Thermoelectric phenomena are a separate topic in physics, in which they consider how temperature can generate electricity, and the latter leads to a change in temperature. One of the first discovered thermoelectric phenomena was the Seebeck effect.
Prerequisites for discovering the effect
In 1797, the Italian physicist Alessandro Volta, conducting research in the field of electricity, discovered one of the surprising phenomena: he discovered that when two solid materials come into contact, a potential difference appears in the contact area. It is called contact difference. Physically, this fact means that the contact zone of dissimilar materials has an electromotive force (EMF), which can lead to the appearance of current in a closed circuit. If now we connect two materials into one circuit (form two contacts between them), then on each of them the indicated EMF will appear, which will be identical in modulus but opposite in sign. The latter explains why there is no current.
The reason for the appearance of EMF is a different Fermi level (energy of the valence states of electrons) in different materials. Upon contact of the latter, the Fermi level is leveled off (in one material it decreases, in another it rises). This process occurs due to the transition of electrons through the contact, which leads to the appearance of EMF.
It should immediately be noted that the magnitude of the emf is insignificant (of the order of several tenths of a volt).
The discovery of Thomas Seebeck
Thomas Seebeck (German physicist) in 1821, that is, 24 years after Volt discovered the contact potential difference, conducted the following experiment. He connected a plate of bismuth and copper, and next to them he placed a magnetic arrow. In this case, as was said 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 arrow began to turn.
Now we know that the cause of its rotation was the Ampere force created by the current conductor, but Seebeck did not know this at that time, so he mistakenly assumed that the induced magnetization of metals arises as a result of the temperature difference.
The correct explanation for this phenomenon was given several years later by the Danish physicist Hans Oersted, who indicated that it was a thermoelectric process, and a current was flowing through a closed circuit. Nevertheless, the thermoelectric effect discovered by Thomas Seebeck now bears his last name.
Physics of ongoing processes
Once again for fixing the material: the essence of the Seebeck effect is to induce an electric current by maintaining different temperatures of the two contacts of different materials that form a closed circuit.
To understand what is happening in the specified system, and why the current begins to flow 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 region due to the alignment of the Fermi levels. The energy of this level in materials changes with increasing or decreasing temperature. The latter fact will lead to the appearance of current if two contacts are closed in a circuit (the equilibrium conditions in the contact zone of metals at different temperatures will be different).
- The process of moving charge carriers from hot to cold areas. This effect can be understood if we recall that electrons in metals and electrons and holes in semiconductors can be considered, to a first approximation, an ideal gas. As is known, the latter increases pressure when heated in a closed volume. In other words, in the contact zone, where the temperature is higher, the "pressure" of the electron (hole) gas is also higher, therefore the charge carriers tend to go to the colder regions 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, โhitsโ an electron (hole) and gives it additional energy.
The three processes noted above ultimately determine the occurrence of current in the described system.
How is this thermoelectric phenomenon described?
Very simply, for this, a certain parameter S is introduced, which is called the Seebeck coefficient. The parameter indicates EMF magnitude is induced if the contact temperature difference is maintained equal to 1 Kelvin (degree Celsius). That is, you can write:
S = ฮV / ฮT.
Here ฮV is the EMF of the circuit (voltage), ฮT is the temperature difference between the hot and cold junctions (contact zones). This formula is only approximately true, since S generally depends on temperature.
The Seebeck coefficient values โโdepend on the nature of the materials that come into contact. Nevertheless, we can definitely say that for metallic materials these values โโare equal to units and tens of ฮผV / K, while for semiconductors they are hundreds of ฮผV / 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, carrier concentration).
Process efficiency
The amazing fact of the conversion 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 if you use the serial connection of a large number of conductors (which is done in the Peltier cell, which will be discussed below).
Secondly, it is a matter of the efficiency of thermoelectricity generation. And this question remains open to this day. The Seebeck effect efficiency 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 raise this efficiency, which can be done by developing materials of a new generation, for example, using nanotechnology.
Using the effect discovered by Seebeck
Despite the low efficiency, it still finds its application. 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 - this is the 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, the temperature of the other end can be calculated. Thermocouples are also used to measure the density of radiant (electromagnetic) energy.
- Generation of electricity on space probes. Human-launched probes for exploring our solar system or space beyond use the Seebeck effect to power the electronics on board. This is done thanks to the radiation thermoelectric generator.
- Application of the Seebeck effect in modern cars. BMW and Volkswagen have announced the appearance of thermoelectric generators in their cars that will use the heat of the 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 above circuit is connected to an external current source. That is, the effects of Seebeck and Peltier are opposite.
The Thomson effect is of the same nature, but it is considered on the same material. Its essence consists in the release or absorption of heat by a conductor through which a current flows and whose ends are maintained at different temperatures.
Peltier cell
When talking about petents for thermoelectric modules with the Seebeck effect, then, of course, the first thing they remember about the Peltier cell. It is a compact device (4x4x0.4 cm) made of a series of n- and p-type conductors connected in series. You can make it yourself. Seebeck and Peltier effects are at the core 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 (โ10%).
It is used to solve such everyday problems as heating or cooling water in a mug or recharging a mobile phone.