Peltier

In 1857, Thomson (Lord Kelvin) placed the whole field on firmer footing by using the newly developing field of thermodynamics (qv) to clarify the relationship between the Seebeck and the Peltier effects. He also discovered what is subsequently known as the Thomson effect, a much weaker thermoelectric phenomenon that causes the generation or absorption of heat, other than Joule heat, along a current-carrying conductor in a temperature gradient. 

The primary thermoelectric phenomena considered in practical devices are the reversible Seebeck, Peltier, and, to a lesser extent, Thomson effects, and the irreversible Eourier conduction and Joule heating. The Seebeck effect causes a voltage to appear between the ends of a conductor in a temperature gradient. The Seebeck coefficient, L, is given by... 

Voltage measurement have been made at very low temperatures using a superconductor as one leg of a thermocouple. Eor a superconductor, S is zero, so the output of the couple is entirely from the active leg. The Thomson heat is then measured at higher temperatures to extend the absolute values of the Seebeck coefficients (7,8). The Thomson heat is generally an order of magnitude less than the Peltier heat and is often neglected in device design calculations. 

The other primary thermoelectric phenomenon is the Peltier effect, which is the generation or absorption of heat at the junction of two different conductors when a current flows in the circuit. Whether the heat is evolved or absorbed is determined by the direction of the current flow. The amount of heat involved is determined by the magnitude of the current, I, and the Peltier coefficients, 7T, of the materials ... 

Silicon drift detectors (SDD, Figs 4.8 and 4.9) now also provide sufficient resolution (FWHM = 0.175 keV) above a sample spot sized 2 x 2 to 100 x 100 mm, and enable high-speed operation (> 10 counts s ). SDD can be combined with microelectronics and applied in portable TXRF models for microanalytical applications [4.30]. They must be cooled by a Peltier element. 

The dewpoint hygrometer detects the dewpoint temperature of air by cooling a surface in contact with the air to the dewpoint temperature. There are several ways to achieve cooling and to observe the formation of condensate on the surface. The early dewpoint hygirometers were cooled simply by applying the vaporization of ether or some other suitable liquid. Condensate formation on the surface was determined visually. Other cooling methods are to use a refrigerant flow in direct or indirect contact with the back of the surface, or to use electricity with a (thermoelectric) Peltier element. 

In many industrial gas measurements, water vapor is present in high concentrations. The sample cell of the measurement instrument and sample line can be heated up to 200 °C to remove water vapor. Sometimes, the sample gas is dried by condensation or by using Peltier gas dryers. 

Some heat pumps, called thermoelectric heat pumps, employ the Peltier effect, using thermocouples. The Peltier effect refers to the evolution or absorption of heat produced by an electric current passing across junctions of two suitable, dissimilar metals, alloys, or semiconductors. Presently, thermoelectric heat pumps are used only in some specialized applications. They have not been developed to a point to make them practical for general heating and cooling of buildings. 

Seebeck s outstanding scientific achievement was the discovei"y of one of the three classical thermoelectric effects, which are the Seebeck, the Peltier, and the Thomson effects. Seebeck s discovery was the first, dating from 1822—1823, followed by that of Jean-Charles-Athanase Peltier in 1832 and that of William Thomson in 1854. Seebeck obseiwed that an electric current in a closed circuit comprised different metallic components if he heated the junctions of the components to different temperatures. He noted that the effect increases linearly with the applied temperature difference and that it crucially depends on the choice of materials. Seebeck tested most of the available metallic materials for thermoelectricity. His studies were further systematized by the French physicist... 

The passage of an electric current through junctions of dissimilar metals causes a fall in temperature at one junction and a rise at the other, the Peltier effect. Improvements in this method of cooling have heen made possible in recent years hy the production of suitable semiconductors. Applications are limited in size, owing to the high electric currents required, and practical uses are small cooling systems for military, aerospace and laboratory use (Figure 2.13). 

The reversible reaction heat of the cell is defined as the reaction entropy multiplied by the temperature [Eq. (15)]. For an electrochemical cell it is also called the Peltier effect and can be described as the difference between the reaction enthalpy AH and the reaction free energy AG. If the difference between the reaction free energy AG and the reaction enthalpy AH is below zero, the cell becomes warmer. On the other hand, for a difference larger than zero, it cools down. The reversible heat W of the electrochemical cell is therefore ... 

The source of the energy of thermoelectric currents is indicated by the observation of Peltier (1884) that heat is absorbed at the... 

Besides the reversible production of heat at the junctions, there is an evolution of heat all round the circuit due to frictional resistance, this Joule s heat being proportional to the square of the current, and hence not reversed with the latter. There is also a passage of heat by conduction from the hotter to the colder parts. But if the current strength is reduced, the Joule s heat, being proportional to its square, becomes less and less in comparison with the Peltier heat, and with very small currents is negligible. We shall further assume that the reversible thermoelectric phenomena proceed independently of the heat conduction, so that the whole circuit may be treated as a reversible heat... 

We first assume that the Peltier effects are the only reversible heat effects in the circuit. Then if 7Ti, 7t2 are the Peltier effects at the hot and cold junctions ... 

The Peltier effect at a single junction is therefore equal to the absolute, temperature of the junction multiplied by the rate of... 

Kohlrausch s theory leaves quite unexplained the fact that no thermoelectric current is set up in a homogeneous wire along which a current of heat is flowing, whilst the theory of Lord Kelvin is difficult to reconcile with the fact that thermoelectric currents cannot be set up in a circuit of liquid metals, although these show the Thomson effect. The latter seems, therefore, to be to a certain extent independent of the Peltier effect. Theories intended to escape these difficulties have been proposed by Planck (1889), and Duhem, in which the conception of the entropy of electricity is introduced. 

The direct measurements of Jahn (1888, 1893) and of Gill (1890) show that the latent heat A arises at the surfaces of contact of the electrodes and electrolyte and is fully accounted for by these Peltier heats at the junctions of conductors. The equation of 197 ... 

The heat of formation of a substance iji a voltaic cell may therefore be calculated from the measured Peltier effects and the electromotive force. 

Pawlewski s rule, 407 Peltier effect, 450, 460 Perpetuuin mobile, 51, 70 Phase, 20 rule, 169, 388, 446 Phosphorescence, 35 Physically small, 38, 69 Plait point, 244... 

Development of a Substitute for the Potassium Chlorate Red Compositions Used in Pyrotechnics , PATR 604 (1935) 3) S. Peltier C. 

Peltier Explosive. A Fr expl invented in 1884 contg K perchlorate 67.1, K nitrate 8.4, sulfur 8.4, fine sawdust 6.0, and extract of campeachy weed 10.1%... 

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