PhD Thesis
Al Jumlat Ahmed, University of Wollongong
Year
2021
Degree Name
Doctor of Philosophy
Department
Institute for Superconducting and Electronic Materials
Abstract
The enormous amount of waste heat from different sources for instance: exhaust system of automobiles, industrial boilers and chimneys, thermal power plants and household cooking stove, can be directly converted into useful electrical energy by thermoelectric generator (TEG). Hence, thermoelectric (TE) has a huge potential to be a simple and environment-friendly technology for direct conversion of waste heat into usable electricity. However, the energy conversion efficiency and lifetime of thermoelectric devices are not adequate for commercial usages.
Energy scientists around the globe are working on different thermoelectric materials and device configurations to overcome these hurdles. Conventional thermoelectric materials such as Bi2Te3, PbTe, and Cu2Se exhibit high thermoelectric performance, however these materials have some shortcomings like poor durability, low operating temperature, limited availability, and high toxicity. On the other hand, metal oxide based thermoelectric materials are highly durable at high temperatures, non-toxic, low-cost, and have minimal environmental impact. However, the performance of metal oxide based thermoelectric materials are still inferior compared to the conventional thermoelectric materials due to their high lattice thermal conductivity.
SrTiO3 and BaTiO3 are intrinsically insulator, however their electrical conductivity can be easily improved by doping. In this study, electron doping in these materials was optimized by adding La in different atomic percentage and they were become n-type semiconducting material. Nanoscale pores were introduced into the bulk samples for further improvement of thermoelectric performance of Sr1-xLaxTiO3 and Ba1-xLaxTiO3. Powder samples with nanoscale pores were synthesized using the polymeric micelles self-assembly method. The commercially ii available Pluronic F127 surfactant was used as soft template for nano scale pores formation. Then the powder samples were rapidly solidified using the spark plasma sintering (SPS) technique to prepare bulk samples with nanostructured pores. The sintering conditions such as sintering temperature, pressure, holding time, heating and cooling rate were optimized to fabricate highly dense nano crystalline bulk samples.
The X-ray diffraction (XRD) peaks shifting and reduction in lattice parameter confirmed that A site of ABO3 crystal system of SrTiO3 and BaTiO3 were substituted by La atoms. Atomic resolution scanning transmission electron microscopy (STEM) images and energy dispersive X-ray spectrometry (EDS) results also showed that La was doped successfully into the lattice. The electrical conductivity of metal oxide materials was improved due to La doping and their showed n-type semiconducting behaviour. The Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy images and transmission electron microscopy (TEM) images revealed that the samples synthesized using the surfactant F127 have nanostructured pores. There was a large reduction in the lattice thermal conductivity in the F127-treated samples arises primarily from the nanoscale pores distribution which introduces anisotropic phonon scattering within the unique nanoarchitecture. It was also observed that the nanoscale pores in the samples significantly improved the Seebeck coefficient (thermopower). The change in phonon charge-carrier interaction and charge-carrier mobility may be responsible for improvement in the thermopower due to nano pores.
Therefore, there was remarkable enhancement in the power factor and the figure of merit (zT) of La doped SrTiO3 and BaTiO3 samples with nanoscale pores.
Recommended Citation
Ahmed, Al Jumlat, Development of Nano-engineered Perovskite-based Thermoelectric Material for Waste Heat Recovery, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2021. https://ro.uow.edu.au/theses1/988