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Highly Mismatched Alloys – Solution for Really Efficient Thermoelectrics

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Thermoelectric devices convert heat into electricity, and are of great use in recovering wasted heat from thermal engines and transforming it into electricity, thus increasing the engine’s overall efficiency, mostly when used in hybrid cars, who have high capacity onboard batteries.

The Seebeck effect lies behind thermoelectric devices, and it was discovered in 1821 by Thomass Johann Seebeck, who observed that a temperature difference between two ends of a metal bar created an electrical current in between, with the voltage being directly proportional to the temperature difference.

By using a Cray XT4 supercomputer named “Franklin”, Junqiao Wu and his team from Berkeley Lab’s Material Sciences Division  proved that introducing oxygen impurities into a unique class of semiconductors known as highly mismatched alloys (HMAs) can dramatically increase the thermoelectric performance of these semiconductors, without degrading its electric conductivity, as it happened in other materials.

“Specifically, we’ve shown that the hybridization of electronic wave functions of alloy constituents in HMAs makes it possible to enhance thermopower without much reduction of electric conductivity, which is not the case for conventional thermoelectric materials,” he says. Collaborating with Wu on this work were Joo-Hyoung Lee and Jeffrey Grossman, both now at the Massachusetts Institute of Technology.

For the thermoelectric material to yield high performances, it has to have a low thermal conductivity and a high electric conductivity. Nano-stuctured materials are being used for that purpose.

In their theoretical work, Wu and his colleagues discovered that this type of electronic structure engineering can be greatly beneficial for thermoelectricity. Working with the semiconductor zinc selenide, they simulated the introduction of two dilute concentrations of oxygen atoms (3.125 and 6.25 percent respectively) to create model HMAs. In both cases, the oxygen impurities were shown to induce peaks in the electronic density of states above the conduction band minimum. It was also shown that charge densities near the density of state peaks were substantially attracted toward the highly electronegative oxygen atoms.

Wu and his colleagues found that for each of the simulation scenarios, the impurity-induced peaks in the electronic density of states resulted in a “sharp increase” of both thermopower and electric conductivity compared to oxygen-free zinc selenide. The increases were by factors of 30 and 180 respectively.

“Furthermore, this effect is found to be absent when the impurity electronegativity matches the host that it substitutes,” Wu says. “These results suggest that highly electronegativity-mismatched alloys can be designed for high performance thermoelectric applications.”

For example, Volkswagen already has plans for introducing a thermal energy recovering system in their future cars, followed by other car manufacturers. The performance increase that Wu and his colleagues found could help car manufacturers develop more efficient hybridization techniques, while they still use ICEs in their cars.

Also, thermoelectric materials could be used in thermoelectric cooling, which implies no moving parts and no unfriendly chemicals for the environment: “Thermoelectric coolers have advantages over conventional refrigeration technology in that they have no moving parts, need little maintenance, and work at a much smaller spatial scale,” Wu says.

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