Thin-film Smart Radiator Tiles With Dynamically Tuneable Thermal Emittance 2005-01-2906

This paper describes recent advances in MPB's approach to spacecraft thermal control based on a passive thin-film smart radiator tile (SRT) that employs a variable heat-transfer/emitter structure. This can be applied to Al thermal radiators as a direct replacement for the existing OSR (optical second-reflector) radiator tiles with a net added mass under 100 gm/m². The SRT employs a smart, integrated thin-film structure based on the nano-engineering of V_1-x-yM_xN_yO_n that facilitates thermal control by dynamically modifying the net infrared emittance passively in response to the temperature of the space structure. Dopants, M and N, are employed to tailor the transition temperature characteristics of the tuneable IR emittance. This facilitates thermal emissivities below 0.3 to dark space at lower temperatures that enhance the self-heating of the spacecraft to reduce heater requirements. As the spacecraft temperature increases above the preselected temperature setpoint, the thermal emissivity of the SRT to dark space gradually increases. Broad IR emittance dynamic tuneabilities exceeding 0.45 have been achieved.

The overall coating structure is substantially simpler than typical electrochromic devices and avoids the use of volatile charge-storage layers. The thin-film SRT methodology has significant advantages over competitive technologies in terms of weight, cost, power requirements, structural simplicity and reliability with no mechanical components, and integration with the space structure.

In the space environment, such as low Earth orbit (LEO), the coating will be subject to various stresses including VUV radiation and atomic oxygen (AO). AO testing in a simulated environment at the Canadian Space Agency (CSA) indicate no resolvable change in the morphology or mass of the SRT coating after exposure to AO equivalent to three years in low Earth orbit (LEO). The thermo-optic characteristics after AO exposure were similar to the “as deposited” characteristics. Work is currently underway to experimentally validate the expected performance for extended use up to 15 years GEO. In preliminary cyclic testing, there was no change in the emittance tuneability after about 3,500 temperature-induced cycles between the low and high emittance states.