Seebeck effect

Thermoelectric phenomenon
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Seebeck effect, also known as the first thermoelectric effect, refers to that Electrical conductor or semiconductor The temperature difference between two substances Voltage Poor Thermoelectric phenomenon Generally, the direction of thermoelectric potential is: at the hot end Electronics From negative to positive.
In the circuit composed of two metals A and B, if the temperature of two contact points is different, current will appear in the circuit, which is called Thermal current The corresponding electromotive force is called thermoelectric potential, and its direction depends on the direction of the temperature gradient.
Seebeck effect can be simply explained as carrier It moves from the hot end to the cold end and accumulates at the cold end, thus forming a potential difference inside the material. At the same time, a reverse charge flow is generated under the effect of the potential difference. When the hot moving charge flow reaches a dynamic balance with the internal electric field, a stable thermoelectric electromotive force is formed at both ends of the semiconductor. The thermoelectric electromotive force of semiconductor is large and can be used as Thermoelectric generation Device.
Chinese name
Seebeck effect
Foreign name
Seebeck effect
Alias
Zebeck effect

Discoverer

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Thomas Johann Seebeck
Thomas Johann Seebeck, born in 1770 Tallinn (Belonging to East Prussia at that time, now Estonia Capital). Seebeck's father was a German of Swedish origin. Perhaps because of this, he encouraged his son to study in Berlin University and University of Gottingen Study medicine. In 1802, Seebeck received a medical degree. Since his chosen direction is physics in experimental medicine, and he spent most of his life in physics education and research, people usually think he is a physicist.

Discovery process

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In the early 1820s, Thomas John Seebeck studied the relationship between current and heat through experimental methods. In 1821, Seebeck connected two different metal wires together to form a current loop. He connected two wires end to end to form a node. He suddenly found that if one of the nodes was heated to a very high temperature while the other was kept at a low temperature, there would be a magnetic field around the circuit. He can't believe that when heat is applied to a junction composed of two metals, there will be a current, which can only be generated by thermal magnetic current or heat Magnetic phenomenon To explain his findings. In the next two years (1822-1823), Seebeck reported his continuous observation to the Prussian Scientific Society, describing this discovery as "metal caused by temperature difference magnetization ”。
Seebeck did find out Thermoelectric effect However, he made a wrong explanation: the reason for the magnetic field around the wire is that the temperature gradient causes the metal to be magnetized in a certain direction, rather than forming a current. The scientific society believes that this phenomenon is due to the temperature gradient causing the current, which in turn creates a magnetic field around the wire. Seebeck was very angry at this explanation. He retorted that scientists' eyes Oster electromagnetics ), so they only use the theory that "magnetic field is generated by current" to explain it, and they can't think of any other explanation. However, Seebeck himself could not explain the fact that if the circuit was cut off, the temperature gradient did not generate a magnetic field around the wire. Therefore, most people agree with the view of thermoelectric effect, which was later confirmed. [1]

principle

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The mechanism of Seebeck effect is different for semiconductor and metal.

Semiconductor effect

The main reason for the Seebeck effect is the diffusion of carriers from the hot end to the cold end. For example, p-type semiconductors, due to the high concentration of holes at the hot end, the holes will diffuse from the high temperature end to the low temperature end; In the case of an open circuit, space charges are formed at both ends of the p-type semiconductor (negative charges at the hot end and positive charges at the cold end), and electric fields appear inside the semiconductor; When Diffusion When the drift effect of the electric field counteracts each other, it reaches a stable state temperature gradient Induced electromotive force—— Thermoelectric electromotive force Naturally, the direction of thermoelectric electromotive force of n-type semiconductor is from the low temperature end to the high temperature end (the Seebeck coefficient is negative). On the contrary, the direction of thermoelectric electromotive force of p-type semiconductor is from the high temperature end to the low temperature end (the Seebeck coefficient is positive). Therefore, the conductive type of semiconductor can be judged by the direction of thermoelectric electromotive force.
It can be seen that there is an electric field in the semiconductor with temperature difference, so the energy band of the semiconductor is tilted, and the Fermi energy level is also tilted; The difference between the Fermi energy levels at both ends is equal to the thermoelectric electromotive force.
In fact, there are two factors that affect the Seebeck effect:
The first factor is carrier Energy and speed. Because the carrier energies at the hot end and cold end are different, this actually reflects the semiconductor Fermi energy level There are differences at both ends, so this effect will also affect Thermoelectric electromotive force Cause impact - enhance Seebeck effect.
The second factor is phonons. Because of the hot end phonon If the number is more than the cold end, the phonons will also diffuse from the high temperature end to the low temperature end, and diffusion process It can collide with the carrier energy transfer To accelerate the movement of carriers—— Phonon traction This effect will increase the accumulation of carriers at the cold end and enhance the Seebeck effect.
The Seebeck effect of semiconductors is significant. Generally, the Seebeck coefficient of semiconductor is hundreds μ V/K, which is much higher than that of metal.

