High temperature rare earth and Si-Ge thermoelectric materials

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1992
Authors
Han, Sang Hyun
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Karl A. Gschneidner, Jr.
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Materials Science and Engineering
Abstract

The preparation of the metastable crystalline high temperature and high pressure polymorphs of R[subscript]2S[subscript]3, where R = Y, Dy, Er, Tm, Yb and Lu, was investigated at room temperature by mechanical milling (MM). For Dy[subscript]2S[subscript]3 the pure metastable high temperature [gamma]-phase was obtained by MM. For Y[subscript]2S[subscript]3, Er[subscript]2S[subscript]3 and Yb[subscript]2S[subscript]3 the pure metastable high pressure [gamma]-phase was obtained, whereas for the Tm[subscript]2S[subscript]3 and Lu[subscript]2S[subscript]3 samples the metastable high pressure [gamma]-phases coexisted with the corresponding equilibrium ambient polymorphic phase. In case of Dy[subscript]2S[subscript]3 and Y[subscript]2S[subscript]3 the [gamma]-cubic phase was present even after 160 hour MM;The Seebeck coefficient, electrical resistivity, and Hall effect have been studied in Cu[subscript]x(Dy[subscript]2S[subscript]3)[subscript]1-x compounds with the [eta]-orthorhombic structure in the composition range 0.006≤ x≤ 0.15 in order to determine their potential as high temperature (25 to 1000°C) thermoelectric materials. In this temperature and composition range Cu-doped Dy[subscript]2S[subscript]3 behaves as a degenerate semiconductor and shows itinerant conduction. The electrical resistivity and the Seebeck coefficient increased with increasing temperature and then reached the maximum values of 4.35 to 7.13 m[omega]-cm and -163 to -177 [mu]V/°C, respectively. The maximum power factor of 7.9 [mu]W/cm-°C[superscript]2 at Cu[subscript]0.039(Dy[subscript]2S[subscript]3)[subscript]0.961 was obtained at 690°C;The electrical activity of phosphorus in Si[subscript]80Ge[subscript]20 by two non-conventional doping processes has been measured over the temperature range 25-1250°C. Both solid state (mechanical alloying) and gaseous phase doping processes were found to extend the electrical activity of phosphorus in Si[subscript]80Ge[subscript]20 alloys beyond the maximum equilibrium activity (2.1x 10[superscript]20/cm[superscript]3) to 2.5 to 2.9x 10[superscript]20/cm[superscript]3 within the temperature range 900-1200°C. It is likely that this extended electrical activity of phosphorus is associated with a high density of defects.

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Wed Jan 01 00:00:00 UTC 1992