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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Thermoelectricity. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur If we measure the heat released during one or another physical process, then we can judge both the presence of the process and the intensity of its flow. It is incomparably more convenient to operate with electrical quantities. Thermoelectric sensors make it possible to carry out measurements in the widest temperature range - from the core of a nuclear boiler to the depths of space. According to the conversion method, sensors can be divided into groups. One group includes sensors that change their ohmic resistance from the influence of heat. These are the so-called thermistors, or thermistors. PTC thermistors (Positive Temperature Coefficient) are semiconductor resistors with a positive temperature coefficient. They sharply increase their resistance when a certain characteristic temperature is exceeded and are used in automotive power networks for protection against current surges, as protection for refrigeration compressors, as self-resetting fuses, and in a number of other cases. NTC thermistors (Negative Temperature Coefficient) are semiconductor resistors with a negative temperature coefficient. Structurally, they are designed in the form of discs and are used for temperature compensation of electronic circuits, limiting the starting current, etc. The resistance change curve is linear only in some areas of temperature change, and the operating temperature range is -40 ... + 200 ° С. In this subgroup, special mention should be made of NTC thermistors TRA-1 and TRA-2, made on the basis of single crystals of artificial diamond, which are distinguished by long-term stability of parameters and uniquely low thermal inertia. Small dimensions (diameter 1,2 mm) allow them to be integrated, for example, into a soldering iron rod. Operating temperature range - 80...600°K. Thermistor-based sensors are volatile, i.e. require measuring voltage. Another large group includes thermocouples, i.e. thermal sensors, in which EMF appears at the point of contact of two dissimilar metals (Fig. 1). Sensors of this type are non-volatile, because when the junction is heated, the emerging thermoEMF is quite sufficient for measurements.
If you connect two ends of conductors made of dissimilar metals and then heat the junction, then at the free ends you can observe the appearance of EMF. The value of contact thermoEMF does not depend on either the contact area or the shape of the conductors, but is determined only by what metals are in contact and what is their temperature. In the practice of using thermocouples, it is customary to distinguish between two conductor connections - hot and cold junctions. A hot junction is a connection located in the heating zone, and a cold junction is outside the measured zone. In this case, the name cold "junction" is purely conditional, because. the electrical circuit is completed through the impedance of the measurement circuit (instrument). If both ends of the cold junction are closed, then the thermoEMF value will be equal to zero. Similarly, if both junctions are heated evenly, then the perturbing forces will be balanced by electric ones. The EMF value is described by a simple formula: ЕТ=KТ(T1-T2), (1) where Kт - constant coefficient. From formula (1) it follows that thermoEMF is proportional to the temperature difference of dissimilar metals. Proportionality factor KT is called specific thermoEMF, and its values for the combination of different metals and their alloys are different. For example, for copper-constantan compound KT= 53 * 10-3 mV/°C, for silver-platinum connection KT= 12 * 10-3 mV/°C. To obtain contact thermoEMF, metals should be joined by welding-fusion with a neutral (carbon) electrode (preferably in an inert gas environment or in a vacuum, in order to prevent even foreign substance molecules from entering the junction). Vacuum deposition bonding on a neutral quartz glass or ceramic substrate gives good results. So the word "spy" in this case is purely conditional. In amateur conditions, you can make a good thermocouple if you weld two wires with a carbon electrode (voltage not higher than 36 V), combining copper, constantan, nichrome, fechral, nickeline and silver. You can use wire racks from an electric lamp. Silicon diodes can be an alternative replacement for both thermistors and thermocouples, and the thermopower developed by them is quite sufficient for practical use. The disadvantage is a large scatter of parameters and complexity in the organization of conclusions. In the 30s ... 50s of the XX century, a large number of thermoelectric generators were produced, operating from various types of coolants (kerosene lamp, kerosene gas and even a fire). Thermal generators were also used at a nuclear power plant. Interest in their widespread use gradually weakened due to the very low efficiency, at best, barely reaching 3%. True, not so long ago, Japanese specialists developed a bracelet generator that runs on the heat of the human body and feeds a transistor receiver. Unfortunately, cheap alkaline cells and nickel-cadmium batteries "closed" the development of thermogenerators. There is another application of thermoelectricity, or rather, a phenomenon discovered in 1834 by the watchmaker Peltier, who drew attention to temperature anomalies that occurred near the junction of two conductors of dissimilar metals when an electric current passed through them. Later, E.Kh. Lenz investigated and explained the nature of this phenomenon. In Lenz's experiment, a drop of water was placed in a recess at the junction of two conductors of bismuth and antimony, which, when a current passed in one direction, froze, and boiled in the other. The phenomenon, first discovered by Peltier, was called the Peltier effect, and thermoelectric elements made on this basis were called Peltier elements (Fig. 2).
In the manufacture of elements, the best results were obtained by connecting pairs of semiconductor materials: lead sulfide, bismuth, antimony, zinc. In Peltier elements, the process of heating and cooling junctions can be considered as a transfer of heat under the influence of an applied EMF from one junction to another and, as it were, an increase in the thermal conductivity of conductors. In Peltier elements, there are hot and cold junctions, but voltage is applied to a closed circuit of dissimilar metals. The hot junction is warmed up and the cold junction is cooled, and the more intensely the released heat is removed, the more the cold junction is cooled. When the polarity of the supply voltage is reversed, the process also changes sign, which can lead to the destruction of the element. To obtain a significant temperature difference, a good coolant is needed to effectively cool the hot junction. Currently (according to the CHIP-DIP catalog) Peltier elements are offered for cooling REA and other purposes where efficiency does not play a significant role. Literature
Author: I.Semenov, Dubna, Moscow region
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