Preparation and Thermoelectric Properties of Microcrystalline Lead Telluride
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aration and Thermoelectric Properties of Microcrystalline Lead Telluride L. D. Ivanovaa, *, Yu. V. Granatkinaa, A. G. Mal’cheva, I. Yu. Nikhezinaa, S. P. Krivoruchkob, M. I. Zaldastanishvilib, T. S. Vekuab, and N. M. Sudakb aBaikov
Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119334 Russia b Sukhumi Physicotechnical Institute, Kodorskoe sh. 665, Sinop, Sukhumi, Russia *e-mail: [email protected] Received November 5, 2019; revised December 24, 2019; accepted March 5, 2020
Abstract—We have worked out conditions for the preparation of microcrystalline n-type lead telluride-based materials doped with lead iodide and investigated their microstructure and thermoelectric properties. The materials were prepared by hot-pressing powders produced by grinding an ingot to a particle size on the order of hundreds of microns in a planetary mill and to a particle size under hundreds of nanometers (mechanical activation) and by melt spinning. Fracture surfaces of the hot-pressed samples were examined on an optical and a scanning electron microscope. All of the samples had a nonuniform microstructure, with both small and larger grains present. In the samples prepared from the powders produced by mechanical activation, nanograins were detected. We have measured the Seebeck coefficient, electrical conductivity, and thermal conductivity of the samples at room temperature and in the range 300–800 K and evaluated their lattice thermal conductivity and thermoelectric figure of merit, ZT. Their lattice thermal conductivity was shown to decrease with decreasing grain size. The highest thermoelectric figure of merit, (ZT)max = 1.32 at 630 K, was offered by the materials produced from the mechanically activated powder. Keywords: lead telluride, grinding in a planetary mill, mechanical activation, melt spinning, hot pressing, microstructure, thermoelectric properties DOI: 10.1134/S0020168520080063
INTRODUCTION Recent years have seen intense studies of materials for direct thermal-to-electrical energy conversion. Lead telluride-based materials are widely used in the fabrication of thermoelectric generators operating in the temperature range 300–800 K. Note that such materials have been studied and utilized since the middle of the 20th century [1–6]. PbTe melts incongruently at 1190 K and has a narrow homogeneity range (with a maximum extent at 1048 K: from 49.994 to 50.013 at % Te). It has a NaCl structure (sp. gr. Oh5 = Fm3m), with interatomic distances of 3.23 Å and ionic–covalent bonds. Its band gap, formed by two subbands, is 0.25 eV (at 300 K). The value obtained for direct transitions is 0.32 eV, and that for indirect transitions is 0.29 eV. It is believed that, in this compound, scattering by acoustic lattice vibrations prevails; that is, the electronic component of its thermal conductivity agrees well with a theoretical curve obtained using the Wiedemann–Franz law under the assumption that r = 0. Dopants capable of ensuring high thermoelectric efficiency of lead telluride are iod
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