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Topological semimetals go thermoelectric

01 Jun 2018 Belle Dumé
Strong magnetic fields help thermoelectrics harvest heat
Strong magnetic fields help thermoelectrics harvest heat

When topological nodal semimetals are placed in a strong magnetic field, they become very good at converting heat current into electric power. That’s the new result from researchers at the Massachusetts Institute of Technology, who say that the efficiency of these materials in fact becomes much higher than the upper limits known to exist for any other class of thermoelectric material known today.

“This is exciting because if we can get really good at converting heat current to electric power by exploiting the thermoelectric effect, this will allow for a lot of useful technologies,” says physicist Brian Skinner, who led this research effort together with colleague Liang Fu. “For example, we could recover the waste heat from a car engine or a power plant and use this heat to power electrical devices. Or we could make new kinds of refrigerators or heaters that are very efficient and have no moving parts.”

Boosting the thermoelectric effect

A temperature gradient applied across a solid material containing free charge carriers (electrons and holes) produces a voltage gradient as the carriers migrate from the hot side of the material to the cold side. The strength of this thermoelectric effect is characterized by its thermopower (or Seebeck coefficient), which is the ratio of the voltage difference to the temperature difference across a material.

Finding materials with a large thermopower is crucial for developing devices that can transform waste heat into useful electric power. Most thermoelectric materials made thus far don’t have a very large thermopower, and so can’t be used in real-world practical applications. This is partly because it is difficult to provide enough thermal energy to electrons so that they can cross the material’s energy bandgap and migrate across the material.

Skinner and Fu have now found that they can overcome this problem by applying a strong magnetic field to doped nodal semimetals. “We studied the generic behaviour of these materials theoretically, and then looked specifically at lead tin selenide (PbSnSe),” explains Skinner. “The thermoelectric properties of this material have been found to have interesting features under intense magnetic fields of 35 Tesla.”

In PbSnSe, electrons and holes move in opposite directions under a strong magnetic field, explains Skinner. “Electrons move towards the cold side of the material and holes towards the hot side. Since both holes and electrons contribute additively to the thermopower under a high magnetic field (rather than subtractively as in the absence of a field), in principle you could get a bigger and bigger voltage out of the same material just by making the magnetic field stronger.”

18% of heat converted into electricity

Thanks to theoretical simulations, the researchers were able to calculate the material’s figure of merit, the ZT, which is a measure of how close a material is to the theoretical limit for generating power from heat. They found that under a magnetic field of around 30 Tesla, PbSnSe can reach a ZT of around 10. This is about five times larger than the value for the best thermoelectric materials available today, they say.

“This means that if the material is heated to about 500 K (around 227 °C), under such a high field, it should be able to convert 18% of that heat into electricity,” adds Skinner. “To compare, materials with a ZT of 2 can only convert 8%.”

But a magnetic field of 30 Tesla is huge, admits Skinner, and for such materials to be practical in everyday applications they would need to work in the 1-2 Tesla range. “To achieve this, we would need extremely clean topological semimetal materials containing few impurities,” says Skinner. “Although PbSnSe is relatively clean, there might be better materials that could generate the same amount of thermopower under a smaller magnetic field.”

The researchers, reporting their work in Science Advances, say they will now be looking at other classes of materials. “In the present study, we investigated the ‘Dirac semimetals’, but there are new classes of materials that have been discovered recently that might be just as good for the applications we have in mind,” Skinner continues. “What we’re hoping is that our results will kick off a flurry of activity investigating thermoelectric effects in semimetals under large magnetic fields.”

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