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1044-U05-09

Enhanced Thermoelectric Figure of Merit in BaxYbyCo4Sb12 Skutterudites Ctirad Uher1, Xun Shi1,2, and Huijun Kong1 1 Physics, University of Michigan, 450 Church St., Ann Arbor, MI, 48109-1040 2 General Motors R&D Center, Warren, MI, 48090 ABSTRACT We report on the development of an efficient n-type skutterudite based on the doublefilled structure of CoSb3. The chosen filler species Ba and Yb have a large phonon contrast and this combination is very effective in lowering the lattice thermal conductivity. The highest figure of merit in excess of 1.3 is obtained near 800K and the figure of merit greater than unity persists down to temperatures of about 550K. The compounds are synthesized with high purity Co (99.998%) and Sb (99.9999%). In order to reduce the cost of the materials, we have also prepared filled skutterudites of similar composition but using less pure Co (99.8%) and Sb (99.999%). The figure of merit of these lower purity compounds reaches ZT = 1 at 800K with the materials’ cost dramatically reduced. INTRODUCTION Over the past decade or so, major efforts worldwide have focused on the development of a new generation of thermoelectrics with efficiencies far exceeding those of Bi2Te3/Sb2Te3 compounds, PbTe, and Si-Ge alloys, the materials that have served as the backbone of thermoelectricity during the past 50 years. Impressive advancements have been made in both bulk and especially thin film forms of novel thermoelectrics [1]. Among the most prospective of the new families of thermoelectrics are skutterudites and, more specifically, their filled structures [2]. Following upon the early work of the Dudkin’s group that clarified bonding in binary skutterudites [3] and the pioneering work of Jeitschko and Brown [4] who synthesized the first filled skutterudite, Slack [5] suggested that the loosely bonded filler species in the skutterudite structure may act as independent oscillators (“rattlers”) that will interact with and scatter the normal modes of the structure and thus degrade the heat transport. Provided the fillers would not dramatically affect the electronic properties, one would have the essence of the Slack’s Electron Crystal-Phonon Glass concept. Indeed, skutterudites, in terms of their exceptionally high carrier mobilities, were always viewed as interesting semiconductors and potentially useful thermoelectrics. Unfortunately, their lattice thermal conductivity was very high also, mitigating their thermoelectric appeal. Thus, identifying a process how to lower phonon thermal conductivity was a key step in making skutterudites viable novel thermoelectric materials. Experimental verification of the Slack’s idea of fillers as rattlers came soon after in measurements by Morelli and Meisner [6], and ever since the filled skutterudites have been at the center of attention [7]. In the late 1990s, major efforts focused on the p-type filled skutterudites of the general form RyCo4-xFexSb12 where the substitution of a Fe atom on the site of a Co atom introduced one hole. The amount of x was then chos