High-efficiency thermoelectric materials could lead to new types of
cooling systems, and new ways to scavenge waste heat for electricity.
Researchers at Rensselaer Polytechnic Institute in Troy, New York, have now developed an easy, inexpensive process to make such materials.
The materials made by the RPI team already perform as well as those
on the market, and the new process, which involves zapping chemicals in a
microwave oven, offers room for improvement. "We haven't even optimized
the process yet," says Ganpati Ramanath, a materials science and engineering professor at RPI. "We're confident that we can increase the efficiency further."
Cooked to order: Zapping raw materials in a microwave oven and
drying the resulting solution produces a black powder (top) made of
hexagonal bismuth telluride nanoplates (bottom).
Thermoelectric materials convert heat into electricity, and vice
versa. They are used in niche applications such as power generation on
spacecraft and temperature-controlled car seats. If they were cheaper
and more efficient, they could perhaps be used to make lightweight
refrigerators, cooling systems for computer chips and buildings, and for
using car exhaust heat to power electronics such as headlights and the radio.
Good thermoelectrics need to conduct electricity well but heat
poorly. One way to boost the heat-transfer efficiency of such materials
is to give them nanoscale features that block the flow of heat without restricting electric current.
Researchers have made nanostructured materials by breaking up crystals
into fine powder. But this process is energy intensive and only results
in high-efficiency p-type thermoelectric materials—the kind rich in
positively charged particles called holes. But both p-type and n-type
materials (which have an abundance of electrons) are needed for
practical devices.
"We've shown that we can make both p- and n-type materials, and we
can do this very scalably and more cost-effectively," Ramanath says. "We
can make gram quantities in minutes."
Ramanath and his colleagues make a solution from raw materials such
as tellurium and bismuth chloride in an organic solvent, and put it in a
domestic microwave oven for two to three minutes. They get a solution
containing hexagonal nanoplates, which they press together and heat to
make nanopellets. By using a solvent containing sulfur, the researchers
get sulfur-doped nanoplates that are n-type.
The technique, presented in a Nature Materials paper
posted online last week, makes p-type materials that are as efficient
as the best ones on the market, while the n-type materials are at least
25 percent more efficient. One of the biggest commercial thermoelectric
device manufacturers is now interested in adopting the new materials and
process.
"This is the first nanostructured n-type mat with a high [efficiency] value," says John Badding, a professor of chemistry at Penn State University.
The key breakthrough of the RPI work, according to Badding, is that
the researchers are building the nanostructured materials from the
bottom up using chemistry. This means they can fine-tune the properties
of the building blocks and their assembly to improve the material's
properties. "The way they're making the material is a big deal," he
says. "The hope is that in the future, this type of approach could lead
to better [efficiency]."
By Prachi Patel
From Technology Review
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