Microstructure Investigation of Non-equilibrium Synthesized Filled Skutterudite CeFe 4 Sb 12
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Microstructure Investigation CeFe4Sb12
of
Non-equilibrium Synthesized
1267-DD05-25
Filled
Skutterudite
Juan Zhou, Qing Jie and Qiang Li Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory Upton, NY 11973, U.S.A. ABSTRACT We have prepared a variety of filled skutterudites through non-equilibrium synthesis by converting melt-spun ribbons into single phase polycrystalline bulk under pressure. In general, better thermoelectric properties are found in these samples. In this work, we performed microstructure characterization of non-equilibrium synthesized p-type filled skutterudite CeFe4Sb12 by X-ray diffraction, scanning electron microscopy and transmission electron microscopy in order to understand the structural origin of the improved thermoelectric properties. It is found that the non-equilibrium synthesized samples have smaller grain size and cleaner grain boundaries when compared to the samples prepared by the conventional solid-state reaction plus long term annealing. While smaller grain size can help reduce the lattice thermal conductivity, cleaner grain boundaries ensure higher carrier mobility and subsequently, higher electrical conductivity at the application temperatures. INTRODUCTION The efficiency of a thermoelectric material is measured by the dimensionless figure of merit ZT (ZT = S2σT/κ), where S, σ and κ stand for the Seebeck coefficient, electrical conductivity and thermal conductivity, respectively [1]. In general, these properties are correlated to one another, so it is very difficult to decouple them and achieve big enhancement in ZT values. So far, substantial increases in ZT values have been achieved only with superlattice structures, e.g. Bi2Te3/Sb2Te3 superlattice thin films exhibit ZT value about 2.4 in at 300 K [2]. However, most of today’s widely used bulk thermoelectric materials such as PbTe [3], (AgSbTe2)x(GeTe)1−x (TAGS) [4], Bi2-xSbxTe3 and Bi2Te3-xSex [3] have their maximum ZT values about 1. Large scale applications require high performance bulk thermoelectric materials to handle large heat loads and be manufactured at an acceptable cost. Filled skutterudites are promising candidate thermoelectric materials for waste heat recovery in the automotive industry due to their high chemical and mechanical stability at intermediate temperatures and the availability of both n-type and p-type materials with relatively high ZT values [5-6]. The lattice thermal conductivities of filled skutterudite structures can be reduced by an order of magnitude through the introduction of “rattlers” [7]. Recently, the melt spinning approach, a non-equilibrium synthesis method, has been used in the preparation of thermoelectric materials with markedly improved transport properties [8-9]. As a result of the melt spinning process, amorphous or fine-grained and homogeneous microstructures are formed upon the rapid quench of a molten alloy at extremely high cooling rates (typically on the order of 105 - 106 K/s). The as-obtained melt-spun ribbons are thermodynamically metast
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