Mohamed Soltani

Passive dynamically-variable thin-film smart radiator device

This paper describes a new approach to spacecraft thermal control based on a passive thin-film smart radiator device (SRD) that employs a variable heat-transfer/emitter structure. The SRD employs an integrated thin-film structure based on V 1 - x - y M x N y O n that can be applied to existing Al thermal radiators. The SRD operates passively in response to changes in the temperature of the space structure. The V 1 - x - y M x N y O n exhibits a metal/insulator transition with temperature, varying from an IR transmissive insulating state at lower temperatures, to a semiconducting state at higher temperatures. Dopants, M and N, are employed to tailor the thermo-optic characteristics and the transition temperature of the passive SRD. The transition temperature can be preset over a wide range from below -30°C to above 68°C using suitable dopants. A proprietary SRD structure has been developed that facilitates emissivities below 0.2 to dark space at lower temperatures to reduce heater requirements. As the spacecraft temperature increases above the selected transition temperature, the thermal emissivity of the SRD to dark space increases by a factor of 2.5 to 3. The thin-film SRD methodology has significant advantages over competitive technologies in terms of weight, cost, power requirements, mechanical simplicity and reliability Preliminary results on an active electrochromic SRD based on the VO 2 system are also presented.

MULTIFUNCTION SMART COATINGS FOR SPACE APPLICATIONS

This paper describes a new multifunction smart coating that can provide atomic oxygen (AO) and electrostatic discharge (ESD) protection, while also improving the thermal control of space structures. The methodology is based on a passive thin-film structure employing VOn transition metal oxides that exhibit a metal to insulator transition. The coating, depending on its formulation, can provide a variable heat-transfer/emitter structure that operates passively in response to changes in the temperature of the space structure, by dynamically varying the ratio of solar absorptance (α) to thermal emittance (ε). This enhances self-heating of the structure at lower temperatures and cooling through thermal radiation at elevated temperatures. Work is currently underway to apply this coating to various polymers and membranes to improve their performance in space. In the space environment, such as low Earth orbit (LEO), the coating will be subject to various stresses including VUV radiation and AO. Atomic oxygen testing in a simulated environment at CSA indicated no resolvable change in the morphology or thickness of the coatings. The thermo-optic characteristics after AO exposure were similar to the “as deposited” films. Additional long-term radiation exposure at the Centre National d’Etudes Spatiales—France (CNES), equivalent to three years in a geostationary orbit (GEO) environment, resulted in a change in the coating ε and α of less than 0.002.

Growth, Characterization and Processing of VO 2 Thin Films for Micro-switching Devices

Single-phase vanadium-dioxide (VO 2 ) thin films have been deposited on various substrates by means of a reactive pulsed-laser-deposition technique. While the preferred orientation is (011) monoclinic for the films deposited on silicon substrates and (020) monoclinic for those deposited on sapphire substrates, the thermochromic properties of the VO 2 layers is found to be fairly independent of the substrate type. It is further shown that W doping and Ti-W co-doping significantly improve the thermochromic properties. Following growth, VO 2 layers were used in the context of the development of micro-switching devices. For this purpose, patterning of the VO 2 layers was investigated using a high-density argon magnetoplasma. Highly anisotropic features have been produced with a high etch rate and a good selectivity over resist. The etch rate for VO 2 /Al 2 O 3 samples is found to be higher than that for VO 2 /Si samples, which is due to the higher number of surface dangling bonds in the (020) phase as compared to the (011) phase.