Seebeck effects

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... 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

Another application of the Seebeck effect is to be found ill detectors of small quantities of heat radiation. These sensitive detectors comprise a thermopile, a pile of thermocoup)les (small pieces of two different metals connected in V form and put into series). Half of the junctions of the thermopile are shielded within the detector, whereas the other half are exposed to... 

Seebeck used antimony and copper wires and found the current to be affected by the measuring instrument (ammeter). But, he also found that the voltage generated (EMF) was directly proportional to the difference in temperature of the two junctions. Peltier, in 1834, then demonstrated that if a current was induced in the circuit of 7.1.3., it generated heat at the junctions. In other words, the SEEBECK EFFECT was found to be reversible. Further work led to the development of the thermocouple, which today remains the primary method for measurement of temperature. Nowadays, we know that the SEEBECK EFFECT arises because of a difference in the electronic band structure of the two metals at the junction. This is illustrated as follows ... 

The production of a current of electricity by heating a junction formed by two dissimilar metals. For temperature measurement the metals are usually in the form of wires see Thermocouple) and the circuit has two junctions, the hot junction which is exposed to the temperature to be measured and the cold junction which is kept at a standard temperature. The thermo-electric effect is also termed the Seebeck Effect after its discoverer. 

Thermoelectric effect discovered by T.J. Seebeck The Seebeck effect is the basis for the thermometers designated as thermocouples... 

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. 

When the two ends of a material containing mobile charge carriers, holes or electrons, are held at different temperatures, a voltage is produced, a phenomenon called the Seebeck effect (Fig. 1.11). The Seebeck coefficient of a material, a, is defined as the ratio of the electric potential produced when no current flows to the temperature... 

Figure 1.11 Seebeck effect. A sample with one end maintained at a high-temperature Tfr and the other at a low-temperature Tc will develop a potential difference A.

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). 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

The determination of the heat flow relies on the so-called Seebeck effect. An electric potential, known as thermoelectric force and represented by E, is observed when two wires of different metals are joined at both ends and these junctions are subjected to dilferent temperatures, 7j and T2 (figure 9.1a). Several thermocouples can be associated, forming a thermopile (figure 9.1b). For small temperature differences, the thermoelectric force generated by the thermopile is proportional to 7j - T2 and to the number of thermocouples of the pile (>/) ... 

The reverse of the Seebeck effect is called the Peltier effect and results from flowing an electric current through the circuits of figure 9.1. If the junctions are initially at the same temperature, a temperature gradient will be developed for instance, in the case of figure 9.1a, one of the junctions will cool and the other will warm. Associated with this electric current there will also be a Joule (resistive) effect, so that the net power (P) produced at each junction is given by... 

The thermocouple utilizes the Seebeck effect. Copper and constantan are the two metals most commonly used and produce an essentially linear curve of voltage against temperature. One of the junctions must either be kept at a constant temperature or have its temperature measured separately (by using a sensitive thermistor) so that the temperature at the sensing junction can be calculated according to the potential produced. Each metal can be made into fine wires that come into contact at their ends so that a very small device can be made. 

Figure 2. Heat conduction (Seebeck effect) batch mixing calorimeter for three samples and one reference channel. After loading and establishing baselines, the assembly is inverted to mix reactants and start heat production. (Reproduced with permission from Ref. 2. 1983, Alan R. Liss, Inc.)...

NiO(200) presents a small excess of oxygen, whereas NiO(250) contains metallic nickel (Table I). Magnetic measurements (15) have confirmed the chemical analysis (19). However, both oxides are p-type semiconductors, as shown by the Seebeck effect measurements. In the case of NiO(250), this result means that, although there is a total excess of nickel, the oxide phase still contains a small excess of oxygen (13). The electrical properties of both oxides are identical. 

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