Thin Interlayers Strengthen Flexible Composite Materials Cool Satellites and Flexible Electronics

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Ultralight, super-flexible, highly insulating: To protect satellites from extreme temperatures, an aluminum-coated polymer film is used. Researchers at Empa have succeeded in making the material even more robust by adding an ultra-thin interlayer. This approach could also improve flexible electronics and medical sensors.

Who doesn’t recognize the archetypal image of a satellite: two extended solar “wings” and a compact body wrapped in a gold- or silver-shimmering foil. It is precisely this foil that researchers from Empa’s “Mechanics of Materials and Nanostructures” laboratory in Thun are studying. Its widespread use already reveals its importance: the foil is essential for satellites and space probes. It is known as multilayer insulation (MLI), or “superinsulation.” It consists of several layers of a robust polymer coated with a thin metal layer—usually aluminum. On Earth, this coated foil is familiar, for example, in the form of emergency blankets.

On board spacecraft, the superinsulation protects the electronics from temperature fluctuations. “For satellites in low Earth orbit, the temperature difference between the sun-facing and sun-shaded sides is around 200 degrees,” says Empa researcher Barbara Putz. “A similar temperature difference also occurs when a satellite passes into the Earth’s shadow or moves back into sunlight—and that happens 16 times a day.” Electronics, however, function best at room temperature. And because they are directly exposed to the conditions of space, the superinsulation itself has to withstand a great deal.

As the polymer base for the thin-film structure, the extremely resilient polyimide is usually used. In addition to its resistance to temperature and vacuum, this plastic is distinguished by the fact that the aluminum layer adheres particularly well to it. “The reason for this is an interlayer only a few nanometers thick that forms during the coating process at the interface between the polymer and the aluminum,” explains Putz. She now wants to investigate this interface in more detail and make targeted use of the interlayer. It is intended not only to enable better superinsulation for future satellites, but also to accelerate the development of flexible electronics on Earth. For this research project, she has received an Ambizione Grant from the Swiss National Science Foundation (SNSF).

Five Nanometers Make the Difference

To better understand the interlayer and its effects on material properties, 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 polymer, the researchers add an aluminum oxide coating just five nanometers thick. Working with such an ultra-thin interlayer is challenging. To ensure clean processing, the researchers use a coating system from the Empa spin-off Swiss Cluster AG, which was founded in 2020 by researchers from the “Mechanics of Materials and Nanostructures” laboratory. The system makes it possible to apply different coating processes sequentially to the same workpiece without removing it from the vacuum chamber.

“Our material combination corresponds to those used in space applications, such as the European Mercury probe BepiColombo or the sunshield of NASA’s James Webb Space Telescope,” says Byloff. “The difference is that in those cases the thin interlayer forms naturally, whereas we produce it in a targeted way, allowing the properties to be tuned.” The space telescope’s sunshield, measuring 69 by 46 feet, also illustrates the demands placed on the material in space. In addition to extreme temperature differences, the insulating layers are exposed to mechanical stresses. “On the one hand, the sunshield was stowed during transport and had to unfold at its destination without the layers tearing or delaminating,” Byloff explains. “On the other hand, particles and space debris can damage the foil. It is crucial that such damage remains localized and does not spread as long cracks across the entire surface.”

From Satellites to Medical Sensors

The researchers put their model film through its paces, subjecting it to tensile tests and thermal shocks and characterizing it chemically and physically. The result: the new interlayer makes the material more stretchable and significantly more resistant to cracking and delamination. Next, the researchers plan to vary the thickness of the layer and apply it to other polymer substrates. “The natural interlayer can form only on a few polymers and only at a thickness of around five nanometers, which limits its usefulness,” says Barbara Putz. “We expect that our artificial interlayer will enable multilayer systems on other polymers that were previously ruled out due to poor coating adhesion.”

Satellite insulation is not the only field in which flexible multilayer systems are in demand. Putz and Byloff also see major potential applications for their research in flexible electronics, which likewise rely on metal-coated polymer substrates. Thin-film components for electronic devices typically consist of multiple layers of different materials. Here too, the mechanical properties could be improved through the targeted use of thin interlayers. This could, for example, advance the development of foldable or rollable devices, as well as smart textiles and conformable medical sensors.

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