Traceable measurement of the Seebeck coefficient of thermoelectric materials up to 850 K
Thermoelectric materials convert heat into electrical energy. The performance of the thermoelectric energy conversion scales with the thermoelectric figure of merit (ZT) of the material used. The figure of merit is defined as ZT = S2σT/κ, where S, σ, and κ are the transport properties Seebeck coefficient, electrical conductivity, and thermal conductivity respectively. T is the thermodynamic temperature. The performances of different thermoelectric materials are compared preferentially by using the values of ZT. Reliable ZT-values requires precise measurements of the transport properties, which are traceable to SI units. Usually different measurement devices, techniques and methods are used to measure S, σ, and κ, which can lead to systematically different results. Therefore, reference materials are a useful tool to verify such measurement devices and measuring procedures.
This paper is focused on Seebeck coefficient measurements. The Seebeck coefficient S is the most important property of ZT. Therefore, a highly accurate measurement of S will lower the total measurement uncertainty of the ZT value. General procedural steps for establishing a reference material for Seebeck coefficients are explained. We describe the measurement method and the measuring system to measure absolute Seebeck coefficients of thermoelectric bulk materials traceable to SI units, which are used for this purpose. All measurement uncertainty contributions to be considered are analysed and quantified. The achievable relative measurement uncertainties of the Seebeck coefficient are in the order of (6-8) % for k = 2. The results of temperature dependent measurements of the Seebeck coefficients of metallic and semiconducting samples are presented.