The Seebeck Effect: Features and Application
The Seebeck effect is the process of the appearance of a potential difference between the connection of two different materials due to the heating of the specified area. This effect was obtained by Seebeck in 1822. It was then that he conducted the experiment of heating a contact of two materials, using bismuth and antimony.
A galvanometer was used to record the resulting changes. Holding the joint of the connected materials, he saw that the magnetic needle had deviated from its initial position. Naturally, the difference was not so noticeable. However, the experiments were repeated again and again, thanks to which it was possible to obtain the desired result.
This effect appeared due to the appearance of an electric driving force in a closed circuit made of different materials. A little later, it turned out that the temperature difference is caused by the appearance of thermopower. And already the consequence of the thermal EMF in a closed circuit is an electric current. Today, this effect is used in many areas. But its greatest application in the modern world can be clearly seen in thermocouples.
How the Seebeck Effect Works?
The Seebeck effect is to create a thermocouple that consists of two dissimilar metals forming a closed circuit with each other. Metals differ from each other by different Seebeck coefficients, as a result of which there is a voltage between the heated conductor of the thermocouple and the non-heated conductor. This voltage is directly proportional to the difference in their temperature values.
Many thermoelectric devices use the Seebeck effect. In most cases, the structure of thermoelectric generators includes thermal batteries, which are recruited from semiconductor thermal elements. They can be connected in parallel or in series. It also includes heat exchangers for heated and non-heated junctions of thermal batteries.
A typical circuit diagram of a thermoelectric generator consists of:
- A semiconductor-type thermoelectric element, which is made of branches of p – and n-type conductivity contacts. These contacts have different signs of the coefficient of thermoelectromotive force.
- Switching plates that have heated and non-heated junctions.
- Active load.
When the thermoelectric element is switched on, a direct current begins to flow to the load in the circuit, which is caused by the Seebeck Effect. It is this same current that leads to the absorption and release of heat on the spikes. To ensure a high EMF coefficient, such semiconductor materials must be distinguished by excellent electrical conductivity. And to obtain a significant temperature difference between heated and non-heated junctions, a low thermal conductivity is sufficient. High-alloy materials are best suited for such parameters.
Operating Principle of Seebeck Effect
The Seebeck effect is that in a closed circuit with cores of different materials, an EMF can appear when their contacts have different temperature values. To put it simply, the parameter of the resulting EMF largely depends on the materials of the conductors used, including the temperatures of the unheated and heated conductor.
In the presence of a temperature gradient in the conductor along the entire length, a phenomenon is observed in which the electrons at the heated end have an order of magnitude higher velocities and energies than in the non-heated end. As a result, electrons appear, which are directed to the cold end. It is on it that the negative charge accumulates. At the heated end, the positive charge accumulates.
The accumulation of charge is observed until the potential difference reaches an indicator at which the electrons do not begin to flow back, as a result of which the potential will come to equilibrium.
The Seebeck effect is characterized by the appearance of various properties:
- There is a potential difference between the contacts. This is explained by the fact that different conductors that are in contact with each other have different Fermi energy. As a result, when the circuit is closed, the electron potentials have the same state, resulting in a potential difference between the contacts. An electric field appears on the contacts, which is localized in the thinnest boundary layer. When the circuit is closed, a voltage appears on the conductors. The direction of the electric field goes in both contacts from the larger to the smaller. If the temperature of the contacts is changed, the voltage will also change. But with a change in the potential difference, the electric field in one of the contacts will also change. As a result, an EMF will appear in the contour. If the conductors have an equal temperature, then the volume and contact EMF in this case will be zero.
- The appearance of phonon entrainment is observed. If there is a gradient in the temperature range in the solid, the number of phonons that are directed to the end of the unheated conductor will increase. They will become more than those that go in the opposite direction. As a result of the collisions with the electrons, the phonons will pull others after them. As a result, a negative charge will accumulate on the heated conductor. While in a heated conductor, positive charges will accumulate until the potential difference is equal to the increase effect. The potential difference at low temperatures can reach parameters hundreds of times higher.
- The appearance of magnon entrainment is observed, but only in conductors made of magnetic materials. The EMF appears due to the entrainment of electrons by magnons.
Practical Application
Such devices are widely used in everyday life. For example, when visiting a sauna, few people think that the temperature in it is maintained using a thermocouple.
That is, a thermocouple is a thermoelectric thermometer that is made of two different metals. They are connected by welding. In this case, one end is placed directly in the sauna, and the other free ends are brought out and connected to the measuring device. When the oven heats the sauna room, the ends of the thermocouple work in completely different temperature values. As a result, a temperature gradient appears, which leads to the appearance of a thermal current, that is, a thermoelectromotive force.
The Seebeck effect is currently used in a wide variety of devices. An example of this can be voltage sensors, temperature sensors, gas pressure sensors, thermoelectric generators and light intensity sensors.
The Future of Seebeck Effect
The Seebeck effect is quite interesting for scientists. Recently, scientists from Ohio have developed a technology that allows you to make the effect incredibly effective. The main disadvantage of modern devices is that this effect does not allow you to generate a significant amount of energy even when using highly alloyed contacts and having a high temperature difference.
Scientists suggest using a non-magnetic semiconductor, which is installed in an external magnetic field with a temperature in the range of 2-20 K. In this case, a giant Seebeck spin effect appears. The use of such thermocouples makes it possible to significantly increase the performance of the devices used, expand their functionality and application.
