Seebeck coefficient
When two electrical conductors form a closed circuit by coming into contact, an electric voltage may arise if the point of contact and the location where the voltage is measured experience different temperatures. This voltage is determined by the equation:
Here, \(S_A\) and \(S_B\) are the Seebeck coefficients, dependent on the material and temperature, with units of [Volt/Kelvin]. For small temperature differences where the Seebeck coefficients remain constant, the relationship \(U = (S_B – S_A) \cdot (T_2 – T_1)\) holds.
The voltage is generated through thermal diffusion, where high-energy electrons at the warmer contact point diffuse to the colder side, causing a continuous electron transport from the positive to the negative conductor. Simultaneously, heat energy is transferred, resulting in a reduced Seebeck effect.
The efficiency of a thermocouple increases with higher electrical conductivity and lower thermal conductivity of the conductor material.
Critical to the conductor’s characteristics is the figure of merit, denoted as “ZT.” This parameter considers temperature, the square of the Seebeck coefficient, thermal conductivity, and electrical conductivity.
The behavior of insulators, metals, and semiconductors can be illustrated through a scheme. Characteristics such as carrier concentration, Seebeck coefficient, and thermal conductivity collectively contribute to the “ZT” or “figure of merit,” offering a comprehensive description of the thermoelectric behavior of any material with a single value.
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