Thermoelectric phenomena are a separate topic in physics, in which they consider how temperature can generate electricity, and the latter lead to a change in temperature. One of the first discovered thermoelectric phenomena was the Seebeck effect.
Prerequisites for opening the effect
In 1797, the Italian physicist Alessandro Volta, conducting research in the field of electricity, discovered one of the amazing phenomena: he discovered that when two solid materials come into contact, a potential difference appears in the contact area. It is called the contact difference. Physically, this fact means that the contact zone of dissimilar materials has an electromotive force (EMF) that can lead to the appearance of a current in a closed circuit. If now two materials are connected in one circuit (to form two contacts between them), then the specified EMF will appear on each of them, which will be the same in magnitude, but opposite in sign. The latter explains why no current is generated.
The reason for the appearance of EMF is a different level of Fermi (energyvalence states of electrons) in different materials. When the latter come into contact, the Fermi level levels off (in one material it decreases, in another it rises). This process occurs due to the passage of electrons through the contact, which leads to the appearance of an EMF.
It should be noted right away that the EMF value is negligible (on the order of a few tenths of a volt).
Discovery of Thomas Seebeck
Thomas Seebeck (German physicist) in 1821, that is, 24 years after the discovery of the contact potential difference by Volt, conducted the following experiment. He connected a plate of bismuth and copper, and placed a magnetic needle next to them. In this case, as mentioned above, no current occurred. But as soon as the scientist brought the flame of the burner to one of the contacts of the two metals, the magnetic needle began to turn.
Now we know that the Ampère force created by the current carrying conductor caused it to turn, but at that time Seebeck did not know this, so he mistakenly assumed that the induced magnetization of metals occurs as a result of the temperature difference.
The correct explanation for this phenomenon was given a few years later by the Danish physicist Hans Oersted, who pointed out that we are talking about a thermoelectric process, and a current flows through a closed circuit. Nevertheless, the thermoelectric effect discovered by Thomas Seebeck now bears his last name.
Physics of ongoing processes
Once again to consolidate the material: the essence of the Seebeck effect is to induceelectric current as a result of maintaining different temperatures of two contacts of different materials, which form a closed circuit.
To understand what happens in this system, and why current starts to run in it, you should get acquainted with three phenomena:
- The first has already been mentioned - this is the excitation of the EMF in the contact area due to the alignment of the Fermi levels. The energy of this level in materials changes as the temperature rises or falls. The latter fact will lead to the appearance of a current if two contacts are closed in a circuit (the equilibrium conditions in the zone of contact of metals at different temperatures will be different).
- The process of moving charge carriers from hot to cold regions. This effect can be understood if we remember that electrons in metals and electrons and holes in semiconductors can, in the first approximation, be considered an ideal gas. As is known, the latter, when heated in a closed volume, increases the pressure. In other words, in the contact zone, where the temperature is higher, the "pressure" of the electron (hole) gas is also higher, so charge carriers tend to go to colder areas of the material, that is, to another contact.
- Finally, another phenomenon that leads to the appearance of current in the Seebeck effect is the interaction of phonons (lattice vibrations) with charge carriers. The situation looks like a phonon, moving from a hot junction to a cold one, "hit" an electron (hole) and imparts additional energy to it.
Marked three processesas a result, the occurrence of current in the described system is determined.
How is this thermoelectric phenomenon described?
Very simple, for this they introduce a certain parameter S, which is called the Seebeck coefficient. The parameter shows if the EMF value is induced if the contact temperature difference is maintained equal to 1 Kelvin (degree Celsius). That is, you can write:
S=ΔV/ΔT.
Here ΔV is the EMF of the circuit (voltage), ΔT is the temperature difference between the hot and cold junctions (contact zones). This formula is only approximately correct, since S generally depends on temperature.
The values of the Seebeck coefficient depend on the nature of the materials in contact. Nevertheless, we can definitely say that for metallic materials these values are equal to units and tens of μV/K, while for semiconductors they are hundreds of μV/K, that is, semiconductors have an order of magnitude greater thermoelectric force than metals. The reason for this fact is a stronger temperature dependence of the characteristics of semiconductors (conductivity, concentration of charge carriers).
Process efficiency
The surprising fact of the transfer of heat into electricity opens up great opportunities for the application of this phenomenon. Nevertheless, for its technological use, not only the idea itself is important, but also quantitative characteristics. First, as has been shown, the resulting emf is quite small. This problem can be circumvented by using a series connection of a large number of conductors (whichis done in the Peltier cell, which will be discussed below).
Secondly, it is a matter of thermoelectricity generation efficiency. And this question remains open to this day. The efficiency of the Seebeck effect is extremely low (about 10%). That is, of all the heat expended, only one tenth of it can be used to perform useful work. Many laboratories around the world are trying to increase this efficiency, which can be done by developing new generation materials, for example, using nanotechnology.
Using the effect discovered by Seebeck
Despite the low efficiency, it still finds its use. Below are the main areas:
- Thermocouple. The Seebeck effect is successfully used to measure the temperatures of various objects. In fact, a system of two contacts is a thermocouple. If its coefficient S and the temperature of one of the ends are known, then by measuring the voltage that occurs in the circuit, it is possible to calculate the temperature of the other end. Thermocouples are also used to measure the density of radiant (electromagnetic) energy.
- Generation of electricity on space probes. Human-launched probes to explore our solar system or beyond use the Seebeck effect to power the electronics on board. This is done thanks to a radiation thermoelectric generator.
- Application of the Seebeck effect in modern cars. BMW and Volkswagen announcedthe appearance in their cars of thermoelectric generators that will use the heat of gases emitted from the exhaust pipe.
Other thermoelectric effects
There are three thermoelectric effects: Seebeck, Peltier, Thomson. The essence of the first has already been considered. As for the Peltier effect, it consists in heating one contact and cooling the other, if the circuit discussed above is connected to an external current source. That is, the Seebeck and Peltier effects are opposite.
The Thomson effect has the same nature, but it is considered on the same material. Its essence is the release or absorption of heat by a conductor through which current flows and the ends of which are maintained at different temperatures.
Peltier cell
When talking about patents for thermo-generator modules with the Seebeck effect, then, of course, the first thing they remember is the Peltier cell. It is a compact device (4x4x0.4 cm) made from a series of n- and p-type conductors connected in series. You can make it yourself. The Seebeck and Peltier effects are at the heart of her work. The voltages and currents with which it works are small (3-5 V and 0.5 A). As mentioned above, the efficiency of its work is very small (≈10%).
It is used to solve such everyday tasks as heating or cooling water in a mug or recharging a mobile phone.
