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Sustainable Energy

New Process Makes Heat-Harvesting Materials Cheaply

Such materials could be used to cool computers and buildings, and harvest waste heat in cars.

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.

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).

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.”

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].”

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