Ionic conductivity in acceptor-doped barium zirconate
- Ionische Leitfähigkeit in Akzeptor-Dotiertem Bariumzirkonat
Draber, Fabian Michael; Martin, Manfred (Thesis advisor); Lüchow, Arne (Thesis advisor)
Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2021
Acceptor-doped BaZrO3 is a very promising material for different applications like electrolysers, fuel cells or for methane conversion cells. Although this material has already been investigated by different groups with theoretical and experimental methods, the understanding for the underlying processes on the atomic level is still limited. This work is supposed to expand understanding, therein. To do so, the explicit goal of this work is to connect macroscopically measurable values like ionic conductivity and microscopic processes like single ion jumps. Trough density functional theory calculations, microscopic properties like defect interactions and migration energies of protons and oxygen vacancies are calculated. These data are used in kinetic Monte Carlo simulations to predict the mobility of both species and the ionic conductivity. Here, the dependence on dopant fraction, degree of hydration and the type of dopant ions is investigated in particular. It is predicted that protons are trapped at low dopant fractions. With increasing dopant fraction, these traps start to overlap and they build complete pathways through the crystal. Especially for yttrium doped barium zirconate, protons are able to move faster inside these pathways than outside. This leads to a strong increase in ionic mobility and conductivity. Comparing these results with experimental data shows good agreement. It is then predicted how the perfect yttrium defect structure to reach a maximum of increase in ionic conductivity is build. Oxygen vacancies play a minor role in yttrium doped barium zirconate because they are much slower than protons. Nevertheless, for a full understanding of the material it is necessary to investigate them as well. Experimental findings that the ionic conductivity is mainly resulting from protons, could be confirmed. On top of that, it was found that oxygen vacancies play a much more important role in barium zirconate with other dopant ions, for example gallium. It is experimentally very challenging to get barium zirconate with either protons or oxygen vacancies. Usually, both are there. Therefore, simulations were performed with both ions mobile at the same time. To our best knowledge, these are the first simulations of this kind. This work is rounded off with some particularities like the investigation of the vehicle mechanism, where one proton and one oxygen ion jump together and in-depth research of the influence of the octahedral tilting in barium zirconate.