Date of Award

8-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Tritt, Terry M

Committee Member

He , Jian

Committee Member

Luo , Jian

Committee Member

Skaar , Eric

Committee Member

Foulger , Stephen H

Abstract

Thermoelectric materials are involved in the direct conversion between thermal and electrical energy. They may be used to generate power from a thermal gradient or to pump heat in or out of a system when driven with DC power. Much of the recent advances in thermoelectric research have been in materials whose thermoelectric efficiency has been optimized at temperatures above room temperature where power generation is the main application.
The question that I seek to answer in this dissertation is to use our standard experimental techniques in order to assess the possibility of a class of transition metal dichalcogenides bases on TiSe2 that are suitable candidates for thermoelectric applications. In addition, a further question is to understand the individual roles of intercalating titanium and nickel into the van der Waal planes of these materials and to investigate their effects on the electrical and thermal transport properties as well as the structure (including microstructure) of these materials.
The work presented herein has focused on deriving a thermoelectric material with a maximum ZT in the temperature range between 100 K and room temperature. Some of the niche applications of thermoelectric cooling in this temperature range include high-speed computing, active cooling of detectors, and most importantly, in the region of 100 to 120 K, the viability of superconducting electronic systems without the need of liquefied gasses.
Transition metal dichalcogenides are layered structures with a weakly bonded van der Waals gap between a-b planes. This gap may be intercalated by many different atomic and molecular species, which may significantly affect the structural and transport properties. Intercalation therefore provides a wide-ranging tuning 'knob' for optimizing the thermal and electrical transport properties of the host material.
Several compounds chosen from this group of materials were synthesized and intercalated with 3d and 4d transition metals. Thermal and electrical transport properties were measured and the best candidate, TiSe2, was chosen as the host matrix for optimization through co-intercalation with nickel and excess titanium. Additionally, the material was further optimized by the substitution of sulfur on selenium sites. While the maximum thermoelectric efficiency as judge by the dimensionless figure-of-merit (ZT) was increased, the temperature of that optimization (Tmax) was also increased to temperatures near and above room temperature where the state-of-the-art Bi2Te3's efficiency has been already optimized and is much higher than those of the materials in this work.
Though the goal of developing a material whose ZT is maximized at temperatures below room temperature was not achieved, the overall thermoelectric efficiency of TiSe2 has been increased by the co-intercalation of nickel and titanium, and then further increased by the substitution of selenium with sulfur. The individual roles of each of the intercalants, as well as the substitution of sulfur, in the manipulation and optimization of the thermal and electrical transport properties have been analyzed and understood in terms of fundamental solid-state theory and principles and are explained herein.

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