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Published on March 26th, 2012 | by Joshua S Hill


Improving Thermoelectric Devices Takes Next Step

March 26th, 2012 by  

Scientists have long been looking for a way to improve our ability to utilize thermoelectric reactions. Researchers looking for just such an improvement believe they have discovered a liquid-like compound whose properties mean that it could be more efficient than traditional thermoelectrics, which in turn may lead to new methods to exploit solar power and create more efficient heating systems in electric cars.

Thermoelectric materials are not a new breed of power, having been used all the way back in the days of the Apollo missions, and all the way through to the currently-en-route Mars rover Curiosity.

In this diagram, the blue spheres represent selenium atoms forming a crystal lattice. The orange regions in between the atoms represent the copper atoms that flow through the crystal structure like a liquid. This liquid-like behavior is what gives the selenium-copper material its unique thermoelectric properties.

It has only been in the past few years, however, that scientists have been using thermoelectric materials to use wasted heat from vehicles or industrial machinery as a potential energy source. From these discoveries, the idea to use thermoelectric materials in heating electric cars has spawned — a notoriously tricky endeavour given the inherent lack of heat generated by an electric engine — as well as the idea to use it as a means of extending the use of solar-generated power.

Researchers led by scientists from the Chinese Academy of Science’s Shanghai Institute of Ceramics in collaboration with researchers from Brookhaven National Laboratory and the University of Michigan, as well as from Caltech, have now described in a paper recently published in the journal Nature Materials their identification of a new type of promising thermoelectric material. The material is made from copper and selenium. This material is physically a solid, but exhibits liquid-like behaviour as a result of the way in which the copper atoms flow through the selenium’s crystal lattice.

“It’s like a wet sponge,” explains Jeff Snyder, a faculty associate in applied physics and materials science in the Division of Engineering and Applied Science at the California Institute of Technology (Caltech) and a member of the research team. “If you have a sponge with very fine pores in it, it looks and acts like a solid. But inside, the water molecules are diffusing just as fast as they would if they were a regular liquid. That’s how I imagine this material works. It has a solid framework of selenium atoms, but the copper atoms are diffusing around as fast as they would in a liquid.”

The science of thermoelectrics is not something they teach you much about in Year 10 science, so I’m going to step aside and let the experts from Caltech shed some more light on the work that they have been doing:

A thermoelectric material generates electricity when there is a temperature difference between one end of the material and the other. For example, if you place a thermoelectric device right next to a heat source—say a laptop battery—then the side closest to the battery will be hotter. The electrons in the hot end will diffuse to the cool end, producing an electric current.

A good thermoelectric material must be good at conducting electricity but bad at conducting heat. If it were good at conducting heat, the heat from the hot end would move to the cool end so fast that the whole material would rapidly reach the same temperature. When that happens, the electrons stop flowing.

One way to improve thermoelectric efficiency, then, is to decrease a material’s ability to conduct heat. To that end, researchers have been developing thermoelectric materials with a mix of crystalline and amorphous properties, Snyder says. A crystalline atomic structure allows electrons to flow easily, while an amorphous material, such as glass, has a more irregular atomic structure that hinders heat-carrying vibrations from traveling.

These heat-carrying vibrations travel via two types of waves. The first type is a longitudinal or pressure wave, in which the direction of displacement—in this case, the jiggling of atoms—is the same as the direction of the wave. The second type is a transverse wave, in which the direction of displacement is perpendicular to the direction of the wave, like when you shake a jump rope up and down, resulting in waves that travel horizontally along the rope.

In a solid material, a transverse wave travels because there is friction between the atoms, meaning that when one atom vibrates up and down, an adjacent atom moves with it, and the wave propagates. But in a liquid, there is minimal friction between the atoms, and a vibrating atom just slides up and down next to its neighbor. As a result, transverse waves cannot travel inside a liquid. Ocean waves are different because they have an interface between the liquid and the air.

The team found that because heat-carrying vibrations in a liquid can travel only via longitudinal waves, a material with liquid-like properties is less thermally conductive. Therefore, a liquid-like material that’s also good at conducting electrically should be more thermoelectrically efficient than traditional amorphous materials, Snyder says.

In the case of the copper-selenium material that the researchers studied, the crystal structure of the selenium helps conduct electricity, while the free-flowing copper atoms behave like a liquid, damping down thermal conductivity. The efficiency of a thermoelectric material is quantified using a number called a “thermoelectric figure of merit.” The copper-selenium material has a thermoelectric figure of merit of 1.5 at 1000 degrees Kelvin, one of the highest values in any bulk material, the researchers say.

NASA engineers first used this copper-selenium material roughly 40 years ago for spacecraft design, Snyder says. But its liquid-like properties—which were not understood at the time—made it difficult to work with. This new research, he says, has identified and explained why this copper-selenium material has such efficient thermoelectric properties, potentially opening up a whole new class of liquid-like thermoelectric materials for investigation.

“Hopefully, the scientific community now has another strategy to work with when looking for materials with a high thermoelectric figure of merit,” Snyder says.

For many reasons, these advances are good news. The potential to extend the efficiency and usability of renewable energies is only going to increase their attractiveness to policymakers. Bringing thermoelectric science in to expand the power generation of solar power — and other renewable and green methods of power generation — is just another in a long line of fantastic advances that will move the world forward towards a clean energy economy.

Source: Caltech
Image Source: Caltech/Jeff Snyder/Lance Hayashida 


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