In the middle of the XIX century two very important discoveries were made. They were made by scientists, whose names these laws are called today. One was Thomas Zeebeck and the other was Jean Peltier. They discovered the same phenomenon that can be called reciprocal. One of them discovered that the application of potential differences causes temperature changes. Another discovered that the application of a temperature gradient causes a potential difference. Today we call these effects the laws of Peltier and Zeebeck. They underlie all thermoelectric phenomena and, of course, thermoelectric materials.
For many years these laws were just laws of physics that enriched science. Demonstrations of the application of these laws have been made. One of the scientists, Lentz, took two wires that were linked and consisted of different chemicals. Flowing current in one direction, he froze a drop of water, passing a drop of water in the other - a drop of water melted.
This lasted for a very long time, until in the mid-1950s, the academician Abram Ioffe formulated the idea that it was possible to use thermoelectrics and thermoelectric materials in one direction to generate cold, and in the other direction - to generate electricity, using the laws of Zeebeck and the laws of Peltier. The idea was not new, but Joffet supported it with the principle of material selection, which is still used in industry. He said that the semiconductor should be narrow-band, made up of very heavy atoms and should not be converted into metal. In this case, the condition of a good thermoelectric material will be met.
When we talk about the material, we have to come up with some system to evaluate its performance or even usefulness. This is the ZT coefficient, which stands for "thermoelectric quality". It does not have a physical dimension, but says that a thermoelectric engineer must be a good conductor of electricity and a bad conductor of heat. It is very difficult to combine in one material, but this is basically possible. The theory does not contain any prohibitions. We had to figure out how to find out how these substances can be used, in which corner of the chemical system in the Mendeleev table it is possible to take such chemical elements so that they can be combined in one substance with the desired properties.
Ioffe invented the material bismuth telluride, its chemical formula - Bi2Te3. It can be a thermoelectric material itself, but in order to increase its efficiency, it is alloyed: a lot of antimony, some amount of selenium and other additives are added. To date, 95% of all thermoelectric materials are based on bismuth telluride.
Where do we use thermoelectric materials? There are many areas, but the market for thermoelectric materials is very small: annual turnover is estimated at less than $2 billion. Nevertheless, there is a certain set of applications. First of all, cooling: you can cool something that doesn't require a lot of power and a lot of cold production. When it comes to thermoelectric chillers of 100 watts or less, they are competitive with compression chillers. Compression chillers are simple: the larger the volume, the greater the efficiency. Thermoelectric materials do not have this relationship.
The advantage of thermoelectric refrigerators over compression refrigerators exists because thermoelectric material and devices based on it can work for a very long time. For some materials, life expectancy estimates range from 20 to 30 years. Such devices do not require constant service: they have no moving parts, no overheating and no mechanical stresses. However, they do not produce very good cold. The efficiency of modern thermoelectric materials producing cold is estimated at about 6-7%. Despite this, bismuth telluride has not been replaced in thermoelectric cooling materials so far. And the search for new materials is a separate task.
As we have already discussed, there are Zeebeck and Peltier effects. And if one of them is responsible for cooling, the other for generating current under the influence of temperature differences. This leads to a new idea, which is that bismuth telluride cannot be avoided for most applications. Bismuth telluride, even if well alloyed, works normally up to temperatures of 100-150 °C. Of course, it can be used: imagine a remote system where there are no electrical networks, and you need to run a sensor that runs on electricity. It is possible that the temperature difference will give you electricity thanks to this thermoelectric material, the sensor will take readings, transmit to the satellite and fall asleep again.
However, we would like to get more out of thermoelectric generators, such as a machine. Imagine that when we fill a car with gasoline, we have to agree that about 25% of the fuel is used to keep the car going, and everything else