Date of Award

8-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Dr. Terry M. Tritt

Committee Member

Dr. John Ballato

Committee Member

Dr. Stephen H. Foulger

Committee Member

Dr. Gary C. Lickfield

Abstract

The direct energy conversion between heat and electricity based on thermoelectric effects is a topic of long-standing interest in condensed matter materials science. Experimental and theoretical investigations in order to understand the mechanisms involved and to improve the materials properties and conversion efficiency have been ongoing for more than half a century. While significant achievements have been accomplished in improving the properties of conventional heavy element based materials (such as Bi$_2$Te$_3$ and PbTe) as well as the discovery of new materials systems for the close-to-room temperature and intermediate temperatures, high-temperature applications of thermoelectrics is still limited to one materials system, namely SiGe. Recently, oxides have exhibited great potential to be investigated for high-temperature thermoelectric power generation. The objective of this Dissertation is to synthesize and investigate both electronic and thermal transport in strontium titanate (SrTiO$_3$) ceramics in order to experimentally realize its potential and to ultimately investigate the possibility of further improvement of the thermoelectric performance of this perovskite oxide for mid- to high temperature applications. Developing a synthesis strategy and tuning various synthesis parameters to benefit the thermoelectric transport form the foundation of this study. It is worth mentioning that the results of this study has been employed to prepare targets for pulsed-laser deposition (PLD) to study the thermoelectric properties of corresponding thin films and superlattice structures at Dr. Husam Alshareef's group at King Abdullah University of Science and Technology (KAUST), Saudi Arabia. Considering the broad range of functionality of SrTiO$_3$, the findings of this work will surely benefit other fields of research and application of this functional oxide such as photoluminescence, ferroelectricity or mixed-ionic electronic conductivity. This Dissertation will ultimately attempt to answer the question, 'Is it possible to further improve the thermoelectric properties of SrTiO$_3$-based ceramics?'. The organization of the Dissertation is as follows: In Chapter 1, the fundamental concepts in the thermoelectric theory is explained. Second, we briefly review the characteristics of 'good' thermoelectric materials and highlight the differences exist between SrTiO$_3$ and conventional thermoelectric materials. In Chapter 2, SrTiO$_3$ is introduced and the electronic and thermal properties arising from its crystal structure are discussed. Chapter 3 is dedicated to the fundamentals of measurements of the electronic and thermal transport properties which are the backbone of the current work. Our experimental results are presented in Chapter 4 and 5. The synthesis and processing techniques to prepare doped SrTiO$_3$ powder and bulk polycrystalline ceramic are presented in Chapter 3. The optimizations of the synthesis and densification parameters involved are presented and discussed in this chapter as well. Significant improvement achieved in the thermoelectric figure of merit of Pr-doped SrTiO$_3$ and the studies performed to understand the results are presented in Chapter 5. Concluding remarks and future work are discussed in Chapter 6.

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