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Kanatzidis' colleague Christopher M. Wolverton, a computational theorist, calculated the electronic structure of tin selenide. He found the electrical properties could be improved by adding a doping material.
"Tin selenide is very unusual, not only because of its exceedingly low thermal conductivity, but also because it has many conduction lanes," said Wolverton, a senior author of the paper and professor of materials science and engineering in the McCormick School of Engineering and Applied Science. "Our calculations said if the material could be doped, its thermal power and electrical conductivity would increase. But we didn't know what to use as a dopant."
Sodium was the first dopant the researchers tried, and it produced the results they were looking for. "Chris' computations opened our eyes to doping," Kanatzidis said. He and Zhao successfully grew crystals of the new doped material.
The researchers also were pleased to see that adding sodium did not affect the already very low thermal conductivity of the material. It stayed low, so the heat stays on one side of the thermoelectric material. Electrons like to be in a low-energy state, so they move from the hot (high-energy) side to the cool side. The hot side becomes positive, and the cool side becomes negative, creating a voltage.
"Previously, there was no obvious path for finding improved thermoelectrics," Wolverton said. "Now we have discovered a few useful knobs to turn as we develop new materials."
The efficiency of waste heat conversion in thermoelectrics is reflected by its "figure of merit," called ZT. In April 2014, the researchers reported that tin selenide exhibits a ZT of 2.6 at around 650 degrees Celsius. That was the highest ZT to date -- a world record. But the undoped material produced that record-high ZT only at that temperature. (There is a ZT for every temperature.)
The new doped material produces high ZTs across a broad temperature range, from room temperature to 500 degrees Celsius. Thus, the average ZT of the doped material is much higher, resulting in higher conversion efficiency.
"Now we have record-high ZTs across a broad range of temperatures," Kanatzidis said. "The larger the temperature difference in a thermoelectric device, the greater the efficiency."
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