Anisotropy and inhomogeneity measurement of the transport properties of spark plasma sintered thermoelectric materials
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Anisotropy and inhomogeneity measurement of the transport properties of spark plasma sintered thermoelectric materials A. Jacquot1, M. Rull2, A. Moure3, J.F. Fernandez-Lozano3, M. Martin-Gonzalez2, M. Saleemi4, M.S. Toprak4, M. Muhammed4, M. Jaegle1 1
Fraunhofer-IPM, Thermoelectric Systems department, Heidenhofstraße 8, 79110 Freiburg, Germany. 2 Instituto de Microelectrónica de Madrid, C/ Isaac Newton 8. Tres Cantos, 28760 Madrid, Spain. 3 Instituto de Ceramica y Vidrio, C/ Kelsen, 5 Madrid 28049, Spain. 4 Functi Instituto de Ceramica y Vidrio onal Materials Division, KTH Royal Institute of Technology, Kista-Stockholm, Sweden. ABSTRACT We report on the development and capabilities of two new measurement systems developed at Fraunhofer-IPM. The first measurement system is based on an extension of the Van der Pauw method and is suitable for cube-shaped samples. A mapping of the electrical conductivity tensor of a Skutterudite-SPS samples produced at the Instituto de Microelectrónica de Madrid is presented. The second measurement system is a ZTmeter also developed at the Fraunhofer-IPM. It enables the simultaneous measurement of the electrical conductivity, Seebeck coefficient and thermal conductivity up to 900 K of cubes at least 5x5x5 mm3 in size. The capacity of this measurement system for measuring the anisotropy of the transport properties of a (Bi,Sb)2Te3 SPS sample produced by KTH is demonstrated by simply rotating the samples. INTRODUCTION Thermoelectric materials enter in the fabrication of thermoelectric generators and coolers. The conversion efficiency is a monotonic growing function of the figure of merit defined as Z = σ S 2 λ , where α is the Seebeck coefficient, σ the electrical conductivity and λ the thermal conductivity. These transport properties are taken all in the same direction. Larger electrical conductivity and Seebeck coefficient in conjunction with a low thermal conductivity results in useful thermoelectric materials. Since some of the best thermoelectric materials do have an anisotropic crystal structure and since plasma sintering (SPS) is uniaxial most or all samples produced by SPS should show to some extend an anisotropy of the transport properties. In addition, inhomogeneity is also expected in SPS samples because the current density, the temperature field and pressure may not be homogeneous during the sintering process. For the above mentioned reasons, it would be useful to map the transport properties of SPS-samples measured along and perpendicular to the sintering direction. Nevertheless, the thermal conductivity is more conveniently measured along a direction which is perpendicular to the measurement direction of the electrical properties with the measurement systems actually available on the market. This methodology may lead to an overestimation of the figure of merit in the case of a strongly anisotropic crystal structure [1-2]. These overestimated figure-of-merits will never be translated into any improvement of the conversion efficiency in real systems. In addition, lots informa
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