This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Researchers from Tokyo Metropolitan University have used a theoretical framework to derive design rules for creating new thermoelectric materials. The efficiency of converting thermal energy to electricity is often determined by parameters that strongly depend on each other, making it difficult to identify ideal recipes.
The team's findings, published in the journal Materials Today Advances, help lay out goals for optimizing "band structure," a map showing the states electrons can take, promising a shift toward rational material design. Around 60% of the energy generated by burning fossil fuels is lost as heat. Efficient ways to convert this "waste heat" to electricity would be an important stepping stone as the world tries to reduce oil and gas consumption.
However, "thermoelectric" conversion remains a big challenge. The physical effect that underlies thermoelectric technology, the Seebeck effect, relies on a current being generated when two different materials are brought into contact and heated. While it is possible to improve conversion efficiency by adding small amounts of a different element ("doping") to modify properties, the factors that determine successful conversion of heat to current often depend on each other in complex ways, making it challenging to nail down quantitative rules for exactly how materials might be tweaked.
In recent work, a team led by Assistant Professor Yuya Hattori of Tokyo Metropolitan University uncovered design rules that apply across a wide range of materials. They used a theoretical framework called Boltzmann transport theory to show that there are common conditions under which thermoelectric conversion efficiency might be maximized. Their work optimizes the "band structure" of materials, a map of the energies that charge carriers like electrons and "holes" ("lack of electrons") can take.
A key feature of band structure is the band gap, the energy gap separating the highest-energy electrons in a material and the lowest-energy states to which they can shift. For example, they discovered that the "bipolar effect," where electrons and holes contribute in opposite directions to the Seebeck effect, reducing performance, becomes activated when the thermal energy of the environment reaches around five times the band gap. The team focused on "band converged" materials, where fine-tuning of composition can create electrons in different environments inside a material with the same or similar energy, all of which might contribute to thermoelectric transport.
They found that the figure of merit for the Seebeck effect was maximized when the energies were precisely matched. They also investigated the optimal chemical potential, the energy required to add a single electron to the material, and how it informs the best amounts of dopant to use for different materials. While universal rules are hard to come by, their work shines a light on key principles that might help identify new recipes for better thermoelectric materials.
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