Angle-independent VO₂ Thin Film on Glass Fiber Cloth as a Soft-Smart-Mirror (SSM)

Designing materials with a negative feedback function is beneficial for achieving temperature regulation inside a greenhouse. VO₂ has been studied extensively because of its low insulator-to-metal transition temperature (IMT). In this study, reflection changes during a VO₂ phase transition were investigated. Glass fiber cloth was used as a substrate, as it is stable and soft. A VO₂ thin film on a glass fiber cloth whose surface contained 96% V⁴⁺ and 4% V⁵⁺ was prepared using an inorganic sol-gels method. The insulator-to-metal transition temperature was decreased by 38 °C, which was observed from the reflection curve detected using an angle-resolved spectrometer. This decrease in IMT occurred mainly because of the presence of V⁵⁺, which causes destabilization of the monoclinic phase of VO₂. When the greenhouse temperature was increased from 30 °C to 40 °C, the reflected intensity of VO₂ on glass fiber cloth decreased by 22% for the wavelength range of 400 nm to 800 nm. In addition, the angle-independent property of the VO₂ thin film was observed using an angle-resolved spectrometer. Owing to its thermo-reflective properties, the thin film can serve as a soft-smart-mirror (SSM) inside a greenhouse to stabilize the temperature, playing a negative feedback role.