Thermoelectric force

Thermo-elektrizitat, /. thermoelectricity, -farbe, /. thermocolor, -kauter, m. thermocautery. -kette, /. thermoelement, -kraft, /. thermoelectric force or power, -lyse, /. thermolysis. 

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 determination of the heat flow relies on the so-called Seebeck effect. An electric potential, known as thermoelectric force and represented by E, is observed when two wires of different metals are joined at both ends and these junctions are subjected to dilferent temperatures, 7j and T2 (figure 9.1a). Several thermocouples can be associated, forming a thermopile (figure 9.1b). For small temperature differences, the thermoelectric force generated by the thermopile is proportional to 7j - T2 and to the number of thermocouples of the pile (>/) ... 

As all the thermocouples are identical, xe /X is constant. Therefore, the thermoelectric force of the pile formed by the n thermocouples connected in series is given by... 

A second improvement in Calvet s calorimeter is that a differential set-up was adopted that aimed to suppress temperature drifts and fluctuations of the heat sink. This was achieved by coupling two calorimetric units in opposition to each other, so the measured thermoelectric force was the difference between the thermoelectric forces of the sample cell and the reference cell. The latter may remain at the temperature of the thermostat while the heat output or input related to the event under investigation occurs in the sample cell. 

All modern heat flow calorimeters have twin cells thus, they operate in the differential mode. As mentioned earlier, this means that the thermopiles from the sample and the reference cell are connected in opposition, so that the measured output is the difference between the respective thermoelectric forces. Because the differential voltage is the only quantity to be measured, the auxiliary electronics of a heat flux instrument are fairly simple, as shown in the block diagram of figure 9.3. The main device is a nanovoltmeter interfaced to a computer for instrument control and data acquisition and handling. The remaining electronics of a microcalorimeter (not shown in figure 9.3) are related to the very accurate temperature control of the thermostat and, in some cases, with the... 

Figure 9.5 Thermogram (plot of the measured thermoelectric force versus time) of the thermal decomposition of Fe(CO)(1,3-C4H6)2 at 418 K. Adapted from [163],...

In a nonisothermal system, an electric current (flow) may be coupled with a heat flow this effect is known as the thermoelectric effect. There are two reciprocal phenomena of thermoelectricity arising from the interference of heat and electric conductions the first is called the Peltier effect. This effect is known as the evolution or the absorption of heat at junctions of metals resulting from the flow of an electric current. The other is the thermoelectric force resulting from the maintenance of the junctions made of two different metals at different temperatures. This is called the Seebeck effect. Temperature measurements by thermocouples are based on the Seebeck effect. 

C. Montemartini and L. Losana studied the e.m.f. of soln. of the salt. A. Hagenbach made observations on the thermoelectric force of the soln. E. Feytis... 

The thermoelectricity theory of thermophoresis has been proposed to explain these experimental results and to describe the effects of pH value and salt concentration and type [7]. This theory predicts that for particles suspended in an electrolyte solution under a temperature gradient, an additional thermoelectric force is generated to exert on the particles by induced electric field due to the thermodiffusion of positive and negative ions. 

Wild found that thermoelectric currents are produced at the contacts of solutions in the tubes at different temperatures, and the thermoelectric forces are stronger than the liquid contact potentials. He used the same solution (e.g. CUSO4) in the lower parts of both tubes, and the same solution (e.g. ZnS04) above. He found a thermoelectric force between two solutions of the same salt of different concentrations. The thermoelectric forces obeyed Volta s law. E. du Bois-Reymond and Wild did not detect a Peltier effect by passing a current through the junctions, but the effect was later observed by Schultz-Sellack. ... 

Figure 14. Thermoelectric force of TajOs specimen as a function of the logarithm of the partial pressure of oxygen [from Kofstad, J. Electrochem. Soc. 109 (1962) 776].

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