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

Periodic Reporting for period 2 - SMART-FLEX (Next generation meta-material based SMART and FLEXible optical solar reflectors)

Reporting period: 2020-01-01 to 2021-12-31

The high-level objective of the project was to develop and bring to preliminary validation in the relevant environment (TRL 5) a radically new type of Optical Solar Reflector OSR (from now on SF-OSR) characterized by being smart and mechanically flexible, whereas:
- Smartness means that SF-OSRs have emittance ε changing with temperature: from high ε in the hot phase — to prevent overheating, to low ε in the cold phase — to prevent overcooling. This characteristic marks the greatest difference from commercial OSRs, which all feature constant high ε.
- Mechanical flexibility means that SF-OSRs can be easily handled and applied via transfer tape to radiator panels both planar and curved. This characteristic is not unique to SF-OSRs, but marks a great difference from top-of-the-range commercial OSRs that are rigid and easy-to-break quartz tiles.
The proposed SF-OSR consists of a meta-material coating deposited on a flexible transfer substrate. The coating is made of two functional blocks, namely a variable emitter topped by a Low Emissivity Solar Reflector (LESR). The variable emitter is a Perfect Metamaterial Absorber (PMA) comprising a meta-layer of doped Vanadium Dioxide VO2 micro-antennas. The LESR is a wideband dielectric filter made of materials that are transparent from the VIS to the thermal IR.
SMART-FLEX is the follow-on of H2020 ‘META-REFLECTOR’, which delivered small SF-OSR breadboards on Titanium foils having good emittance contrast Δε > 0.4 but too high solar absorbance α > 0.4 and too high Transition Temperature TMIT ~ 65 °C. The specific objectives of SMART-FLEX were to:
- Reduce α (target α ≤ 0.24) while maintaining εHOT ≥ 0.8 and Δε ≥ 0.4;
- Reduce TMIT and its width ΔTMIT (targets TMIT ~ 25 °C, ΔTMIT≤ 30°C);
- Make the coating static-dissipative (target Sheet Resistance Rs ≤ 10E9 Ohm/);
- Implement the coating on polymer film;
- Demonstrate the scalability of the technology to larger areas by delivering demonstrators of size ≥ 80 mm x 80 mm (target 100 x 100 mm), patterned by Nano Imprint Lithography NIL;
- Validate the durability of SF-OSRs to space ageing through Thermal Vacuum Cycles (TVAC) and through tests with UV radiation and charged particles at doses equivalent to two years in GEO orbit;
- Validate the durability of SF-OSRs to AIT operations through adhesion tests after cutting and bending.
The project explored a variety of solutions and produced hundreds of samples in different configurations. In the end, it converged on a so-called Configuration A which was used to build seven larger-area demonstrators 100 mm x 100 mm on Kapton film. Samples were characterized and tested extensively in terms of properties at BoL and resistance to ageing and manipulations. Results were as follows:
- α = 0.24 ± 0.02 (as expected);
- εHOT = 0.77 ± 0.03 (as expected);
- Δε = 0.32 ± 0.02 (low);
- TMIT = 27°C (as expected);
- ΔTMIT = 26°C (as expected);
- Rs > 1E12 Ohm/square (high);
- Substrate: Kapton FPC 3 MIL (as expected);
- Size of demonstrators: 100 mm x 100 mm (as expected);
- T.O. durability: properties stable after TVAC and p+, effects of UV not tested (partially as expected);
- Mechanical durability: Adhesion good after cutting, bending, humidity and TVAC (as expected).
Results are in good alignment with the expected values, apart from three deviations. Two deviations (high Rs, no UV testing) are the effect of partial budget reallocation from integration and testing activities to development activities. The third, Δε ≈ 0.3 is the effect of a non-optimal choice of materials.
Results were shared with the thermal engineers of Thales Alenia Space TAS and very well received. In particular, it was observed that, without improvements, the technology could already be of interest for applications in which the radiator sees the sun only at a grazing angle. In the general case, the T.O. performance still needs some improvement, but efforts should be directed more to lowering α below 0.2 than to raising Δε. In fact, Δε = 0.3 is already good enough as it generates about 40% saving in the heating power budget for the cold case, compared to a standard coating ε = 0.8.
The Smart-Flex Consortium has already outlined an evolution plan to improve the technology further, and identified the topics on which the R&D efforts should be focused. Furthermore, in line with TAS recommendations, the Consortium is already planning new ageing tests on existing samples, from UV tests to tests in space.
The study of variable emittance materials and coatings for the thermal control of spacecrafts has been an active field of research for the last 20 years. Initially, the preferred solutions had perovskite manganites as the thermochromic material. However, it was soon realized that manganites have very high crystallization temperatures, and pretty low TMIT. The focus was then shifted to continuous-thin-film coatings based on VO2, which requires lower temperatures for crystallization, and provides transition at room temperature if properly doped. One striking characteristic of all the proposed coatings, however, is that they have either extremely complex design, or poor T.O. properties. Most papers and patents focus on Δε, while they hardly mention α, which tends to be too high (above 0.4). High α derives in part from the poor quality of the LESR (which may be difficult to implement with good optical and mechanical properties), in part from the use of a continuous VO2 layer, that has high absorbance across the UV-VIS-NIR spectrum. One strategic advantage of Smart-Flex is that it utilizes a patterned layer of VO2: α decreases with the fill factor of the pattern, and tends by design to be lower than in continuous thin-film solutions. A second advantage derives from a good choice of materials and processes. These advantages translate into experimental results that place Smart-Flex at the state of the art among the smart solutions for thermal control, both in terms of current T.O. performance and prospective evolution, and for the fact that it is the only technology implemented on a flexible substrate.
The Smart-Flex technology can find application on space platforms of any kind and use in replacement of traditional OSRs, as it offers advantages in terms of temperature-variable emissivity and mechanical flexibility. These advantages are appreciable not only at the component level, but also and especially at the system level. In particular, temperature variable emissivity 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, and/or reduced masses deriving from the integration of louvers. At the proposal preparation stage, Smart-Flex appeared most attractive for small satellites, that cannot take on board massive and power-hungry active emittance control systems. More recently, however, it has been underlined the relevance of the technology also for the new telecom satellites which use electrical orbit raising. Electrical orbit raising to geostationary orbit takes several months and is a cold case because the satellite is not fully operational. The available electrical power is preferably dedicated to electrical propulsion rather than for heating the satellite, and the implementation of temperature variable OSRs would have a great added value.