This dissertation aims at a deeper understanding of the microstructural factors that origin and influence non-ohmic behaviour in ZnO ceramics. The third chapter presents results and discussion of the classical simulation results. We have carried out calculations on a large number of twist and tilt grain boundary systems and we have investigated the energetics and segregation properties of cobalt and barium to the Σ7 twist grain boundary core. We have determined the energies of a large number of twist and tilt boundaries and seen how they fit into known models. The structural units of the Σ13 tilt boundaries were identified and systems for the <i>ab initio </i>calculations selected. The electronic structure calculations on twist grain boundaries are presented in chapter four, along with the results for segregation of antimony and bismuth to the Σ7 and Σ13 boundaries. We have found a limit doping that is approximately the same for very different twist and tilt boundaries and suggests a maximum for the density of interfacial impurity states. The impurity states found were of a donor type and, although they were approximately in the right position in the band gap, they did not correspond to the acceptor states associated with varistor behaviour. The fifth chapter describes the results on tilt grain boundaries and doping with antimony, bismuth and defect complexes. We have investigated the shallow interface states that result from the tilt grain boundaries, the states introduced by oxygen interstitial impurities, the effect of Zn vacancies and the electronic structure of defect complexes. The sixth chapter presents a model intergranular film boundary. The charges, interface states and energetics are analysed as well as the possible origin of the compensation mechanism in intergranular thin films. We suggest that these films can be electrically active. The seventh chapter contains unrelated work on solid phases of Met-Car analogue clusters. We have calculated a number of clusters assembled phases and related their properties to the superconductive characteristics of <i>C</i><sub>20</sub> solids.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598591 |
| Date | January 2004 |
| Creators | Domingos, Helder Sousa |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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