Cullen Boyle

Date of Graduation

2017

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xueyan Song

Committee Co-Chair

Yun Chen

Committee Member

Bruce Kang

Committee Member

Hailin Li

Committee Member

Dustin McIntyre

Committee Member

Jacky Prucz

Abstract

Thermoelectric (TE) materials have the unique ability to convert temperature differences directly into electricity due to the Seebeck effect. Thermoelectric generators have a variety of applications including waste heat recovery for power plants and automotives. For high-temperature waste-heat recovery, misfit layered calcium cobaltite, Ca3Co4O9, is one of the most promising p-type TE oxides offering good durability in air with low cost and minimized environmental impact. A challenge for developing polycrystalline calcium cobaltite TE materials is to improve its energy conversion efficiency for large scale applications. The energy conversion efficiency of TE materials is characterized using the figure-of-merit ZT, which is defined as ZT = S2rho -1K-1T, where S, rho, S 2rho-1, K-1, and T are Seebeck coefficient, electrical resistivity, power factor, thermal conductivity, and temperature respectively. State of the art commercial conventional TE materials, such as bismuth telluride (Bi2Te3) and lead telluride (PbTe), possess values of ZT ≈ 1, which corresponds to an energy conversion efficiency of 10%. Calcium cobaltite single crystals show extraordinary TE performance with an extrapolated ZT value of 0.87 at 973 K.4 By contrast, the TE performance of polycrystals is reported to be only ~20% of that from the single crystal and with the commonly measured ZT of ~0.2 at ~900 K. This dissertation focuses on improving the performance of polycrystalline Ca3Co4O9 ceramics for large scale applications. The present work demonstrates the feasibility of increasing the values of Seebeck coefficient, S and power factor of calcium cobaltite Ca3Co4O9 ceramics through dopant grain boundary (GB) segregation. For the first time in the field of thermoelectrics, various dopants which include Bi, Ba, and co-dopants Bi-Ba were proved to segregate to the GBs and dramatically increased the ZT of Ca3Co4O9, and a high ZT of 0.52 was achieved for Ca3Co4O9 ceramics through dopants grain boundary segregation. The method of introducing the dopants to the grain boundaries, and their origin of the performance enhancement of thermoelectric oxide ceramics were thoroughly investigated and presented in this dissertation.

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