Electrochromic Devices for Controlling Infrared Emissivity

Advanced devices of this type would be incorporated into adaptive infrared camouflage.

Electrochromic devices that exhibited adjustable infrared emissivity were developed in a research program that was part of a larger effort to develop adaptive infrared camouflage. Inasmuch as objects that one seeks to conceal by use of camouflage are generally hotter than their surroundings, the basic idea of adaptive infrared camouflage is to reduce the infrared emissivity of an object of interest by a controllable amount so that, as viewed via an infrared camera, the object blends into the background.

Electrochromic materials are used in self-dimming rear-view mirrors in automobiles. An electrochromic material changes in color, reflectivity, and other optical properties by undergoing an electronic transition of some form. An electrochromic material of the type considered in this research program is a special polymeric material, the optical properties of which are changed by electrochemical doping and undoping.

A basic conceptual electrochromic device of the type considered in this research is an electrochemical cell (see figure) that includes a electrochromic polymer, the back surface of which is coated with a pigment that is an infrared analog of common white visible pigments. The cell also includes electrodes for applying the bias potential to effect controlled doping or undoping. When the polymer is undoped, it transmits radiation that is reflected by the “white” pigment backing, resulting in low infrared emissivity. When the polymer is sufficiently doped, it strongly absorbs infrared (but not visible) light, resulting in high infrared emissivity.

Guided by the device concept described in the preceding paragraph, the research program included synthesis of suitable electrochromic polymers, fabrication of prototypes of infrared “white” pigments, and fabrication of electrochromic devices. The major accomplishments of this research program were the following:

• Fabrication and testing of a prototype infrared “white” pigment in the form of a photonic crystal comprising ordered layers of 3.9-μm-diameter silica microspheres;
• Fabrication of an electrochromic device that operated in the important atmospheric- transmission band of wavelength between 8 and 12 μm;
• Fabrication of an electrochromic cell that contained no metallic components;
• Demonstration of an infrared electrochromic cell that could be electrically adjusted to obtain an emissivity contrast of 80 percent; and
• Demonstration of electrochromic switching in a response time of 67 ms, which is comparable to a video frame time.

This work was done by Eli Yablonovitch, Fred Wudl, and Bruce Dunn of the University of California, Los Angeles; John R. Reynolds and David B. Tanner of the University of Florida; and Ray H. Baughman and Anvar A. Zakhidov of the University of Texas, Dallas for the U.S. Army Research Office.
ARL-0041