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J. Phys. Chem. C 2010, 114, 13126–13131
Thermoelectric Properties Evolution of Spark Plasma Sintered (Ge0.6Pb0.3Sn0.1)Te Following a Spinodal Decomposition Yaniv Gelbstein,* Yoav Rosenberg, Yatir Sadia, and Moshe P. Dariel Department of Materials Engineering, Ben-Gurion UniVersity, Beer-SheVa 84105, Israel ReceiVed: April 24, 2010; ReVised Manuscript ReceiVed: June 25, 2010
The pseudoternary (Ge,Pb,Sn)Te system is characterized by demixing to both Pb- and Ge- rich telluride submicrometer and nanosized domains. The present study is concerned with the thermoelectric (TE) properties of the quasi-ternary (Ge0.6Pb0.3Sn0.1)Te compound at 390 °C, during the demixing process over lengthy periods of time up to 695 h. The dimensionality of the various physical metallurgy patterns evolved was correlated to the lattice thermal conductivity, κL, values. Excluding an initial period ( 1, where R-Seebeck coefficient, and σ, κ are the electrical and thermal conductivities, respectively, have been known as state of the art materials for T < 500 °C for several decades.1 Advances over the past decade show that it is possible to enhance ZT in nanoscale systems, based on these alloys, by using phonons scattering at interfaces, for reduction of the lattice thermal conductivity, κL.2-4 Many of the improvements of the TE performance have been demonstrated in epitaxial, multilayer thin-film geometries, or in individual nanostructures (e.g., nanowires).5 Large scale TE power generation applications involve large temperature gradients and high power densities that can be only obtained in bulk TE materials. It is not surprising therefore that nanostructuring in the latter is currently an active topic of research. Nanostructuring methods in bulk materials, for κL reduction, include grain size reduction, twin and domain boundaries generation, interfacial nanocoatings, embedded nanoinclusions, lamellar/multilayer structures, and others (a comprehensive review article is included in ref 4). A recently reported thermodynamically driven process that generates a nanostructure in an initially normally structured TE material is based on the spinodal decomposition.2 Spinodal decomposition is a mechanism for phase separation which leads to a characteristic modulated structure that can be exploited to control the microstructure on the nanometer scale.6 In this mechanism, small compositional fluctuations (c) reduce the Gibbs free energy (∆G) and promote a gradual phase separation. A necessary (though not sufficient) condition is that d2G/dc2 < 0 (between the inflection points) in special thermodynamic cases * To whom correspondence should be addressed. E-mail: yanivge@ bgu.ac.il.
characterized by two minima in the compositional dependence of ∆G, the free energy function.7 The spinodal instability leads to periodic modulations of composition. In some elastically anisotropic alloys, the spinodal structure tends to be aligned along elastically soft directions. Nanostructures for spinodal decomposed systems were reported for TiO2-SnO2,6 Mn-Cu,8 and Cu-Ni-Sn9 alloys. Recently, a reduction of the lattice thermal conductivity and a corresponding ZT enhancement in Pb1-xSnxTe-PbS10 and Gex(SnyPb1-y)1-xTe11-14 systems was attributed to nano patterns resulting from the spinodal decomposition. The evolution of the micro/submicrometer structure of the (Ge0.6Pb0.3Sn0.1)Te compound has been investigated previously by means of X-ray diffraction and optical and electron microscopy in order to evaluate its stability over different temperatures and time periods.14 As far as we know, no such investigation has previously been made for the evaluation of the corresponding TE properties, in the course of the demixing process. Such an evaluation is crucial for estimating the potential of the system to be involved in practical thermoelectric applications over long periods of time as is usually required. Recently, optimally doped pseudobinary Ge- rich GexPb1-xTe alloys have been proposed as potential candidates to improve the thermoelectric performance of p-type legs,15,16 with a maximal ZT value of ∼1.8 for the composition of 3 mol % Bi2Te3 doped Pb0.13Ge0.87Te.16 It was also reported that alloying of (GeTe)x(PbTe)1-x compounds with SnTe can result in a spinodal decomposition with changed periodicity that depends on the heat treatment.17 In order to further probe the improvement potential of the ZT values of the (GeTe)x(PbTe)1-x compounds by nanostructuring due to the spinodal decomposition, two compositions Ge0.5Sn0.25Pb0.25Te and Ge0.6Sn0.1Pb0.3Te were prepared by Spark Plasma Sintering.11-14 Both of these alloys exhibited extremely low κL values of ∼0.8 W/mK at room temperature, as compared to a value of ∼1.6 W/mK that was found for pure GeTe. The maximal ZT value of ∼1.2 at
10.1021/jp103697s 2010 American Chemical Society Published on Web 07/14/2010
Thermoelectric Properties of (Ge0.6Pb0.3Sn0.1)Te 450 °C,12 was reported for Ge0.5Sn0.25Pb0.25Te, underlying the high potential of this composition as a p-type leg in practical thermoelectric applications. The Ge0.6Sn0.1Pb0.3Te composition, although attaining a lower ZT maximal value of ∼0.7, was further investigated as a case study, by means of X-ray diffraction and microscopy. The study was focused toward monitoring the structural evolution during the various demixing stages of spinodal decomposition, nucleation of new phases and growth of these phases.13,14 This composition was chosen for further investigation, due to it is thermodynamical location well inside the miscibility gap of the system, for enhancement of the demixing process. It was previously concluded, that although growth of the Ge- and Pb- rich domains up to 3-4 µm was obtained after lengthy heat treaments at 500 °C, submicrometer domains, which potentially can still be effective for phonons scattering, were still discernible at lower tempertaures, lengthy heat treatment of 390 °C. In the present work, the thermoelectric properties, namely, Seebeck coefficient, R, the electrical resistivity, F, and the thermal conductivity, κ, were monitored after heat treatment at 390 °C, for different periods of time up to 695 h, and correlated to the compositions and nano/submicrostructure, obtained by SEM, HRTEM, and XRD.
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Figure 1. HRTEM of solution treated (Ge0.6Pb0.3Sn0.1)Te at 600 °C for 1 h, followed by rapid ice water quenching.
Experimental Section
Results and Discussion
Synthesis of the Ge0.6Sn0.1Pb0.3Te alloy was performed by melting of the source materials (purity of 5N), at appropriate concentrations, under vacuum (10-5 Torr) in a rocking furnace at 800 °C for 1 h followed by water quenching. Spark Plasma Sintering (SPS) (type HP D 5/1 FCT Systeme GmbH) was applied, after milling to
