If two electrical conductors are in contact with each other forming a closed cycle, there can result an electric voltage if the touching point and the point where the voltage is measured have different temperatures. The resulting voltage is given by the equation:
SA and SB are the Seebeck-coefficients, which are depending on material and temperature. T1 and T2 represent the different temperatures. The Seebeck-coefficient has the unit [Volt/Kelvin]. If the temperature difference is very small and the Seebeck-coefficients stay constant, U=(SB-SA)*(T2-T1) will be assumed.
The voltage is produced by thermal diffusion due to high-energy electrons at the warm contact point that are diffusing to the negative side. This causes a constant electron transport from the positive to the negative conductor while also heat energy is transferred resulting in a lower Seebeck effect.
The efficiency a thermocouple is higher the higher the electrical conductivity and the smaller the thermal conductivity of the conductor material is.
Decisive for the characteristics of a conductor is the figure of merit. This parameter, also known as “ZT” includes the temperature, the Seebeck-coefficient (square), the thermal conductivity and electrical conductivity.