REPOSITORY > RESULTS

Abstract

The rapid development and globalization in the 21st century are still mainly powered by the fossil fuels with the negative environmental impact and limited natural resources. A large and re-usable energy source is the “waste heat” and its harvesting is possible with the thermoelectric generators, which have the ability to transform heat into an electric potential through the Seebeck effect. Oxide-based materials are very suitable for the high-temperature applications due to their good chemical and temperature stabilities in air. The ZnO - In2O3 system with the series of the ZnkIn2Ok+3-type homologous phases with a superlattice structure offers an excellent platform for tailoring the thermoelectric properties of the n-type ZnO-based ceramics.
At first, this PhD study was focused on the formation mechanism of the homologous phases in the (ZnO)kIn2O3 polycrystalline ceramics and its influence on the sintering, microstructure development and thermoelectric properties. Results indicate the homologous phase formation at temperatures above 1000 °C and its strong relation to the local Zn-to-In ratios in the starting powder mixture. Accordingly, in the compositions with high addition of In2O3 (k=5) the equilibrium homologous phase Zn5In2O8 forms directly and the growth of plate-like grains is uniform due to soon reached compositional homogeneity. At lower additions of In2O3 (k=11, and 18) the Zn5In2O8 and Zn7In2O10 phases are formed in In2O3-rich areas and grow on expense of the high k phases formed in the In2O3-deprived regions till the composition of the equilibrium homologous phase for starting k value is not reached, which requires longer sintering times at higher temperatures. Exaggerated grain growth manifests in these samples as long as compositional homogeneity is not reached through all the sample. Hence, their grain size is larger in comparison to the sample k = 5. It also showed that a higher degree of structural and compositional homogeneity results in better thermoelectric characteristics of the (ZnO)kIn2O3 ceramics.
The study of Al-doping, contrary to previous literature reports, clearly showed that Al substitutes In on its both primary sites in the Zn5(In1-xAlx)2O8 homologous phase, the octahedral sites in the basal-plane inversion boundaries and the trigonal bi-pyramidal sites in the zig-zag inversion boundaries. This results in a linearly increased shrinkage of the unit cell with the increasing additions of Al, decreased charge carrier concentration and effects on their mobility, resulting in reduced electrical conductivity and just slightly decreased ZT values.
Preparation of the (ZnO)kIn2O3-based ceramics requires long sintering times at high temperatures. Hence, their processing was studied using the microwaves sintering (MW) and the field assisted sintering (FAST) as fast and energy efficient methods. The initial heating in MW sintering was caused by the more effective In2O3 coupling with MW, transferring the heat to the less effective MW absorber ZnO. It resulted in the formation of the ZnkIn2Ok+3 homologous phases and well densified ceramics with thermoelectric properties akin to the conventionally sintered samples, however at about 70% lower input of energy. The FAST process resulted in the strongly enhanced sintering and formation of the ZnkIn2Ok+3 homologous phases; i.e. the Zn5In2O8 phase formed already after 5 minutes at 1000 °C, while conventional sintering required 22 h at 1200 °C. Furthermore, the applied pressure together with the electric field resulted in modest texturing and 50 % higher electrical conductivity in comparison to the conventional samples.