H2020 project Meta-Reflector highlights metamaterials value
Metamaterials could improve the lifetime of satellites.
Metamaterials could be defined as artificial materials that have unique characteristics not found in the natural world. Enhanced optical absorption is one of these. Normally, light absorptions come from a material’s chemical composition. Once the material geometric dimension goes below the wavelength of the light source, its optical characteristics become more dependent on its physical size, not just on the chemical composition. With a metamaterial, if you pattern the material or make a product with small features, the final object’s light absorption power can take on a much higher capability. In optics, it is possible to have perfect light absorption, or selected wavelength absorption for particular colours.
Deflecting heat sources
The H2020 project Meta-Reflector is a functional coating for a satellite’s thermal management over its lifetime. It is being developed by the University of Southampton, UK, Consorizo CREO, Thales Alenia Space and NIL Technology. This coating is commonly referred to as an optical solar reflector (OSR), which can almost fully reflect solar radiation in UV/Visible range and at the same time, achieve infrared emittance for background readings. Infrared light emission is needed for radiation cooling purposes, as it is an effective cooling method in vacuum.
To date, OSRs on satellites have been made with aluminium-coated quartz. Similar to glass, the quartz in particular is fragile and planar so it cannot be bent. It takes hours to mount a 5x5cm² tile onto a satellite and days to complete the assembly, which is risky as the tiles are easily damaged. There are mechanical vibrations during a satellite launch, and the testing and handling apparatus attached to the tiles make it easy to break them. As part of the device, the research team has achieved a metamaterial-based coating with an equivalent performance to the original. The new coating is 25 micrometres thick and very light compared with existing coatings. It is flexible and therefore robust against mechanical vibrations during test and launch.
Manufacturing the part
The coating is made of a stack of layers on flexible substrates with aluminum metal, a silicon dioxide (SiO2) spacer and a transparent conductive oxide on the outer layer. This latter is transparent for visible light, but it absorbs infrared light. It is made of a thin aluminium-doped zinc oxide of 100nm, and plays a key role in infrared emittance. Atomic layer deposition (ALD) is used to create this uniform and reproducible film on the top side of the outer coating.
To put the coating together, the bottom is aluminium, the middle is SiO2 and the top is the transparent conductive oxide. The aluminium reflects visible reflections, while the spacer layer in the middle creates a constructive interference to enhance the infrared absorption. We created a square feature with the transparent conductive oxide layer on top. These form a meta-surface, which can further enhance the infrared absorption of the device.
We formed the device through ALD, lithography and plasma etching. In our first demonstration of size-up, we were able to produce a tile of 8x8cm2 using a nanoimprint lithography technique and have the capability to further scale-up.
Testing and performance
Thales Alenia Space performed tests on the new tiles with thermocycling, which mimics the conditions in orbit by repeatedly heating then cooling the environment, for up to 100,000 cycles, and they also performed this in a vacuum. The film was also exposed to UV radiation in a controlled environment to test for ageing and robustness against humidity. This is mainly for the purpose of storage on Earth, where humidity may affect the device, which is not an issue in space.
Performance is commonly defined by two numbers, alpha and epsilon. Alpha is the UV/Visible range absorption for solar radiation – the lower the absorption, and the higher the ability to reflect the solar radiation. Epsilon is the infrared emittance, which is roughly defined by emittance at the temperature of 300, room temperature. For space requirements, these numbers need to be between 0.1–0.2 for alpha and 0.7–0.8 for epsilon. From our work, we have been able to achieve alpha below 0.2 and an epsilon of approximately 0.8.
During the ageing tests, there has been a slight drop from 0.8, but we are still optimising the material for performance improvement.
Another idea as part of the project is SMART meta-reflectors. Here, the infrared absorption/emittance is achieved through vanadium dioxide (VO2). This material has a unique capability – when temperature goes up, it’s a conductor, but when the temperature goes down, it’s an insulator.
When you transfer that information into optics, that means when the temperature goes up, it has an infrared emittance. When temperature goes down, it has little emittance. So if you use this material to replace what we used (aluminum-doped zinc oxide), that means when the satellite is heated up by the sun, the material will perform radiation cooling. When the satellite is blocked by the Earth, in the shadow region, the temperature will go down. We don’t want the temperature down further by radiation cooling as we want it to stay in a mild range. In this case, we have a new performance parameter, epsilon difference between high and low temperatures, which stands for its tunable capability of radiation cooling.
But by introducing the smart reflector, this material has a big challenge – VO2 has a significant UV/Vis absorption, which results in poor solar absorption. By introducing the VO2 meta-surface instead of a planar VO2 film, that means we reduce the VO2 coverage by removing some unwanted areas you can see in the square and in turn, reduce the unwanted UV/Vis absorption. So when we remove some part of it, it means we will improve the alpha and, for the affected infrared absorption, meta-surface can compensate it through its enhancement of infrared absorption. This technique will provide a trade-off between the epsilon difference and the alpha.
Financial benefits of metamaterials
If the protective coating in particular is used, the cost is about €2,000/m2 compared with the current material which costs about €8,000/m2. This is mostly because of weight as less fuel is needed for mass reduction of the whole satellite.
Many hold the idea that nanotechnology is extremely expensive and can be only applied to state-of-the-art high-end electronics. Admittedly, the device size is equivalent to the cost in the semiconductor industry and extreme efforts are being made to reduce device size through scaling in past decades to reach the well-known Moor’s law.
However, certain technologies have been targeting scale-up in the past decade. For example, LCD panel sizes has been scaled up to more than 2x2m², making a 165cm (65-inch) LCD panel less than £300. With recent development with nanoimprint lithography and roll-to-roll production technique, low-cost mass production becomes possible and even more affordable. These make these nanotechnology-based meta-surface or meta-material products more competitive than conventional alternatives in term of performance and cost. Coming back to the satellite with the aluminium-coated quartz tiles, a metamaterial-based coating that is lighter and can perform a task more efficiently and reliably would save more money over the life of the satellite. Besides this application, the material might be suitable for cooling purposes such as air-conditioners on top of buildings.
Sun Kai is a University of Southampton Senior Research Fellow, Faculty of Engineering and Physical Sciences.