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Chapter 3: Introduction
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.
