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Next generation meta-material based SMART and FLEXible optical solar reflectors

Periodic Reporting for period 1 - SMART-FLEX (Next generation meta-material based SMART and FLEXible optical solar reflectors)

Reporting period: 2019-01-01 to 2019-12-31

The goal is to develop a radically new type of Optical Solar Reflector (OSR) that combines the space durability of quartz OSRs with the easy handling and application of flexible OSRs, and with the temperature variable emissivity of Smart Radiator Devices. The proposed Smart & Flexible OSR (from now on SF-OSR) is a very thin thermochromic meta-material coating grown on the space-facing surface of a polymer foil, that can be easily glued to a radiator panel via transfer tape. The SF-OSR has low solar absorbance α, and changes passively its emissivity ε with temperature: high ε in the hot phase — to prevent overheating, and low ε in the cold phase — to prevent overcooling .

The coating consists of two functional blocks: a Perfect Metamaterial Absorber PMA that changes ε with temperature, topped by a dielectric Low Emissivity Solar Reflector LESR that reduces α and ensures protection against the space environment. The PMA is a three-layer structure designed to absorb/emit selectively the thermal InfraRed (IR) radiation and consisting of a metal back-reflector, a dielectric spacer, and a patterned top layer of conductive nano-antennas. Temperature variable emissivity derives from the use of thermochromic doped Vanadium Dioxide as the material of the PMA nano-antennas. The LESR is an interferential filter built with materials that combine good mechanical stability with good transparency in the thermal IR spectrum —to enable radiative exchange between the variable emitter and the outer space . The coating as a whole is fabricated via thin film deposition techniques and Nano Imprint Lithography (NIL), this ensuring the scalability of the process to large areas at fair costs.

SMART-FLEX is the follow-on of the H2020 project ‘META-REFLECTOR’ ( ), in which the feasibility of the technology was demonstrated. More specifically, META-REFLECTOR delivered small SF-OSR breadboards on Titanium foils having good emissivity contrast (Δε > 0.4) but still too high solar absorbance (α > 0.4) and too high Metal-Insulator Transition Temperature (TMIT ~ 65 °C). The specific objectives of SMART-FLEX are to:
- Reduce solar absorbance (target α ≤ 0.2) without negative impact on emissivity contrast;
- Reduce transition temperature (target TMIT ~ 20 °C);
- Develop a fabrication process compatible with space-compliant polymer foils;
- Deliver demonstrators of minimum active area 80 x 80 mm (target 100 x 100 mm);
- Validate the technology with ageing tests corresponding to minimum two years operation in GEO orbit.
Work in the first year of the project was primarily aimed at:
- Defining the preliminary design of the device;
- Developing the materials and fabrication processes needed to build the device.

Results can be summarized as follows:

- Design. Numerical simulations were aimed at identifying a set of device parameters that give the desired thermo-optical (TO) performance: α ≤ 0.2; εhot ≥ 0.8; Δε ≥ 0.4. PMA and LESR were first modeled and optimized separately, then simulated jointly. The optimum PMA was found with a dielectric spacer of about 900 nm matched to a square array of TCantennas of size = 2700nm and gap = 1000nm. This configuration gives α ~ 0.6 and ε ~ 0.4. For the LESR, efforts were aimed at maximizing solar reflectance with the minimum number of layers. The trade-off was found with a dielectric filter of thickness less than 2000 nm. When the LESR is added on top of the PMA, α improves from 0.4 to about 0.2 while ε undergoes only modest degradation.

- TC material and antennas. Two alternative process were developed to grow Tungsten-doped thermochromic Vanadium Dioxide. For both processes, conditions could be found to produce TC layers having strong metal-insulator transition centered at TMIT ~ 20 °C. The layers could be patterned as per the optimized square design, using a simple lift-off process. Initial breadboards built with these patterned layers gave Δε up to 0.45 and perspective room for improvement .

- Low emissivity materials and LESR. The high and low index materials needed for the LESR were developed using sputtering and co-sputtering techniques. Good transparency could be achieved both in the VIS and in the thermal InfraRed range, matched to good resistance to mechanical solicitations and to air humidity. These materials were used to build LESRs on both rigid and flexible devices. While process optimization is still ongoing, the best hits to date are SF-OSR breadboards having α = 0.30.
In recent years, research on smart surfaces having low alpha and high or temperature variable emissivity has been primarily aimed at applications similar to SMART-FLEX, that is thermal control of spacecrafts. Most of the proposed solutions are continuous-thin-film smart coatings that utilize either VO2 or perovskite manganites as thermochromic material. One striking characteristic of these coatings is that they have either extremely complex design, or poor thermo-optical properties. Several papers and patents focus on emissivity and emissivity contrast between the cold and the hot state, while they hardly mention alpha, which tends to be too high. High alpha derives from the use of continuous thermochromic layers, that ar bad optical materials both in the metal and in the dielectric phase. Meeting the alpha requirement calls for the implementation of more complex structures that are at risk of mechanical failure and adhesion losses. The advantage of SF-OSRs is threefold:
Lower alpha. SF-OSRs integrate the TC material as a patterned layer. Since solar absorption scales with the fill factor of the pattern, by design the alpha of a SF-OSR tends to be lower than traditional variable emitters.
Higher emissivity contrast. Simulations and preliminary experimental verification show that PMA structures with a VO2 pattern may have up to 0.1 higher emissivity contrast than analogous continuous three layer structures.
Thinner coating. Since spectral response depends more on the geometry of the VO2 pattern, than on the design of the multi-layer, SF-OSR metamaterial coatings are thinner than traditional coatings, which makes them less prone to mechanical failure on flexible substrates.

SF-OSRs can find application on space platforms of any kind and use in replacement of traditional OSRs, as they offer advantages in term of smart emissivity, which means reduced heat losses in the cold phase, whereas the thermal control subsystem is designed primarily as a cooler during the hot phases. Reduced heat losses translate into reduced electric budget at the heaters level. The saved power can be reused for additional payloads, and enable hardware simplifications, by reducing the number of heaters, cables and control units, and, especially, by eliminating the use of heavy active thermal control equipment like louvers.
SF-OSRs will result most attractive for smallsats (minisats, microsats, nanosats, and cubesats, of mass between 1 and 500 Kg), that cannot take on board massive and power hungry active emissivity control systems. This opens interesting business opportunities in what is today the most lively of the space market segments.
Finally, SF-OSRS could be used to reduce the thermal gradients and temperature swings of surfaces external to the satellite body, from antennas, to star-trackers, to the rear side of solar panels, to observation instrument structures.