Date of Graduation

Summer 2020

Degree

Master of Science in Chemistry

Department

Chemistry

Committee Chair

Fei Wang

Keywords

crystal structures, physical properties, semiconductive transition-metal silicides, thermoelectric, doping, incommensurate, superconductive

Subject Categories

Inorganic Chemistry | Materials Chemistry

Abstract

Semiconductive transition-metal silicides (e.g., ReSi) express weak thermoelectric properties. These properties can improve through n-type or p-type doping (e.g., Al), resulting in altered crystal structures. Binary rhenium silicide (ReSi1.75) exhibits promising thermoelectric properties and an intriguing crystal structure. Previous research has shown ReSi1.75 changes from a commensurate supercell MoSi2-type structure to an incommensurate superspace group structure, when doped with aluminium to ReAl0.074Si1.67. ReSi1.75 expresses a figure of merit (ZT) of 0.70 at 800°C, which improves to a ZT of 0.95 at 150°C for ReAl0.02Si1.75. Because of the interesting properties expressed by rhenium silicide and the Al-doped variants, my research involves al-doping of rhenium silicides at various doping ratios, using the (18-n) electron rule, to determine how crystal structures and physical properties are affected. Using a non-thermal plasma arc furnace, (MRF SA200-1-VM), Al-doped rhenium silicides were synthesized. Half of the samples were annealed at 1000°C for a week, while the other half remained As Cast. Crystal structures and the bulk composition was determined using SXRD, PXRD and neutron diffraction, which was refined using Jana 2006. The resulting crystallographic data showed a transition from an incommensurate composite superspace group structure when Si-rich, to a P4/nmm space group for ReAlSi, and finally to a I4/mmm space group for the Al-rich samples. Neutron diffraction showed Al and Si were segregated with a 1:1 doping ratio but became mixed through increased Al-doping; these findings were substantiated using computational chemistry. Both ReAlSi and ReAl1.2Si0.8 became superconductive below 3.5K, with transition temperatures occurring above 4K. Though the resistivity, at temperatures above 3.5K, was ten-times higher for ReAlSi compared to ReAl1.2Si0.8.

Copyright

© Victoria DeCocq

Open Access

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