The Seebeck Effect

When the circuit is open, there appears an electric potential difference called the Seebeck electromotive force, denoted emfoT and expressed in V. This voltage is a complex function of both the temperature difference and the type of conductors [i.e., = F(AT,A, B)]. 

In practice, the Seebeck electromotive force is related to the temperature difference by a polynomial equation, where the polynomial coefficients (i.e., c , c, Cj, C3, etc.) are empirical constants determined by experiment and that characterize the thermocouple selected. 

However for a small temperature difference, the Seebeck electromotive force can be assumed to be directly proportional to the temperature difference  

In practice, the thermoelectric power of a conductor A is usually reported for a temperature difference of 100 C between hot and cold junction and by fixing the second material B. The second material used as the standard is most of the time pure platinum and, less frequently, copper or even lead. Hence, the thermoelectric power is reported in modern tables in mV versus Pt or mV versus Cu, respectively. Therefore to convert a thermoelectric power measured with a given scale into another scale, the following simple equation can be used  

The order of magnitude for thermoelectric power is commonly in the range of several mV/K for semiconductors and of several gV/K for most metals and alloys (Table 9.1). On the other hand, for semiconductors, the theoretical thermoelectric power can be assessed using the following equation, where is the electronic density in the conductor and C the molar heat capacity at constant volume  

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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... 

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 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. 

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. 

Finally, a very simple molecular electronic component is benzene-1,4-dithiol, which is readily used as a linker between gold electrodes in the same way as the cobalt terpyridine complex shown in Figure 11.40. Benzene dithiol has been used to demonstrate the thermoelectric effect in molecular electronic systems as a linker between a gold surface and the gold tip of a modified atomic force microscope. Thermoelectricity (termed the Seebeck effect) is the generation of an electrical potential... 

The most often used temperature detectors in renewable energy and most other processes are the thermocouples (TCs). Their operation is based on the principle known as the Seebeck effect. T. J. Seebeck discovered that heating the junction of dissimilar metals generates a small, continuous electromotive force (EMF). The name is a combination of thermo and couple denoting heat and two junctions, respectively. The dissimilar TC wires are joined at the hot (or measurement) end and also at the cold junction (reference end),... 

In 1821 Thomas Seebeck discovered that when two different types of metal wires were joined at both ends and one of the ends was heated or cooled, a current was created within the closed loop (this current is now called the Seebeck effect). Specifically, heat energy was transformed into measurable electrical energy. 

Thermocouples, or thermal junctions, or thermoelectric thermometers have two intermetallic junctions between two different metals (or semimetals, or semiconductors) A, B in a loop (Fig. 10.21). When these two junctions are held at different temperatures (T i, and T2), then a potential difference A Vis set up this is the Seebeck98 effect. For instance, for a Cu-constantan thermocouple, with T2 = 300 K and T, 273.15 K, AV = 1.0715 mV. Its converse is the Peltier99 effect If a current at a fixed voltage is applied in a loop like in Fig. 10.21, then a temperature difference AT can be maintained (thermoelectric heaters and coolers). The Seebeck effect arises because, before the junctions are made, the two metals have different Fermi levels after the junctions are made, electrons will flow from the higher-level metal to the lower-level metal, until a single Fermi level results across the junction. 

Seebeck effect — is the potential difference that results when the joins of two different metals are at different temperatures and induces a movement of charge through the conductors. The Seebeck effect is the opposite of the Peltier effect (see - Peltier heat). 

In a nonisothermal system, an electric current (flow) may be coupled with a heat flow this effect is known as the thermoelectric effect. There are two reciprocal phenomena of thermoelectricity arising from the interference of heat and electric conductions the first is called the Peltier effect. This effect is known as the evolution or the absorption of heat at junctions of metals resulting from the flow of an electric current. The other is the thermoelectric force resulting from the maintenance of the junctions made of two different metals at different temperatures. This is called the Seebeck effect. Temperature measurements by thermocouples are based on the Seebeck effect. 

Consider the Seebeck effect resulting from two junctions maintained at two different temperatures as shown in Figure 7.5. Assume that points 1 and 4 are at the same temperature T0. These points are connected to a potentiometer so that the electromotive force E can be measured with zero current Je = 0. Under these conditions and using the reciprocal rules, Eq. (7.287) yields... 

While the Seebeck effect enables TE devices to be used for power generation, the Peltier effect allows them to be used for cooling. In Peltier effect devices, heat flows in the same direction as majority carriers. The appropriate metric for this apphcation is called the TE efficiency, t], which is simply the ratio between the load s power input and the net heat flowrate. Essentially, rj gives the fraction of Camot efficiency attainable... 

The Seebeck effect is involved in the generation of thermoEMF in a closed electric circuit that is composed of different metals whose junctions are maintained at different temperatures. This phenomenon is widely used, for example, for measuring the temperature with the help of thermocou pies. In this case, the thermoEMF is caused by the redistribution of the current carriers through the conductors due to the existence of the tern perature gradient. 

In 1948 Verwey and his co-workers (88) established that lithium ions incorporated into nickel oxide produced an equivalent number of Ni + ions and so enhanced the electrical conductivity. Later, from measurements of the Seebeck effect, Parravano (89) confirmed that in the presence of lithium the Fermi level of nickel oxide is indeed depressed in accordance with the increased concentration of positive holes. For trivalent additions, Hauffe and Block (90) have shown that the incorporation of small amounts of Cr + ions decreases the conductivity of nickel oxide one infers accordingly that the hole concentration is decreased and that the Fermi level is raised. This is therefore an attractive situation with which to examine the influence of the height of the Fermi level on catalytic activity. The most appropriate n-type oxide for analogous studies is zinc oxide. 

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