History and Properties of Thermoelectric Devices

In 1834 Jean Peltier discovered that when electrical current passes through a closed circuit made up of two dissimilar metals, heat energy is absorbed at one dissimilar metal junction and discharged at the other junction. This discovery is the basis for all thermoelectric devices, and is quantified by a property known as the Seebeck coefficient, named after Thomas Seebeck who discovered the reverse effect in 1821 (as used in thermocouples and for power generation in thermoelectrics). In addition to the Seebeck coefficient, two other properties are important for optimization of thermoelectric devices: thermal conductivity and electrical resistivity. The ideal thermoelectric material has a high Seebeck coefficient and low electrical resistivity, so little heat is generated from the electrical current flow, and low thermal conductivity, so little heat is transferred from the hot junction to the cold junction.

Figure 1.

Semiconducting materials, (in conjunction with copper inter-connecting pads), have been found to offer the best combination of Seebeck coefficient, electrical resistivity, and thermal conductivity. Semiconducting materials provide another benefit, the ability to use electrons or "holes" (the absence of an electron in a crystal matrix) to conduct current. This last property is useful in assembling many thermoelectric junctions in series to reduce the overall current flow in the device to manageable levels. Figures 1 and 2 show the construction of the typical thermoelectric device used today.

Figure 2.

The electrical properties of semiconducting materials can change dramatically with temperature. As a result, semiconducting materials can only function as thermoelectric materials within a specific temperature range, that varies depending upon the material. Figure 3 shows the effectiveness of the three most commonly used semiconductor materials for thermoelectric devices, as measured by the, "Figure of Merit", Z, where: Z = (Seebeck Coefficient) 2/ (Thermal Conductivity* Electrical Resistively). Higher figures of merit yield better thermoelectric performance.

Figure 3.

The most commonly used semiconductor material for cooling applications, Bismuth Telluride (Bi2Te3), reaches its peak performance at approximately 70 °C and has an effective operating range of -100 °C to +200 °C. Lead Telluride (PbTe), the next most commonly used material, is typically used for power generation and is not as efficient as Bi2Te3 in cooling applications. PbTe reaches its peak figure of merit at 350 °C and has an effective range of 200 °C to 500 °C. PbTe is typically used for power generation because its higher operating temperatures yields more efficient power generation when the heat is rejected into ambient air. Silicon Geranium (SiGe) is rarely used as a thermoelectric material and would only be viable for power generation at very high temperatures.