The goal of META-REFLECTOR was to develop a radically new type of Optical Solar Reflector (OSR), consisting of an extremely thin, fully inorganic, metamaterial coating deposited on the first, space-facing surface of a flexible foil. The new type of OSR that we envision (from now on a meta-OSR) combines the performance and durability of quartz tile OSRs with the flexibility and ease of use of Silver /Teflon foils. On demand, it can integrate thermochromic materials to regulate emissivity as a functon of temperature.
The coating that META-REFLECTOR has proposed consists of a minimum of three layers: a metal back reflector, a dielectric spacer, and a 2D array of nano-antennas made of a Transparent and Conductive Oxide (TCO). A cladding can be added over the antennas to improve thermo-optical properties and provide protection against Electro Static Discharging (ESD).
In the jargon of the metamaterial community, our coating is a type of Perfect Metamaterial Absorber (PMA), in which the 2D conductive array is designed to generate close-to-unit broadband absorption in the thermal IR (which means high emissivity ε) and high reflectivity in the VIS range (which translates into low solar absorbance α). In a PMA, the position and width of the resonant absorption depends at a large extent on the shape, size and spacing of the nano-antennas, rather than on the number and thickness of the layers as in a classical interferential filter, which explains why a PMA can be extremely thin.
Thinness is the key to implementing the coating on foils. In META-REFLECTOR we have fabricated demonstrators on 3 MIL polyimide and 1 MIL Titanium, and achieved high resistance against handling and bending tests. The patterns that we have developed have simple geometry and can be easily fabricated by Nano Imprint Lithography (NIL), a technique that is scalable to large areas at fair costs.
The thermo-optical properties of a meta-OSR are constant or variable with temperature depending on the material used for the nano-antennas. At a good extent, we have focused efforts on constant high emissivity meta-OSRs, using Indium Tin Oxide (ITO) for the antennas. If ITO is replaced by thermochromic Vanadium Dioxide (VO2), we obtain a smart meta-OSR, that features high emissivity at high temperature, and low emissivity at low temperature, so avoiding heat losses in the cold phase. Using VO2, we have already fabricated breadboards on Titanium foils having high emissivity in the hot state, strong emissivity contrast, and high durability against thermal and mechanical stress.
