Thermopower temperature dependence

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... 

The pressure-dependent electrical resistivity of the heavy-fermion compound YbNi2B2C (see also Section 4.12) could be explained by competing contributions from crystal-electric-field splitting and Kondo effect (Oomi et al., 2006). The pressure-dependent room-temperature thermoelectric power of YNi2B2C exhibits a peak around 2 GPa, which was explained by changes in the Fermi-surface topology (Meenakshi et al., 1998). A possible correlation with a small peak in the temperature-dependent thermopower around 200 K (Fisher et al., 1995 Section 3.4.3) needs further investigation. 

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 thermopower of single-chain conductors pertaining to the (TMTSF)2X series ranges around 20 xV/K at room temperature [66] it follows a quasi-linear temperature dependence and extrapolates to a value near zero at 0 K (Fig. 15b). The use of a one-dimensional tight-binding formulation [67a],... 

Figure 15 (a) Temperature dependence of the Hall voltage in the relaxed state of (TMTSF)2C104 at T = 0.35 K. The outset of a FISDW phase is visible at 4 T. (From W. Kang, private communication.) (b) Thermopower data of (TMTSF)2C104. (After Ref. 67b.)... 

The statistical shift of E and the possible temperature dependence of the conduction energy, which complicate the analysis of the conductivity, also enter into the thermopower equations. However, o(J) and S T) can be combined into a function Q T) which eliminates the statistical shift, since the Fermi energy position drops out of the combined expression (Beyer and Overhof 1979),... 

Figures 13 and 14 show the crystal structure and the temperature dependence of electrical conductivity measured along the one-dimensional axis, fc-axis, of TTF-TCNQ [53]. The conductivity increases with decreasing temperature down to about 60 K below which the conductivity is characterized by thermally activated nature. The metallic properties are ascertained by much experimental evidence such as optical reflectivity, spin-magnetic susceptibility, and thermopower [54]. In the insulating state similar measurements also suggest the presence of a band gap at the Fermi level. These measurements suggest the metal-insulator transition to oceur at 53 K.

It should be noted that in situ conductivity measurements of ECP films allow for an estimation of the absolute values ofconductivity, although high values of conductivity are not the only feature of a metallic state. Ex situ direct current electronic conductivity measurements of the films should be carried out in order to examine any temperature dependence of the conductivity and thermoelectric power over a wide range of temperatures, starting with very low values (of a few K) [30]. Typical metals have negative temperature coefficients for their electronic conductivity, and positive temperature coefficients for their thermopower, which contrasts with... 

The electronic transport properties point to both possible features i.e, the metallic and non-metallic ones (e.g. [93,96]). The temperature dependence of conductivity is predominantly of a non-metallic kind (i.e. conductivity increases with temperature in the case of metals it decreases with temperature), but at higher temperatures it often changes to a metallic sign. However, the thermopower of conductive polymers is... 

The earlier theimopower data for PAni/PVC blends [80] are similar to that for the blends in Figure 11.56, except for negative values at very low temperatures. In the only other data on PAni blends, the theimopower of PAni-CSA/PMMA blends measured by Yoon el al [102] is very close to that of the PAni/PMMA blends investigated here, in magnitude as well as temperature dependence (except for small negative values seen at very low temperatures). These data further emphasise the similarity of PAni blend thermopowers. The theimopower of the pure PAni does appear to be significantly smaller than that of the blends small thermopowers (of either sign) have been seen for some PAni samples by other authors [93,94,95]. 

At an early stage it was recognised [12] that ICPs display a temperature dependence of thermopower that identifies them as metals (according to Kaiser with similarities to amorphous metals). It was therefore interesting for us to know what the thermopower of blends of non-conductive matrix polymers with PAni was like. (See Section 7.)... 

As early as 1987 [137], therefore, we measured the thermopower of a PAni blend (at saturation conductivity concentration, at that time around 35%) with PVC and recognised its temperature dependence as linear, though in view of our limited experimental facilities in this field we were only able to measure a tsJ of about lOOK, and were above all not able to measure sufficiently far in the direction of absolute zero. 

Although the PAni-PVC blends display a greater thermopower than the pure PAni, the figures are still well below what can be expected for semiconductors. Above 100 K the temperature dependence of S is described relatively well by a linear approach (which when extrapolated to T = 0 results in a positive value of So)-... 

