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Chapter 3: Conducting Polymer / High-Temperature Superconductor Interface Properties

There has been a significant volume of research related to the formation and characterization of organic conducting polymers.1-37 Moreover, the formation of electrically conducting polymer / high-temperature superconductor interfaces has been the subject of several papers.38-49 In this chapter, background information in section 3.1 is provided on the topic of the polymerization of conducting polymers. The remainder of the chapter presents new research on the growth of polypyrrole on high-temperature superconducting substrates.

There is considerable interest in forming hybrid conducting polymer / superconductor composites. These polymers may help increase the processability of the ceramic superconductors and well as provide environmental protection for these highly reactive materials. As new electronic materials utilize high-temperature superconductors, electrical contacts must be formed. Since semiconductor systems like Si and GaAs are incompatible with the cuprate superconductors, conducting polymers may be employed. Important electronic interactions such as the proximity effect are also of interest. Research into the induction of superconductivity in organic polymeric conductors is also a promising area of study due to the tunable nature of their electrical properties.

For these polymer / superconductor systems to be used, you must first develop a way to assemble the components while maintaining good interfacial characteristics. There are several goals involved in the formation of good interfaces between conducting polymers and high-temperature superconductors. First, a uniform coating of the polymer must be achieved. The polymer must have the desired properties distributed uniformly throughout the polymer. This includes the electrical properties. Since doping of conducting polymers typically involves diffusion of anions or cations throughout the polymer matrix, uniform doping can be difficult to achieve. Diffusion usually takes place at the exterior surface of the polymer that is in contact with an electrolytic solution to supply ions for the doping process. Diffusion of the dopant species must continue completely through the bulk region of the polymer and into the polymer / superconductor interface. Although solid-state diffusion at room temperature of ionic species through polymers is faster than similar diffusion through ionic crystals, the kinetics can hinder the formation of uniform electrical properties throughout the polymer.

Another issue that must be considered in a conducting polymer / high-temperature superconductor interface is the contact between the polymer and superconducting material. With a wide array of deposition techniques possible, the degree of contact can be varied. Although this has yet to be achieved because the polymers are ammorphous, the ideal contact would be true epitaxial growth of the polymer on the superconducting material. Unfortunately, epitaxial growth50 occurs only in ordered crystalline systems where the substrate layer is built upon so that the growth material is built up in a crystalline fashion. The ideal interface would have the contact area between the two materials at ~100%. In the other extreme, there could be a few contact points that exist at the interface. Polymer growth is likely to nucleate only in localized areas as seen in Illustration 3.1. In this case, these contact points anchor the materials together. Under these circumstances, the electrical and thermal conduction will be regulated through these contact points. The contact area is likely to be made up of an amorphous layer with random interfacial contacts.

The last major factor influencing this interface is how pristine the interface is between these two materials. Since most high-temperature superconductors are highly susceptible to chemical damage from water, CO2, and acids,44, 51-57 the formation of a corrosion layer on the superconductor could drastically affect the interfacial properties. For example, if the superconductor is chemically damaged resulting in the formation of an insulating layer, the electrical conduction across the interface would be hindered, even with a pristine conducting polymer. Another possibility is that polymer deposition may require a catalytic layer deposited on the superconductor substrate to initiate polymerization of monomeric species. The catalyst would then be present at the interface.

Steps must be taken to control the interfacial properties of conducting polymer / high-temperature superconductor assemblies. Fundamental studies of electron transfer between the two materials necessitate control of the interfacial properties. If future devices are to employ conducting polymer / high-temperature superconductor assemblies, manipulation of the interface will play a key role in their success.

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Updated on: April 15, 2010 8:26 PM