Multilayer Tuneable Emittance Coatings, with Higher Emittance for Improved Smart Thermal Control in Space Applications

MPB has developed advanced technologies based on smart radiator thin-film tiles (SRTs) employing V1-x-yMxNyOn, for passive dynamic thermal control of space structures and payloads. In preparation for a flight demonstration of this technology, the SRT has successfully passed major ground tests and its performance validated for extended use in the harsh space environment, with a target of up to 15 years GEO. Preliminary ground testing of the SRD tiles and assembly was completed and presented in previous papers. A set of environmental tests was performed in order to validate the coating resistance and performance stability in space for a single layer SRT, including extended thermal cycling and thermal shock testing. This layer demonstrated good emittance tuneability (Δe), with relatively high solar absorptance (α) at the larger thicknesses required for a high Δe. Recently MPB developed and presented (ICES 2009) a multilayer thin-film structure to decrease the net solar absorptance (α), while maintaining high emittance tuneability. The approach uses a relatively simple thin dielectric stack selective reflector based on SiO2 (e.g. SiO2(λ/4)/VO2 (λ/4) to provide peak reflectance at λ=500 nm, the spectral position of the peak solar AM0 radiation. Reducing the net solar absorptance, without any significant decrease of the emittance tuneability, was recently demonstrated. Six samples based on a three-layer structure on Al (Substrate Al/VO2/SiO2(λ/4)/VO2(λ/4)) were studied. Thermal vacuum cycling (up to 4000 cycles) and thermal shock test (17 cycles between Liquid Nitrogen and 165°C) validated the stability of the emittance tuneability and solar absorptance. The best sample obtained has an emittance tuneability (Δe) of 0.36 (e-low = 0.38, = 0.74), and a solar absorptance of 0.32. Smart Tuneable Emittance Devices have a relatively low e-high ( 0.85), thus requiring a larger reflecting surface and limiting their commercial applications. In this paper MPB will present an innovative design to increase the e-high range, using an additional layer of TiO2.