Thermoelectric electromotive forces

To measure the Seebeck coefficient a, heat was applied to the sample which was placed between the two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature difference (JT <2 E) between the both ends of the sample. The Seebeck coefficient a of the compound was determined from the E/JT. The electrical resistivity p of the compound was measured by the four-probe technique. The repeat measurement was made rapidly with a duration smaller than one second to prevent errors due to the Peltier effect [3]. The thermal conductivity k was measured by the static comparative method [3] using a transparent Si02 ( k =1.36 W/Km at room temperature) as a standard sample in 5x10 torr. 

The Seebeck Effect The production of an electromotive force in a thermocouple under conditions of zero electric current. Thermoelectric power is the change in voltage at a thermocouple as a function of temperature. 

In a thermocouple, heating one junction of a bimetallic couple and cooling the other produces electromotive force in the circuit. This observation was originally was made by Seebeck in 1821. Besides the use of thermocouples, transistor electronics and semiconductors are important areas of interest for thermoelectric phenomena. Thermocouples made of semiconductors can develop relatively large electromotive potentials and are used to convert heat into electricity. 

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. 

On the other hand, for slow reactions, adiabatic and isothermal calorimeters are used and in the case of very small heat effects, heat-flow micro-calorimeters are suitable. Heat effects of thermodynamic processes lower than 1J are advantageously measured by the micro-calorimeter proposed by Tian (1923) or its modifications. For temperature measurement of the calorimetric vessel and the cover, thermoelectric batteries of thermocouples are used. At exothermic processes, the electromotive force of one battery is proportional to the heat flow between the vessel and the cover. The second battery enables us to compensate the heat evolved in the calorimetric vessel using the Peltier s effect. The endothermic heat effect is compensated using Joule heat. Calvet and Prat (1955, 1958) then improved the Tian s calorimeter, introducing the differential method of measurement using two calorimetric cells, which enabled direct determination of the reaction heat. 

The measurement of thermoelectric properties employed the DC method with high speed and high resolution[2] to remove fully errors occurring by Peltier effect. The thermo-electromotive force o was measured as a function of temperature difference AT at both ends of a specimen. The thermoelectric power a was obtained from a slope of o-A Tcurve and expressed as an absolute value. 

Temperature measurement using thermocouples is based on the thermoelectric effect. Two dissimilar metals are joined together at a junction where an electromotive force (emf) is generated according to the Seebeck effect. The emf level depends on the junction temperature. The Peltier effect causes an emf to be generated when the dissimilar metals are connected to an electrical circuit. 

The ionization constants at various temperatures and thermodynamic functions for sulphamic acid and several other aminosulphonic acids have been determined in formamide from electromotive force measurements40. The initial thermoelectric power (TEP) for hydrogen electrode thermocells has been determined for aqueous solutions of sulphamic acid at 29 °C over the concentration range 0.001 to 0.01 molal41. 

If the system is no longer isothermal, it is possible to demorrstrate that the experimentally measrtred potential difference is the contact potential difference added to the electromotive force of thermoelectric origin. 

If the thermal gradient at the electrodes is weak, which is usually the case, the terms a(T, -T2) and a (T3 -T4) ate neghgible. Moreover, since the conductor connecting the two sohds R and S is metalhc, its thermoelectric power a is weak and the term a (Tj — T3) is small. This electromotive force, for systems at a temperature lower than 360°C, was estimated at a few mV by J.P. Beaufils. 

Because we are only interested in the variations of the woik functions, and since the electromotive force of thermoelectric origin is stable when the temperature conditions are also stable, the problem concerning its influence on the contact potential difference information becomes irrelevant... 

Thermoelectric power n. Measured by the electromotive force produced by a thermocouple for unit difference of temperature between the two junctions. It varies with the average temperature and is usually expressed in microvolt per °C. It is customary to list the thermoelectric power of the various metals with respect to lead. 

In Tables 3.1-140-3.1-142 and Fig. 3.1-206, characteristic data are shown absolute thermoelectric power, thermo-electromotive force of pure Ag as well as Ag—Au, Ag—Pd, and Ag—Pt alloys at different temperatures against a reference junction at 0°C [1.217, 235,236],... 

Thermoelectric Properties. Tables 3.1-206-3.1-209 [1.216,217] and Figs. 3.1-269, 3.1-270 [1.216,218] give data of absolute thermoelectric power, thermal electromotive force of pure Pd and Pd alloys at different tenqreratures. Special alloys for thermocouples with high corrosion resistance are shown in Table 3.1-210 [1.217]. 

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