Dependence of the Elastic Moduli of Skutterudites on Density and Temperature
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Dependence of the Elastic Moduli of Skutterudites on Density and Temperature G. Rogl1,3 , S. Puchegger2, M. Zehetbauer3, A. Grytsiv1, P. Rogl1 1 Institute of Physical Chemistry, University Vienna, Währingerstr. 42, A-1090 Wien, Austria 2 Faculty of Physics, University Vienna, Boltzmanngasse 5, A-1090 Wien, Austria 3 Faculty of Physics, Physics of Nanostructured Materials, University Vienna, Boltzmanngasse 5, A-1090 Wien, Austria ABSTRACT The dependency of Young’s and shear modulus on relative density and temperature was determined for three series of skutterudites: p-type MmyFe3CoSb12 (y = 0.70, 0.80), p-type DDyFe3CoSb12 (y = 0.65, 0.68) and n-type Sr0.07Ba0.07Yb0.07Co4Sb12. For all three groups of materials the dependency of Young’s modulus, E, on the density and temperature is linear: E(d) = 4.1(1)×d – 268(3) GPa (d is the relative density in %), ΔE/ΔT = 0.20(1) GPa/K, thus allowing extrapolation of E as a function of temperature and towards 100% density. INTRODUCTION Albeit filled skutterudites exhibit excellent thermoelectric (TE) properties, technological use requests reliable mechanical parameters at operation temperatures. Thermoelectric performance is characterized by the dimensionless figure of merit ZT = S2T/(ρλ), where S is the Seebeck coefficient, ρ is the electrical resistivity, and λ is the thermal conductivity at the temperature T. Among the most promising TE-materials are double, triple and multifilled skutterudites, ExRyT4Sb12, where E and R are rare-earth or alkaline earth elements, didymium (“DD”: 4.76% Pr, 95.2% Nd) or mischmetal (“Mm”: 50.8% Ce, 28.1% La, 16.1% Nd, 5.0% Pr) and T is a transition metal preferably from the iron group [1-10]. DD and Mm are natural double/multi-fillers and cheaper than the respective rare-earth metals, an important fact in respect to large-scale commercial applications. Among the most advanced TE candidates are p-type DD0.65Fe3CoSb12 (ZT = 1.2 at 750 K) or n-type Sr0.08Ba0.08Yb0.08Co4Sb12 (ZT = 1.4 at 800 K). Considering extensive thermal cycling, thermoelectric materials should be robust from the thermo-mechanical point of view to achieve long-term service. Therefore mechanical characterization is important. In this paper three types of skutterudites with ZT>1 have been investigated by resonant ultrasound spectroscopy to acquire the elastic moduli of skutterudites as a function of density and temperature: p-type MmyFe3CoSb12 (y = 0.70, 0.80), p-type DDyFe3CoSb12 (y = 0.65, 0.68) and n-type Sr0.07Ba0.07Yb0.07Co4Sb12. EXPERIMENT The samples were prepared via a melting reaction technique followed by milling (hand-milled (HM) and ball-milled (BM)) and hot pressing (HP). Details of preparation and composition are given in our previous papers [7-11]. In order to obtain samples with different densities, hot-pressing conditions were made dependent on pressure, P, and/or temperature, T.
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Resonant ultrasound spectroscopy, RUS, [12] was used to determine elastic properties via the eigenfrequencies of the samples and the knowledge of sample mass and dimensions. The cylin
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