Semester

Spring

Date of Graduation

2012

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xueyan Song

Committee Co-Chair

Ever J Barbero

Committee Member

Daneesh Simien

Abstract

Power plants, turbine engines, automobiles and air conditioning, among others, release enormous amounts of heat into the environment, thereby wasting vast amounts of thermal energy. The typical U.S. power plant is only about 33% efficient, using three units of fuel to produce one unit of electricity. The rest is released as waste heat into the atmosphere. In standard diesel-powered vehicles, up to 65% of the fuel is lost as waste heat, which is rejected through the cooling and exhaust systems at temperatures up to 900°C. A potential way to improve the sustainability of our energy infrastructure and electricity base is through waste heat recovery using solid state thermoelectric power generators, which can directly convert temperature differentials into electrical power. Oxide materials, such as newly-developed nontoxic Ca3Co 4O9, are particularly promising for thermoelectric applications because of their relatively high stability, even at high temperatures in air. The main challenge for enabling oxide thermoelectric materials is to improve their conversion efficiency, which is currently lower than that of conventional, low temperature thermoelectric materials. The conversion efficiency is usually evaluated by the dimensionless figure-of-merit ZT = S2T/rhokappa, where S, T, rho and kappa are Seebeck coefficient (or thermoelectric power), absolute temperature, electrical resistivity, and thermal conductivity, respectively.;The main purpose of this project is to optimize the synthesis condition for powder precursor and pelletizing condition for bulk ceramic, to obtain the pure Ca3Co4O9 with high energy conversion efficiency. In the meanwhile, for practical applications, it is essential to understand the thermal stability of sintered Ca3Co4O 9 materials in air, the formation of other cobaltite Ca-Co-O phases, and their property and structure evolution upon long term high temperature annealing and over-heating above the reported Ca3Co4O 9 decomposition temperature of 926°C, as indicated in the CaO-CoO phase diagram. In details, the following is studied in this thesis: (1) Effect of calcinations temperature for the precursor powders on the thermoelectric properties of Ca3Co4O9; (2) Anisotropic thermoelectric properties in a misfit-layered calcium cobaltite affected by the conditions of pellets preparation. (3) Nanostructure and thermoelectric properties evolution of polycrystal Ca3Co4O9 upon over-heating condition.;The above study will be instrumental for fabricating the polycrystal Ca3Co4O9 with optimized microstructure and enhanced thermoelectric properties. Upon the completion of the microstructure and properties optimization of pure polycrystal Ca3Co4O 9, investigation will be carried out to further improve the thermoelectric performance of Ca3Co4O9 through doping impurity and engineering the nanostructure.

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