Thermoelectric principle

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

A Mass/Heat Flow Sensor Combining Shear Mode Resonators with Thermoelectrics Principles and Applications, IEEE Frequency Control Symposium. IEEE, Tampa, FL, 2003,1062-1065... 

It would evidently be premature to hazard a guess whether the combination of the thermoelectric principle with the nuclear reactor will become economical and whether the low pressure, high-mean-free-path stem, or the high pressure, low mean-free-path stem will eventually prevail. Both have strengths and weaknesses. One can say, however, that the thermoelectric heat conversion is an intriguing and intellectually attractive idea which will stimulate further ideas which, in turn, may render them attractive also from the economic point of view. 

The Entropy Principle gives, on the assumptions as to reversibility of thermoelectric effects ... 

Although the electrolysis of molten salts does not in principle differ from that of aqueous solutions, additional complications are encountered here owing to the problems related to the higher temperatures of operation, the resultant high reactivities of the components, the thermoelectric forces, and the stability of the deposited metals in the molten electrolyte. As a result of this, processes taking place in the melts and at the electrodes cannot be controlled to the same extent as in aqueous or other types of solutions. Considerations pertaining to Faraday s laws have indicated that it would be difficult to prove their applicability to the electrolysis of molten salts, since the current efficiencies obtained are generally too small in such cases. 

The components of a thermoelectric cooler are indicated by the cross section of a typical unit shown in Fig. 1. Therm oelectiic coolers such as this are actually small heat pumps that operate on the physical principles well established over a century ago. Semiconductor materials with dissimilar characteristics are connected electrically in senes and thermally in parallel, so that two junctions are created. The semiconductor materials are n- and /i-type and are so named because either they have more electrons than necessary to complete a perfect molecular lattice structure (ri-type), or not enough electrons to complete a lattice structure (/7-type). The extra electrons in the -type material and the holes left in the /7-type material are called carriers and they are the agents that move the heat energy from the cold to the hot junction. 

Thermocouples operate on the principle of thermoelectricity (Seebeck effect). When the junction of two dissimilar electronic conductors - typically two metals, two semimetals, or semiconductors - is held at temperature T (the so-called cold junction ), a measurable voltage develops between it and the hot (Tz) junction. This thermoelectric voltage (V) of the two conductors can be measured according to (3.9). 

Liess, M. and Steffes, H. (2000) The modulation of thermoelectric power by chemisorption a new detection principle for microchip chemical sensors. /. Electrochem. Soc. 147, 3151-3153. Tran-Minh, C. and Vallin, D. (1978) Anal. Chem. 50, 1874. 

Nolas, G.S., Sharp, J., Goldsmid, H.J., 2001. Thermoelectrics Basic Principles and New Materials Developments. Springer Verlag, New York. 

MacDonald, D.K.C., 1962. Thermoelectricity An Introduction to the Principles. Wiley, New York. 

It is believed that Galileo invented the liquid-in-glass thermometer around 1592. Thomas Seebeck discovered the principle behind the TC—the existence of the thermoelectric current—in 1821. The same year Sir Humphry Davy noted the temperature dependence of metals, but C. H. Meyers did not build the RTD until 1932. Today, some 20 different types of temperature sensors are available, and Table 3.160 lists the temperature ranges and accuracies of a number of them. 

This is the basic principle of thermoelectric pyrometry. The electromotive forces developed by thermocouples are small, usually a few thousandths of a volt. To measure such small electromotive forces special types of sensitive voltmeters (millivolt-meters) or indicators are required. For any particular type of couple these instruments may be graduated to read temperature directly instead of electromotive force. 

The dynamic thermocouple, also known as the "Herbert-Gottwein" thermocouple, measures the temperature at the rubbing interface of two dissimilar metals by making it the hot junction of a thermoelectric circuit. The utilization of this principle for the measurement of the chip-tool rubbing temperature in metal cutting appeared on the scene in 1925/1926 [6, 7, 8] and for the measurement of temperature in ordinary rubbing about 10 years later [9]. 

As measuring vapor pressure depression using VPO requires extreme sensitivity, a thermoelectric method is used based on the following principle. One drop of pure solvent is placed on one of two matched temperature-sensitive thermistors located in a chamber saturated with vapor solvent, at constant temperature. On the other thermistor, a drop of polymer solution is placed. The solvent condensation on the solution drop will heat it up until its vapor pressure matches that of the pure solvent drop [35]. 

PELTIER EFFECT - When direct current is passed through two adjacent metals one junction will become cooler and the other will become warmer. This principle is the basis of thermoelectric refrigeration. 

The exponential decay of the power with time may be integrated to measure the heat produced. Cooling in the thermopiles may be induced by passing a current in the reverse direction allowing a more efficient means of temperature control, reducing experimental time. Such principles are used in Calvet or thermoelectric heat pump calorimeters. 

This introductory section will describe the various issues that are motivating research on direct thermoelectric gas sensors before presenting a brief introduction to thermoelectric power for the general reader. The principles of direct and indirect thermoelectric gas sensors are then outlined, while early research work is reviewed in the final subsection. 

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