Thermoelectricity for dummies

I am a Ph.D., and as all doctors, I worked on a very narrow and specific subject for more than three years. When people ask me what I did during that time, they may get scared by the answer. After I say that I study new thermoelectric composites based on CoSi they look at me with eyes that say “No, please, don’t start a 45 min speech where I will understand nothing, I beg you”. This is a fascinating topic and it often turns out to be an interesting conversation. So I’m going to explain briefly what is thermoelectricity and why you should care about it.

What is a thermoelectric material?

It is a material with the ability to create a different of electric tension when it undergoes to a gradient of temperature and vice-versa. When I take a bar of a thermoelectric material and I attach it to a battery, one-half will get cooler and the other warmer that the initial temperature. If I put one end of the same bar in contact with ice and the other with boiling water, it will work as a battery.

When are those materials used in everyday life?

Thermoelectric devices are used for three kinds of devices: coolers, heaters and as electricity sources. They are used to cool electronics, as IR visors, and maybe one day your laptop. They are the cooling elements for specific refrigerators such as the small portable refrigerators you plug in your car and those used in scientific laboratories. They are also used to heat the seat of some fancy cars and to power satellites. 

So does my refrigerator have thermoelectric materials in it?

No, thermoelectric refrigeration isn’t very effective, which means that it waste more energy than classic refrigeration systems. But thermoelectric refrigerators do not have moving parts, which means no noises, and they easily last more than a decade. It is also possible to make very small refrigeration devices, with dimensions similar to a computer chip and they are extremely precise, which means that they can keep the exact temperature you program, within 0.1 °C precision.

And what about power sources?

These are not very effective too, but they are one of the best options for space probes, in particular, those that goes in the further part of the solar system. A radioactive nucleus produces heat for extremely long times, and Space is cold, so if you put a thermoelectric material between these two you will have an extraordinary source of electricity, which works also far away from the sun. This is smaller and more reliable than solar panels.

Why you need new thermoelectric materials?

Thermoelectric materials aren’t very effective, so part of the scientific community is trying to find more efficient materials. And they are finding great alternatives to the most common material Bi2Te3. Another part of the community, as my team and me, took another approach. We selected materials with worst performances but based on low-cost, abundant, and non-toxic elements. If we put a thermoelectric heater in the seats of each car, within a few years we will have used all the Tellurium present on Earth! If thermoelectric devices are expensive or toxic, they will never substitute the cheaper options.

We also considered polymers, which can really change the game. Thermoelectric polymers are lightweight, can be flexible and can be printed on plastic substrates. If we can increase just a little their performances amazing things may happen. For example, we could produce a wearable tissue able to power microdevices, such as sensors to monitor our health. Our skin is at 37°C and the air is often between 15 and 25°C. This difference of temperature could already produce micro currents to power small sensors.

It sounds fascinating, but why did you choose to study this topic?

Thermoelectric materials are brainteasers. To achieve high performances you need high electrical conductivity, such as in metals, but also higher thermopower and low thermal conductivity, such as in electrical insulators. Optimize these properties at the same time is a fascinating challenge. Incredible strategies have been used: reducing the size of particles below 100 nm, creating complex ordered structures, create “complex” materials formed by numerous elements in the same structure.