The Design and Development of a Smart Multilayer Coating with Variable Emissivity Capability for Space Vehicle Thermal Control Systems

The primary means of heat rejection for space vehicles are limited to radiation due to the absence of the earth's atmosphere for convective heat transfer. Thermal radiators, solely, cannot regulate the temperature and often require supplemental heaters, heat pipes, and control systems to maintain the spacecraft temperature within a desirable range. A thermochromic multilayer structure with reversible optical properties, (e.g., emittance and reflectance) based on environmental temperature, may be exploited to provide a passive thermal control system for space vehicles. The current study aims to design and develop a smart multilayer structure that contains thermochromic material that allows for positive emittance switching at room temperature-based phase-transition temperature. The materials, thickness, and sequence for each layer is determined such that the structure can provide a wide dynamic range of emissivity with temperature in the mid-infrared wavelength region to ensure the effectiveness of the proposed passive thermal control system for the space vehicles. The thermochromic layer considered for the present study is Vanadium Dioxide whose phase-transition temperature is 68°C. To achieve an emittance switching temperature close to the room temperature, this study also strives to reduce the phase-transition temperature of the thermochromic system using doping. The proposed multilayer structure is grown using thin film deposition techniques consisting mainly of Magnetron sputtering with optimal operating conditions. After fabricating the proposed structure, an experimental apparatus is designed and developed to directly measure the emitted power from the coating under different operating conditions. Using the proposed test setup based on calorimetric approach, the total hemispherical emittance can directly be explored.