Tracking the structure of waste-heat recovery systems

Transport, household appliances and electronic devices produce waste heat- lots of it. Thermoelectric materials, which can convert a heat gradient into a voltage, are a possible solution for recovering and using waste heat. However, their exact mechanism is still unclear. This is despite decades of applications, including in the power packs of the Voyager space probe and Mars rover, for example.

Now, a team of researchers led by Université Bourgogne Franche-Comté, Oak Ridge National Laboratory (ORNL), Columbia University, Brookhaven National Laboratory and the ESRF has developed new scattering techniques to address this question for the material GeTe. This is part of a family of so-called IV-VI materials, which are some of the oldest and best performing thermoelectrics.

The thermoelectric effect relies upon a heat gradient; therefore, it requires materials that resist heat flow, while preserving electrical conductivity. In the case of GeTe, this was believed to be due to intrinsic nanoscale disorder, despite its apparently simple table-salt structure.

The team used the brilliant high-energy X-rays of the ESRF’s ID15 beamline to investigate GeTe, using the X-ray Pair Distribution Function technique.  "The high-energy X-rays available on the ID15 beamline were crucial for this study, as they allowed us to chart the real-space atomic displacements relevant to thermoelectricity in exquisite detail", says Simon Kimber, post-doctoral researcher at the ESRF at the time and first author of the publication. And he adds: "All of the X-ray measurements were performed in collaboration with my peers, and this paper is a testament to the excellent scientific atmosphere and international spirit of collaboration at the ESRF".

At high temperatures (~550 °C), which correspond to device operating conditions, researchers observed local structural deviations. At first glance, these seem to break symmetry locally, i.e. to represent static disorder. However, complementary measurements performed with neutrons at ORNL showed that these displacements were actually dynamic, and induced by the high temperature.

To resolve this apparent contradiction, the researchers turned to molecular dynamic simulations performed using the supercomputers at ORNL. This showed that, on average, the atomic structure of GeTe looks highly crystalline. However, snapshots on a picosecond time-sale reveal an intricate pattern of atomic vibrations, which disrupt heat flow.

The X-ray data contained a further mystery: The data showed that the crystal structure of GeTe was much more rigid in certain directions than along others. Theoreticians from Argonne National Laboratory, the University of Chicago, and the University of Costa Rica explained this final piece of the jigsaw. Their model shows that the local fluctuations seen by X-ray scattering actually reinforce chemical bonding in certain directions. This explains the mystery of GeTe’s properties. Dynamic disorder is found at high temperature, which suppresses heat conduction. However, at the same time it reinforces the overlap of atomic orbitals along the unit cell edges. This favours electrical conduction. The model is surprisingly general, and can be applied to many other materials.

Simon Billinge, professor at Columbia University and co-author of the publication, concludes that "wide adoption of thermoelectrics in devices in households is tantalizingly close, but still out of reach. A better fundamental understanding of the basic science behind what makes current thermoelectrics good will help us take the next step in increasing performance. At that point, every household will have a quiet, efficient solid-state refrigerator, a development comparable to the solid-state lighting revolution we just went through. Advancing this basic understanding is what I find so exciting about this study”.

Reference:

Kimber, S.A.J., et al. Nat. Mater. (2023). https://doi.org/10.1038/s41563-023-01483-7

Top image: Evidence for disorder from synchrotron pair distribution function measurements. The picture is a fragment of the disordered structure required to fit the data. The Ge sites (purple) need to be displaced and split from their positions. Credits: S. Kimber