| dc.description.abstract | There is a current global movement toward renewable sources and away from non- renewable energy sources. This can be attributed to the limited resources and harmful effects, both economic and environmental, of non-renewables. In order to harness the renewable energy sources, and make their usage a feasible solution to the world’s energy crisis, cheap and efficient energy conversion devices must be fabricated. The working efficiency of these devices must be improved to ensure their continued popularity and one such way to achieve this goal involves optimising the diffusion process within the multi-crystalline components.
One of the main components of an electrochemical device is the electrolyte and requires high ionic conductivity with no electronic conductivity for the device to function. The electrolyte material in many electrochemical devices has a fluorite structure, and like all materials, it contains defects, both localised and extended. These defects affect the diffusion within the material, in a manner that is not fully understood. The main motivation for the work presented in this thesis is to investigate the effect multiple defects have on the ionic diffusion within fluorite-structured materials, using molecular dynamics with a highly accurate polarisable force field.
We began by examining the diffusion properties of calcium fluoride (CaF2), which was selected as a model fluorite material, due to its characteristically fast ionic diffusion. The bulk diffusion within this material was investigated to demonstrate that the method used here is capable of replicating literature results and to establish a base with which the effects of defects could be compared. Six surface orientations and seven grain boundary orientations of CaF2 were investigated, and their ionic diffusion, as a function of depth, was examined for each. A peak in the diffusion, parallel to the surfaces and grain boundaries, when compared to the bulk, was observed at each surface and grain boundary, although the magnitude differed, depending on the orientation. The effect on ionic diffusion of straining the bulk system and surface slabs was investigated, as strain can be introduced into materials via lattice mismatch. An increase in tensile strain was found to increase the diffusivity of the system, while an increase in compressive strain had a detrimental effect on the ionic diffusion.
The information gained here on the structure and diffusive properties of CaF2, and also the effect of defects, can be applied to other fluorite structured systems, such as in slower diffusing oxide ion materials, which are commonly used in electrochemical devices. A popular material used for the electrolyte in these devices is yttria-stabilised zirconia (YSZ). Bulk YSZ with 8%, 10% and 12% yttria concentration were investigated. The effect of six YSZ surface orientations was compared to the bulk and to what was observed for corresponding surface orientations in CaF2. Unlike CaF2, most YSZ surface orientations demonstrated a decreased diffusivity at their surfaces, compared to the bulk. This was postulated to be as a result of the segregation of the charge compensating vacancies generated in YSZ. This additional layer of complexity was investigated to determine their role in the diffusion within the system. The diffusivity at the YSZ surfaces was found to be dependent on both the surface orientation and a balance between the number of O2− anions available for diffusion, and the number of oxygen vacancies present for these mobile anions to diffuse into.
Ionic diffusion within a material can be affected in a number of ways, and the effects must be fully understood before the diffusion process can be optimised. The knowledge of this diffusion process can then be used to generate highly efficient diffusion based electrochemical devices, that could be the answer to the world’s energy crisis. | en |
