The development of high figure-of-merit thermoelectric materials

摘要

Since the beginning of the industrial revolution in the late 18th century, there's an abrupt increase in need for energy and resources. Despite of advantages it offers, waste heat or energy generated by machines or vehicles exhaust is evolved into the surrounding atmosphere, resulting in the evolution of the Greenhouse effect. With rampant global warming, thermoelectricity - an environmentally benign technology that recovers waste heat and converts it directly into useful electricity or vice versa - has gradually become an interesting topic for researchers. Regardless, it has yet to be widely commercialised mainly due to its inferior energy conversion efficiency to the conventional power generators and cooling devices. The aim of this project was to explore ways of enhancing the efficiency of thermoelectric materials.NaC0204 is one of the state-of-the-art p-type thermoelectric materials to date due to its superior thermoelectric properties and stability at elevated temperatures. As a result of its layered structure, NaC0204 is highly anisotropic in thermoelectric properties. Literatures revealed that the thermoelectric properties along the in-plane direction are superior to that along the out-of-plane direction, which emphasise the significance of texture. In this work, the texturing mechanisms and the texturing process as a function of sintering duration were studied in detail and the synthesis procedure of highly textured NaC0204-; materials was optimized. The results indicate that the extent of texture increased with increasing process duration and then reached the maximum when the materials were sintered at 900°C for 12 hrs. The texturing kinetics obeys an exponential relationship, suggesting that the texturing processing is dominated by self-diffusion mechanism. The experimental evidence also shows that the extent of texture decreased when the annealing time exceeded 12 hrs. The mechanisms of such deterioration were highly due to the nature of grain growth in ceramics.Given that the Co ions in NaxC020 4-; have different valence between 3+ and 3.48+ depending on the Na content and have spins, it is expected that oxygen vacancies formed in the lattice would cause charge-spin interaction, which would have a profound effect on the thermoelectric properties. As part of current study, the oxygenation kinetics and possible mechanisms were investigated. Room temperature X-ray diffraction study shows a decrease in FWHM and a certain left shift of spectra for the oxygenated samples, suggesting an increase in d-spacing and lattice distortion as a result of oxygen deficiency. The extent of oxygen saturation increased with decreasing annealing temperature. The high oxygenation flux in the first three hours of annealing was possibly due to lower energy requirement for oxygen to occupy its vacant sites than that to interstitially occupy a site. These experimental results could be used to further study the oxygen dependence of thermoelectric performance. Moreover, the investigation of temperature effects on lattice structure of NaC02O4-; with/without oxygenation was carried out to understand oxygen contribution to the lattice structure and also thermal expansion behaviours. Refined high temperature XRD results revealed that lattice parameter a decreased and c increased over the temperature range investigated after oxygenation due to Coulomb interaction between the constituent ions. Linear thermal expansion coefficient along the a- b-axis direction is circa 1.105 x 10-5K-1 for the sample before oxygenation and 1.06 x 10-5K-1 after oxygenation. An almost three-fold increase was observed for the thermal expansion coefficient along the c-axis direction for the samples before and after oxygenation, suggesting that higher oxygen concentration suppresses the thermal expansion coefficient in all directions.