Converting waste heat into energy - Composite material yields 10 times voltage output

a scanning transmission electron microscope image of a nickel-platinum composite material. At left, the image is overlaid with false-colour maps of elements in the material, including platinum (red), nickel (green) and oxygen (blue).
Engineers from The Ohio State University have used magnetism on a composite of nickel and platinum to amplify voltage output 10 times or more - not in a thin film, as they had done previously, but in a thicker piece of material that more closely resembles components for future electronic devices.

A growing area of research called solid-state thermoelectrics aims to capture waste heat, produced by electrical and mechanical devices, inside specially designed materials to generate power and increase overall energy efficiency.

“Over half of the energy we use is wasted and enters the atmosphere as heat,” explained Stephen Boona, postdoctoral researcher at Ohio State. “Solid-state thermoelectrics can help us recover some of that energy.

“These devices have no moving parts, don't wear out, are robust and require no maintenance,” he continued. “Unfortunately, to date, they are also too expensive and not quite efficient enough to warrant widespread use. We're working to change that.”

In 2012, the same Ohio State research group, led by Joseph Heremans, demonstrated that magnetic fields could boost a quantum mechanical effect called the spin Seebeck effect, and in turn boost the voltage output of thin films made from exotic nano-structured materials from a few microvolts to a few millivolts.

In this latest advance, the researchers have increased the output for a composite of two common metals, nickel with platinum, from a few nanovolts to tens or hundreds of nanovolts. The researchers say this smaller voltage is in a much simpler device that requires no nanofabrication and can be scaled up for industry.

Heremans, a professor of mechanical and aerospace engineering and the Ohio Eminent Scholar in Nanotechnology, said that, to some extent, using the same technique in thicker pieces of material required that he and his team rethink the equations that govern thermodynamics and thermoelectricity, which were developed before scientists knew about quantum mechanics. While quantum mechanics often concerns photons, Heremans' research concerns magnons - waves and particles of magnetism.

Research in magnon-based thermodynamics was up to now always done in thin films and even the best-performing films produce very small voltages.

Previously, his team described hitting electrons with magnons to push them through thermoelectric materials. Now, it has shown that the same technique can be used in bulk pieces of composite materials to further improve waste heat recovery.

Instead of applying a thin film of platinum on top of a magnetic material as they might have done before, the researchers distributed a small amount of platinum nanoparticles randomly throughout a magnetic material - in this case, nickel. The resulting composite produced enhanced voltage output due to the spin Seebeck effect. This means that for a given amount of heat, the composite material generated more electrical power than either material could on its own. Since the entire piece of composite is electrically conducting, other electrical components can draw the voltage from it with increased efficiency compared to a film.

While the composite is not yet part of a real-world device, Prof Heremans expects the proof-of-principle established by this study will inspire further research that may lead to applications for common waste heat generators, including car and jet engines.

Tom Austin-Morgan

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