| Description |
Thermoelectric-based devices are promising in the area of energy generation, waste-heat harvesting, refrigeration, temperature controller, and wireless sensors. The thermoelectric efficiency is directly related to a dimensionless figure of merit ZT. As ZT approaches to an infinitely large quantity, the thermoelectric process is approximate to a reversible conversion with the Carnot efficiency. ZT is dependent on materials properties (i.e., Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ)). Usually, a prospective thermoelectric material should possess large S, high σ, and low κ. In my research, those parameters were measured by several custom-made experimental systems, which are shown in Chapter 3. I investigated the thermoelectric properties of two oxide materials: Ca3Co4O9 (CCO) and ZnO. In Chapter 4, the porous polycrystalline bulks of CCO were prepared by adding wood powders in the CCO pellets. Wood was burned off in the sintering process creating many pores in the pellets. Large open pores can significantly reduce κ, but also induce the carriers' localizations. The oxygen deficiency formed during the wood burn-off process contributed to a decrease in carrier concentrations, which posed an effect on S. Chapter 5 shows the studies on CCO thin films grown by the pulsed laser deposition. The grain sizes were reduced by controlling the thickness, which led to an improvement in S due to the energy filtering effect. In Chapter 6, the thermoelectric properties of ZnO were studied by dually doping the pure compounds with various trivalent elements. It is iv revealed that the microstructures of ZnO varied with different doping elements, which resulted in apparent changes in the electrical and thermal conductivities. Besides, I also fabricated the spintronics consisting of a bilayer of ferromagnet insulator substrate (YIG) and alloy films (Ta), which is capable of making a thermal-to-electrical energy conversion at room temperature. One significant advantage of this system is the interrelated parameters σ and κ can be successfully decoupled through the spin Seebeck effect. In Chapter 7, I attempted to improve the generation of voltages by adjusting the composition of the metallic layer, which is responsible for the conversion from an injected spin current into a charge voltage. |
