Thermoelectric potential

Keywords Thermoelectric potential field, particles motion, trajectory of electrons. 

According to the pioneering experiments of Seebeck, who discovered the thermoelectric potentials in 1823, we may arrange the metals in a series such that each metal becomes positively charged at the hot join with respect to the preceding metal of the series. The same summation law holds for the thermoelectric potential difference between any two metals as in the voltaic sequence, namely that it is equal to the sum of the potential differences between the intermediate members of the series. Thus, in the series A, B, C, A etc., the potential difference AD between A and D is equal at the same temperature to AB BC+CD. The thermoelectric series is approximately as follows ... 

Different observers do not always agree as to the exact order of the metals in the series, as the value of the thermoelectric potential difference is affected in a marked degree by the temperature and the purity of the metal. 

The thermoelectrical behaviour of many alloys is typical. Their thermoelectric potential is often much higher than that of the pure metals of which they are composed. These facts cannot be deduced from thermodynamics, which in general can tell us nothing new about constants which are characteristic of the chemical nature of substances. We must have recourse here to special theories, just as in the calculation of the osmotic pressure of solutions. We may mention that the electronic and molecular theories of R. Schenck I and A. Bernoulli J have done valuable service in this direction. 

The thermoelectric potential, te> is due to the heat transport of the electrons and arises if an electron conductor (usually a wire) is in nonisothermal condition. Eje is a function of the temperature gradient and, for the most common wire materials, is usually up to a few millivolts. Eje can be calculated over a wide range of temperatures for most of the wire materials such as Pt, Ag, Cu, Fe, Ni, and so on [13]. 

According to Lister [1981] deposition may be induced by thermoelectric potentials. The passage of heat normal to a surface (heat flux) can induce a "thermoelectric" effect if the liquid stream flowing across the surface contains a sufficient concentration of charged particles. The induced EMF will generate a particle flux according to the equation... 

Recently, Griffith and his co-workers (5) have shown, from a study of thermoelectric potentials, that although chromic oxide is a p-type semiconductor in oxygen, it becomes n-type in pure hydrogen. This is presumably associated with a partial reduction of the surface to chromous oxide (< , 6). A similar result had been reported earlier for chromia-alumina catalysts both by Chaplin, Chapman, and Griffith (7) and by Weisz, Prater, and Rittenhouse 8), but in view of the ra-type semiconductor properties of pure alumina (9,10), it is not clear whether measurements on chromia-alumina in hydrogen give information on the chromia or the alumina. 

Of particular interest in this experiment is the fact that the resistivity passes through a maximum as increasing amounts of hydrogen are reacted. In view of the results of Chapman, Griffith, and Marsh (5), it seems reasonable to associate this behavior with the transition from p-type to n-type semiconduction as the hydrogen pressure is increased however, confirming measurements of the Hall effect or thermoelectric potential as a function of hydrogen reacted would be necessary to establish this conclusion. 

J. D. F. Marsh North Thames Gas Board, England) We have measured the thermoelectric potential of chromia reduced at 500° in H2 and found that it is an n-type semiconductor even if this H2 is saturated with water at room temperature, that is, under conditions where bulk chromous oxide is not stable. Thus, addition of water to dry reduced catalyst does not cause a shift to beyond the maximum resistivity, as postulated in the last paragraph of the paper (Lecture 22), and the increase in resistivity follows naturally from the observed decrease in the amount of chemisorbed hydrogen. 

Figure 26-20. Practical signincance of thermoelectric potentials. The relative potentials, tRpt< of a platinum electrode in melts satisfying Equation (26-33) were obtained from standiud thermoelectric potentials, of a zirconia electrode and temperature-dependent emfs, E, of cell (VI) according to Equation (26-37). The temperature dependence of Pp, is determined by the standard Seebeck coefficients of the melts and is positive (a), negative (b), and, depending on the temperature, both negative and positive (c).

When two junctions of a material are maintained at different temperatures, then a potential difference or thermal emf develops between them. This phenomenon is referred to as the Seebeck effect 18), and the thermal emf is called the thermoelectric potential . The type of conductivity may be deduced by the sign of . 

