Thermoelectricity is a kind of thermoelectric phenomenon, which refers to the reversible conversion between thermal energy and electrical energy in conductors.
Two different conductors are connected to form a closed circuit. If there is a temperature difference between the two contacts, the so-calledThermoelectric electromotive force。Such a circuit is called a thermocouple or thermocouple.Conversely, when the current flows through the closed circuit, the contact will release or absorb heat respectively.This phenomenon is called Peltier effect.[1]
Also called thermoelectric effect.When two different conductors or semiconductors with different conductive types are connected according to the attached figure, if the temperature of two nodes is different, an electromotive force will be generated between the two nodes.It was first discovered by German physicist Seebeck in 1821, so it is also called Seebeck phenomenon.
Thermoelectric couple
The current generated by connecting two ends is called temperature difference current (formerly called thermal current).In thermoelectric phenomena, the properties of metals (or semiconductors) can be arranged into a sequence, called thermoelectric sequence (formerly called thermoelectric sequence).When any two metals are selected from the sequence to make a thermoelectric element, the node current with high temperature flows from the metal in front of the sequence to the metal in the rear of the sequence.The thermoelectric sequence of several common metals is as follows:
The thermoelectric effect of metal is small, and it is often used for temperature measurement and control (such as thermocouple thermometer);Semiconductor has a large thermoelectric effect, which can be used to manufacture thermoelectric generators.In 1834, French scientist Peltier discovered that if a current flows through the above closed circuit, it will become cold (exothermic) at one node and hot (endothermic) at the other node. This phenomenon is called Peltier effect.The Peltier effect of semiconductors is relatively significant, which can be used to manufacture refrigerators.
Main categories
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Materials with significant thermoelectric effects.There are three different thermoelectric effects in solid or liquid conductors:Seebeck effectPeltier effect and Thomson effect.Materials with different effects can have different uses.
Seebeck effect
In order to connect two different conductors into a closed loop, if the temperature (ToneAnd Ttwo）At the same time, there will be current in the circuit.Electromotive force scale causing current generationThermoelectric electromotive force。
Peltier effect
It is the inverse effect of Seebeck effect, that is, when the current passes through the circuit composed of two different conductors, one node absorbs heat and the other node releases heat.
Thomson effect
When the current passes through a uniform conductor with temperature gradient, in addition to irreversible Joule heat, the conductor also absorbs or emits a little heat.The Seebeck effect can be used to measure the radiant energy, make filament thermocouples and connect several pairs of thermocouples in series to form a thermoelectric stack, which has been widely used in spectrum, remote sensing, laser and other technical fields.
thermoelectric materials
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Thermocouple materials include silver and bismuth, manganese and copper nickel alloys, copper and copper nickel alloys, bismuth and bismuth tin alloys, etc.The thermoelectric generator can also be made by using several pairs of thermoelectric couples in series (or parallel)[2]。
Thermoelectric
The hot and cold junction temperature (T) is required for making the thermoelectric generatorb、Tc）The difference shall be increased as much as possible, and the quality factor Z (a function of thermoelectric electromotive force rate, conductivity and thermal conductivity) of thermoelectric materials shall be as high as possible.
The best thermoelectric power generation materials at different temperatures are different: Bi for P type below 300 ℃twoTethree-SbtwoTethree；Type N is BitwoTethree-BitwoSethree; PbTe, PbTe SnTe, PbTe PbSe, GeTe, AgSbTe, etc. at 300~600 ℃;GeSi alloy and MnTe are available at 600~1000 ℃.
Thermoelectric generatorIt can use solid, liquid, gaseous fuels, solar energy, nuclear energy, waste energy and other energy sources, and is suitable for power supply of satellites, offshore lighthouses, etc.The thermoelectric generator can be made by using Peltier effect.At present, the materials used aresemiconductorThe best performance is BitwoTethreeBased solid solution materials.
Semiconductor coolerNo mechanical rotating parts, no refrigerant, no noise, can be miniaturized, and the current direction can be changed from refrigeration to heating. It is an ideal pollution-free refrigerator, which can be used for cooling in refrigerators, refrigerators, cold drinks, cold and hot boxes, scientific testing instruments, and medical equipment.
Specific application
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Thermocouple, also known as thermocouple, is measured byThermoelectric electromotive forceAn important device for measuring temperature.Experiments and theories prove that if the third metal conductor C is connected in series between two metals A and B, and both ends of C maintain the same temperature T0, the thermoelectric electromotive force is independent of the material of C. This feature makes it easy for the thermoelectric couple to connect with other measuring instruments (such as potentiometer) to measure the electromotive force.
The temperature measurement range of thermocouple is very wide, which can be used in the range of - 200~2000 ℃. From the low temperature of liquid air to the high temperature in the steelmaking furnace, thermocouple can be used to measure.Thermocouples have high sensitivity and accuracy of temperature measurement, which can reach below 10-3K. In particular, thermocouples made of platinum and rhodium alloys have high stability and are commonly used as standard thermometers.
The area and heat capacity of the temperature measuring end of the thermocouple are very small, which can measure the temperature or micro heat in a small range. This is of great significance to the study of metallographic changes, chemical reactions and temperature measurement of small organisms.
The temperature measuring end of the thermocouple is sealed in a vacuum tube, and the end is spot welded with a blackened metal sheet, which can absorb radiant heat more effectively, and the sensitivity is greatly improved. It is an important detection device for measuring light radiation and infrared.Connect many thermocouples in series to form a thermoelectric stack, which can increaseThermoelectric electromotive forceSo as to improve the temperature measurement sensitivity.
