Degree Type

Dissertation

Date of Award

1992

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

Karl A. Gschneidner, Jr.

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.

DOI

https://doi.org/10.31274/rtd-180813-12819

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Sang Hyun Han

Language

en

Proquest ID

AAI9311494

File Format

application/pdf

File Size

154 pages

Included in

Metallurgy Commons

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