New model goes against type to predict thermoelectric materials

The promise of thermoelectric materials as a source of clean energy has driven the search for materials that can efficiently produce substantial amounts of power from waste heat. Now, in a paper in Science Advances, researchers report the discovery of a new explanation for asymmetrical thermoelectric performance. This is the phenomenon that occurs when a thermoelectric material that is highly efficient in a form that carries a positive charge, known as ‘p-type’, is far less efficient in the form that carries a negative charge, known as ‘n-type’, or vice versa.

Zhifeng Ren, professor of physics at the University of Houston (UH), director of the Texas Center for Superconductivity at UH and corresponding author of the paper, and his team have developed a model to explain the previously unaddressed disparity in performance between the two types of formulations. They then used this model to predict promising new materials for generating power using waste heat from power plants and other sources.

The researchers already knew that thermoelectric efficiency depends on the performance of the material in both forms, p-type and n-type. But most materials either don't exist in both formulations, or one type is more efficient than the other.

It is possible to build effective thermoelectric devices using just a p-type or n-type compound, but it is easier to design a device that contains both types. Ren said the best performance would come when both types exhibit similar properties.

Jun Mao, a post-doctoral researcher at UH and co-author of the paper, said they determined that the asymmetrical performance of some thermoelectric materials is linked to the charge moving at different rates in the two types of formulation. "If the charge movement of both the positive charge, for p-type, and the negative charge, for n-type, is similar, the thermoelectric performance of both types is similar," he said.

Knowing that, they were able to use the mobility ratio to predict the performance of previously unstudied formulations.

"When the thermoelectric performance for one type of a material has been experimentally studied, while the other type has not yet been investigated, it is possible to predict the ZT by using the identified relationship between the asymmetry and weighted mobility ratio," the researchers wrote in the paper. ZT, or the figure of merit, is a metric used to determine how efficiently a thermoelectric material converts heat to electricity.

Hangtian Zhu, a post-doctoral researcher at UH and another co-author, said the next step is determining how to formulate the corresponding type of material, once a material with a high efficiency in either p-type or n-type is found. That can require experimentation to determine the best dopant – researchers tweak performance by adding a tiny amount of an additional element to the compound, known as ‘doping’ – to improve performance.

According to Zhu, that's where the new understanding of asymmetrical performance comes in. By predicting which compounds will have high performance in both types, researchers can be encouraged to continue looking for the best combination, even if early efforts did not succeed.

The researchers have already synthesized one of their predicted materials, a zirconium-cobalt-bismuth compound. This had a measured heat-to-electricity conversion efficiency of 10.6% at both the cold side, about 303K (86°F), and the hot side, about 983K (1310°F), for both the p-type and the n-type.

This story is adapted from material from the University of Houston, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.