Named after his discoverer, the Seebeck effect describes the occurrence of an electrical voltage induced by a temperature gradient. While the theoretical interpretation in Seebeck's pioneering paper [1] is surpassed by his general discovery by far, he also gave an overview of several material combinations usable in thermocouples as illustrated in Fig. 2.1.
Two rods of different materials are soldered together and the soldered points
are held at the temperatures
and
, respectively maintaining a
temperature difference
and thus an according temperature gradient
along the rods. On device level, the given temperature difference causes a
certain voltage measured at the device's contacts
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(2.1) |
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(2.2) |
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(2.3) |
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(2.4) |
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(2.5) |
While the Seebeck coefficient of most metals is in the range of
-
, values of
and more are obtained with
semiconductors. Both metals with positive and negative Seebeck coefficients
exist. The choice of according material combinations depends on the intention
of use. For example in measurement applications, high total Seebeck
coefficients are less important than a linear behavior in the desired
temperature range. In semiconductors, the Seebeck coefficient can be varied
by appropriate doping. While n-type semiconductors have negative Seebeck
coefficients, the ones of p-type materials are positive. Quantitative values
obtained in semiconductors can be obtained by analysis of carrier transport.
Based on Boltzmann's equation, expressions for the coefficients are derived
throughout Chapter 3.
M. Wagner: Simulation of Thermoelectric Devices