Surprising material could play role in saving energy

April 17th, 2014
One strategy for addressing the world's energy crisis is to stop wasting so much energy when producing and using it, which can happen in coal-fired power plants or transportation. Nearly two-thirds of energy input is lost as waste heat.

Now Northwestern University scientists have discovered a surprising material that is the best in the world at converting waste heat to useful electricity. This outstanding property could be exploited in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings.

An interdisciplinary team led by inorganic chemist Mercouri G. Kanatzidis found the crystal form of the chemical compound tin selenide conducts heat so poorly through its lattice structure that it is the most efficient thermoelectric material known. Unlike most thermoelectric materials, tin selenide has a simple structure, much like that of an accordion, which provides the key to its exceptional properties.

The efficiency of waste heat conversion in thermoelectrics is reflected by its figure of merit, called ZT. Tin selenide exhibits a ZT of 2.6, the highest reported to date at around 650 degrees Celsius. The material's extremely low thermal conductivity boosts the ZT to this high level, while still retaining good electrical conductivity.

The ZT metric represents a ratio of electrical conductivity and thermoelectric power in the numerator (which needs to be high) and thermal conductivity in the denominator (which needs to be low).

Potential areas of application for the high-temperature thermoelectric material include the automobile industry (a significant amount of gasoline's potential energy goes out of a vehicle's tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places where large combustion engines operate continuously (such as in large ships and tankers).

"A good thermoelectric material is a business proposition—as much commercial as it is scientific," said Vinayak P. Dravid, a senior researcher on the team. "You don't have to convert much of the world's wasted energy into useful energy to make a material very exciting. We need a portfolio of solutions to the energy problem, and thermoelectric materials can play an important role."

Dravid is the Abraham Harris Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science.

Details of tin selenide, probably among the world's least thermally conductive crystalline materials, are published today (April 17) by the journal Nature.

The discovery comes less than two years after the same research group broke the world record with another thermoelectric material they developed in the lab with a ZT of 2.2.

"The inefficiency of current thermoelectric materials has limited their commercial use," said Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences. "We expect a tin selenide system implemented in thermoelectric devices to be more efficient than other systems in converting waste heat to useful electricity."

The material, despite having a very simple structure, conducts heat so poorly that even moderate thermoelectric power and electrical conductivity are enough to provide high thermoelectric performance at high temperature.

The researchers did not expect to find tin selenide to be such a good thermoelectric material.

"Lidong Zhao, the first author of the paper, deserves a lot of credit for looking at tin selenide," said Kanatzidis, who also holds a joint appointment at Argonne National Laboratory. "He is a good example of the curious people we try to attract to Northwestern."

Zhao, a postdoctoral fellow in Kanatzidis' research group, grew crystals of tin selenide and measured the crystal in three directions, along each axis. He found that the thermal conductivity was "ridiculously low" along the a-axis but also along the other two axes.

"The results are eye-opening because they point in a direction others would not look," Dravid said. "This material has the potential to be applied to other areas, such as thermal barrier coatings."

Kanatzidis and Zhao identified the potential of the material intuitively by looking at its crystal structure. They confirmed its exceptional thermoelectric properties and then turned to Dravid and Christopher M. Wolverton to uncover how the crystal was behaving and why.

"We found that the bonds between some atoms in this compound are very weak and lead to exceptionally soft, floppy atomic vibrations," said Wolverton, a senior author of the paper and a professor of materials science at the McCormick School.

Wolverton, an expert in computational materials science related to energy applications, showed that the accordion-like structure and weak bonds lead to atoms that vibrate very slowly.

"These very weak vibrations are responsible for the inability of the material to conduct heat," Wolverton said. "Our theory provides the scientific basis as to why the material behaves the way it does and also provides us with a new direction to search for even higher-efficiency materials."

"Tin selenide reminds us of that popular TV commercial for a memory foam mattress in which a person can jump on one side of the mattress while a glass of wine a few feet away is unperturbed—the vibrations do not reach the glass because of the mattress' material," Kanatzidis said.

"Similarly, in tin selenide, heat cannot travel well through this material because its soft, accordion-like structure doesn't transmit vibrations well," he said. "One side of tin selenide gets hot—where the waste heat is, for example—while the other side remains cool. This enables the hot side to generate useful electricity."

"Our discovery underscores why the Department of Energy EFRC program works," Kanatzidis said. "A multidisciplinary team, such as ours, can look at a problem from many different angles, with sustained funding increasing the chances of a scientific breakthrough. And we have a special ambience here—the spirit of Northwestern is interdisciplinary."

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