The thermoelectric properties and microstructure of two families of misfit type layered structure cobaltites were investigated for thermoelectric applications. Firstly, Bismuth strontium cobaltite ceramics with the formulations Bi2+xSr2Co2Oy (x=0, 0.1 and 0.2), Bi1.74Sr2Co1.8Oy and Bi2Sr2Co1.8Oy were produced using solid-state reaction (MO) method. The same powders were also used to produce ceramics by Spark Plasma Sintering (SPS) fabrication technique. SEM, high resolution XRD and HRTEM techniques has been employed to characterise the microstructure and crystal structures of the ceramics. Figure of merit (ZT) was also determined from measurement of electrical resistivity, Seebeck coefficient and thermal conductivity. Together with the above, calcium cobaltite of formulation Ca3-xBixCo3O9 (x=0 and 0.3) was also produced via MO and SPS routes. The same characterisation techniques were used for characterisation of calcium cobaltite. For Bi2+xSr2Co2Oy ceramics, it was found that SPS fabrication is essential to obtain high density samples. Excess bismuth has a major role in the adjustment of the microstructure and thermoelectric properties. The room temperature microstructure contains two minor phases with compositions of CoO and Bi0.75Sr0.25O1.26. The crystal structure of the main phase was successfully indexed and refined as misfit type structure having monoclinic symmetry with I2/a space group. A high ZT of 0.12 was achieved in both x=0.1 and 0.2 MO samples. For Bi2Sr2Co1.8Oy ceramics, the microstructure contains only one minor phase, Bi0.75Sr0.25O1.26. A high ZT of 0.16 was obtained at 900 K for this composition. For Bi1.74Sr2Co1.8Oy, it was not possible to obtain high density ceramics by MO route and SPS fabrication was necessary. However, SPS sample showed a low ZT of 0.04 at 900 K.For ceramics of formulation Ca3-xBixCo3O9 (x=0 and 0.3), it was difficult to obtain high density calcium cobaltite ceramics by MO route and SPS fabrication was found to be essential. In addition to improved density, SPS produced textured microstructure. Similar to bismuth strontium cobaltite, excess bismuth played a major role in microstructure development and thermoelectric properties. Single phase and high density Ca3Co4O9 ceramics were obtained by SPS. A minor phase of Bi2Ca2Co2Oy was found in the microstructure of Ca3-xBixCo3O4 (x=0.3) samples. A high ZT of 0.25 was obtained for Ca3Co4O9 SPS samples at 900 K through improvement of power factor. In-situ synchrotron XRD in the temperature range of 300-1223 K was performed on both Bi2Sr2Co2Oy and Ca3Co4O9 to obtain their high temperature structural characteristics. The crystal structure of both compounds remains unchanged till 1223 K. For, Bi2Sr2Co2Oy the CoO and Bi0.75Sr0.25O1.26 minor phases disappear above 1073 K and a new minor phase containing (Bi-Sr-O) or (Bi-Co-O) starts forming. On heating, the lattice volume and coefficient of thermal expansion change linearly for both compounds. Thermal expansion coefficient was found to be 0.0000353-0.0000343 and 0.0000296-0.0000288 K-1 over the temperature range of 300-1223 K for Bi2Sr2Co2Oy and Ca3Co4O9 respectively.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:728002 |
| Date | January 2017 |
| Creators | Kulwongwit, Nuth |
| Contributors | Freer, Robert |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/ptype-misfit-layered-structure-cobaltite-for-thermoelectric-applications(c967ef03-5dd2-444d-b99b-1a1f784d10f0).html |
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