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Next generation meta-material based Optical Solar Reflectors

Periodic Reporting for period 1 - META-REFLECTOR (Next generation meta-material based Optical Solar Reflectors)

Reporting period: 2016-01-01 to 2017-12-31

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.
"Design
The design of meta-OSRs consists primarily of an Aluminum back reflector, a SiO2 dielectric spacer, and an array of square islands in Aluminum Zinc Oxide -AZO- or Indium Tin Oxide –ITO–, completed with a dielectric UV reflector (UVR) and a thin TCO layer, for a total thickness of ≈ 2 µm. The UVR has a positive effect on both alpha and on epsilon. The ITO layer is effective at ensuring electro static dissipation.
The design of smart meta-OSRs combines the usual Aluminum back-reflector and SiO2 dielectric spacer with an array of VO2 squares. As in the case of meta-OSRs, and more so, integration of an UV reflector would be beneficial for alpha . However, smart meta-OSRs would require a special UVR made of low emissivity materials, to improve alpha without compromising emissivity contrast. This special UVR could not be realized in the course of the project because of lack of time and resources.

Materials and deposition processes
The project led to t new processes for the growth of AZO by Atomic Layer Deposition (ALD), and of VO2 by RF sputtering of V metal and thermal oxidation in oven at 375°C.

Patterning and overall fabrication processes
Nano Imprint Lithography was used to fabricate meta-OSRs via both lift off and etch. Patterns were written into a Silicon master, then transferred into a flexible stamp, and from the stamp into the foil. SEM images reveal good quality patterns that match design specifications.
Electron Beam Lithography was instead used to produce smart meta-OSRs . Etching turned out to be the only viable approach, because of the high temperature at which VO2 must be annealed. The process still needs some optimization, particularly on Titanium foils, where partial etching of the antennas or incomplete etching of the gaps have been observed. Pattern quality is definitely better on Silcon wafers.

Demonstrators
Several meta-OSR demonstrators were delivered at the end of the project on polyimide foils 3 MIL, with active area 8 cm x 8 cm. Emissivity is greater than 0.80 and meets the application requirement. Solar absorbance is around 0.25 and therefore a bit over the acceptance threshold of 0.20 BOL.
Fabrication of smart meta-OSRs was hindered by a long shutdown of the EBL machine. A few demonstrators were delivered on silicon wafers, others on 1 MIL Titanium foils o1"" x 1"". Emissivity contrast shows some variability that reflects the variable quality of the patterns. Good quality pattern generate hot state emissivity around 0.8 and strong emissivity contrast around 0.45 which comfortably meet the application requirements. Solar absorbance α could not be brought below 0.4 as expected from the model.

Tests and validation
All the demonstrators passed adhesion tape, bending, thermal and humidity tests with no visible coating delamination or failure, and with no appreciable variation of thermo-optical or electrical properties.

Final Technology Assessment
More R&D work is needed to improve the thermo-optical performance of the proposed meta-material coatings. At the same time, results appear very promising, and have so far evidenced no intrinsic limit of the technology. The main issue that remains to be solved is alpha, that is today too high. However, we have already identified solutions and development paths capable to bring solar absorbance within the requirement α ≤ 0.2."
In the medium term, our technology can find application on space platforms of any kind and use in replacement of traditional OSRs. This is more true for smart meta-OSRs, as they offer radical advantages in term of temperature variable emissivity, which means reduced heat losses in the cold phase, whereas the thermal control subsystem is designed firstly as a cooler during the hot phases. Preliminary thermal simulations show that the electric budget at the heaters might be reduced of more than 50% during eclipses in safe mode. The saved power can be reused for additional payloads, and enable hardware simplifications, especially by eliminating the use of heavy active thermal control equipment like louvers. Smart meta-OSRs will result most attractive for smallsats , and for missions with prolonged cold phases, as those planned for the exploration and economic exploitation of Mars and the Moon. Finally, smart meta-OSRS could be used to reduce the thermal gradients and thermal excursions of surfaces external to the satellite body.

Other advantages, common to both constant and variable emissivity meta-OSRs include:
- Flexibility of the foil, which simplifies AIT operations compared to quartz tiles, with no risk of breakage.
- Improved space durability compared to Ag / FEP foils, because of the presence on the first surface of a fully inorganic solar reflector, which protects the polymer from interaction with UVs.
Fig. 6: meta-OSR demonstrator 100 mm x 100 mm during bending tests
Fig. 3: smart meta-OSR design.
Fig. 5: smart meta-OSR on 1 MIL Titanium 30 mm x 30 mm
Fig. 4: meta-OSR patterns by Nano Imprint Lithography
Fig. 1: meta-OSR concept.
Fig. 2: meta-OSR design.