New material offers ecofriendly solution to converting waste heat into
energy
Purified tin selenide has extraordinarily high thermoelectric performance


Date:
August 2, 2021
Source:
Northwestern University
Summary:
Scientists have demonstrated a high-performing thermoelectric
material in a practical form that can be used in device
development. The material - - purified tin selenide in
polycrystalline form -- outperforms the single-crystal form in
converting heat to electricity, making it the most efficient
thermoelectric system on record. A key application target of
thermoelectric devices is capturing industrial waste heat, with
potentially enormous energy savings.



FULL STORY ========================================================================== Perseverance, NASA's 2020 Mars rover, is powered by something very
desirable here on Earth: a thermoelectric device, which converts heat
to useful electricity.


==========================================================================
On Mars, the heat source is the radioactive decay of plutonium, and
the device's conversion efficiency is 4-5%. That's good enough to power Perseverance and its operations but not quite good enough for applications
on Earth.

A team of scientists from Northwestern University and Seoul National
University in Korea now has demonstrated a high-performing thermoelectric material in a practical form that can be used in device development. The material -- purified tin selenide in polycrystalline form -- outperforms
the single-crystal form in converting heat to electricity, making it
the most efficient thermoelectric system on record. The researchers were
able to achieve the high conversion rate after identifying and removing
an oxidation problem that had degraded performance in earlier studies.

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 is 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 Northwestern's Mercouri Kanatzidis,
a chemist who specializes in the design of new materials. "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 more widespread application." Kanatzidis, the Charles
E. and Emma H. Morrison Professor of Chemistry in the Weinberg College
of Arts and Sciences, is a co-corresponding author of the study. He has
a joint appointment with ArgonneNational Laboratory.



========================================================================== Details of the thermoelectric material and its record-high performance
will be published Aug. 2 in the journal Nature Materials.

In Chung of Seoul National University is the paper's other
co-corresponding author. Vinayak Dravid, the Abraham Harris Professor of Materials Science and Engineering at Northwestern's McCormick School of Engineering,is one of the study's senior authors. Dravid is a long-time collaborator of Kanatzidis'.

Thermoelectric devices are already well defined, says Kanatzidis, but what makes them work well or not is the thermoelectric material inside. One
side of the device is hot and the other side 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 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.

'Something diabolical was happening' In 2014, Kanatzidis and his team
reported the discovery of a surprising material that was the best in the
world at converting waste heat to useful electricity: the crystal form
of the chemical compound tin selenide. While an important discovery,
the single-crystal form is impractical for mass production because of
its fragility and tendency to flake.



==========================================================================
Tin selenide in polycrystalline form, which is stronger and can be cut
and shaped for applications, was needed, so the researchers turned to
studying the material in that form. In an unpleasant surprise, they
found the material's thermal conductivity was high, not the desirable
low level found in the single- crystal form.

"We realized something diabolical was happening," Kanatzidis said. "The expectation was that tin selenide in polycrystalline form would not have
high thermal conductivity, but it did. We had a problem." Upon closer examination, the researchers discovered a skin of oxidized tin on the
material. Heat flowed through the conductive skin, increasing the thermal conductivity, which is undesirable in a thermoelectric device.

A solution is found, opening doors 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. The researchers then could produce
tin selenide pellets with no oxygen, which they then tested.

The true thermal conductivity of the polycrystalline form was 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.

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.

Northwestern owns the intellectual property for the tin selenide material.

Potential areas of application for the 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).

========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Megan Fellman. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Chongjian Zhou, Yong Kyu Lee, Yuan Yu, Sejin Byun, Zhong-Zhen Luo,
Hyungseok Lee, Bangzhi Ge, Yea-Lee Lee, Xinqi Chen, Ji Yeong Lee,
Oana Cojocaru-Mirédin, Hyunju Chang, Jino Im, Sung-Pyo
Cho, Matthias Wuttig, Vinayak P. Dravid, Mercouri G. Kanatzidis,
In Chung.

Polycrystalline SnSe with a thermoelectric figure of merit
greater than the single crystal. Nature Materials, 2021; DOI:
10.1038/s41563-021- 01064-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210802114928.htm

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