VO2-based thin-film radiators with variable thermal emissivity

VO2 is a thermochromic material well suited for smart radiation devices due to its dramatic change in infrared reflection near its phase transition temperature (-68 degrees C). In this work, we have demonstrated a layered thin-film radiator, which consists of a TiN bottom infrared mirror, an Al2O3 dielectric spacer and a VO2 top absorber layer, to achieve thermal emissivity control using a phase transition of the VO2 layer. An analytical modeling approach is used to optimize the optical response of the layered radiators with varying the thickness of Al2O3 spacers and VO2 layers for maximum emissivity change (delta epsilon) between 25 degrees C and 80 degrees C. These modeling results show that the radiators composed of the optimized thickness of VO2 (30 - 50 nm) and Al2O3 (600 - 800 nm) can provide the highest emissivity change (delta epsilon -0.48) between these two temperature states. Experimental results validate that the radiator with a 50 nm thick VO2 layer and a 600 nm of Al2O3 layer exhibits a maximum emissivity change (delta epsilon-0.46) under the same temperature range. Our experimental results agree very well with the modeling results obtained from the same radiator design. These results are of crucial importance for designing mechanically and thermally stable radiators for spacecraft thermal control due to the stability of both TiN and Al2O3 materials.