Reduction of the Lattice Thermal Conductivity in Immiscible PbS-PbTe Systems
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1267-DD06-03
Reduction of the lattice thermal conductivity in immiscible PbS-PbTe systems Simon Johnsen,1 Steven N. Girard,1 Iliya Todorov,2 Duck Young Chung2 and Mercouri G. Kanatzidis1,2 1 Northwestern University, Department of Chemistry, 2145 Sheridan Rd, Evanston, IL 60208, U.S.A. 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA ABSTRACT The synthesis and properties characterization of several PbS1-xTex x = 0-0.16 samples are presented. Notably it is shown how a local minimum occurs in the thermal diffusivity for the PbS1-xTex samples at x ∼ 0.03. The thermoelectric properties of doped PbS1-xTex with x = 0.03 are reported and the properties are compared to the pure PbS and PbTe end members. The electronic contribution to the total thermal conductivity is analyzed for PbS1-xTex x = 0.03 and it is shown how the lattice thermal conductivity is significantly lowered compared to single crystalline PbS. INTRODUCTION Current state of the art thermoelectric materials are often tellurium based compounds.1-6 The scarcity of the latter can be problematic if thermoelectric modules reach mass markets. Cheaper alternatives containing abundant elements are therefore sought. PbS is a narrow band gap semiconductor with fair thermoelectric properties. A significant lattice thermal conductivity limits the thermoelectric performance. Recently we showed how nucleation and growth and spinodal decomposition in the PbTe rich part of the pseudo-binary phase diagram of PbTe-PbS leads to bulk nanocomposite materials with low thermal conductivities and enhanced thermoelectric performance relative to that of PbTe.3, 7 In this paper the thermoelectric properties of the PbS-rich part of the pseudo-binary PbS-PbTe phase diagram are investigated. The diagram is dominated by an immiscibility dome which leads to a two phase system. For appropriate cooling rates this could lead to bulk nanocomposites known to be effective phonon scatterers.2, 6, 8 Special focus is given to find a minimum in the thermal diffusivity for varying x in PbS1-xTex. EXPERIMENTAL PbS and PbTe binaries were synthesized using elemental Pb (99.99 % American Elements), elemental S (99.99 % 5N+), and Te (99.999% 5N+). Subsequently, the binaries were mixed in stoichiometric ratio in a quartz ampoule and PbCl2 was added as dopant. The quartz ampoule was sealed under vacuum and reacted at 1150 °C and air quenched to room temperature. The samples were then annealed for 3 days at 500 °C. Carbon coating of the quartz ampoule eliminated reaction with the quartz tube and was found to improve the mechanical strength of the samples. The as-cast ingots were cut into a coin of ∼Ø8 mm and 1-2 mm thickness for thermal diffusivity measurements and ∼10x3x2 mm parallelepipeds that were used for measurements of the Seebeck coefficient and the electrical conductivity. Examples are shown in Figure 1. Thermal diffusivity was measured on the abovementioned coins cut from the sample using a Netzsch LFA457 employing a Cowan model with pulse correction. Linear co
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