What is the Seebeck Effect?

The Seebeck effect is also called the first pyroelectric effect, which refers to a thermoelectric phenomenon that causes a voltage difference between two substances due to the temperature difference between two different electrical conductors or semiconductors. Generally, the direction of thermoelectric potential is: electrons flow from negative to positive at the hot end.

The mechanism that produces the Seebeck effect is different for semiconductors and metals.

After the Seebeck effect was discovered, people found applications for it. Using the Seebeck effect, it can be made

The domestic thermoelectric material measurement started late, but has developed rapidly. Seebeck coefficient measurement systems are mainly self-made and imported instruments. The main international manufacturers are Japan's ULBAC-RIKO, Germany's linseis, and the Netherlands' Kryoz Technologies. Japan's ULBAC-RIKO entered the Chinese market earlier. Early users adopted more Japanese products, but it did not set up after-sales service in China.

Later, German linseis entered the Chinese market.

In the low temperature range (75-298k), Kryoz Technologies' Cryolab series is mainly used.

Other research institutions also have some self-built Seebeck coefficient measurement systems. Unfortunately, they have not been commercialized on a large scale. Some domestic universities have also built seebeck coefficient test systems by themselves, but the accuracy is not high. ^[1]

The thermoelectric effect is a thermoelectric phenomenon that occurs due to the contact between different kinds of solids. It has three main effects: Seebeck effect, Peltier effect, and Thomson effect.

Seebeck effect If the two ends of the conductor (or semiconductor) A and B are in close contact with each other to form a loop, if different temperatures T1 and T2 are maintained at the two junctions, a temperature difference electromotive force will be generated in the loop due to the temperature difference. The current flowing in the loop is called temperature difference current. The above-mentioned device consisting of two conductors (or semiconductors) with uniform physical properties is called a temperature difference thermocouple (or thermocouple), which was discovered by French scientist Seebeck in 1821. of. It was later discovered that the temperature difference electromotive force has the following two basic properties: The intermediate temperature law, that is, the temperature difference electromotive force is only related to the temperature of the two nodes, and has nothing to do with the temperature of the wire between the two nodes. Intermediate metal law, that is, the temperature difference electromotive force formed by the contact between A and B conductors has nothing to do with whether a third metal C is connected between the two nodes. As long as the two junction temperatures T1 and T2 are equal, the temperature difference electromotive force between the two junctions is also equal. It is precisely because of and that the thermoelectric phenomenon is widely used today.

Peltier effect In 1834, Peltier found that when current passed through the nodes of different metals, there was a phenomenon of heat absorption Qp at the nodes. Heat or heat is determined by the direction of the current. Qp is called Peltier heat. The rate at which it is generated is directly proportional to the intensity of the current passed, i.e.

Among them, 12 is called the Peltier coefficient, and its size is equal to the amount of heat absorbed and discharged each time a unit current is passed through the node. The reason why electric current absorbs and exotherms when a current passes through a node made of two different metals is that a Peltier electric heat is gathered at the node. The Peltier heat is absorbed by this electromotive force when it performs positive or negative work on the current. Exhausted heat. Considering that different metals have different electron concentrations and Fermi energy EFs, after the two metals are in contact, unequal amounts of electron diffusion will be caused at the junction, causing an electric field to be established between the two metals at the junction, thus establishing a potential difference ( Of course, the above explanation only considers certain aspects of the phenomenon of temperature difference electricity, and the actual situation is much more complicated). It can be seen that the Peltier electromotive force should be a function of temperature, and the Peltier electromotive force of different junctions may have different temperature dependence. The above view can also be used to explain that when the current is reversed, the absorption and discharge of Peltier heat by the two junctions should be reversed, and therefore reversible. Generally, the Peltier potential of a metal junction is on the order of V, and a semiconductor junction can be several orders of magnitude larger than it.

(3) The Thomson effect In 1856, W. Thomson (Kelvin) analyzed the Seebeck effect and the Peltier effect by thermodynamics and predicted that there should be a third type of thermoelectric phenomenon. Later, it was experimentally discovered that if a current is passed through a uniform conductor with a temperature gradient, in addition to generating irreversible Joule heat in the conductor, a certain amount of heat must be absorbed or emitted. This phenomenon is named the Thomson effect. The heat released is called Thomson fever. The difference between Thomson heat and Peltier heat is that the former absorbs and radiates heat along the conductor (or semiconductor), and the latter absorbs and radiates heat at the nodes. Thomson heat is also reversible, but measuring Thomson heat is much more difficult than measuring Peltier heat because it is more difficult to distinguish Thomson heat from Joule heat.

Thermoelectric phenomenon of thermoelectric generator is mainly used in three aspects: temperature measurement, thermoelectric generator and thermoelectric cooling.

Temperature difference power generation uses the Seebeck effect to convert thermal energy into electrical energy. When the two junctions of a pair of thermocouples are at different temperatures, the thermoelectromotive force across the thermocouple can be used as a power source. A semiconductor thermocouple is commonly used; this is a DC power generation device made of a group of semiconductor thermocouples in series and parallel. Each thermocouple is composed of an N-type semiconductor and a P-type semiconductor in series. The connected ends of the two are in contact with a high-temperature heat source, and the non-junction ends of the N-type and P-type semiconductors are in contact with a low-temperature heat source through a wire. There is a temperature difference between the cold end and the cold end, so that the negative charge of the cold end of P accumulates and becomes the cathode of the generator; the cold end of N has the positive charge accumulation and becomes the anode. If it is connected to an external circuit, a current flows. This type of generator is not very efficient. In order to obtain a large power output, many pairs of thermocouples are often used in series and in parallel to form thermocouples.

Temperature difference electric refrigerator According to the Peltier effect, if a power source is connected to a circuit composed of temperature difference electric materials, one node will release heat and the other node will absorb heat. If the exothermic junction maintains a certain temperature, the other junction will begin to cool, which will produce a cooling effect. The semiconductor temperature difference electric refrigerator is also formed by a series of semiconductor temperature difference couples connected in series and parallel. Due to the very small size of the temperature difference electric refrigeration, there is no moving part (and therefore no noise), there are few safety faults in operation, and the current can be adjusted to correctly control the temperature. It can be used in submarines, thermostats of precision instruments, cooling of small instruments, storage and transportation of plasma ^[1] .