Thermoelectric properties of Bi-FeSb 2 nanocomposites: Evidence for phonon-drag effect
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Thermoelectric properties of Bi-FeSb2 nanocomposites: Evidence for phonondrag effect Mani Pokharel, Machhindra Koirala, Huaizhou Zhao, Kevin Lukas, Zhifeng Ren, and Cyril Opeil Department of Physics, Boston College, Chestnut Hill MA 02467
Abstract The thermoelectric properties of Bi-FeSb2 nanocomposites are reported. The electrical resistivity and the Seebeck coefficient measurements show a significant dependence on bismuth concentration. Our results reveal that the shifting of the Seebeck peak in FeSb2 nanocomposites is purely a grain size-effect. The thermal conductivity data indicates a presence of an electronphonon interaction. Over all, our analysis of the the thermoelectric properties of Bi-FeSb2 nanocomposites provide additional evidence for phonon-drag in FeSb2.
Introduction FeSb2 has drawn extensive research efforts in the recent years [1-7] because of its colossal Seebeck coefficient of -45000 μVK-1 at ~ 10 K, which makes this compound a potential candidate for thermoelectric cooling applications. Despite the huge power factor (PF) of 2300 μWK-2cm-1 at ~ 10K, the ZT values for single crystals are limited by the large value of the thermal conductivity. In our earlier work [6, 7], we successfully reduced the thermal conductivity by three orders of magnitude using a nanostructuring approach. However due to the suppressed peak value of the Seebeck coefficient, ZT values for nanocomposite samples were still far too low for use in practical applications. In general, the Seebeck coefficient is the sum of two independent contributions.
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The diffusion term arises due to the spontaneous diffusion of carriers in the presence of thermal gradient whereas the phonon term is associated with the preferential scattering of carriers by phonons from the hot to the cold end, also called the phonon-drag effect. The origin of the huge Seebeck coefficient in FeSb2 is not completely understood yet. A huge electron diffusion caused
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by a strong electron-electron correlation has been suggested by many authors [1-5] as a possible cause. Other authors [8, 9], however, emphasized the non-electronic origin and suggested phonon-drag as a possible mechanism. In this paper, we study the thermoelectric properties of Bi-FeSb2 nanocomposites. The preliminary results provide additional evidence for phonon-drag effects in FeSb2.
Experiment and Methods FeSb2 was synthesized by ingot formation through melting and solidification inside an evacuated quartz tube. The ingot was then ball-milled for 15 hours to obtain a fine powder. Bipowder in the amount of 2, 6 and 10 % by weight was added to the FeSb2 powder the mixture was ball-milled for additional 6 hours. The final powder was then hot pressed at 200 oC for 2 minutes. A pressure of about 70 MPa was applied during pressing. The electrical resistivity (ρ), Seebeck Coefficient (S), and thermal conductivity (κ) of the samples were all measured on a Physical Property Measurement System (PPMS) from Quantum Design.
Results and Discussion Figure (1) shows the te
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