Technology Advances
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TECHNOLOGY ADVANCES
Infrared Transparency and Copperlike Conductivity Combined in Polymer Coatings Infrared-transparent polymer coatings that have the conductivity of copper have been developed by Fractal Systems Inc. The development of durable, mid-infraredtransparent, electrically conductive coatings is of importance for applications such as solid-state lighting, coatings for optical devices, and electromagnetic shielding, as well as the windows and domes of forward-looking IR sensors, IR search and track sensors, IR transmitting seekers, and space-based IR systems. The mid-IR spectral region of interest is 1–1.5 µm, 3–5 µm, and 8–12 µm, with a desired transparency of >80%. No materials have satisfied both the optical and the electrical requirements for these applications. Existing conductive, visibly transparent materials such as indium tin oxide (ITO) are not sufficiently transparent in the IR region. They break down in the IR above 1 µm at the thickness needed for providing the high conductivity for electromagnetic interference (EMI) shielding. A metallic mesh degrades the optical signal passing through the window, particularly with respect to off-axis optical characteristics of the detector. The
Figure 1. Impedance (Z) plots of various high-conductivity polymers with copper as a reference (second curve from the top). Negative impedance means metallic conductivity; lower impedance, more negative on the log scale, means higher conductivity. Other curves are polyaniline (top), polypyrrole perpendicular to the orientation (two curves immediately below the Cu reference), and polypyrrole parallel to the orientation direction (bottom).
Figure 2. Fourier transform infrared spectrum of a polypyrrole film.
mesh needs to filter out the electromagnetic energy and also should pass through the appropriate IR energy without distortion. Doped semiconductors tend to oxidize with time, resulting in degradation of IR characteristics (i.e., reduced transparency and increased reflection) and are difficult to manufacture. The Fractal Systems coatings are based on the use of intrinsically conductive polymers (ICPs) processed using a modified electrophoretic technique. High copperlike conductivity and high visible and IR transparency have been achieved with some of these polymers through both processing and bandgap tuning. The polymers are stable for several weeks; steps have not yet been taken to assess their long-term stability. The ICP thin-film coatings of submicron thickness are pinholefree and very homogeneous. Because of their high conductivity, their EMI shielding effectiveness is sufficient to provide protection. The coatings are prepared by electrochemical polymerization of the monomer(s) or by processing of already synthesized polymers. They are applied using traditional chemical processes at atmospheric conditions. The result is an increase in conductivity of four orders of magnitude (shown in Figure 1) relative to that of conventionally synthesized ICPs by chemical or electrochemical means, which is typically in the
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