Temperature‐Modulated Radiative Coatings for Dynamic Thermal Management of Spacecraft

This research spans the design, fabrication, characterization, and experimental demonstration of a variable emittance coating for passive spacecraft thermal control applications. Variable emittance devices have considerable potential to enable a diverse assortment of missions by modulating a spacecraft radiator’s heat rejection according to changes in spacecraft environment or internal heat load. For example, variable emittance coatings could potentially eliminate the need for dual loop thermal control in human spacecraft missions. Currently the ISS uses a dual loop thermal control architecture to take advantage of favorable transport properties for the external loop while avoiding toxic fluids for the internal cabin loop. Similarly, variable emittance coatings could reduce or eliminate the need for survival heaters on robotic missions during cold phase operation. Thermochromic variable emittance coatings, which vary their emittance according to temperature, could lead to significant reductions in a spacecraft thermal control system’s mass, volume, system complexity, and required power, making them an especially promising approach for variable emittance capabilities. Variable emittance radiator technologies were identified in the 2015 NASA Technology Roadmap (TA14.2.3.7) as a critical technology to be developed in the next 20 years. A Fabry-Perot resonance cavity structure was proposed in the first phase of this research, which consisted of a vanadium dioxide thin film, lossless silicon spacer, and aluminum substrate. Vanadium dioxide is a thermochromic insulator-to-metal phase transition material that can be incorporated into nano-engineered multilayered coatings to yield variable radiative properties. The proposed Fabry-Perot coating is designed to have high IR emittance at high temperatures and low IR emittance at low temperatures, with a total emittance change of approximately 0.40. A novel process to fabricate the required vanadium dioxide thin films was first developed to enable fabrication of the proposed coating. The material and optical characteristics of the fabricated films were investigated to assist with modeling efforts. Then the multilayer variable emitter coating was fabricated using physical vapor deposition techniques. The fabricated variable emitter achieved a total emittance change of 0.40, measured with Fourier-transform infrared (FTIR) spectroscopy. Further, a calorimetry based thermal vacuum set-up was developed to measure the variable heat rejection capability of the fabricated emitter in a space-like environment. The measured heat rejection was in excellent agreement with the theory, showing a sharp increase as the coating was heated beyond its transition temperature range. In addition to the demonstration of the designed variable emitter, the feasibility of using vanadium dioxide for a variable emittance device was investigated. The vanadium dioxide thin films underwent a low number of thermal of cycles to determine if vanadium dioxide is sensitive to thermal cycling. Similarly, the temperature stability of both the fabricated emitter and the vanadium dioxide thin films was studied and the samples were found to have stable radiative properties from 77 K (cryogenic temperatures) up to 200 °C. Empirical expressions to describe the hysteresis behavior of the vanadium dioxide under partial heating and subsequent cooling were also derived as part of this fellowship research. Finally, a representative human spacecraft model was developed to determine the optimal transition temperature range for human spaceflight applications.