Metal effect

Since the carrier concentration and Fermi energy level position of the metal basically do not change with temperature, the Seebeck effect of the metal must be very small, and generally the Seebeck coefficient is 0~10 μ V/K。
Although the Seebeck effect of metals is very small, it is still considerable under certain conditions; In fact, the metal thermocouple that uses the metal Seebeck effect to detect high temperature is a common component.
The mechanism of metal Seebeck effect is relatively complex, which can be analyzed from two aspects:
① The diffusion of electrons from the hot end to the cold end. However, the diffusion here is not concentration gradient (Because the electron concentration in the metal has nothing to do with temperature), it is caused by the higher energy and speed of electrons at the hot end. Obviously, if this effect is major, the coefficient of Seebeck effect thus generated should be negative.
② The influence of electron free path. Because there are many free electron However, Fermi is the main contributor to conductivity energy level The so-called conduction electrons within the range of 2kT nearby. And these electronic Mean free path And subject to scattering( phonon Scattering, impurity and defect scattering) and Density of energy states It depends on the change of energy.
If the average free path of the hot end electrons increases with the increase of the electron energy, then the hot end electrons will have larger energy on the one hand and larger average free path on the other hand, so the transport of the hot end electrons to the cold end is the main process, which will produce the Seebeck effect with negative Seebeck coefficient; This is the case for metals Al, Mg, Pd, Pt, etc.
On the contrary, if the average free path of the hot end electrons decreases with the increase of the electron energy, then although the hot end electrons have large energy, their average free path is very small, so the electron transport will be mainly from the cold end to the hot end, which will produce the Seebeck effect with a positive Seebeck coefficient; This is true for metals such as Cu, Au, Li, etc. Calculation formula of Seebeck effect potential difference:
And
Seebeck coefficient of two materials. If
And
It does not change with temperature, and the above formula can be expressed as follows:
Seebeck later measured some metal materials and arranged 35 metals into a sequence (i.e. Bi Ni Co Pd-U-Cu-Mn-Ti-Hg-Pb Sn-Cr-Mo-Rb-Ir-Ag-Zn-W-Cd-Fe-As-Sb-Te -...), and pointed out that when any two metals in the sequence form a closed loop, the current will flow from the metal with higher order to the metal with lower order through the hot joint.

application

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After the discovery of Seebeck effect, people found a place for its application. With Seebeck effect Thermoelectric couple (thermocouples, that is Thermocouple )To measure the temperature. As long as the proper metal is selected as the thermocouple material, the temperature can be easily measured from - 180 ℃ to+2000 ℃. With such a wide measurement range, the temperature of alcohol or Mercury thermometer so far behind that one can only see the dust of the rider ahead. Thermocouple thermometer , and can even measure temperatures up to+2800 ℃!
Two different metal wires of the thermocouple are welded together to form two nodes. The loop voltage VOUT is the difference between the junction voltage of the hot node and the junction voltage of the cold node (reference node). Because VH and VC are generated by the temperature difference between the two junctions, that is, VOUT is a function of the temperature difference. Scale factor α The ratio of voltage difference to temperature difference is called Seebeck coefficient.

Measuring instrument

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The measurement of thermoelectric materials in China started late, but developed rapidly. The seebeck coefficient measurement system mainly consists of 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, and early users used more Japanese products, but it did not set up after-sales services in China.
Later, German linseis entered the Chinese market.
In the low temperature range (75-298k), Cryolab series of Kryoz Technologies is mainly used.
In addition, various research institutions also have some self built Seebeck coefficient measurement systems, which unfortunately cannot be commercialized in scale. Some domestic universities have also built their own seebeck coefficient measurement systems, but the accuracy is not high [1]