The predictions OS.Sgi of the correct temperature dependence of the resistivity and the correct signs for both the Hall and thermopower tensors is a remarkable accomplishment for the band structure approaches. The discrepancies in the magnitudes of the calculated transport coefficients compared with experiMnt seem to us relatively minor. We therefore wish to oppose strongly the view suggested by some that band theory is completely irrelevant. It is very important that accurate single crystal measurements on these and other materials be performed to stimulate further theoretical progress. 

The temperature dependent thermopower of the copper voltage leads is required in order to obtain the samples absolute thermopower. For temperatures below 9IK, sample 1 is superconducting and therefore has zero thexmopower, which allowed a direct determination of the absolute thermopower of the copper voltage leads. The copper thermopower values from this self-calibration agree with the results of Crisp et al. ( ), as corrected by the new thermopower scale of Roberts ( ), throughout... 

Fig. 1. Schematic representation of the temperature dependence of the Fermi energy Ef. Conductivity and thermopower measurements yield apparent Fermi-level positions i RO).

The temperature dependence of thermopower for several non-magnetic RI compounds is shown in fig. 8 (LaAy, fig. 9 (YAI2, LuAl ), and fig. 10 (YCu2, LuCu2). 

The temperature dependence of the thermopower S of RCU2 compounds is shown in figs. 47-49 (Mikovits, 1981). As mentioned above the light RCU2 compounds are paramagnetic and the rest are antiferromagnetic (see section 3.2.1.2). 

This compound is ferromagnetic (To = 74 K, cubic MgCu2 structure) and exhibits a kink in the resistivity (see section 3.2.1.10) and the absolute thermopower (fig. 53) near the Curie temperature Tj. Zorid et al. (1973) have shown that these are correlated discontinuities in the temperature dependence of the thermopower and the resistivity at in GdNi2. This is shown in fig. 54. These anomalies have been described theoretically by Zorid et al. (1973) and the agreement between the results shown in fig. 54 and the theoretical conclusion described in section 3.1.3.4 is good. 

At present there is no detailed understanding of the temperature dependence of the thermal conductivity of these RAI2 compounds. However, we believe that the systematic investigation of the temperature dependence of A across the rare earths of this section can give us an indication of the effect of local magnetic moments on the thermal conductivity. This is analogous to the situation for the thermopower. 

The temperature dependence of the thermopower of (Gd, Y)4Co3 compounds is given in fig. 83 for several Gd concentrations. Gratz et al. (1980d) have shown that the Curie temperature of these compounds varies almost linearly with Gd... 

Fig. 26. Temperature dependence of thermopower, S against T, for CeCujSij for r>1.5K (Franz et al. 1978). Low-T (<1 K) data in the normal state (not shown) exhibit an additional anomaly similar to that in fig. 20 (Steglich et al. 1984),...

Fig. 2 shows the temperature dependence of the electrical resistivity of (DIET)2(BF4)3 as a typical example. The compound behaves as a metal at room temperature and l low. At lower temperature, a very broad phase transition is indicated in the resistivity data. Fig. 3 summarizes results of the thermopower measurement on this material at different temperatures. The latter data clearly reveal a phase transition at around 50 K. Similar results have been obtained from DIET salts with other anions like HSO4" or N03", as shown in Fig. 4 for the compound (DIET)j (N03)y, the full structure of which could not be solved so far. 

We have studied the resistance and thermopower behavior of the p -phase [9]. From the temperature-dependent resistance curve, one can see clearly a metal-semiconductor phase transition at about 140 K (Fig. 2), whereas on the temperature-dependent thermopower curve, only a very small (but distinct) kink appears at the same temperature (Fig. 3). To interpret these seemingly conflicting phenomena, a two-energy band model was used [9]. In this model, the conductivity is due to a combination of the two bands, and the thermopower is due to a competition of the two bands. From the room-temperature X-ray diffraction, which shows the iodine atoms arranged randomly, we speculate that, at room temperature, there should be an energy gap at the Fermi surface, but that the random arrangement of iodine atoms smears the gap. Thus, the crystal stays metallic at room temperature. 

The -phase has a relatively higher room temperature conductivity [8]. Estimated from our measurement, Ort falls in the range of 5 x 10 -- 5 x 10 S cm-i for all samples. Fig. 6 shows the temperature-dependent resistance curve, and Fig. 7 shows the influence of thermal cycles on its transport behavior. It can be seen that (i) there exists a transition temperature, below which the resistivity increases almost linearly (ii) it is somewhat unstable in both the transition temperature and the behavior (slope), above and below the transition. In contrast to the resistance measurement, the thermopower of the A, -phase is rather stable [8]. Fig. 8 shows the temperature-dependent thermopower of the sample. These particular... 

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