Heat-resistant nanomaterials can help convert heat into electricity

A new nanophotonic material has broken records for high-temperature stability, which could lead to more efficient electricity production and open up a variety of new possibilities in controlling and transforming thermal radiation.

Developed by a team of material and chemical science engineers led by the University of Michigan, the material is flux-controlled and stable at temperatures up to 2,000 degrees Fahrenheit in air, an almost double improvement over current methods.

The material uses a phenomenon called destructive interference to reflect infrared energy while allowing shorter wavelengths to pass through. This can reduce heat waste in thermophotovoltaic cells, which convert heat into electricity but cannot use infrared energy, by reflecting infrared waves back into the system. The material could also be useful in photovoltaics, thermal imaging, environmental barrier coatings, sensing, camouflage from infrared monitoring devices and other applications.

Micrographs show no noticeable deterioration before and after heat treatment of the material. Image credit: André Lehnert
Andre Lehnert
Andre Lehnert

“It is similar to the way a butterfly’s wings use wave interference to obtain its color. Butterfly wings are made of colorless materials, but these materials are structured and textured in a way that absorbs some wavelengths of white light but reflects others, resulting in the appearance of color.” Andre LehnertUM assistant professor of chemical engineering and co-author of the study in Nature Photonics.

“This material does something similar with infrared energy. The hard part was preventing the breakdown of that color-producing structure under high temperatures.”

This approach represents a significant departure from the current state of engineered thermal emitters, which typically use foams and ceramics to reduce IR emissions. These materials are stable at high temperatures but provide very limited control over the wavelengths they allow to pass through. Nanophotons could provide more tunable control, but previous efforts were not stable at high temperatures, and often melt or oxidize (the process that forms rust on iron). In addition, many nanophotonic materials maintain their stability only in a vacuum.

His artist's rendering shows the material that reflects infrared light while allowing other wavelengths to pass through.  Image credit: André Lehnert
His artist’s rendering shows the material that reflects infrared light while allowing other wavelengths to pass through. Image credit: André Lehnert

The new material solves that problem, beating the previous record for heat resistance among air-stable photonic crystals by more than 900 degrees Fahrenheit outdoors. Additionally, the material is tunable, allowing researchers to modify it to modulate energy for a variety of potential applications. The research team predicted that applying these materials to existing TPVs would increase efficiency by 10% and believed that much larger efficiency gains would be possible with further improvement.

The team developed the solution by combining expertise in chemical engineering and materials science. Lennert’s chemical engineering team set out to find materials that wouldn’t mix, even if they started to melt.

“The goal is to find materials that maintain nice, clear layers that reflect light the way we want them to, even when things get super hot,” said Lennert. “So we looked for materials with very different crystal structures, because they tend not to mix.”

They assumed that a mixture of rock salt and perovskite, a mineral made of calcium and titanium oxides, fit the bill. Collaborators at UM and the University of Virginia ran supercomputer simulations to make sure the combination was a good bet.

John Heron
John Heron

John Heron, study co-author and assistant professor of materials science and engineering at UM, and Matthew Webb, a doctoral student in materials science and engineering, then carefully deposited the materials using pulsed laser deposition to achieve micro-layers with smooth interfaces. To make the material more durable, they used oxides instead of traditional photovoltaic materials; Oxides can be layered more precisely and are less likely to decompose under high temperatures.

“In previous work, conventional materials oxidized under high heat, losing their orderly layered structure,” Heron said. “But when you start with the oxides, this decomposition has already taken place. It leads to increased stability in the final layered structure.”

After testing confirmed that the material worked as designed, Sean McShere, first author of the study and a doctoral student in materials science and engineering at UM, used computer modeling to identify hundreds of other combinations of materials that would likely work as well. While commercial implementation of the material tested in the study may take years, the fundamental discovery opens a new door of research into a variety of other nanomaterials that could help future researchers develop a range of new materials for a variety of applications.

The research was supported by the Department of Defense, Defense Advanced Research Projects Agency with grant number HR00112190005.