Variable Visible-to-Infrared Optical and Emissivity Properties of VO₂ and W-VO₂ Thin Films for Passive Smart Radiative Device Applications

Numerous technologies have been developed to regulate the temperatures of several space equipment within the allowable tolerance temperature range from -150 °C to 150°C to meet the operation requirements by rejecting or injecting Joule heating. A spacecraft is a typical example of such devices, which can be subjected to different thermal environments upon lunch, from the Earth’s surface to outer space, for various missions. The thermal environment of these systems is controlled by dissipating heat through either passive or active processes. Many traditional spacecraft systems employ heat pipes, thermal switches and heaters to regulate the spacecraft temperature. Others use advanced thermal control devices such as the electric louvre based on microelectromechanical technologies comprised of a bimetallic and spring-actuated mechanism to manage the heat when it falls or rises above the allowable temperature range during operation. Even though there has been tremendous progress in smart radiative devices (SRD) over the years for thermal control designs via mass and size reduction with minimum or no electrical energy required using perovskites. The SRD consists of variable emissions to regulate the temperature by changing the environmental temperature concerning the phase transition property of the thermochromic materials. In addition, the SRD must have low emissivity at low temperatures to reduce heat loss and high emissivity at high temperatures allows it to dissipate heat through thermal radiation. A phase change material such as VO₂(M1) with a phase transition temperature of 68°C starts from a low-temperature semiconductor state to a high-temperature metallic state. This is accompanied by structural transformation from a low-temperature monoclinic (M) to a high-temperature rutile (R) phase. For instance, optical characteristics of spacecraft smart radiative devices have been demonstrated and reported, which consist of a multilayer thin film of VO₂ on silica or aluminium-silver substrate.<br/>This presentation presents a direct method of fabricating SRD W-doped VO₂ thin films onto silica and silicon substrates from W-doped V₂O₅ precursor using a high repetition rate (75 kHz) femtosecond-PLD. Surface morphology and crystal structure characterisation of the SRD were investigated. In addition, visible to mid-infrared optical properties such as temperature-dependent solar absorptance, reflectance index, and variable emissivity of the phase change material VO₂ SRD device will be systematically investigated and discussed.