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Researchers from Northwestern University in the US and Seoul National University in Korea have demonstrated a high-performing thermoelectric material in a practical form that can be used to develop devices that convert heat to useful electricity. 

The material — purified tin selenide in polycrystalline form — outperforms the single-crystal form of selenide in converting heat to electricity and sturdiness, making it the most efficient thermoelectric system on record. 

The researchers were able to achieve a high conversion rate after identifying and removing an oxidation problem that had degraded performance in earlier studies.

In a paper published in the journal Nature Materials, the scientists explain that the polycrystalline tin selenide could be developed for use in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings. 

A key application target would be capturing industrial waste heat — such as from power plants, the automobile industry, and glass- and brick-making factories — and converting it to electricity. More than 65% of the energy produced globally from fossil fuels is lost as waste heat. 

“Thermoelectric devices are in use, but only in niche applications, such as in the Mars rover,” said Mercouri Kanatzidis, co-author of the study, in a media statement.

“These devices have not caught on like solar cells, and there are significant challenges to making good ones. We are focusing on developing a material that would be low cost and high performance and propel thermoelectric devices into a more widespread application.”

According to Kanatzidis, thermoelectric devices are already well defined, but what makes them work well or not is the thermoelectric material inside. One side of the device is hot and the other side is cold. The thermoelectric material lies in the middle. Heat flows through the material, and some of the heat is converted to electricity, which leaves the device via wires.

The researcher explained that the material needs to have extremely low thermal conductivity while still retaining good electrical conductivity to be efficient at waste heat conversion. And because the heat source could be as high as 400-500 degrees celsius, the material needs to be stable at very high temperatures. These challenges and others make thermoelectric devices more difficult to produce than solar cells.

Making the right material

Before reaching the current state of development, the tin selenide in polycrystalline form material was found to have high thermal conductivity, which is not desirable in a thermoelectric device.

Upon closer examination, the researchers discovered a skin of oxidized tin on the material. Heat flowed through the conductive skin, increasing the thermal conductivity. 

After learning that the oxidation came from both the process itself and the starting materials, the Korean team found a way to remove the oxygen, which allowed them to produce tin selenide pellets with no oxygen.

The true thermal conductivity of the polycrystalline form was then measured and found to be lower, as originally expected. Its performance as a thermoelectric device, converting heat to electricity, exceeded that of the single crystal form, making it the most efficient on record. 

In the paper, the researchers explain that the efficiency of waste heat conversion in thermoelectrics is reflected by its ‘figure of merit,’ a number called ZT. The higher the number, the better the conversion rate. The ZT of single-crystal tin selenide earlier was found to be approximately 2.2 to 2.6 at 913 Kelvin.

In this new study, the researchers found the purified tin selenide in polycrystalline form had a ZT of approximately 3.1 at 783 Kelvin. Its thermal conductivity was ultralow, lower than the single-crystals.

“This opens the door for new devices to be built from polycrystalline tin selenide pellets and their applications explored,” Kanatzidis said.