High temperature thermal energy harvesting has attracted much attention recently. In order to achieve stable operation at high temperatures there is emerging need to develop efficient and oxidation-resistant materials. Most of the well-known materials with high dimensionless figure of merit (ZT) values such as Bi2Te3, PbTe, skutterudites, and half-Heusler alloys, are not thermally stable at temperatures approaching 500°C or higher, due to the presence of volatile elements. Oxide thermoelectric materials are considered to be potential candidates for high temperature applications due to their robust thermal and chemical stability in oxidizing atmosphere along with the reduced toxicity, relatively simpler fabrication, and cost. In this dissertation, nanoscale texturing and interface engineering were utilized for enhancing the thermoelectric performance of oxide polycrystalline Ca3Co4O9 materials, which were synthesized using conventional sintering and spark plasma sintering (SPS) techniques. In order to tailor the electrical and thermal properties, Lu and Ga co-doping was investigated in Ca3Co4O9 system. The effect of co-doping at Ca and Co sites on the thermoelectric properties was quantified and the anisotropic behavior was investigated. Because of the effective scattering of phonons by doping-induced defects, lower thermal conductivity and higher ZT were achieved. The layered structure of Ca3Co4O9 has strong anisotropy in the transport properties. For this reason, the thermoelectric measurements were conducted for the samples along both vertical and horizontal directions. The ZT value along the vertical direction was found to be 3 to 4 times higher than that along the horizontal direction. Metallic inclusions along with ionic doping were also utilized in order to enhance the ZT of Ca3Co4O9. The texturing occurring in the nanostructured Ca3Co4O9 through ion doping and Ag inclusions was studied using microscopy and diffraction analysis. Multi-length scale inclusions and heavier ion doping in Ca3Co4O9 resulted in higher electrical conductivity and reduced thermal conductivity. The maximum ZT of 0.25 at 670°C was found in the spark plasma sintered Ca2.95Ag0.05Co4O9 sample. In literature, limited number of studies have been conducted on understanding the anisotropic thermoelectric performance of Ca3Co4O9, which often results in erroneous estimation of ZT. This study addresses this limitation and provides systematic evaluation of the anisotropic response with respect to platelet microstructure. Textured Ca3Co4O9/Ag nanocomposites were fabricated using spark plasma sintering (SPS) technique and utilized for understanding the role of microstructure towards anisotropic thermoelectric properties. The thermoelectric response was measured along both vertical and horizontal direction with respect to the SPS pressure axis. In order to achieve enhanced degree of texturing and increase electrical conductivity along ab planes, a two-step SPS method was developed. Ag nanoinclusions was found to increase the overall electrical conductivity and the thermoelectric power factor because of improved electrical connections among the grains. Through two-step SPS method, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C in Ca3Co4O9/Ag nanocomposites was achieved.
Lastly, this dissertation provides significant progress towards understanding the effect of synthesis method on thermoelectric properties and evolution of textured microstructure. The anisotropy resulting from the crystal structure and microstructural features is systematically quantified. Results reported in this study will assist the continued progress in developing Ca3Co4O9 materials for practical thermoelectric applications. / PHD / Among the wide range of renewable energy sources, wasted thermal energy has attracted worldwide interest as it is freely available from most of the industrial and natural processes. Among various choices for converting thermal energy into electricity, thermoelectric devices are attractive as they are solid state, noiseless, no moving parts, and can be easily integrated with most of the heat sources. Thus, there has been significant efforts to develop high efficiency thermoelectric energy harvesting devices. However, currently available thermoelectric materials are not thermally stable in oxidizing environments because of heavy metals’ evaporation and reactivity. In order to overcome this limitation of thermoelectric materials, in this dissertation, the focus is on developing calcium cobalt oxide (Ca₃Co₄O₉) materials through innovation in the processing, composition design, and modulation of the thermal transport mechanism by exploiting the anisotropy.
Ca₃Co₄O₉ is promising candidate for high temperature thermoelectric applications due to its thermal and chemical stability in oxidizing atmosphere, reduced toxicity, easy fabrication, and low cost. Its main disadvantages are the high thermal conductivity and low electrical conductivity. In order to tailor the electrical and thermal properties, Lu and Ga co-doped Ca₃Co₄O₉ were synthesized and characterized. The thermoelectric measurements were conducted along both vertical and horizontal directions with respect to pressure axis during spark plasma sintering. Layered structure of Ca₃Co₄O₉ induces strong anisotropy in the transport properties which indicates that textured microstructure will result in better properties. Texturing and interface engineering were employed to control the grain orientation and thereby improve the electrical and thermal properties. In textured and nanostructured Ca₃Co₄O₉, Ag inclusions along with ionic doping was utilized to enhance the thermoelectric performance.
In literature, the importance of the anisotropy in Ca₃Co₄O₉ has been less emphasized, which has restricted accurate thermoelectric evaluation of this material for practical application. In order to address this issue, first textured Ca₃Co₄O₉/Ag nanocomposites were fabricated using spark plasma sintering (SPS) techniques and next detailed investigation was conducted on correlation between microstructure and anisotropic thermoelectric properties. The power factor of the Ca₃Co₄O₉/Ag nanocomposites at high temperatures was almost 50% enhanced, as compared to the pure Ca₃Co₄O₉, which resulted in 50% improvement in ZT both horizontal and vertical directions. The samples with texturing along the vertical direction were used to perform the long-term durability test and almost same value of resistivity was maintained after a long-term heating.
Two-step SPS method was developed to improve the in-plane electrical conductivity. Through this newly proposed synthesis process, 28% improvement in the average ZT values below 400°C and 10% improvement above 400°C was obtained in Ca₃Co₄O₉/Ag nanocomposites. Using a wide range of composition and synthesis process, the anisotropy and microstructural effects clarified in this study provides promising pathway towards enhance the thermoelectric performance of Ca₃Co₄O₉ materials.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/98542 |
| Date | 30 November 2018 |
| Creators | Song, Myung-Eun |
| Contributors | Materials Science and Engineering, Priya, Shashank, Maurya, Deepam, Aning, Alexander O., Guido, Louis J., Reynolds, William T. Jr. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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