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New research advances field of thermoelectrics as waste heat recovery technology

Fundamental to the laws of thermodynamics, many systems, like car engines, produce heat as a by-product of their desired operation. It’s called waste heat.

But instead of going to waste, the heat sometimes can be utilized by another process to generate power and increase overall energy efficiency. Most waste heat recovery processes are driven by thermoelectric solid-state devices, in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances—known as the Seebeck effect.

Professor Joseph HeremansProfessor Joseph HeremansLed by Mechanical and Aerospace Engineering Professor Joseph Heremans, Ohio State engineers have demonstrated that magnetism can be used in composite materials to improve waste heat recovery. Their research paper, “Observation of spin Seebeck contribution to the transverse thermopower in Ni-Pt and MnBi-Au bulk nanocomposites,” appears in the December 12 issue of Nature Communications.

“Over half of the energy we use is wasted and enters the atmosphere as heat,” said co-author Stephen Boona, a postdoctoral researcher at Ohio State who designed the study. “Solid-state thermoelectrics can help us recover some of that energy we’re already producing but not using. These devices have no moving parts, don’t wear out, are robust and require no maintenance. Unfortunately, to date, they are also not quite efficient enough to warrant widespread use. We’re working to change that.”

A National Academy of Engineering member, Heremans is a pioneer in spin Seebeck research, which the Ohio State team has proven previously can, under certain conditions, be a more efficient thermoelectric process. The spin Seebeck effect describes a phenomenon where thermal non-equilibrium leads to the creation of magnetic waves—magnons—within magnetic materials. The spinning magnons polarize electrons in adjacent nano-scale metallic films, leading to a voltage in the metal.

The Ohio State team’s most recent work expands on the use of magnetism to drive electrons in a temperature gradient. Until now, research demonstrations in this field were done exclusively with thin films of metals like platinum deposited on insulating magnets. These structures are useful in physics experiments for isolating related phenomena from one another in order to understand how they work. However, the films are so thin that they are not practical for most real-world energy conversion applications because they have little useful volume.

In their breakthrough, the researchers have made great strides toward practicality by showing that thin films are not necessary to observe the effect. Instead of a thin film of platinum on top of the magnetic material, they distributed platinum nanoparticles randomly throughout the material. The composite material containing the platinum nanoparticles produced enhanced voltage output due to the spin Seebeck effect. This means that for a given temperature difference, the composite material generates more electrical power than either material can on its own. Since the entire device volume is electrically conducting, the electrical power can be extracted with increased efficiency. The idea is very general and can be applied to a variety of material combinations, enabling entirely new approaches that don’t require expensive metals like platinum or delicate processing procedures like thin-film growth.

“It required rethinking the thermoelectric equation completely,” said Heremans. “The engineering has to be done differently so you can get the necessary cost benefits from using these metals.”

Until this research was performed, the spin Seebeck effect had only been analyzed in thin film structures. While not yet a real-world device, Heremans is confident the proof-of-principle established by this study will inspire further research that may lead to applications for common waste heat generators like automobile and jet engines.

The research has received funding from the National Science Foundation’s Materials Research Science and Engineering Program and the U.S. Army’s Multidisciplinary University Research Initiative (MURI). The nanocomposite materials were synthesized with the help of the study’s co-author Koen Vandaele, a visiting scholar from Ghent University in Belgium. Also involved in the study were co-authors and microscopy experts Isabel Boona and Professor David McComb from Ohio State’s Center for Electron Microscopy and Analysis (CEMAS), who used CEMAS’s state-of-the-art materials characterization equipment to validate the findings.

High-angle annular dark-field (HAADF) image of a Ni 2wt% Pt composite material overlaid with false-color elemental mapping of Pt (red), Ni (green) and O (blue) determined by X-ray energy-dispersive spectroscopy in a scanning transmission electron microscope. In both images, the Ni particles are coated in a thin layer of NiO and decorated with well-dispersed Pt nanoparticles.