Seebeck coefficient, thermoelectric

The Seebeck coefficients (thermoelectric powers) of Na WOs have been measured over a wide range of x values at room temperature (300° K.). At this temperature, the residual resistance, p0, and thermal resistance, pt, are comparable, the value of p0 being between pt and 2pt. Nevertheless, one would expect to a first approximation (10) that S = (1/3) (ir2k2T/e ), where S is the Seebeck coefficient, k is Boltzmann s constant, e is the electronic charge, and f is the Fermi energy. For free electrons, the Fermi energy f = (h2/2m ) (3n/87r)2/3 where h is Planck s constant, m is the effective mass, and n is the density of free electrons. Since n is proportional to x, f varies as x2/3 and S varies as xr2/3. 

TABLE 16.12 Nominal Seebeck Coefficients (Thermoelectric Power), aAB ( iV7°C) [36]... 

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... 

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). 

The thermoelectric effect is due to the gradient in electrochemical potential caused by a temperature gradient in a conducting material. The Seebeck coefficient a is the constant of proportionality between the voltage and the temperature gradient which causes it when there is no current flow, and is defined as (A F/A7) as AT- 0 where A Fis the thermo-emf caused by the temperature gradient AT it is related to the entropy transported per charge carrier (a = — S /e). The Peltier coefficient n is the proportionality constant between the heat flux transported by electrons and the current density a and n are related as a = Tr/T. 

Ce[ 64Asi2 is probably a narrow gap semiconductor, but little low temperature data are available for this compound. The resistivity of a polycrystalline sample indicates a small gap on the order of 0.01 eV (Grandjean et al., 1984). The high temperature thermoelectric properties of this compound were investigated by Watcharapasorn et al. (2002). They found semimetallic behavior with a room temperature resistivity of 0.49 m 2 cm, a Seebeck coefficient of 40 pV/K, and a thermal conductivity of 3.8 W/mK. The maximum value for ZT, the thermoelectric figure of merit, was estimated to be 0.4 at 850 K. 

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],... 

This expression shows that the imposition of a temperature difference dT in the absence of any current produces a difference d i in electrochemical potential i.e., d ife) = —z/a, dT. This effect is known as the thermoelectric effect, and the ratio d i/e)/dT, or A(f,/e)/Ar = —ZiCii is known as the Seebeck coefficient (1823), or thermoelectric power.Experimentally, the difference of electrochemical potential may be measured by a voltmeter under open circuit conditions, and dT, measured by means of thermocouples a, is thereby experimentally determined. As defined here for p-type (n-type) material the measured Seebeck coefficient is a positive (negative) quantity. For, and both increase in the direction of increasing hole or electron concentration, which is in a direction opposite to the increase in temperature. Comparison with (6.9.2) shows that Ui = ZiSgfe. Then Eq. (6.9.6) becomes... 

Consider thermoelectric measurements that are carried out under adiabatic conditions for which = 0. Relate the resulting electrical conductivity and Seebeck coefficient to the quantities introduced above. 

Efficient thermoelectric devices require the use of high figure of merit thermoelectric materials. The thermoelectric figure of merit, ZT, can be expressed as oQ-oTIk, where a is the Seebeck coefficient, o the electrical conductivity, T, the temperature, and k, the thermal conductivity. Among the various thermoelectric materials, bismuth telluride (Bi2Te3) has been the main focus of research because of its superior ZT near room temperature [58]. 

In this paper, thermoelectric figure of merit for n-type sintered materials of SiGe and PbTe has theoretically been estimated considering effects of grain boundaries on carrier mobility and Seebeck coefficient based on a model. 

The thermoelectric properties were measured at room temperature along the direction perpendicular to the pressing direction. The samples with dimensions of 2 X 2 X15 mm and of 4x4x4 mm were cut out of the compound for the measurements of Seebeck coefficient a and thermal conductivity k and of the electrical resistivity p, respectively. Then, their surfaces were polished with a series of SiC polishing papers of up to 2000 and further polished on a polishing cloth impregnated with AI2O3 powders of 0.3 //m size. 

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. 

As the thermoelectric characteristics of FGM samples, the electrical resistivities (p) and Seebeck coefficients (a) were measured in a method shown schematically in Fig. 3. 

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