November 25-30, 2012 | Boston
Meeting Chairs: Chennupati Jagadish, Thomas Lippert, Amit Misra, Eric Stach, Ting Xu
Filling vacancies in the skutterudite structure with rare earth atoms has been used extensively in cobalt and iron based skutterudites. State of the art filled n-type CoSb3 skutterudite has a ZT approximately 30% greater than unfilled, substitutionally doped CoSb3 at equivalent carrier concentration. IrSb3 is a potentially attractive thermoelectric material because it is more refractory than cobalt-based skutterudites and can therefore take advantage of higher operating temperatures, as well as fill some of the gap in performance in segmented device configurations between state of the art filled iron- and cobalt-based skutterudite antimonides and higher temperature thermoelectric materials such as Yb14MnSb11 and La3-xTe4. In the past, it has been found quite challenging to prepare highly doped n-type IrSb3 compositions by doping with impurities such as Pd and Pt. Peak ZT values obtained at elevated temperatures only ranged from 0.1 to 0.15 at best. In contrast, alkaline and rare earth filling of IrSb3 skutterudite has produced greater carrier concentrations and carrier mobilities than ever achieved through substitutional doping alone. As a result, a large increase in ZT values is reported here, with a peak of nearly 0.9 at 1000 K.
Filled skutterudites are one of the most promising materials for thermoelectric (TE) power generation applications in the intermediate temperatures, due to their superior TE and thermomechanical performance as compared to other materials [1-2]. In the past, we have demonstrated that n-type skutterudites can be optimized so that their maximum TE figure of merit reaches 1.7 at 850 K . TE performance of the p-type, however, is lagging behind, which hinders the optimization of skutterudites-based TE module development. The underlying reasons for this are related to the skutterudites electronic band structures, which results in higher thermal conductivity for the p-type at elevated temperatures due to bipolar lattice thermal conduction; and lower power factor because of the heavy valence bands unlike the conduction bands with beneficial 3-fold degeneracy. In this talk, I will review our recent theoretical and experimental effort on modifying the valence bands of p-type skutterudites and highlight means of improving their TE properties. 1. X. Shi, Jiong Yang, J. R. Salvador, M. Chi, J. Cho, H. Wang, S. Bai, J. Yang, W. Zhang, and L. Chen, “Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports”, J. Am. Chem. Soc. 133, 7837 (2011). 2. J. R. Salvador, J. Yang, A. A. Wereszczak, H. Wang, and J. Y. Chi, “Temperature Dependent Tensile Fracture Stress of n- and p-Type Filled-Skutterudite Materials”, Sci. Adv. Mater. 3, 1 (2011).
Filled skutterudite materials are very promising for mid-temperature thermoelectric power generation and waste heat recovery, because of their good thermoelectric and mechanical properties. Traditional preparation method needs a very long time annealing (usually 7 to 14 days) to form the right skutterudite phase. The annealing is especially critical for p-type filled skutterudites, since Fe4Sb12 need filler atoms to enter the cage to form a stable phase. In this work, we prepared Ce/Nd double filled p-type skutterudite material by directly ball-milling alloyed and quenched ingot. The results show that, by breaking the ingot into nano-sized particles, ball-milling greatly reduced the distance which filler atoms need to travel and hence accelerated the phase formation. With appropriate ball-milling and handling, pure p-type filled skutterudite phase can be obtained by hot-pressing the ball-milled powder for just 5 minutes, although the powder is still a mixture of FeSb2 and Sb phases. The samples prepared by this way have the same high quality as the samples prepared by the traditional way, and show a peak ZT value above 1 at 750 K. This method greatly saves the processing time and is suitable for large scale industrial production. This research was supported by Bosch and the Solid State Solar-Thermal Energy Conversion Center (S3TEC), and Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001299/DE-FG02-09ER46577 (GC and ZFR).