VO2-based superposed Fabry-Perot multilayer film with a highly enhanced infrared emittance and emittance tunability for spacecraft thermal control

Thermal control coating for spacecraft based on thermochromic film attracts increasing interest due to their ability of self-adaptive emittance switch and less resource consuming compared with traditional thermal control coatings. However, practical applications of thermochromic film for spacecraft are constrained by the low infrared emittance at a high temperature and narrow emittance tunability. In this work, a thermochromic film with simple structure, nearly perfect infrared emission and large emittance tunability is proposed for the application of spacecraft thermal control. The thermochromic film is a VO2-based superposed Fabry-Perot (FP) multilayer film, which is constructed by encapsulating three thin VO2 layers in four lossless BaF2 spacer on the Al substrate. The infrared emittance and emittance tunability of the superposed FP film is dramatically enhanced by the three superposed VO2-BaF2-Al FP resonances at wavelengths of 9, 15 and 20 µm, respectively. For VO2 layers under metallic state, the spectral normal emittance of the superposed FP film is close to unity in the entire mid-infrared spectral range, while for VO2 layers under dielectric state, the film is highly reflective. For the typical growth techniques of the VO2 layers considered here, the emittance tunability of the superposed FP film can exceed 0.70 with total normal emittance larger than 0.91 at high temperature, simultaneously. The largest total normal emittance of the superposed FP film can reach 0.95 with emittance tunability of 0.78. In addition, the infrared emission and emittance tunability performances of the superposed FP film remain excellent for incident angles up to 60°. This work proposes a simple structure with highly enhanced infrared emittance and emittance tunability that outperforms the existing thermochromic films, which could accelerate the application of thermochromic films in the field of spacecraft thermal control.