Transport Properties and Microstructure of Indium and Cerium added Cobalt-Antimony based Skutterudites
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Transport Properties and Microstructure of Indium and Cerium added Cobalt-Antimony based Skutterudites A. Sesselmann1, T. Dasgupta, C. Stiewe and E. Müller Institute of Materials Research, German Aerospace Center (DLR), D-51170 Köln, Germany ABSTRACT Indium and cerium added cobalt-antimony based skutterudites with different filling fractions were synthesized using different annealing synthesis parameters. Phase homogeneity and microstructure of the resulting as-cast material were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The skutterudite material was further compacted using a current-assisted short-term sintering device. Temperature dependent measurements of the Seebeck coefficient, electrical and thermal conductivity were carried out on the compacted specimens in the temperature range of 350 K700 K. Results indicate significant differences in the transport properties between the slowly cooled and quenched as-cast materials and also with different filling fractions. Based on the measured transport properties the dimensionless figure of merit ZT was calculated for different filling fractions of indium and cerium. Among these compositions a ZTmax of 1.1 at 700 K was obtained. INTRODUCTION Efficient conversion of a heat difference into electrical energy by thermoelectric generators demands a high Seebeck coefficient and electrical conductivity combined with a low thermal conductivity of the thermoelectric material. These criteria which were found by Altenkirch [1] can be expressed by the dimensionless figure of merit ZT, which is defined ZT=TσS2/κ, where T is the absolute temperature, σ the electrical conductivity, S the Seebeck coefficient and κ the thermal conductivity, respectively. Among novel thermoelectric materials, skutterudites have shown notably high ZT values at elevated temperatures between 350 K to about 700 K [2,3]. The basic compound is the binary skutterudite structure M4Pn12, where M is a transition metal and Pn a pnicogen element, which exhibits one large icosahedral void per formula unit (R1M4Pn12). This Wyckoff 2a position can be filled to a certain level by a range of different atoms, such as alkaline/rare earths or electropositive elements (e.g. Tl, Sn) [4,5]. Thus a solely suppression of the thermal conductivity is attained and consequently a vast improvement in ZT. Further investigations on filled skutterudites indicate that inserting of two or more different elements can further refine the thermoelectric properties of this material system [6]. The addition of indium and cerium of cobalt-antimony based skutterudites accompanies a remarkable suppression of the thermal conductivity and a concomitant increase in electrical conductivity. For cerium it has been demonstrated that it can occupy the empty lattice site in the skutterudite matrix and as a consequence enhance the thermoelectric figure of merit [4]. For indium however it is still under debate, whether InSb nanosized precipitates or indium as a solute in the
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