Cryothermal vacuum measurement of thermochromic variable-emittance coatings with heating/cooling hysteresis for spacecraft thermal management

In this work, the dynamic heat rejection of a VO₂-based nanophotonic variable-emittance coating is demonstrated in a cryothermal vacuum experiment that simulates a cold space environment. The fabricated coating displays a significant augmentation in radiated heat flux, from 136 W/m² at the onset of the VO₂ insulator-to-metal phase transition to 444 W/m² at the end, which is in excellent agreement with theoretical modeling. Additionally, the temperature stability of both VO₂ thin films and the fabricated nanophotonic emitter are assessed for cryogenic temperatures (down to −196 °C) and high temperatures (up to 450 °C). The VO₂ thin film shows little change in transmittance from cryogenic temperatures up to temperatures of 200 °C. The variable-emittance coating likewise shows no change in behavior at cryogenic temperatures, while the emittance remains stable up to 200 °C. The partial heating/cooling hysteresis behavior is also investigated for VO₂ thin films on silicon and the variable-emittance coating, where a theoretical model quantifying the hysteresis effect is developed for both. The substantial increase in heat rejection upon VO₂ phase transition at higher temperatures indicates great promise for the nanophotonic variable-emittance coating to be used for spacecraft thermal control with excellent temperature stability and resistance to thermal cycling.