Polymer Thin Interlayers Protect Satellites from Extreme Temperatures

Superlight, superflexible, superinsulating: To protect satellites from extreme temperatures, an aluminum-coated polymer foil is used. Researchers at Empa Thun have made the material even more resilient by using an ultra-thin interlayer.

Who doesn't know the archetypal image of a satellite: two outstretched solar "wings" and a compact body wrapped in gold or silver shimmering foil. Researchers from the "Mechanics of Materials and Nanostructures" lab at Empa in Thun are precisely focused on this foil.

The film in question is so-called "Multilayer Insulation," also known as superinsulation. It consists of multiple layers of a durable polymer coated with a thin layer of metal—typically aluminum. On Earth, the coated film can be encountered in the form of emergency blankets. Onboard spacecraft, the superinsulation protects electronics from temperature fluctuations. "For satellites in low Earth orbit, the temperature difference between the sun-facing and sun-shaded sides is around 300 °F," says Empa researcher Barbara Putz. "However, electronics function best at a room temperature of 77 °F." Since it is directly exposed to space conditions, the superinsulation itself must be highly durable.

Accelerating the Development of Flexible Electronics on Earth

Polyimide, a robust material, is typically used as the polymer base for the thin-film structure. In addition to its temperature and vacuum resistance, it is notable for its excellent adhesion to aluminum coatings. "The reason for this is a nanometer-thin intermediate layer that forms between the polymer and the aluminum during the coating process," explains Putz. The researcher now aims to examine this intermediate layer in greater detail—and apply it strategically. The layer is not only expected to enable better superinsulation 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.

0.2 Microinches 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 0.002 inches-thick polyimide film coated with 6 microinches of aluminum. The researchers apply a coating of aluminum oxide, measuring just five nanometers, between the metal and the polymer. Working with such a thin intermediate layer is challenging. 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 allows several coating processes to be sequentially applied to the same workpiece without removing it from the vacuum chamber.

"Our material combination corresponds to the one used in space applications, for example, on the European Mercury probe BepiColombo or the sunshield of NASA's James Webb Space Telescope," says Byloff. "Only there the oxide interlayer forms naturally, while we produce it deliberately, allowing us to tailor the properties."

The 69 by 46 feet sunshield of the space telescope also highlights the demands placed on materials in space. In addition to the large temperature differences, the insulating layers are also exposed to mechanical stresses. "On the one hand, the sunshield was stowed during the telescope's launch 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 film. It is crucial 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 film, subjecting it to stretching experiments, 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 plan 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, which have so far been unsuitable due to poor coating adhesion."

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

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