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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.

A Basic Conceptual Electrochromic Device for producing controlled infrared emissivity includes all of the components shown here, except that the lowermost infrared-absorber layer is optional, depending on the intended application. This view is partly schematic and not to scale.
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

Topics:
Electronic equipment Fabrication Hazardous materials Hazards and emergency operations Manufacturing processes Materials Optics Polymers Radiation Reaction and response times Research and development
This Brief includes a Technical Support Package (TSP).
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Electrochromic Devices for Controlling Infrared Emissivity

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    Defense Tech Briefs Magazine

    This article first appeared in the August, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 4).

    Read more articles from the archives here.

    Overview

    The "Electrochromic Adaptive Infrared Camouflage" report, authored by Eli Yablonovitch and covering the period from August 1999 to January 2005, presents significant advancements in the development of technology aimed at controlling infrared emissivity for camouflage applications. The research was conducted under the auspices of the U.S. Army Research Office and involved collaboration among several prominent universities, including the University of California, Los Angeles, and the University of Florida.

    The primary objective of the research was to create surfaces that could artificially adjust their infrared emissivity, allowing objects, such as vehicles or personnel, to blend seamlessly into their surroundings and become less detectable by infrared cameras. This capability is particularly valuable in military contexts, where stealth and concealment are critical.

    Key accomplishments highlighted in the report include the fabrication and testing of the first inverse opal infrared "white" pigment, which represents a novel approach to infrared camouflage. Additionally, the researchers developed the first electrochromic cell that operates within the crucial 8μm-12μm atmospheric transmission window, enhancing the effectiveness of the camouflage technology. They also achieved the creation of an all-polymer electrochromic cell that contains no metallic components, marking a significant step towards more versatile and lightweight camouflage solutions.

    One of the standout achievements was the demonstration of 80% contrast in an infrared electrochromic cell, which can be electrically tuned to adapt to different environments. Furthermore, the research team successfully showcased electrochromic switching at a speed of 67 milliseconds, effectively achieving a video rate of response, which is essential for real-time adaptability in dynamic environments.

    The report emphasizes the importance of electrochromics, a technology that allows materials to change color through electrochemical processes, thus enabling controlled reduction of infrared emissivity. This reduction is crucial since objects of interest, such as military assets, are typically warmer than their surroundings, and the ability to lower their infrared signature can enhance stealth capabilities.

    Overall, the research outlined in this report represents a significant step toward practical applications of adaptive infrared camouflage, with the potential to revolutionize how military personnel and equipment can operate undetected in various environments. The findings contribute to the broader field of electrochromic materials and their applications in stealth technology.

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