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thermocouples to modules

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The Seebeck effect

The discovery of thermoelectricity dates back to Seebeck [1] (1770-1831). Thomas Johann Seebeck was born in Revel (now Tallinn), the capital of Estonia which at that time was part of East Prussia. Seebeck was a member of a prominent merchant family with ancestral roots in Sweden. He studied medicine in Germany and qualified as a doctor in 1802. Seebeck spent most of his life involved in scientific research. In 1821 he discovered that a compass needle deflected when placed in the vicinity of a closed loop formed from two dissimilar metal conductors if the junctions were maintained at different temperatures. He also observed that the magnitude of the deflection was proportional to the temperature difference and depended on the type of conducting material, and does not depend on the temperature distribution along the conductors. Seebeck tested a wide range of materials, including the naturally found semiconductors ZnSb and PbS. It is interesting to note that if these materials had been used at that time to construct a thermoelectric generator, it could have had an efficiency of around 3% - similar to that of contemporary steam engines.

The Seebeck coefficient is defined as the open circuit voltage produced between two points on a conductor, where a uniform temperature difference of 1K exists between those points.

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The Peltier effect

It was later in 1834 that Peltier[2] described thermal effects at the junctions of dissimilar conductors when an electrical current flows between the materials. Peltier failed however to understand the full implications of his findings and it wasn't until four years later that Lenz[3] concluded that there is heat adsorption or generation at the junctions depending on the direction of current flow.

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The Thomson effect

In 1851, Thomson[4] (later Lord Kelvin) predicted and subsequently observed experimentally the cooling or heating of a homogeneous conductor resulting from the flow of an electrical current in the presence of a temperature gradient. This is know as the Thomson effect and is defined as the rate of heat generated or absorbed in a single current carrying conductor subjected to a temperature gradient.

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The three thermoelectric effects above are related by the Kelvin relationships[5], assumed to be valid for all materials used in thermoelectrics.
 

Thermoelectric Materials

It was later in 1909[6] and 1911[7] that Altenkirch showed that good thermoelectric materials should possess large Seebeck coefficients, high electrical conductivity and low thermal conductivity. A high electrical conductivity is necessary to minimise Joule heating, whilst a low thermal conductivity helps to retain heat at the junctions and maintain a large temperature gradient. These three properties were later embodied in the so-called figure-of-merit, Z. Since Z varies with temperature, a useful dimensionless figure-of-merit can be defined as ZT.

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Figure-of-merit, Z

The figure-of-merit of a thermoelectric material is defined as :

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where alpha.gif (1046 bytes) is the Seebeck coefficient of the material (measured in microvolts/K), sigma.gif (1021 bytes) is the electrical conductivity of the material and lambda.gif (1004 bytes) is the total thermal conductivity of the material.

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Metals

Although the properties favoured for good thermoelectric materials were known, the advantages of semiconductors as thermoelectric materials were neglected and research continued to focus on metals and metal alloys. These materials however have a constant ratio of electrical to thermal conductivity (Widemann-Franz-Lorenz law) so it is not possible to increase one without increasing the other. Metals best suited to thermoelectric applications should therefore possess a high Seebeck coefficient. Unfortunately most possess Seebeck coefficients in the order of 10 microvolts/K, resulting in generating efficiencies of only fractions of a percent.

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Semiconductors

It was during the 1920's that the development of synthetic semiconductors with Seebeck coefficients in excess of 100 microvolts/K increased interest in thermoelectricity. At this time it was not apparent that semiconductors were superior thermoelectric materials due to their higher ratio of electricall conductivity to thermal conductivity, when compared to metals.

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A.F.Ioffe

Abram F. Ioffe

As early as 1929 when very little was known about semiconductors, Abram Fedorovich Ioffe (1880-1960) showed that a thermoelectric generator utilising semiconductors could achieve a conversion efficiency of 4%, with further possible improvement in its performance. By the 1950's, Ioffe and his colleagues [8] had developed the theory of thermoelectric conversion, which  forms the basis of all modern thermoelectric theory.
A large number of semiconductor materials were being investigated by  the late 1950's and early 1960's , several of which emerged with Z values significantly higher than in metals or metal alloys. No single compound semiconductor evolved that exhibited a uniform high figure-of-merit over a wide temperature range, so research focused on developing materials with high figure-of-merit values over relatively narrow temperature ranges. Of the great number of materials investigated, those based on bismuth telluride, lead telluride and silicon-germanium alloys emerged as the best for operating to temperatures of about 450K, 900K and 1400K respectively.

simple thermoelectric generator

Illustration of thermoelectric generation (Seebeck effect)

Thermoelectric
Generation

The simplest thermoelectric generator consists of a thermocouple, comprising a p-type and n-type thermoelement connected electrically in series and thermally in parallel. Heat is pumped into one side of the couple and rejected from the opposite side. An electrical current is produced, proportional to the temperature gradient between the hot and cold junctions.

simple thermoelectric cooler

Illustration of thermoelectric cooling
(Peltier effect)
Thermoelectric
Cooling

If an electric current is applied to the thermocouple as shown, heat is pumped from the cold junction to the hot junction. The cold junction will rapidly drop below ambient temperature provided heat is removed from the hot side. The temperature gradient will vary according to the magnitude of current applied.

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The Thermoelectric Module

 

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A typical thermoelectric module is shown left. The module consists of pairs of p-type and n-type semiconductor thermoelements forming thermocouples which are connected electrically in series and thermally in parallel.

 

  • In cooling mode, an electrical current is supplied to the module. Heat is pumped from one side to the other (Peltier effect), the result is that one side of the module becomes cold.
  • In generating mode, a temperature gradient is maintained across the module. The heat flux passing through the module is converted into electrical power (Seebeck effect).

 

References

1. Seebeck, T.J., 1822, Magnetische Polarisation der Metalle und Erzedurch Temperatur-Differenz. Abhand Deut. Akad. Wiss. Berlin, 265-373.

2. Peltier, J.C., 1834, Nouvelles experiences sur la caloriecete des courans electriques. Ann. Chem., LVI, 371-387.

3. See Ioffe, A.F., 1957, Semiconductor Thermoelements and Thermoelectric Cooling, Infosearch, London.

4. Thomson, W., 1851, On a mechanical theory of thermoelectric currents, Proc.Roy.Soc.Edinburgh, 91-98.

5. See Goldsmid, H.J., 1960, Applications of Thermoelectricity, London, Methuen.

6. Altenkirch, E., 1909, Physikalische Zeitschrift, 10, 560-580.

7. Altenkirch, E., 1911, Physikalische Zeitschrift, 12, 920-924.

8. Ioffe, A.F., 1956, Poluprovoduikovye Termoelementy, Moskow-Leningrad.

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