Multilayer Coatings and Optical Materials for Tuned Infrared Emittance and Thermal Control
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ABSTRACT Many thermal control applications require thin film coatings that emit or absorb strongly at near infrared and infrared wavelengths. One of the primary applications for these coatings is thermal control for surfaces and structures of spacecraft, which are exposed to solar radiation during at least 60% of their orbit, causing wide temperature fluctuations. Another recent application for this type of coating is infrared emissive imaging employing a fiber optic infrared scene projector. While single layer coatings can provide high emissivity in a broad wavelength band, multilayer coatings can be used to obtain higher emissivities over a narrow wavelength band. This band can be tuned to a specific range of temperatures and wavelengths. Coatings developed for thermal control have a reflective base layer, either ZrN or a refractory metal boride or silicide. These materials have increased durability compared to metal layers. The multilayer coating deposited over the based layer consists of an A12 0 3/SiO 2 stack with high emittance at 300 K (9.8 pim), and solar reflectance near 0.6. Multilayer tuned infrared absorber/emitter coatings are applied to fiber optic infrared scene projectors. The coatings consists of a three layer Si3N4/Cr/Si 3N 4 absorber tuned at the 1.06 ýtm laser wavelength, and a six layer Cr/dielectric/Cr/dielectric/Cr/dielectric coating which emits strongly in either the 3 - 5 jim or the 8 - 12 gim infrared wavelength bands. Absorption bands of the coatings are independently tunable. All coatings are deposited by reactive DC and RF magnetron sputtering onto 2.5-in fiber optic faceplates. Either Si3N 4, Si, or ZnS thin film dielectric materials were used in the emitter coatings. With an input laser power of 15 W, the coatings emit at a black body temperature 529 K, which compared well with predicted performance. INTRODUCTION Thin film coatings that absorb and emit optical radiation can be used to enhance the thermal emissive and absorptive properties of surfaces. Recent applications include solar control, thermal control of space structures, and infrared emissive displays. The coatings are designed to absorb or emit radiation to increase or decrease the temperature of a surface [1], reduce the transmittance of a window [2], or provide a detectable visual image [3]. For solar control, a high emissivity at visible and near infrared wavelengths, and high reflectance at infrared wavelengths is usually desired. Space structures experience large temperature fluctuations that must be moderated either by coatings applied to their surface or by compositional modification of the structure itself. Heat from power sources in these structures must also be dissipated. A new application for these high emittance coatings is for infrared displays, such as a fiber optic infrared scene projector [4]. Many types of thin film materials and coating layer designs are used to achieve the desired surface emittance performance. Single-layer coatings have been used extensively to provide emittance in a broad infrared wavel
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