Microstructures and nanostructures in long-term annealed AgPb 18 SbTe 20 (LAST-18) compounds and their influence on the
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sanne Perlt and Thomas Höche Leibniz Institute of Surface Modification (IOM), D-04318 Leipzig, Germany
Paula Bauer Pereira and Raphaël P. Hermann Forschungszentrum Jülich GmbH, IFF, JNCS and JARA-FIT, D-52425 Jülich, Germany Faculty of Sciences, University of Liège, B-4000 Liège, Belgium (Received 22 December 2010; accepted 20 April 2011)
This article reports on the role of annealing on the development of microstructure and its concomitant effects on the thermoelectric properties of polycrystalline AgPbmSbTe2+m (m 5 18, lead–antimony– silver–tellurium, LAST-18) compounds. The annealing temperature was varied by applying a gradient annealing method, where a 40-mm-long sample rod was heat treated in an axial temperature gradient spanning between 200 and 600 °C for 7 days. Transmission electron microscopy investigations revealed Ag2Te nanoparticles at a size of 20–250 nm in the matrix. A remarkable reduction in the thermal conductivity to as low as 0.8 W/mK was also recorded. The low thermal conductivity coupled with a large Seebeck coefficient of ;320 lV/K led to high ZT of about 1.05 at 425 °C for the sample annealed at 505 °C. These results also demonstrate that samples annealed above 450 °C for long term are more thermally stable than those treated at lower temperatures.
I. INTRODUCTION
In recent years, efficient use of energy has attracted a great deal of attention from leading economies due to the looming shortage of fossil fuels and concerns about climate change. The direct conversion of waste heat released by industrial processes and automobile engines into electrical energy has led to increasing research interest in thermoelectricity.1 The development of thermoelectric (TE) generators operating at intermediate temperatures, i.e., up to 400–500 °C at the hot junction, is crucial because large amounts of waste heat from existing technical processes and energy machinery are available within this temperature range. For this purpose, PbTe-based complex chalcogenides and silicides2 are among the most promising TE converter materials. To develop TE materials with high performance, i.e., with a large dimensionless figure of merit ZT 5 (S2r/j)T (T is the absolute temperature), one needs to tune electronic and phononic properties to combine a high electrical conductivity, r, and high thermopower (Seebeck coefficient), S, with a low thermal conductivity, j. The recently developed quaternary AgPbmSbTe2+m (m 5 18, lead–antimony–silver–tellurium, LAST-18) compound has gained much attention due to its high reported ZT a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.142 1800
J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
http://journals.cambridge.org
Downloaded: 14 Mar 2015
values,3–5 and is thus considered as a highly promising intermediate temperature TE material. The enhancement of ZT in LAST-18 compounds was attributed to the remarkable decrease in lattice thermal conductivity, which ascribed to the scattering of heat-carrying phonons by coherent nanostructures rich in Ag
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