Seebeck thermopower

The Seebeck coefficient is frequently called the thermoelectric power or thermopower, and labeled Q or S. Neither of these alternatives is a good choice. The units of the Seebeck coefficient are not those of power. The symbol Q is most often used to signify heat transfer in materials. The designation S can easily be confused with the entropy of the mobile charge carriers, which is important because the Seebeck coefficient is equivalent to the entropy per mobile charge carrier (see Supplementary Material S3). 

A second historical line which, is of paramount importance to the present understanding of solid state processes is concerned with electronic particles (defects) rather than with atomic particles (defects). Let us therefore sketch briefly the, history of semiconductors [see H. J, Welker (1979)]. Although, the term semiconductor was coined in 1911 [J. KOnigsberger, J, Weiss (1911)], the thermoelectric effect had already been discovered almost one century earlier [T. J. Seebeck (1822)], It was found that PbS and ZnSb exhibited temperature-dependent thermopowers, and from todays state of knowledge use had been made of n-type and p-type semiconductors. Faraday and Hittorf found negative temperature coefficients for the electrical conductivities of AgzS and Se. In 1873, the decrease in the resistance of Se when irradiated by visible light was reported [W. Smith (1873) L. Sale (1873)]. It was also... 

Dc, ac, impedance, and thermoelectric power of the compounds 33-38 in Fig. 9 have been investigated in detail. The measured temperature dependence of the thermoelectric power of 33-38 in thin film varied approximately exponentially with temperature. Compared to 38, the absolute value of the thermopower for the film of 34 is larger by nearly a factor of 3. The positive sign of Seebeck coefficient confirms that thin films of the compounds behave as a p-type semiconductor [46],... 

Couples comprised of two different pure metals have low Seebeck coefficients since the absolute thermopowers of pure metals are in the microvolt per degree Celsius range (superconductors have zero absolute thermopowers). The difference between the absolute thermopowers of each metal in a couple yields the observed TE power of the couple. However, in metals (with half-filled bands) the electrons and holes have a cancelling effect the TE voltage produced is small. This makes them unsuitable for use in most TE apphcations with the exception of thermocouples that are used for temperature measurements. Semiconductors, by contrast, can be doped with an excess of electrons... 

The second, more general, treatment is based on nonequilibrium thermodynamics. Fluxes and forces are connected by a matrix. The diagonal elements (the main effects) of this matrix are well-known - for example, the diffusion coefficient (which is the connection between a particle flux under a concentration gradient) or the thermal conductivity (which relates the temperature gradient with the heat flux). One of the non-diagonal elements is the Seebeck coefficient (= thermopower, ]), which relates a temperature gradient with a particle flux. Based on this, general equations are obtained that describe the heat and particle flow in a thermal and concentration profile ... 

In contrast, in direct thermoelectric gas sensors (DTEGs) the Seebeck coefficient (thermopower, rj) of the gas sensitive material itself changes when the concentration of the analyte varies in the ambient atmosphere. The density of the free electrons and/or defect electrons (holes) - or, in other words, the Fermi level - is directly affected by a changing analyte gas... 

Compared with the relatively simple resistance measurement, DTEGs require a more sophisticated set-up. The measurand thermopower (Seebeck coefficient) is defined by ... 

In Equation [7.7], is the Seebeck coefficient of the gas sensitive film, AVgst is the measured thermovoltage of the gas sensitive film, and AT is the temperature difference at the junctions between the gas sensitive layer and the conductor tracks. Due to the fact that the conductor tracks also add a thermovoltage, the thermopower of the gas sensitive layer has to be corrected by the thermopower of the conductor track material (here, platinum), J7p,. 

DTEGs are an alternative to resistive gas sensors. Accurate, rapid and long-term stable gas sensors have been presented in this chapter. The main advantage of DTEGs is the measurand thermopower or Seebeck coefficient . In contrast to conductometric gas sensors, the measurand thermo-power is not influenced by changes in the geometry of the gas sensitive... 

Electrical conduction and heat transport are closely linked, the connection being described by three thermoelectric coefficients, the Seebeck coefficient (or thermopower), the Peltier coefficient and the Thomson coefficient, all of which have relevance to thermoelectric power generation and refrigeration. In perovskites, the most reported values are for the Seebeck coefficient. The magnitude and sign (+ or -) of the Seebeck coefficient are related to the concentration and type of mobile charge carriers present. For band-like perovskites, the magnitude of the Seebeck coefficient is proportional to the density of states, either in the conduction band, for electron transport, or the valence band for hole transport. 

One of the unique properties the Si- and Ge-containing MAX phases is their small and temperature independent Seebeck coefficients [86, 87]. Using ah initio calculations, Chaput et al. [94] calculated the thermopower to be negative alongthe c axis and positive in the basal planes. The small value experimentally observed was thus ascribed to a compensation between the thermopowers of the two nonequivalent crystallographic axes. Yet, while certainly reasonable, this prediction awaits experimental verification on epitaxial thin films or large single crystals. 

For some compounds the resistivity and thermopower were measured from 78 to 360 K. The Seebeck coefficient decreases with increasing temperature, while the resistivity exhibits the normal temperature dependency (table 31). 

In Figure 4.11, the values of S at 300 K are reported as a function of x. Also shown is the evolution of S x) calculated with the Heikes formula, and with the spin and orbital degeneracy calculated by Marsh et The data are perfectly fitted by the Marsh and Parris formula,showing that an enhancement of thermopower can be obtained by optimising the spin and orbital degeneracies. Combining the resistivity and Seebeck... 

The measurement of thermopower is based on the Seebeck effect, which consists of the generation of a difference in the electrical potential aeross imposed temperature gradient. For a nondegenerate semiconductor, in which one type of electronic charge carrier predominates, one may formulate interrelationships between thermopower (S) and the concentration of electronic carriers. For n-type and p-type regimes the following expressions may be written ... 

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