Absolute Seebeck coefficient

Wang, T.P. 1992. Absolute Seebeck coefficients of metalhc elements. Temperature, Its Measurement and Control in Science and Industry, American Institute of Physics, 6(2) 509—514. 

Because the third law of thermodynamics requires S = 0 at absolute zero, the following equation is derived, which enables the determination of the absolute value of the Seebeck coefficient for a material without the added complication of a second conductor ... 

Voltage measurement have been made at very low temperatures using a superconductor as one leg of a thermocouple. Eor a superconductor, S is zero, so the output of the couple is entirely from the active leg. The Thomson heat is then measured at higher temperatures to extend the absolute values of the Seebeck coefficients (7,8). The Thomson heat is generally an order of magnitude less than the Peltier heat and is often neglected in device design calculations. 

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],... 

The minus sign of the Seebeck coefficient indicates an n-type semiconductor as shown in Figure 18.6. The absolute values increase except for the specimen (M = Cu, X = 0.1), where the metallic Cu is supposed to exist in the crystal. The Seebeck coefficient in the specimen of Cu (0.01 mol%) increased against temperature although the electrical resistivity did not change much, implying a good characteristic for the TE material. 

We will now discuss the Seebeck coefficient in the system Fej xMxO (M — Ti). The experimental result is shown in Figure 18.10. In the case of X = 0 the standard Seebeck coefficient is very high because of a large energy gap according to Eq. (2). The Ti-substitution decreases an absolute value of the Seebeck... 

Three analytical tools were used to characterize the compounds. The first is powder x-ray diffraction methods using a 114.6-mm. diameter camera. The photographs were in general poor for germanium telluride and, as a result, the parameters were determined from low-angle reflections only. The second procedure involved room temperature Seebeck coefficient data (taken versus copper and converted to absolute values) which qualitatively vary inversely as the log of the carrier concentration. Finally, Hall measurements were taken on 1.6 X 0.5 X 0.1 cm. plates in a manner already described (6). 

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 Seebeck coefficient a and figure of merit Z for B4C-B ceramics as a function of C content are given in Fig. 7 and Fig. 8, respectively. The a was always positive, and its absolute value increases with increasing carbon content except for B4C+5B. a (0.30 0.38 mV/K), whose maxima was observed at 20 at.% C (B4O sintered at 2250 °C, showed opposite tendency of electric resistivity (7X 10 6 X10" Qm), whose minimum was at B4C+8B sample fired at 2250°C. Though the figure of merit Z is evaluated from electrical resistivity, thermal conductivity and Seebeck coefficient, the Z values showed maximum of 2.4 X 10 K at B4C+8B composite fired at 2250 °C. Therefore, the electric resistivity affects more than the Seebeck coefficient. 

The performance of TE materials is characterized by a dimensionless figure of merit ZT = S oT/k (S, a, k, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature. 

In the classical language, which does not rely on the pole-dipole distinction, the Seebeck coefficients defined at this level are called absolute coefficients. This logically leads to attributing the adjective relative to the dipolar Seebeck coefficient. The link between these coupling factors is, as one may read in the expanded Formal Graph (Graph 12.21),... 

Where a o is referred to as the electrical power factor, with a, the Seebeck coefficient, a the electrical conductivity and X. is the total thermal conductivity. The figure-of-merit is often expressed in its dimensionless form ZT wWe T is absolute tenperature. 

Representative Values of Absolute Seebeck Thermoelectric Coefficients of Some Materials Used in Industrial Electronic Circuits... 

The electrical conductivity (a) of the single-doped AxCai-xByMni.yOa-g thin-films increased when a higher valence ion was substituted for the Ca or Mn ion, presumably due to the increased electron carrier and the accelerated hopping conduction by Mn and Mn . The absolute value of the Seebeck coefficient was decreased by substituting the A or B ion for the Ca- or Mn-site in all sample. The decreased Seebeck coefficient in CaNio.o5Mno.9503-8 was presumed to be due to the decrease of the electron density by the substitution with a valence number smaller than that of the Mn ion. The thermoelectric behavior of thin-films was similar to that of sintered bodies. 

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