vol. 17, No. 3, Mar 2002
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foams of varying relative density and cell size. A unit cell model was then used to investigate the dependence of the conductivity upon these parameters. II. MATERIALS
A set of open cellular aluminum (6101) samples was obtained from ERG Inc. The electrical properties of an aluminum cellular metal sam
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