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arXiv2026

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

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.

cond-mat.mtrl-sci↗

arXiv2025

Reducing Temperature Swing and Rectifying Radiative Heat Transfer for Passive Dynamic Space Thermal Control with Variable-Emittance Coatings

Dynamic radiative thermal control is crucial for normal operation and energy saving of spacecraft that copes with changing thermal environment involving heat dissipation to cold deep space, external heating from the Sun and nearby planet, and internal heating from onboard electronics. Variable-emittance coatings, whose infrared emittance can be tuned passively by temperature or actively by external stimuli, could provide a viable solution. In this work, we experimentally demonstrate self-adaptive dynamic radiative heat transfer with variable-emittance coating based on thermochromic VO2 in space-like thermal environment with a coldfinger and a custom-made sample mount inside a vacuum cryostat. Black Actar and highly reflective tungsten mirror are used to calibrate the parasitic head load and heat flux sensor sensitivity, while multiple static-emittance samples made of silicon wafers with different doping levels are measured for validation of the experimental method and for direct comparison with the variable-emittance VO2 coating. With the coldfinger at 80 K to mimic external radiative scenarios in space, the tunable coating exhibits 6-fold enhancement in radiative thermal conductance upon VO2 phase transition for promoted heat dissipation, in addition to reduced temperature swing by almost 20degC compared to the static emitters. With the coldfinger at 25degC as internal radiative scenarios in space, similar 6-fold heat dissipation from the variable-emittance coating is also observed, while radiative heat transfer is much suppressed with a constant radiative thermal conductance when the coldfinger is hotter than the tunable coating at 25degC, leading to a thermal rectification factor of 1.8 experimentally achieved.

physics.app-ph↗