Polymer Thin Intermediate Layers Protect Satellites from Extreme Temperatures

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EMPA Eidgenössische Material- Prüfungs-und Forschungsanstalt

GGB Heilbronn GmbH

Super light, super flexible, super insulating: To protect satellites from extreme temperatures, an aluminum-coated polymer film is used. Researchers at Empa Thun have made the material even more durable by using an ultra-thin intermediate layer.

Who doesn't know the archetypal image of a satellite: two extended solar "wings" and a compact body wrapped in golden or silver shimmering foil. It is precisely this foil that researchers from the "Mechanics of Materials and Nanostructures" lab at Empa in Thun are working on.

The foil is so-called "Multilayer Insulation," also known as super insulation in German. It consists of multiple layers of a durable polymer coated with a thin metal layer, typically aluminum. On Earth, this coated foil is encountered in the form of emergency blankets. On spacecraft, the super insulation protects electronics from temperature fluctuations. "For satellites in low Earth orbit, the temperature difference between the sun-facing and the sun-shaded side is about 150 degrees (approx. 302°F)," says Empa researcher Barbara Putz. "However, electronics function best at room temperature of 25 degrees Celsius (approx. 77°F)." Since it is directly exposed to space conditions, the super insulation needs to withstand quite a bit.

Accelerating the development of flexible electronics on Earth

The durable polyimide is typically used as the polymer base for the thin-film structure. In addition to its temperature and vacuum resistance, it is also characterized by the fact that the aluminum layer adheres particularly well to it. "The reason for this is an intermediate layer just a few nanometers thick, which forms between the polymer and the aluminum during coating," explains Putz. The researcher now aims to study this intermediate layer in more detail—and use it purposefully. The layer is intended not only to enable improved super insulation for future satellites but also to accelerate the development of flexible electronics on Earth. For this research project, she received the "Ambizione Grant" from the Swiss National Science Foundation (SNSF) in 2020.

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Five nanometers make the difference

To precisely understand the intermediate layer and its effects on material properties, Barbara Putz and her doctoral student Johanna Byloff chose a simple model system: a 50-micrometer-thick polyimide film coated with 150 nanometers of aluminum. Between the metal and the plastic, the researchers apply an aluminum oxide coating only five nanometers thick. Working with such a thin intermediate layer is demanding. To ensure clean processing, the researchers use a coating machine from the Empa spin-off Swiss Cluster AG, founded in 2020 by researchers from the "Mechanics of Materials and Nanostructures" laboratory. The device enables multiple coating processes to be applied sequentially to the same workpiece without removing it from the vacuum chamber.

"Our material combination corresponds to that used for space applications, for example, in the European Mercury probe BepiColombo or the sun shield of NASA's James Webb Space Telescope," says Byloff. "The oxide intermediate layer forms naturally there, while we produce it deliberately, allowing the properties to be controlled."

The 21 by 14-meter sunshield of the space telescope also illustrates the demands placed on materials in space. In addition to the large temperature differences, the insulating layers are also subjected to mechanical stress. "On the one hand, the sunshield was stowed during the launch of the telescope and had to unfold at its destination without the layers tearing or separating from each other," explains Byloff. "On the other hand, particles and space debris can damage the foil. It is important that the damage remains localized and does not spread as long cracks across the entire surface."

From satellites to medical sensors

The researchers thoroughly tested their model foil, subjecting it to tensile tests, temperature shocks, and chemical and physical characterization. The result: the intermediate layer makes the material more stretchable and significantly more resistant to cracks and shear forces. Next, the researchers want to vary the thickness of the layer and apply it to other polymer substrates. "The natural intermediate layer forms only on polyimide and only at a thickness of five nanometers, which limits its usefulness," says Barbara Putz. "We expect that our artificial intermediate layer will enable multilayer systems on other polymers that previously were not viable due to poor adhesion of the coating."

Satellite insulation is not the only area where flexible multilayer systems are in demand. Putz and Byloff also see a major application for their research in the field of flexible electronics, which also relies on metal-coated polymer substrates. Thin-film components for electronic devices typically consist of multiple layers made from different materials. However, mechanical properties could also be improved there through the targeted use of thin intermediate layers. This could enable foldable or rollable devices, smart textiles, and flexible medical sensors.