Seebeck Effect
Introduction
The Seebeck effect is a phenomenon in thermoelectricity where a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. Named after the German physicist Thomas Johann Seebeck, who discovered this effect in 1821, it is the basis for thermocouples and thermopile sensors.
History
In 1821, Seebeck discovered that a compass needle would be deflected when a closed loop was formed of two metals joined in two places with a temperature difference between the junctions. This deflection was caused by a current driven through the loop by the temperature difference, which Seebeck initially believed was due to magnetic fields caused by the temperature gradient. However, it was later shown that the effect was due to the charge carriers in the material (electrons or holes) diffusing from the hot side to the cold side, creating a voltage difference.
Physical Principle
The Seebeck effect is a result of the diffusion of charge carriers (electrons or holes) in a material from a region of high temperature to a region of low temperature. This diffusion is driven by the temperature gradient, and results in a net charge imbalance between the hot and cold regions, creating a voltage difference or electromotive force (EMF).
The magnitude of the Seebeck effect (the Seebeck coefficient) is a material property, and is measured in volts per kelvin (V/K). The Seebeck coefficient is not constant for a given material, and can vary with temperature, the type and concentration of charge carriers, and the presence of impurities or defects in the material.
Mathematical Description
The Seebeck effect can be mathematically described by the Seebeck coefficient (S), which is defined as the ratio of the induced thermoelectric voltage (V) to the temperature difference (ΔT) across the material:
S = V/ΔT.
The Seebeck coefficient is a tensor property, meaning it can vary with the direction of both the temperature gradient and the electric field. In isotropic materials, however, the Seebeck coefficient is a scalar and does not depend on direction.
Applications
The Seebeck effect has a wide range of applications, particularly in temperature sensing and thermal energy harvesting.
Thermocouples
A common application of the Seebeck effect is in thermocouples, which are widely used as temperature sensors. A thermocouple consists of two dissimilar metals joined at two points. A temperature difference between the junctions creates a voltage difference, which can be measured and correlated to the temperature.
Thermoelectric Generators
The Seebeck effect is also the operating principle behind thermoelectric generators (TEGs), which convert thermal energy directly into electrical energy. TEGs are used in a variety of applications, from power generation in space probes to waste heat recovery in industrial processes.
Peltier Coolers
While not a direct application of the Seebeck effect, Peltier coolers operate on a closely related principle (the Peltier effect), where an electric current is used to create a temperature difference. Peltier coolers are used for electronic cooling and more recently in portable coolers.