Open 2 Thermoelectric effects

There are also two well-known thermoelectric effects resulting from the joining of dissimilar materials (forming a junction) the Seebech effect, on which thermocouples are based, and the Peltier effect, used for thermopiles. The Seebech effect results when the two junctions of the dissimilar materials are held at different temperatures. The Seebech coefficient, e, is defined as the open-circuit voltage generated per unit temperature differential of the two junctions ... 

This expression shows that the imposition of a temperature difference dT in the absence of any current produces a difference d i in electrochemical potential i.e., d ife) = —z/a, dT. This effect is known as the thermoelectric effect, and the ratio d i/e)/dT, or A(f,/e)/Ar = —ZiCii is known as the Seebeck coefficient (1823), or thermoelectric power.Experimentally, the difference of electrochemical potential may be measured by a voltmeter under open circuit conditions, and dT, measured by means of thermocouples a, is thereby experimentally determined. As defined here for p-type (n-type) material the measured Seebeck coefficient is a positive (negative) quantity. For, and both increase in the direction of increasing hole or electron concentration, which is in a direction opposite to the increase in temperature. Comparison with (6.9.2) shows that Ui = ZiSgfe. Then Eq. (6.9.6) becomes... 

Fig. 5.41 Conductivity (filled-in circles) as a function of the oxygen partial pressure for La2Cu04 at high temperatures. (The thermoelectric effect also exhibits the same dependence (open squares) this is not discussed in the text.) [179]...

The diffusion (Ed), thermal diffusion (Etd)> thermoelectric ( te)> and streaming (2 str) potentials are the potentials that are not desirable in the potentiomet-ric measurements but can occur. If these phenomena have an effect, they should be taken into account to correctly translate the measured open-circuit potential to thermodynamic properties of the electrochemical system using a generalized Nemst equation ... 

The circuits schematized below show two possible utilizations of the Seebeck effect, one (left) in closed circuit (and therefore with a potential difference equal to zero) producing a current from thermal energy and the other (right) in open circuit, therefore in the absence of current, called thermocouple and used for measuring temperature differences. These circuits are both made up of two soldered joints of two materials having distinct thermoelectric properties. Case study J8 is devoted to the description and modeling of the thermocouple, also called thermoelectric junction, which is recalled here for comparison. 

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