Chapman et al. (182-184) extended the work of Bevan et al. (19) by establishing a temperature difference of about 20° between the two platinum electrodes, and using the Seebeck effect (22,185) they determined the thermoelectric potential (g). From the sign of it was ascertained that chromia is a p-t3rpe (oxygen excess or positive hole) semiconductor in an oxygen atmosphere, and an -type (metal excess or electron) semiconductor in a hydrogen atmosphere. 

The preceding data were all obtained at temperatures below 1000°. Fischer and Lorenz (186,187) measured the thermoelectric potential of chromia in an atmosphere of 10 Torr of oxygen from 750 to 1750°, and found it to possess a constant negative value up to about 1250°, and to decrease in magnitude (become less negative) rapidly with increasing temperature above 1250°. 

As mentioned above, the thermoelectric potential is almost constant below 1250° and its magnitude decreases rapidly above this temperature. Fischer and Lorenz 186) attribute the intrinsic conductivity above 1250° to the reaction... 

Chapman et al. (184) studied the electric conductivity chromia-alumina catalysts pretreated at 500°. Activation energies were calculated from the straight lines obtained from the temperature dependence of the conductivity data between 400 and 500° by means of the equation... 

The dependences of the electrical conductivity, activation energy, and thermoelectric potential on the chromium concentration are shown in Figs. 33, 34, and 35, respectively. 

Fig. 35. Dependence of the thermoelectric potential on the chromium concentration for a series of chromia-alumina catalysts. Its value is —10.0 in Oa at 11% Or. The data are from 184).

The electrical behavior of the chromia-alumina series after oxidation was completely different from that just described. The thermoelectric potential was always negative, so all the samples were p-type semiconductors. The conductivity gradually decreased from 6.3 x 10 for pure chromia to 2 X for pure alumina, without passing through... 

Thermoelectric potentials are quite small and should be determined at zero current so that a high precision potentiometer is required. Considerable care must be exercised to avoid parasitic e.m.f s which may be present at terminals and other connections. The e.m.f. produced per unit temperature may be increased by using multijunctions but the curve of current (on which sensitivity depends) versus number of Junctions eventually flattens to the point that further increase in thermal bulk is no longer worthwhile. Scatchard et alP have used a 128 junction thermocouple to measure depressions of the triple point of water with a reported precision of 10" K. Brown and Prue obtained a precision of about 10" K with a 24 junction couple. They used the simple circuit shown in Fig. 2.8.4 to amplify the e.m.f. by a factor of 10. 

The right-hand term in parentheses equals the Gibbs energy of reaction (13) with the opposite sign. Hence, the emf of an electrochemical cell (a cell with eliminated diffusion and thermoelectric potential drops is implied, see later) corresponds to the reaction Gibbs free energy ... 

The thermoelectric potential, Ej, is due to the heat transport of the electrons and arises if an electron conductor (usually a wire) is in nonisothermal condition. 

Table I. Thermoelectric Potential Differences for Gold-Cobalt, Constantan and "Normal Silver vs. Copper Thermocouples...

Table II. Thermoelectric Potential Differences in fiv for Several Thermo- couple Combinations...

The research on gold-cobalt, constantan, and "normal silver versus copper has been completed. The work on chromel versus alumel, iron versus constantan, and new materials of greater reliability is still in progress. For gold-cobalt, constantan and "normal" silver versus copper the thermoelectric powers are plotted in Fig. 1 and the thermoelectric potential differences are given in Table I. The total thermoelectric potential differences between 4° and 20 K, 20 and 76 K, and 76 and 273 K are given in Table II for each of the systems listed above. 

Currently, two approaches have been employed for potentiometric measiuements at elevated temperatures (1) the use of an internal reference electrode operating within the high-temperature environment, and (2) the use of an external reference electrode working at room temperature, but connected to the high-temperature environment by a non-iso-thermal electrolyte bridge. The first approach requires solving the well-known problem of the diffusion potential whereas the latter approach involves the additional problem of estimating the thermal liqmd-jimction and thermoelectric potentials. 

---