New material could produce record of electricity when heated in high magnetic fields

In a recent MIT study, scientists find a way to improve thermoelectric materials that can spontaneously produce electricity when one side of the material is heated.

The MIT physicists find a method to strongly boost thermoelectricity’s potential.

The material they model with this method is five times more efficient, and could potentially generate twice the amount of energy, as the best thermoelectric materials that exist today.

This means people can use energy more efficiently, such as powering a car partly from the heat that its engine gives off or getting a portion of home’s electricity from the heat that a power plant emits.

A material’s ability to produce energy from heat is based on the behavior of its electrons in the presence of a temperature difference.

When one side of a thermoelectric material is heated, it can energize electrons to leap away from the hot side and accumulate on the cold side.

The resulting buildup of electrons can create a measurable voltage.

Materials that have so far been explored have generated very little thermoelectric power, in part because electrons are relatively difficult to thermally energize.

In most materials, electrons exist in specific bands, or energy ranges.

Each band is separated by a gap — a small range of energies in which electrons cannot exist. Energizing electrons enough to cross a band gap and physically migrate across a material has been extremely challenging.

In the study, the team decided to look at the thermoelectric potential of a family of materials known as topological semimetals.

In contrast to most other solid materials such as semiconductors and insulators, topological semimetals are unique in that they have zero band gaps — an energy configuration that enables electrons to easily jump to higher energy bands when heated.

Scientists had assumed that topological semimetals, a relatively new type of material that is largely synthesized in the lab, would not generate much thermoelectric power.

When the material is heated on one side, electrons are energized, and do accumulate on the other end.

But as these negatively charged electrons jump to higher energy bands, they leave behind what’s known as “holes” — particles of positive charge that also pile up on the material’s cold side, canceling out the electrons’ effect and producing very little energy in the end.

But the team wasn’t quite ready to discount this material. In an unrelated bit of research, the researchers had noticed a curious effect in semiconductors that are exposed to a strong magnetic field.

Under such conditions, the magnetic field can affect the motion of electrons, bending their trajectory.

In a study from Princeton University, a team who attempted to fully characterize a type of topological material known as lead tin selenide, had also measured its thermoelectric properties under a magnetic field.

Among their many observations of the material, the researchers had reported seeing an increase in thermoelectric generation, under a very high magnetic field of 35 tesla (most MRI machines, for comparison, operate around 2 to 3 tesla).

In the current study, the researchers used properties of the material from the Princeton study to theoretically model the material’s thermoelectric performance under a range of temperature and magnetic field conditions.

In their theoretical modeling, the group calculated lead tin selenide’s ZT, or figure of merit, a quantity that tells you how close your material is to the theoretical limit for generating power from heat.

The most efficient materials that have been reported so far have a ZT of about 2.

The team found that, under a strong magnetic field of about 30 tesla, lead tin selenide can have a ZT of about 10 — five times more efficient than the best-performing thermoelectrics.

They calculate that a material with a ZT equal to 10, if heated at room temperature to about 500 kelvins, or 440 degrees Fahrenheit, under a 30-tesla magnetic field, should be able to turn 18% of that heat to electricity, compared to materials with a ZT equal to 2, which would only be able to convert 8 percent of that heat to energy.

The group acknowledges that, to achieve such high efficiencies, currently available topological semimetals would have to be heated under an extremely high magnetic field that could only be produced by a handful of facilities in the world.

For these materials to be practical for use in power plants or automobiles, they should operate in the range of 1 to 2 tesla.

This should be doable if a topological semimetal were extremely clean, meaning that there are very few impurities in the material that would get in the way of electrons’ flow.

The team says that lead tin selenide, the material they focused on in their study, is not the cleanest topological semimetal that scientists have synthesized.

In other words, there may be other, cleaner materials that may generate the same amount of thermal power with a much smaller magnetic field.

The team has filed a patent for their new thermolelectric approach and is collaborating with Princeton researchers to experimentally test the theory.

The research is supported by the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center of U.S. Department of Energy, and by Office of Basic Energy Sciences of U.S. Department of Energy.

The lead author is Brian Skinner, a postdoc in MIT’s Research Laboratory of Electronics.

Skinner’s co-author on the paper is Liang Fu, the Sarah W. Biedenharn Career Development Associate Professor of Physics at MIT.

The study is published in Science Advances.