- Physicists have developed a theoretical model that can make thermoelectric materials 5 times more efficient than what we have today.
- A topological Semimetals, lead tin selenide, under an extremely strong magnetic field, can transform 18% of heat into electricity.
How nice it would be if you could recharge the battery of your car from the waste-heat that its engine puts off. Or what if you could get some electricity from the heat generated by coal power plant or nuclear reactor. Well, such scenarios are possible if one could find a way to enhance the efficiency of thermoelectric materials.
Over the last six decades, researchers have examined numerous materials to define their thermoelectric efficiency – capacity with which they can transform heat into electrical energy. So far, researchers have found materials with mediocre efficiencies, which cannot be used in practical applications.
Now, a team of physicists at MIT has discovered a method to improve the potential of thermoelectric materials. They claim that this new technique is 5 times more efficient than existing ones, and could possibly produce 100 percent more energy than today’s best thermoelectric material(s).
Finding New Thermoelectric Material: Topological Semimetals
The ability of a material to generate electric current from heat energy is based on how its electrons behave under high temperature. When you heat a solid material, the temperature gradient forces the electrons to move from the hot side to the cold side. The final buildup of electrons creates a small amount of voltage.
The strength of the thermoelectric effect is defined as the ratio between the temperature difference and voltage difference. Finding materials with high thermoelectric strength is a challenging task, mainly because it’s quite hard to thermally energize the electrons present in the materials.
Typically, electrons in materials exist in certain energy ranges or bands, and there is a gap between each band that separates them. This gap represents a specific energy range in which electrons can’t exist. To produce voltage you need to provide enough energy to electrons (in the form of heat) so that it can cross a band gap.
Image: Chelsea Turner / MIT
Other than insulators and semiconductors, researchers analyzed topological semimetals that have no (or zero) band gaps. This means exciting electrons is relatively easy in these materials.
But there is one major problem – when material is heated, the electrons that carry negative charge jump to higher energy bands, leaving positive charge particles called ‘holes’. These holes pile up on the cold side of the material, and thus the effect of electrons gets canceled, lowering the amount of energy produced.
Unusual Effect Under Strong Magnetic Field
While studying previous researches, authors observed the effects of a strong magnetic field on semiconductors. They found that electrons bend their trajectory under magnetic field. And this type of effect kept them wondering what would happen if a magnetic field is applied to topological semimetals.
Researchers tested a topological semimetal called lead tin selenide under an extremely strong magnetic field, and they saw an elevation in thermoelectric generation. They used Princeton research to theoretically model the thermoelectric performance of the material under a certain range of magnetic field and temperature.
E=electric field, B=magnetic field, V(d)=Magnitude (E/B)
What happens under a high magnetic field is holes and electrons travel in opposite directions. The holes move towards the hot area, while electrons go towards the colder side. Applying stronger magnetic field on the same material would result in higher voltage.
How Much Power It Could Generate?
Using this theoretical approach, the team measured the ZT of lead tin selenide – it shows the theoretical limit of the material for producing electricity from heat. For instance, today’s best thermoelectric materials have a ZT of approximately 2.
On the other hand, lead tin selenide, under 30 tesla magnetic field, reaches a ZT of 10. Moreover, if the material is heated to 227 degree Celsius under the same magnetic field, it transforms 18% of that heat into electricity. Whereas, materials of ZT of 2 turns only 8% of that heat into electricity.
However, applying a magnetic field of 30 Tesla isn’t practical in almost all applications. To put this into context, most MRI machines function between 2 to 3 Tesla.
According to the researchers, we can apply 3 Tesla magnetic field on the best topological semimetal available to this date. It would not increase the thermoelectric power by a factor of 2, but could increase efficiency up to 50 percent, which still would be a very impressive result.