N-type Lead-Chalcogenide Thermoelectric Materials Alloyed with Tin
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1044-U04-10
N-type Lead-Chalcogenide Thermoelectric Materials Alloyed with Tin Jan Koenig, Alexandre Jacquot, and Harald Boettner Thermoelectric systems, Fraunhofer IPM, Heidenhofstr. 2, Freiburg, 79112, Germany ABSTRACT N-type conduction was observed in epitaxial Pb1-xSnxSe and Pb1-xSnxTe thin films grown by MBE with an increasing charge carrier concentration as well as an increasing thermopower value related to an increasing tin content reaching up to 12at%. The lattice thermal conductivity decreases as expected due to alloy scattering but the carrier mobility is almost stable. All these effects increase strongly the thermoelectric properties of Pb1-xSnxSe and Pb1-xSnxTe. The n-type conduction arises from a metal excess in the stoichiometry range. The same type of conduction has also been found in bulk samples. Bulk single crystals of Pb1-xSnxSe and Pb1-xSnxTe are grown by the unseeded vapor growth technique. A rather constant mobility and a decreasing thermal conductivity with increasing tin content are measured in the n-type bulk samples. A figure of merit of ZT=0.9 was measured at 325°C for a Pb0,965Sn0,035Se sample with a carrier concentration of about 9.3E18 cm-3, i.e. already better than known n-type PbTe-based materials for this temperature range. INTRODUCTION Thermoelectric materials are used in thermoelectric generators for a direct conversion of heat energy into electricity. They are heavily doped semiconductors. Compatible materials of nand p-type have to be found to build a thermoelectric generator. Since the transport properties of thermoelectric materials are a function of the temperature, their conversion efficiency defined by α 2σ ZT = T , (1) the figure of merit λ where α is the thermopower, σ is the electrical conductivity, λ is the thermal conductivity, T is the absolute temperature, is maximal only on a restricted temperature range. Doped lead telluride and its solid solution have shown the best performance in the mid-temperature range (500-700K) [1][2]. Up to now, lead telluride doped with PbI2 is the best n-type material with a thermoelectric dimensionless figure of merit of about 0.8 at approximately 350°C. Pb1-xSnxTe or PbTe doped with Na are compatible p-type materials in this temperature range. These p-type materials are replaced by TAGS, because their figure of merit is higher (~1.4). Therefore it would be interesting to find a better n-type material compatible with these high efficient p-type materials. The most successful approach to increase the figure of merit is more than 50 years old and was proposed and was applied successfully by Joffe [3]. Joffe proposes to mix two isoelectronic crystals having also similar crystal structures. The objective was to increase the quotient of the carrier mobility µe and the lattice thermal conductivity λL. In fact, the figure of merit can be written as a function of the elementary charge q, the carrier concentration n, the carrier mobility µe and the electronic λe and lattice part λL of the thermal conductivity α 2q n µe ZT = T (2) λe + λ L
and i
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