Thermoelectric materials are being used widely to convert heat into electricity for different cutting-edge technologies, like energy recovery in cars. Still, they are mostly inefficient and can’t cope with the relatively high amounts of heat produced by an internal combustion engine, which limits their applicability.
That is why researchers around the world try to find better materials for them. A team of scientists from important US research institutes such as the Brookhaven National Laboratory, Columbia University, Argonne National Laboratory, Los Alamos National Laboratory, Northwestern University and the Swiss Federal Institute of Technology, while studying lead chalcogenides (lead paired with tellurium, selenium or sulfur), have discovered how these behave at an atomic scale and how they could offer great thermoelectric properties.
Brookhaven physicist Emil Bozin, first author on the paper, was the first to notice the odd behavior in the data, and he worked tenaciously to prove it was something new and not a data artifact. “If we had just looked at the average structure, we never would have observed this effect. Our analysis of atomic pair distribution functions gives us a much more local view – the distance from one particular atom to its nearest neighbors – rather than just the average,” Bozin says. The detailed analysis revealed that, as the material got warmer, these distances were changing on a tiny scale – about 0.025 nanometers – indicating that individual atoms were becoming displaced.
Studying the behavior of individual atoms, something never done before, they found out that lead chalcogenides can act as very good thermally insulating materials in some cases. If you drop the water’s temperatures below 0 degrees Celsius, you’ll get water crystals. If you go even further, the crystals will rearrange or become displaced to lower overall symmetry.
That’s not what happens to lead chalcogenides when you cool them extremely. They do not change their crystalline structure on cooling, at all, but when you heat them, displacements start to happen, with atoms flipping back and forth, like tiny dipoles. This seems to be the key to the material’s thermoelectric properties.
“The randomly flipping dipoles impede the movement of heat through the material in much the same way that it is more difficult to move through a disorderly wood than an orderly apple orchard where the trees are lined up in rows. This low thermal conductivity allows a large temperature gradient to be maintained across the sample, which is crucial to the thermoelectric properties,” says Simon Billinge, a physicist at Brookhaven Lab and Columbia University’s School of Engineering and Applied Science.
A next step, in the view of the researchers, would be finding new materials that could convert even better and that are much friendlier than lead: “Our next step will be searching for new materials that show this novel phase transition, and finding other structural signatures for this behavior,” Billinge said. “The new tools that allow us to probe nanoscale structures are essential to this research.
“Such studies of complex materials at the nanoscale hold the key to many of the transformative technological breakthroughs we seek to solve problems in energy, health, and the environment,” he added.