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PARTIALLY-FILLED SKUTTERUDITES: OPTIMIZING THE THERMOELECTRIC PROPERTIES 1

G.S. Nolas, 2M. Kaeser, 2R.T. Littleton IV, 2T.M. Tritt, 3H. Sellinschegg, 3D.C. Johnson and 4E. Nelson. 1 R&D Division, Marlow Industries, Inc., 10451 Vista Park Road, Dallas, Texas 75238 2 Department of Physics, Clemson University, Clemson, South Carolina 29634 3 Department of Chemistry, University of Oregon, Eugene, Oregon 97403 4 US Army Research Laboratory, Adelphi, Maryland 20783 ABSTRACT The skutterudite family of compounds continues to be of interest for thermoelectric applications due to the low thermal conductivity obtained when filling the voids with small diameter, large mass interstitials such as trivalent rare-earth ions. In the last few years there has been a substantial experimental and theoretical effort in attempting to understand the transport properties of these compounds in order to optimize their thermoelectric properties. One such approach involves partially-filling the voids in attempting to optimize the power factor while maintaining low thermal conductivity. In this report experimental research on skutterudites with the voids partially filled with heavy mass lanthanide and alkaline-earth ions is reported. INTRODUCTION The continuing effort in improving the thermoelectric properties of skutterudite compounds has resulted in attempts to fill the voids in the crystal structure with ever differing atoms.[1] To this end new synthesis approaches are also underway in order to form ever more varied compounds in this diverse materials system. Novel approaches to skutterudite compound synthesis[2, 3] that can result in the preparation of many compounds that could not be successfully formed employing "traditional" synthesis techniques are also of interest. Whether employing these novel synthesis approaches or traditional ones, an approach that has been reported to be an optimization route is partial void filling.[4,5,6] The goal in this research is in obtaining compounds with low thermal conductivity while maximizing the electronic properties. THERMOELECTRIC POTENTIAL Figure 1 illustrates the maximum dimensionless figure of merit, ZT=TS2 6HHEHFNFRHIILFLHQW

 ZKHUH6LVWKH

LVWKHHOHFWULFDOFRQGXFWLYLW\DQG WKHWKHUPDOFRQGXFWLYLW\ IRUVNXWWHUXGLWH

compounds at room temperature. The plot was constructed by gathering and ploting experimental data of CoSb3 at different doping levels from the literature[1] and inserting different values for the ODWWLFHWKHUPDOFRQGXFWLYLW\ g. The different optimum carrier concentrations can be directly attributed to the difference in the effective masses of n (me) and p-type (mp) specimens (me ~ 10mp). The n-type material is therefore optimized at a higher carrier concentration. As seen in WKLVILJXUHLWLVFOHDUWKDW a g in order to obtain skutterudites with a higher ZT than that of Bi2Te3-alloy materials presently used in thermoelectric devices (with ZT ~ 1). One important note is that the ZT values at higher temperatures are higher than those shown in Figure 1, implying that