Date of Award

8-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Advisor

Tritt, Terry M.

Committee Member

Rao , Apparao M.

Committee Member

Sosolik , Chad E.

Committee Member

Marinescu , Catalina D.

Abstract

Research into materials that have high efficiencies of thermoelectric heat-energy conversion has been at a plateau since the middle of the last century. During this time, efficiencies have been engineered high enough for several interesting niche applications but not high enough for widespread adaptation into traditional power generation or refrigeration technologies. The past decade has seen considerable advancement as a number of theoretical works have suggested that lower dimensional structures could hold the key for enhanced efficiency, and several experiments have provided the proof of principle needed to inspire just such a research direction. The benefit of low dimensional structures for thermoelectric efficiency comes from both the potential enhancement of the electronic properties due to quantum confinement effects as well as from the potential for increased scattering of heat-carrying phonons. Widespread application of these principles for technological application requires the creation of composites of nanostructures that can be manufactured easily with dimensions on the bulk materials scale. A good starting point for such materials research is to manufacture composites of materials that are currently known to have high thermoelectric efficiencies by incorporating nanostructures into a bulk matrix.
The goal of this project is to create nanocomposites using bismuth telluride, a compound known to have one of the highest thermoelectric efficiencies at room temperature, as a matrix material. Various methods of synthesizing sufficient quantities of bismuth telluride nanostructures were attempted, including pulsed laser vaporization, chemical vapor deposition, and solvothermal synthesis. The method of solvothermal synthesis was found to be the simplest approach for producing high yields of bismuth telluride nanostructures. In the initial stages of the project, cold pressing was tested as a means of compaction, but in the end a uniaxial hot pressing technique was adopted in order to consolidate the nanostructures into the bulk matrix.
Nanocomposites were produced using both n-type and p-type bismuth telluride compounds as the matrix material, into which nanostructures of Bi2Te3, BiSb, Bi2S3, as well as Au and Ag nanoparticles and C60 were incorporated. The preferred consolidation technique utilized a 3-axis mechanical mixer, followed first by cold and then hot pressing of the bulk-nano mixtures. The composites were studied with respect to their microstructure and elemental composition, as well as with regard to their thermal and electrical transport properties. The effects of the nanoparticle additions upon the efficiencies of the materials are presented, and the viability of improving the thermoelectric performance of this class of materials by this method is considered.

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