2. Fundamentals of Thermoelectric Devices

THE HISTORY OF THERMOELECTRIC DEVICES began back in 1821, when Seebeck discovered the deviation of a compass needle when keeping the two junctions of different metals at different temperatures [1]. Today's understanding of his discovery, the formation of a potential difference due to the temperature slope, which further causes an electric current was named after his discoverer, the Seebeck effect. Thirteen years later, Peltier observed that an electrical current driven through a junction of two different metals causes a temperature change at the junction [2]. However, it took until 1838 to discover an important property of Peltier's experiment, when Lenz realized, that both material combination and current direction control if the junction is heated or cooled [3]. This behavior is known today as the Peltier effect. The theoretical explanation as well as the connection between the Seebeck and the Peltier effect has been performed by Thomson (later Lord Kelvin) within the framework of thermodynamics in 1851 [4]. Furthermore, he predicted the third thermoelectric effect, later named Thomson effect.

An important foundation for the theory of thermoelectric materials was built by Altenkirch [5,6]. He concluded that high quality thermoelectric materials are characterized by high Seebeck coefficients and electrical conductivities, while thermal conductivities should be low. Based on this attributes, the thermoelectric figure of merit has been formulated later, which became an important cornerstone of the systematic search for novel thermoelectric materials. Due to the availability of first artificially manufactured semiconductors, Ioffe intensified the research on semiconductor based thermoelectric devices in the mid of the last century and formulated the basis of modern thermoelectric theory [7,8]. The efficiency of thermoelectric generators could be raised to about $5\,\%$ due to the favorable material properties of semiconductors compared to metals.

Intense material research in the sequel lead to the successive discovery of materials with increasing thermoelectric figures of merit suitable for several temperature ranges. Independent of the materials used, the basic structure of thermoelectric generators has been established as a combination of n-type and p-type semiconductor rods, which are arranged thermally parallel and electrically serial. In the following, this chapter gives an overview about the thermodynamic foundation as well as today's and possible future applications.

Subsections

• 2.1 Thermoelectric Phenomena
  • 2.1.1 Seebeck Effect
  • 2.1.2 Peltier Effect
  • 2.1.3 Thomson Effect
  • 2.1.4 Thermodynamic Relations
  
  
• 2.2 Applications of Thermoelectric Devices
• 2.3 Future Applications

M. Wagner: Simulation of Thermoelectric Devices