Electrical conductivity of discontinuous filament-reinforced unidirectional composites
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The electrical conductivity of discontinuous filament–reinforced unidirectional composites was investigated. The discontinuous reinforcement used was copper filament, and the matrix materials included solder and epoxy. It was found that the composite conductivity increases with filament length and eventually becomes a constant. Such behavior is described by introducing an interface impedance and the principle of rule of mixture. It was found both experimentally and theoretically that there exists a critical filament length that must be exceeded so the composites will exhibit conductivity as composites having continuous reinforcing filaments.
I. INTRODUCTION
Although composite materials evolved as structural materials, other applications have been identified. Among them, one is the use of high thermal conductivity composites for heat dissipation in electronics, and another is electrically conductive composites for electromagnetic interference (EMI) shielding and antistatic purpose.1–5 In both applications, continuous or discontinuous filamentous reinforcement is used. The filaments are randomly oriented or aligned in a particular direction.6–8 According to the percolation theory, the volume fraction of the reinforcement has to be greater than a critical value so that electrical or thermal conduction can occur.9 Percolation theory is basically a geometrical theory that describes the structure of random particles or filaments in a epoxy matrix as a function of their volume fraction. It depicts that it is only when the volume fraction of the particle or filament exceeds a critical value that the particle or filament can come into contact and form clusters. As a result, electrical conduction can occur due to the connection of the clusters. Under this circumstance, the composite electrical conduction has been related, theoretically and experimentally, to the filament’s aspect ratio (diameter to length) and matrix properties.6,10,11 In this study, we investigated the electrical conductivity of discontinuous filament–reinforced unidirectional composites and related the composite conductivity only to the filament length by introducing an interface impedance. II. EXPERIMENTAL
Two types of unidirectional composites were prepared. One used epoxy resin as the matrix, and the other used solder (PbSn ⳱ 60/40). All the composites have a single 940
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 4, Apr 2000 Downloaded: 18 Mar 2015
copper filament and a fiber volume fraction of 8%. There are two types of filamentous arrangements in the matrix, as shown in Fig. 1. One is merely a continuous filament, leading to continuous filament reinforced composites [Fig. 1(a)]. The other type consists of segments, with various lengths, of copper filaments butt-joined by either solder or epoxy, depending on the matrix used, so that all the segments are well aligned in the matrix [Fig. 1(b)]. Such a filamentous arrangement gives discontinuous filament reinforced composites. The average distance between two adjacent filamentous segmen
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