N112-145 TITLE: Infrared-Transparent, Electrically Conductive Coating

TECHNOLOGY AREAS: Air Platform, Materials/Processes, Weapons

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OBJECTIVE: Demonstrate an infrared-transparent, electrically conductive coating for electromagnetic shielding of sensor windows and domes. Transmission should be at least 90% in the 3-5 micron wavelength region. Sheet resistance should be less than 10 ohms per square. The coating must be chemically stable in the atmosphere and in sunlight. It is desirable that the coating be able to operate at temperatures of at least 600C and be resistant to erosion by rain and solid particles in the atmosphere.

DESCRIPTION: Electromagnetic shielding of electro-optic sensor electronics in military systems is currently provided by electrically conductive metal grids applied to the sensor window or dome. Grids provide excellent electrical shielding, but compromise the optical system through geometric blockage, diffraction, and undesired reflection of light. In addition, grids are difficult to deposit on curved shapes. Grids are poorly resistant to erosion damage by rain and particle impact on the external surface of a window or dome. A continuous thin-film coating that has both electrical conductivity and optical transparency could provide adequate electromagnetic shielding and superior optical performance. The conductive layer must be part of an anti-reflection stack of layers to maximize optical transmission. For external coatings, the outer layers of the anti-reflection stack must also provide erosion resistance.

Physics draws a fine line between electrical conductivity and infrared transparency of a continuous thin-film coating. Increasing electrical carrier mobility while decreasing carrier concentrations provides the possibility of obtaining adequate electrical conductivity while pushing optical absorption to the longest possible wavelengths. Increased effective mass of the charge carriers in p-type semiconductors also increases the wavelength for the onset of infrared absorption.

PHASE I: Demonstrate an infrared-transparent, electrically conductive coating with strong potential to achieve less than 10% absorption in the 3-5 micron wavelength region and a sheet resistance less than 10 ohms per square. The coating should be deposited on 25-mm-diameter disks of an infrared-transparent substrate such as spinel to allow infrared optical properties to be measured.

PHASE II: Optimize the coating for minimum optical absorption and maximum electrical conductivity. Design and demonstrate an anti-reflection layer structure to provide >90% transmittance in the 3-5 micron wavelength region. If possible, incorporate hard layers in the anti-reflection stack to provide erosion resistance. Conduct sand and rain erosion testing of the coating if it is designed to be on an external surface. Demonstrate stability of the coating in the atmosphere and in sunlight.

PHASE III: Demonstrate commercial production capability for coating windows and domes.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Optically transparent, electrically conductive coatings could become components of photovoltaic cells.

REFERENCES:
1. L. F. Johnson, M. B. Moran "Infrared Transparent Conductive Oxides" Proc. SPIE 2001, Volume 4375, p 289.

2. H. Kawazoe, M.Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, and H. Honsono, "p-Type Electrical Conduction in Transparent Thin Films of CuAlO2," Nature, 1997, Volume 389, p. 939.

3. F. A. Benko and F.P. Koffyberg, "Opto-electronic Properties of p- and n- Type Delafossite CuFeO2," J. Phys. Chem. Solids 1987Volume 48, p. 431.

4. M. Joseph, H. Tabata, and J. Kawai, J. "p-Type Electrical Conduction in ZnO Thin Films by Ga and N Codoping," Jpn. J. Appl. Phys. Part 2: Lett.1999, Volume 38, p. L1205.

KEYWORDS: conductive optical coating; optical coating; electrically conductive coating; coating; sensor window

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