Qiang Li. Photo: Brookhaven National Laboratory |
Scientists from the Chinese
Academy of Science’s Shanghai
Institute of Ceramics, in collaboration with scientists from the United States
Department of Energy’s (DOE) Brookhaven National Laboratory, the University of Michigan, and the California Institute
of Technology, have identified a new class of high-performance thermoelectric
materials—materials that convert heat to electricity or vice versa. In the
material described in a paper published in Nature Materials, liquid-like copper
ions carry electric current around a solid selenium crystal lattice. The
research offers a new strategy and direction for developing high-efficiency
thermoelectric materials, which have a wide range of potential applications—including
systems that convert solar energy and/or waste heat from engines and industrial
processes directly into useable electricity.
“About 60% of the power we get from primary energy sources today
goes to waste, mostly in the form of heat,” said Brookhaven physicist Qiang Li,
leader of Brookhaven’s Advanced Energy Materials Group and a co-author on the
paper. “If we can recover that waste heat as electricity, it would be a huge
economic benefit.”
In the best thermoelectric materials, electric charge carriers
such as electrons move easily from an area of higher temperature to lower
temperature, as long as a difference in temperature is maintained. One key to
maintaining the heat gradient, even at the high temperatures found in
automobile engines, is to use materials with low thermal conductivity.
Many groups have made advances in thermoelectrics by introducing
glass-like substances into crystalline semiconductors to disrupt the regular
structure and dampen vibrations along the crystalline lattice. Since a large
part of heat moves via these vibrations, the dampening effect reduces thermal
conductivity.
But a solid glass still propagates some heat through transverse,
shear vibrations. A liquid does not.
“Our approach of using liquid-like ions instead of a solid glass
is a new strategy to decrease thermal conductivity below that of a glass,” said
Li. “It not only reduces the ‘mean free path’ of lattice vibrations—how far
these vibrations can travel—but also eliminates some of the vibrational modes
completely.”
The reduction in thermal conductivity results in a
thermoelectric material with a “figure of merit”—a measure of its performance—that
is among the highest for any bulk material, even at temperatures of 1,000 K
(nearly 730 C). That’s a higher operating temperature than bismuth-tellurium-based
thermoelectric materials used in limited applications today. And unlike those
materials, the new copper-selenium material contains no rare-earth elements.
This research therefore provides a new strategy and direction
for developing high-efficiency thermoelectric materials for more widespread
applications.
“Over the past decade, there has been an increasing sense of
urgency in the field of thermoelectrics, driven primarily by the demand for
alternative energy and waste heat recovery,” said Li. “Thermoelectric
generation systems convert heat into electric energy directly without producing
carbon dioxide gas, radioactive substances, or other emissions. Improving the
performance and use of these materials could therefore play an immensely
important role in the worldwide effort to recover waste heat and develop
alternative energy technologies to reduce our dependence on fossil fuels and
reduce greenhouse gas emissions.”
