Thermochromic VO2-based smart radiator devices with ultralow refractive index cavities for increased performance

VO2 thermochromic coatings show great potential as the main component in smart radiator devices (SRD) for spacecraft, most notably for micro- and nano-satellites. Indeed, the inherent metal to insulator transition (MIT) of VO2 allows such a coating to act as a lightweight thermal regulator, eliminating the need for heavy and failure-prone mechanical louvers. However, spacecraft industry standards require an emissivity variation Δε of at least 60%, a value which, to our knowledge, has yet to be demonstrated. To reach and surpass this value, we first apply a modeling approach to optimize the optical properties and thicknesses of the individual constituent films of a typical SRD with the following architecture: mirror | dielectric resonant cavity | VO2. This study then highlights various possible avenues to enhance the performance of the devices, one of these being the use of an infrared transparent ultralow refractive index dielectric material for the resonant cavity. This theoretical prediction is then confirmed by the deposition of various prototype devices implementing a CaF2 cavity layer with n @ 10 μm = 1.17 and with measured values of Δε in excess of 60% for the full 3 to 25 μm wavelength range. In fact, we demonstrate a prototype device with a maximum Δε value of 66%, thus bringing this technology one step closer to implementation.