Thermoelectric phenomenon

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Thermoelectric effect is a thermoelectric phenomenon caused by the contact of different kinds of solids. It mainly has three effects: Seebeck effect, Peltier effect and Thomson effect.
(1) Seebeck effect If the two ends of conductor (or semiconductor) A and B are closely contacted to form a loop, if the temperature T1 and T2 at the two connections are kept at different temperatures, the thermoelectric electromotive force will be generated in the loop due to the temperature difference. The current flowing through the loop is called temperature difference current, which is composed of two uniform physical properties conductor The above device composed of (or semiconductor) is called thermoelectric couple (or thermocouple), which was discovered by French scientist Seebeck in 1821. It was later discovered that, Thermoelectric electromotive force There are also two basic properties as follows: ① the law of intermediate temperature, that is, the thermoelectric electromotive force is only related to the temperature of two nodes, and has nothing to do with the temperature of the conductor between two nodes. ② The law of intermediate metal, that is, the thermoelectric electromotive force formed by the contact of A and B conductors is independent of whether the third metal C is connected between the two nodes. As long as the two node temperatures T1 and T2 are equal, the thermoelectric electromotive force between the two nodes is also equal. Because of the two properties of ① and ②, Thermoelectric phenomenon Now it will be widely used.
(2) Peltier effect In 1834, Peltier found that when the current passes through the nodes of different metals, there is a phenomenon of heat absorption and release Qp at the nodes. Endothermic or exothermic is determined by the current direction, and Qp is called Peltier heat. The rate of its generation is proportional to the current intensity it passes through, that is
among Π It is called Peltier coefficient, and its value is equal to the heat absorbed and released per unit current on the node. The reason why the current will absorb and release heat when it passes through a node composed of two different metals is that a Peltier electric heat is accumulated at the node. The Peltier heat is the heat absorbed and released when the electromotive force does positive or negative work on the current. Considering that different metals have different electron concentrations and Fermi energy EF, the contact of two metals will cause unequal electron diffusion at the node, resulting in the establishment of electric field , thus establishing Potential difference (Of course, the above explanation only considers the generation of Thermoelectric phenomenon The actual situation is much more complicated). It can be seen that the Peltier electromotive force should be a function of temperature, and the dependence of Peltier electromotive force of different junctions on temperature can also be different. The above point of view can also be used to explain that when the current is reversed, the absorption and discharge of two pairs of Peltier heat should be reversed, so it is reversible. Peltier of general metal junction potential by μ V, and semiconductor junction can be several orders of magnitude larger than it.
⑶ Thomson effect In 1856, W. Thomson (Kelvin) predicted that there should be a third thermoelectric phenomenon after he analyzed Seebeck effect and Peltier effect with thermodynamics. Later, someone found experimentally that if the current flows through a uniform conductor with temperature gradient Joule heat In addition, it also absorbs or emits a certain amount of heat. This phenomenon is called Thomson effect, and the heat absorbed and released is called Thomson heat. The difference between Thomson heat and Peltier heat is that the former follows conductor (or semiconductor) as distributed heat absorption and release, and the latter absorbs and releases heat at nodes. Thomson heat is also reversible, but measuring Thomson heat is much more difficult than measuring Peltier heat, because it is difficult to distinguish Thomson heat from Joule heat.
⑷ The thermoelectric phenomenon of thermoelectric generator is mainly applied to temperature measurement, thermoelectric generator and thermoelectric refrigeration.
Thermoelectric power generation uses Seebeck effect to convert heat energy into electric energy. When the two junctions of a pair of thermocouples are at different temperatures Thermoelectric electromotive force Can be used as power supply. Semiconductor thermocouples are commonly used; This is a DC generating 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 end connected by the two is in contact with the high-temperature heat source, while the non junction ends of the N-type and P-type semiconductors are connected with the heat source Contact, because there is a temperature difference between the hot end and the cold end, negative charges accumulate at the cold end of P and become the cathode of the generator; The cold end of N has positive charge accumulation and becomes anode. If connected to an external circuit, there will be current flowing through it. This generator is not efficient. In order to get a larger power output, many pairs of Thermoelectric Even series and parallel connection form a thermoelectric stack.
(5) According to Peltier effect, if a power supply is connected to the circuit composed of thermoelectric materials, one node will emit heat and the other node will absorb heat. If the exothermic node maintains a certain temperature, the other node will begin to cool, thus producing a cooling effect. The semiconductor thermoelectric cooler is also composed of a series of semiconductor thermoelectric couples in series and parallel. Due to its small size, thermoelectric refrigeration has no movable parts (and therefore no noise), fewer operating safety faults, and can adjust the current to correctly control the temperature. It can be used in submarine, thermostatic bath of precision instruments, cooling of small instruments, plasma storage and transportation, etc [1]