The present invention relates to a method for fabricating metal-containing polymer fibers or sheets having high electrical conductivity, and more particularly, to a method which utilizes an in-situ chemical, electrochemical, or thermal reduction of metal salts infiltrated into such polymer fiber or sheets.
Electrical wires are made of highly conductive metals such as copper and silver. These metals afford the highest conductivity (approximately 6.times.10.sup.5 S cm.sup.-1 at room temperature) for power, signal and EMI shielding applications. However, their high density, e.g. 10.5 g cm.sup.-3 for silver and 8.96 g cm.sup.-3 for copper, is undesirable for applications in space and aerospace vehicles where weight savings are important. Attempts have been made to replace the 22 gauge copper wire currently used in aerospace vehicles with a smaller 26 gauge or 30 gauge wire, but the thinner wires do not have the necessary mechanical strength and durability and therefore cannot be used. During the past twenty years, considerable research effort has been spent on developing conducting polymers for optoelectronic applications. Conjugated polymers such as polyacetylene, polythiophene and polypyrrole have been introduced with electrical conductivity up to 10.sup.5 S cm.sup.-1 by chemical and electrochemical doping. However, these highly conductive doped conjugated polymers are environmentally unstable and therefore have few practical applications.
In recent years, a new class of conductive fibers (Aracon.RTM. available from DuPont) has been developed by cladding Kevlar.RTM. (DuPont) fibers with highly conductive metals such as nickel, copper and silver. Because the interior Kevlar.RTM. fiber has a tensile strength of 425 Ksi, Young's modulus of 12-25 Msi, density of 1.4 g cm.sup.-3 and diameter of 15 .mu.m, the Aracon.RTM. fibers offer benefits over copper wires in flexibility, weight savings (60% in braid and 26% in cable), strength and durability, tailored electrical/mechanical properties, and more uniform EMI shielding. However, the Aracon.RTM. fibers have several disadvantages resulting from the fact that the metals are coated only on the surface of the Kevlar.RTM. fiber. For example, the fibers may suffer from potential fatigue or delamination in thermal or mechanical cycles. Second, the fibers have a poor EMI shielding below 25 MHz because the interior Kevlar.RTM. fiber is an insulator.
Accordingly, there is still a need in the art for a method which allows highly conductive metals to be incorporated into a polymer matrix such as a fiber or sheet to form a lightweight material containing a low volume fraction of metal but which has high metallic conductivity.