Cryothermal Measurements of Variable-Emittance Coatings with Lower Phase Transition Temperatures for Space Thermal Control

Abstract

Space thermal control is critically important to ensure proper operation of on-board equipment in a regulated temperature range. Passive thermal control with variable-emittance coatings (VECs) could help save power consumption in a dynamically changing space thermal environment. Vanadium dioxide (VO2) based VECs have been studied for space thermal control but its intrinsic phase transition around 68°C limits its wider space applications where lower temperature ranges are expected. In this work, we experimentally demonstrate enhanced radiative heat dissipation in space-like thermal environment via cryothermal measurements with VECs of undoped and tungsten doped VO2. The fabricated undoped VEC exhibits a large emittance change of 0.6 across the phase transition, while the 1 at.% tungsten doped one shows an appreciable emittance variable of 0.4 with phase transition temperature lowered by 25°C. A vacuum cryothermal setup is developed with a liquid nitrogen cooled coldfinger to mimic cold space thermal background and a custom-designed sample mount suspended by nylon wires. After careful calibration and validation, greatly enhanced radiative heat dissipation upon VO2 phase transition up to 3.5 times with transition temperature lowered by 25°C from 1 at.% tungsten doping is clearly observed from the cryothermal tests. In the actual space thermal environment, radiative heat flux could further increase across phase transition from 160 W/m2 to 650 W/m2 with undoped VO2 coating from 55°C to 80°C, and from 175 W/m2 to 493 W/m2 with 1 at.% tungsten doped VEC from 30°C to 55°C.