Thermoelectrics and the Movement of Electrons

Often the big picture, like a river winding down a canyon or flashes of lightning in the sky, can be understood by looking at the small-scale behavior of each component, like molecules of water rushing to lower elevation or electrons seeking to equalize potential. I’ve always liked that approach to understanding nanoscale physics phenomena, like electricity or heat. But if you think about it, since electricity is the movement of charge carriers in response to an electric field, and hotter particles also move around more, shouldn’t there be some interaction between the two? Can something develop an electric current as a result of being hot? Or can driving electricity result in the generation of heat?

If you’ve ever felt an electronic device heat up in your hand or on your lap, then you know the answer is yes! Running an electric current through some materials, like resistors, generates waste heat, which is a big practical problem for electronics manufacturers. But there are some materials that can do the opposite, converting heat to electricity. Extracting usable electricity from waste heat is especially impressive when you think about the reduction in entropy involved; turning the high-entropy disordered heat back into an ordered electrical potential is a strong local reduction of entropy. This is how 90% of the electricity in the world is generated, but at a low efficiency. Materials where a temperature difference creates an electrical potential are called thermoelectrics, and in addition to being really practically important, thermoelectrics are a great illustration of how important it is to understand what’s happening at the nanoscale.

The most common thermoelectric device is one where two different metals are pressed together, creating a junction. Each metal is a conductor, and will have its own electrons, which can freely move across the junction. But the electrons experience different forces in each conductor: they may find it easier or more difficult to move through the material, based on the physical properties of the metal itself. So applying a voltage across the junction will affect the electrons in each material differently, and can cause one metal’s electrons to move faster than the other’s. This difference in electron speeds, or a difference in how easily the electrons transfer their energy to the atomic nuclei in the metal, or just slow diffusion of electrons across the junction, can all lead to a temperature difference between the two metals. Thus heat can be produced at the junction, and it can even be removed given the right material properties. Heat generation or removal at an electrical junction is called the Peltier effect, and is the basis of some nanoscale refrigerators and heat pumps.

Conversely, if a temperature difference already exists across a junction of two conductors, you can imagine the faster moving electrons in the hotter material, interacting with the slower moving electrons in the colder material at the junction. For the right combination of material properties, an electrical potential will be induced by the differing temperatures, which is called the Seebeck effect. But it’s the same mechanism as the Peltier effect above, namely that both heat and electric fields induced the movement of charge carriers, and so of course the two effects have some interaction with each other.

It’s not just metals that exhibit thermoelectric behavior, though. Semiconductors can also be used as thermoelectrics, and actually have a broader range of thermoelectric behavior because their carrier concentration varies more widely than that of metals. Heat and electric field affect the charge carriers in every material, it’s just that some materials have properties that result in a more interesting and usable phenomenon.

Thermoelectric materials can be used as heat pumps and refrigerators, as I mentioned above. But the thermoelectric effect can also be used to measure temperature, by putting two metals that react differently to temperature together and then measuring the induced electric potential. This is how thermocouples work, which are incredibly common. And it all comes from the fact that both heat and electricity cause motion at the nanoscale.

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