Optimizing Thermoelectric Efficiency of La 3-x Te 4 with Calcium Metal Substitution
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Optimizing Thermoelectric Efficiency of La3-xTe4 with Calcium Metal Substitution Samantha M. Clarke1,2*, James M. Ma1,3*, C.-K. Huang1, Paul A. von Allmen1, Trinh Vo1, Richard B. Kaner2,3, Sabah K. Bux1, and Jean-Pierre Fleurial1** 1
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90092, U.S.A. 3 Department of Materials Science and Engineering, University of California, Los Angeles, CA 90092, U.S.A. *These authors contributed equally to this paper 2
ABSTRACT La3-xTe4 is a state-of-the-art high temperature n-type thermoelectric material with a previously reported maximum zT~1.1 at 1273 K. Computational modeling suggests the La atoms play a crucial role in defining the density of states for La3-xTe4 in the conduction band. In addition to controlling charge carrier concentration, substitution with Ca2+ atoms on the La3+ site is explored as a potential means to tune the density of states and result in larger Seebeck coefficients. High purity, oxide-free samples are produced by ball milling of the elements and consolidated by spark plasma sintering. Powder XRD and electron microprobe analysis are used to characterize the material. High temperature thermoelectric properties are reported and compared with La3-xTe4 compositions. A maximum zT of 1.3 is reached at 1273 K for the composition La2.22Ca0.775Te4. INTRODUCTION Thermoelectric (TE) power sources have consistently demonstrated their extraordinary reliability and longevity in support of the National Aeronautics Space Administration’s (NASA) deep space science and exploration missions. Proven state-of–practice “heritage” (TE) materials exhibit only modest thermal-to-electric energy conversion performance, resulting in relatively low system-level conversion efficiencies of 6 to 6.5%. These heritage materials have been known since the late 1950’s and 1960’s, so that even the recently developed multi-mission radioisotope thermoelectric generator (MMRTG) builds upon 40-year old thermoelectric converter technology. However, there is great potential for large gains in performance thanks to recent advances in materials synthesis, the discovery of novel complex structure compounds, the ability to engineer with increasing precision micro- and nanostructure features coupled with improved scientific understanding of electrical and thermal transport in such engineered materials and the means to perform in-depth theoretical simulations with fast turnaround time. NASA’s Radioisotope Power Systems Technology Advancement Program is pursuing the development of more efficient thermoelectric technologies that can increase performance by a factor of 2 to 4X over state-of-practice systems. The use of advanced materials, n-type and ptype filled skutterudites and rare earth compounds, has already resulted in doubling TE couplelevel conversion efficiency up to 15% at the beginning of their life. One of these rare earth compounds that demonstrates a high performance at
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