Seebeck effect

History

The Seebeck effect is a Thermoelectric effect, discovered by the German physicist Thomas Johann Seebeck in 1821. This one noticed that a metal needle is deviated when it is placed between two conducting of different nature bound by junctions at their ends and is subjected to a thermal Gradient (see figure).

He explained this phenomenon by the appearance of a Magnetic field, and thus believed to provide an explanation to the existence of the Terrestrial magnetic field. It is only well later that the electric origin of the phenomenon was included/understood: a Potential difference appears with the junction of two materials subjected to a difference in temperature. The most known use of the Seebeck effect is the temperature measurement using Thermocouple S. This effect is also at the base of the generation of electricity by Thermoelectric effect.

Principles

The following figure shows the basic thermoelectric circuit:

Two conducting materials of different nature has and B are connected by two junctions in X and W. In the case of the Seebeck effect, a difference in temperature dT is applied between W and X, which involves the appearance of a Potential difference FD between Y and Z. In open circuit, the Seebeck coefficient of the material couple, S_ab, or thermoelectric capacity is defined by:

$S_ {ab} = \ frac {FD} {dT} \,$

So for T_W > T_X the potential difference is such as V_Y>V_Z, then S_ab is positive.

The Seebeck coefficient of each material is related to the coefficient of the couple by the relation:

$S_ {ab} = S_a-S_b \,$

The Seebeck coefficient is expressed in V.K^-1 (or more generally in µV.K^-1 within sight of the values of this coefficient in usual materials).

William Thomson (Lord Kelvin) showed that the Seebeck coefficient is related to the coefficients Peltier and Thomson according to:

* $\ Pi_ {ab} =S_ {ab} T \,$

* $\ tau_a- \ tau_b=T \ frac {dS_ {ab}} {dT} \,$

where Π is the Peltier coefficient and τ the Thomson coefficient.

Measure Seebeck coefficient

In practice, the Seebeck coefficient can be measured only for one material couple. It is thus necessary to have a reference. This is made possible by the property of the superconductive materials to have a coefficient Seebeck S null. Indeed, the Seebeck effect is related to the transport of Entropie by the charge carrier within material (electrons or holes), but they do not transport entropy in the superconductive state. Historically, the value of S_ab measured until the critical temperature of Nb₃Sn (T_c=18K) for a Pb-Nb₃Sn couple made it possible to obtain S_Pb until 18K. The measurement of the Effet Thomson until the room temperature then made it possible to obtain S_Pb on all the range of temperature, which made Plomb a material of reference.

Experimental device

The principle of the determination of the Seebeck coefficient rests on the determination of a Potential difference induced by a difference in known temperature (see diagram).

A sample whose Seebeck coefficient is unknown (S_inconnu) is fixed between a thermal bath at the temperature T, which evacuates heat, and a foot-warmer at the T+dT temperature which provides heat to the sample. This one is thus subjected to a Gradient of temperature, and a Potential difference appears. Two Thermocouple S of comparable nature , generally an alloy or+fer, chromel or Constantan, whose Seebeck coefficient is known (S_ref) are fixed on the sample at the points has and B. These thermocouples at the same time make it possible to measure the potentials V_a and V_b and the temperatures T_a and T_b. The Seebeck coefficient of materials is then obtained by the relation:

$unknown S_ {} =S_ {ref.} + \ frac {V_a-V_b} {T_a-T_b} \,$

Seebeck coefficient of some metals with 300K

See too

Category: Thermoelectricity

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