Thermoelectric materials—substances that exhibit the so-called thermoelectric effect, in which a temperature difference creates an electric potential or vice versa—have been known for decades and employed in applications including power generation and refrigeration. Despite their commercial use, however, scientists have remained somewhat baffled as to why some of these materials work well for thermoelectric applications, while others made up of similar elements do not.
Now, a team of researchers at MIT, Rutgers University and the University of Notre Dame say they have found a theoretical explanation for this difference, which could lead to the discovery of new, better thermoelectric materials.
According to the researchers, the solution was discovered through ongoing research on phase-change materials, which are being examined as a potential replacement for flash memory in electronic devices. These materials switch between a solid and liquid state in response to a change in temperature and are capable of absorbing or emitting massive amounts of energy. Analysis of phase-change materials has shown this change in state occurs because of a kind of chemical bonding known as resonant bonding—a type of bond in which electrons flip back and forth between several adjacent atoms. Though resonant bonds’ effects on electrical and optical properties have been studied, says MIT graduate student Sangyeop Lee, no one had previously looked at their effect on thermal properties.
“There is little communication between people doing phase-change research and those doing thermoelectric research,” Lee said, adding that interdisciplinary meetings at MIT were key to the new study published in Nature Communications.
Researchers found that the electrons’ “flipping” in resonant bonding lead to long-range interactions between atoms, producing the material’s low thermal conductivity. This effect could explain known discrepancies between similar materials of low and high thermal conductivity.
“This paper thus gives new insight and fundamental understanding into the nature of heat flow in materials, and it also provides a new paradigm that opens the door” in the search for new materials for both thermoelectric energy conversion and phase-change memory applications, David Broido, a professor of physics at Boston College who was not connected with this study, said.
“This is an example where communication between people with different backgrounds can lead to new opportunities and boost understanding,” Lee said.
Further research is required to identify new materials that could be useful for thermoelectric devices, says Gang Chen, the Carl Richard Soderberg Professor of Power Engineering at MIT, as an ideal material must combine low thermal conductivity and high electrical conductivity.
