Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-01-19
2001-05-08
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S439000, C524S440000, C524S441000, C524S609000, C524S611000
Reexamination Certificate
active
06228922
ABSTRACT:
BACKGROUND OF THE INVENTION
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 6×10
5
S cm
−1
at room temperature) for power, signal and EMI shielding applications. However, their high density, e.g. 10.5 g cm
−3
for silver and 8.96 g cm
−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
5
S cm
−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® available from DuPont) has been developed by cladding Kevlar® (DuPont) fibers with highly conductive metals such as nickel, copper and silver. Because the interior Kevlar® fiber has a tensile strength of 425 Ksi, Young's modulus of 12-25 Msi, density of 1.4 g cm
−3
and diameter of 15 &mgr;m, the Aracon® 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® fibers have several disadvantages resulting from the fact that the metals are coated only on the surface of the Kevlar® 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® 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.
SUMMARY OF THE INVENTION
The present invention meets those needs by providing a method for fabricating highly conductive metal-containing polymers such as fibers and sheets in which metal precursors are infiltrated into the polymer and then reduced by chemical, electrochemical, or thermal means.
According to one aspect of the present invention, a method for forming a highly conductive metal-containing polymer is provided comprising the steps of providing a polymer, immersing the polymer in a solution containing a metal precursor, and then converting the metal precursor to metal such that conductive metal is entrapped in the polymer.
The metal precursor is preferably selected from the group consisting of organic or inorganic salts of copper, silver, aluminum, gold, iron and nickel. In a preferred embodiment, the metal precursor solution comprises an aqueous silver nitrate solution.
The polymer is preferably selected from the group consisting of poly(p-phenylene benzobisthiazole), poly (p-phenylene benzobisoxazole), poly(p-phenylene benzobisimidazole), poly(imidazoisoquinolines), poly(p-phenylene terephthalamide) and nylon. Preferably, the polymer comprises poly(p-phenylene benzobisthiazole).
The polymer may be provided in the form of a fiber or sheet. Where the polymer is provided in the form of a fiber, the fiber is preferably infiltrated with metal after being produced through an extrusion/coagulation process, i.e., the fiber is coagulated in a solution such as phosphoric acid and is wet prior to being immersed in the metal precursor solution. Alternatively, a fiber in dry form may be infiltrated by swelling the fiber in a solvent solution prior to being immersed in the metal precursor solution.
In a preferred embodiment of the invention, the method utilizes a chemical reduction in which the converting step comprises immersing the polymer in an aqueous solution containing a reducing agent. In this embodiment, the reducing agent preferably comprises sodium borohydrate.
In an alternative embodiment of the invention, the method utilizes a thermal reduction in which the converting step comprises heating the polymer at a temperature of about 250° C.
In yet another alternative embodiment of the invention, the method utilizes an electrochemical reduction in which the converting step includes providing iron in a concentrated HCl solution (anode), where the polymer immersed in the metal precursor solution comprises a cathode.
After the metal is infiltrated into the polymer, the polymer is preferably further heat treated at a temperature of between about 300° C. to 500° C. to enhance conductivity. The heat treated polymer preferably has a room temperature conductivity of up to about 10
4
to about 10
5
S cm
−1
.
In addition to exhibiting high electrical conductivity, the resulting metal-containing polymers also exhibit good thermal and thermooxidative stability, mechanical flexibility, durability and strength, are lightweight. The polymers may be used in applications such as signal and power transfer and EMI shielding as well as satellite antennas and microelectronics applications. For example, the replacement of metal signal wires in existing aircraft and satellites with conductive metal-containing polymer fibers will result in a substantial weight savings, leading to enhanced system performance. The method of the present invention may also be used to fabricate metallized polymer sheets with high electrical conductivity for satellite charge dissipation and semiconductor on-chip applications or with high surface reflectivity for inflatable space membrane antenna and collector applications.
Accordingly, it is a feature of the present invention to provide a method for producing metal-containing polymers having high electrical conductivity. This, and other features and advantages of the present invention will become apparent from the following detailed description and the appended claims.
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Edward W. Tokarsky et al, “Metal Clad Aramid Fibers for Aerospace Wire and Cable” Jul. 18 & 19, 1995.
Lee Jar-Wha
Vaia Richard A.
Wang Chyi-Shan
Cain Edward J.
Killworth, Gottman Hagan & Schaeff, L.L.P.
Lee K Wyrozebski
The University of Dayton
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