An Experimental Investigation Towards Improvement of Thermoelectric Properties of Strontium Titanate Ceramics

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\u27s 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, \u27Is it possible to further improve the thermoelectric properties of SrTiO$_3$-based ceramics?\u27. 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 \u27good\u27 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

Solid state generators and energy harvesters for waste heat recovery and thermal energy harvesting

This review covers solid state thermal to electrical energy converters capable of transforming low grade heat directly into electricity for waste heat recovery and thermal energy harvesting. Direct solid state heat engines, such as thermoelectric modules and thermionic converters for spatial temperature gradients, are compared with pyroelectric energy harvesters and thermomagnetic generators for transient changes in temperature. Temperature and size limitations along with the maturity of the technologies are discussed based on energy density and temperature range for the different generator technologies. Despite the low energy conversion efficiency with solid state generators, electric power density ranges from 4 nW/mm2 to 324 mW/mm2. The most promising sector to implement changes while reducing the primary energy consumption and saving resources, is the processing industry along with stationary and mobile electronics

Large Thermoelectric Power Factor in Pr-Doped SrTiO_3−δ Ceramics via Grain-Boundary-Induced Mobility Enhancement

We report a novel synthesis strategy to prepare high-performance bulk polycrystalline Pr-doped SrTiO₃ ceramics. A large thermoelectric power factor of 1.3 W m^–1 K^–1 at 500 °C is achieved in these samples. In-depth investigations of the electronic transport and microstructure suggest that this significant improvement results from a substantial enhancement in carrier mobility originating from the formation of Pr-rich grain boundaries. This work provides new directions to higher performance oxide thermoelectrics as well as possibly other properties and applications of this broadly functional perovskite material