Monitoring thermo-optical properties of Multilayer Tuneable Emittance Coatings for Smart Thermal Control in Space

摘要

MPB has developed advanced technologies based on smart radiator thin-film tiles (SRDs) employing Vanadium Dioxide (VO_2) doped with two metals M and N (V_(2-x-y)M_xN_yO_2), for the passive dynamic thermal control of space structures and payloads. The SRD has successfully passed the major ground tests and validated its performance for extended use in the harsh space environment, with a target of up to 15 years GEO, in preparation for a flight demonstration of this technology. The best SRD samples obtained have an emittance tuneability (△ε) of about 0.36 (ε-low = 0.38, ε-high = 0.74), and a solar absorptance of 0.32. These SRD Devices have a somewhat lower high-temperature emittance "ε-high" (< 0.75) compared to common Optical Solar Reflector (OSR) tiles with a fixed emissivity (e) of 0.8 to 0.88. This requires a slightly larger radiator surface for a given power dissipation. To decrease the net solar absorptance (α), while maintaining high emittance tuneability, a multilayer thin-film structure was developed, based on a relatively simple thin dielectric stack selective reflector based on glass (SiO_2) (e.g. SiO_2(λ/4)/VO_2(λ/4)) to provide peak reflectance at λ=500 nm. There is ongoing work to further improve the tuneability of the SRD emissivity. This is being assisted by detailed characterization of the thermo-optical properties and the associated switching of the VO_2-based SRD. The emissivity of VO_2 on Al substrates exhibits an inverse characteristic with temperature to that of VO_2 on Si, SiO_2 (glass), and Sapphire. The analysis with Raman spectroscopy and X-ray Photo-Emission Spectroscopy (XPS) shows that the VO_2 is in metallic state at high temperature, even its emissivity is lower, due potentially to the reflectance through an interface that forms between the Al and the VO_2 during the VO_2 deposition on the Al substrate. In a step towards the deposition on Optical Solar Reflectors (OSR), we deposited the VO_2 over a thin Aluminum layer (100-200 nm) prepared on a Si substrate. This thin Al layer would replace the relatively thick Al plate currently used (0.3-0.5 mm thick). The VO_2 deposited on the thin-film Al had similar properties and emittance tuneability to that deposited on thicker Al substrates. These trial experiments were performed on a high performance off-axis Pulsed Laser Deposition recently set-up at INRS-EMT. Successful deposition of tuneable VO_2 on gold (Au) thin layer was demonstrated for the first time, on a metal different than Al.