Thermoelectric power, temperature

The basic ideas of thermoelectricity have been known for nearly two centuries, but until well after the Second World War the primary use was for temperature measurement (qv) using metallic wires. Then, upon improvements in semiconductor technology, thermoelectric power generation and refrigeration came under serious consideration. 

Thermocouples are unsurpassed for making temperature-difference measurements. The thermoelectric power of thermocouple materials makes them adequate for use at hquid-air temperatures and above. At 20 K and below, the thermoelectric power drops to a few lV/K, and their use in this range is as much art as science. 

Fig. 6. Temperature dependence of the thermoelectric power of three SWCNT samples [11].

Calvet and Persoz (29) have discussed at length the question of the sensitivity of the Calvet calorimeter in terms of the number of thermocouples used, the cross section and the length of the wires, and the thermoelectric power of the couples. On the basis of this analysis, the micro-calorimetric elements are designed to operate near maximum sensitivity. The present-day version of a Tian-Calvet microcalorimetric element, which has been presented in Fig. 2, contains approximately 500 chromel-to-constantan thermocouples. The microcalorimeter, now commercially available, in which two of these elements are placed (Fig. 3) may be used from room temperature up to 200°C. 

Unfortunately, the thermoelectric power vanishes when temperature tends to zero pairs as the classic Cu/constantan thermocouples show very low sensitivity below 10 K (see Fig. 9.2). 

Obviously, the small thermoelectric powers produced by metal pairs at low temperatures cannot be measured using a reference temperature of 0°C, where the thermoelectric power is high a 4.2 K bath is a typical reference. 

Fig. 9.3. Thermoelectric power of Pd doped with Fe (in ppm) as a function of temperature. The value for the pure sample has been multiplied by 10 for sake of clarity [64].

The Seebeck Effect The production of an electromotive force in a thermocouple under conditions of zero electric current. Thermoelectric power is the change in voltage at a thermocouple as a function of temperature. 

At relatively high temperatures thermocouple thermometers are most commonly used to measure temperature. The thermoelectric power of three frequently used thermocouples is compared in Figure 10.2. The choice of thermocouple depends on the temperature range, the chemistry of the problem in question, sensitivity requirements and resistance towards thermal cycling. The temperature range and typical uncertainty of some of the most commonly used thermocouple thermometers are given in Table 10.2. 

Figure 10.2 Thermoelectric power versus temperature response of three frequently used thermocouples.

During operation the voltage developed at the thermopile output is proportional to the thermoelectric power of each of the two different materials and to the temperature difference between the warm and cold junction (Seebeck effect). 

A thermopile can also be used as a chemical sensor if one of the two materials is a catalytic metal for a given volatile compound. In this case it is necessary to keep the warm and cold junctions at constant temperature. During absorption of the volatile compound on behalf of the catalytic material the thermoelectric power may change, giving rise to an output voltage which can be related to the concentration of the volatile compound. A typical example is the thermopile as hydrogen sensor, where one of the two materials is palladium, a standard hydrogen catalyzer. 

To gauge the magnitude of the thermoelectric power effect, the thermoelectric power coefficient is defined simply as the potential difference developed, dV, per unit temperature difference, dT ... 

There are simply two copper probes maintained at a 10"C temperature difference, which then gives rise to a potential, V, between the copper probe and the material being investigated. The thermoelectric power coefficient of the material is then calculated as ... 

Figure 6. Thermoelectric power coefficient as a function of H atoms/LaNi5 for hydrogen charged LaNij at room temperature (green) and 190°C (blue).15...

In addition, the oxidation of [Bu4N][2b] resulted in a neutral complex as a polycrystalline sample. This material displays metallic properties with a room temperature eleetrieal eonductivity of 6 S cm" and a thermoelectric power of 5.5 V K". This is reported to be the first example of such metallic properties observed in a moleeular system based on a neutral species. ... 

The thermopower or thermoelectric power is the electrostatic potential difference between the high and low temperature regions of a material with an impressed thermal gradient and zero electric current flow. The sign gives an indication of the sign of the charge carriers - positive for hole carriers. 

Figure 19 Temperature dependence of the thermoelectric power for the La-Ba-Cu-O material with the fraction of Ba either x =0.15 (a) or x = 0 (b). Ref. 71.

Fig. 10.22 Electrical conductivity (a) and thermoelectric power (b) of liquid Se J x alloys as functions of reciprocal temperature. 

The electrical resistivity data on crystals of indium(III) oxyfluoride indicate a nearly temperature independent conductor (3.6 X 10 2 fl-cm. at room temperature and 1.8 X 10-2 fl-cm. at liquid-helium temperature) with high negative thermoelectric power (—230 juV./°C.). These properties are similar to those observed for some conductive forms of indium(III) oxide. 

The thermoelectric power tensor A is not symmetric because A = a 1f3, and although the ORRs require a to be symmetric, this is not true of the off-diagonal 71(2) 8. Equation (24) shows that when there is no temperature gradient, a flow of electric current produces heat (the Peltier effect), the magnitude of which is determined by A8... 

Thermoelectric power in single crystalline metal phthalocyanines (MPc) at room temperature Material Thermoelectric power Reference... 

Both the room-temperature thermoelectric power and the phonon-drag component at low temperatures increase with hydrostatic pressure due to a decrease in the volume fraction of strong-correlation fluctuations in an itinerant-electron matrix. 

At higher temperatures, 700-1000 K, a positive thermoelectric power a(T) is essentially temperature-independent and the evolution of its magnitude with S is consistent with polaronic conduction described by the statistical component... 

Fig. 26. Temperature variation of the resistivity p(T) and thermoelectric power a( I ) for LaMnC>3 jg, after Topfer et al. (1996).

The compositional dependence of the structural O -O transition at 7jt and the 0 -R transition at Tor can be clearly followed by monitoring the temperature dependence of the resistance (Mandal et al., 2001) monitoring the variation with x of the higher-order transition at T from the resistance curve R(T) is more subtle and has been accomplished with further aid from the thermoelectric power a(T) measured on single crystals (Zhou and Goodenough, 2000). The transition from polaronic to itinerant electronic behavior in the paramagnetic R-rhombohedral phase has not been studied. 

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