This thesis is partly concerned with investigating news ways of applying microwaves in a chemical synthesis; it is also concerned with the mechanism of such processes, including microwave-assisted diffusion, structural changes induced by microwave fields and the effect of pressure on microwave-driven reactions. In some cases we also studied the mechanism of reactions driven by more traditional sources of heat. A novel microwave reactor was designed and constructed to enable us to perform mechanistic studies of the hydrothermal synthesis of iron oxide particles by small angle neutron scattering (SANS); this was performed in parallel with small angle X-ray scattering (SAXS) and EXAFS on samples heated conventionally. SANS measurements showed that after an initial burst of nucleation, particles of 50 Å mean diameter were formed in the first 30 minutes, evolving to a final size of 100 Å after 7.5 hours. The SAXS measurements reported a burst in particle growth as the sample was heated above 50-60°C, producing particles up to 250-300 Å in diameter. However, wide-angle X-ray scattering data showed that the latter samples did not exhibit any Bragg scattering, implying that they had no medium-long range crystalline order when suspended in the growth medium. EXAFS data revealed the presence of very small particles of haematite in solution, which seems to add weight to the hypothesis that haematite particles are produced first through the growth of small particles which then aggregate to larger clusters. Studies were also performed of the microwave-assisted diffusion of cations into channelled iron and manganese oxides, and showed that the choice of experimental conditions -particularly the nature of the host material - have a significant effect upon the final product. In several cases, microwave heating accelerated diffusion and could also lead to novel routes to insertion compounds. Further studies on the influence of microwave heating on the structure and dynamics of materials involved the construction of further pieces of apparatus to perform in situ diffraction during microwave irradiation. Neutron diffraction was performed on aspirin to probe proton disorder, and high-resolution powder X-ray diffraction was conducted on the fast-ion conductor beta alumina to look at thermal excitation of the relatively mobile sodium ions during microwave heating. In both cases we demonstrated that the sample could be heated and studied <i>in situ, </i>and structural changes were revealed through changes in the Debye-Waller factors of particular atoms in both materials. The experiments on aspirin indicated that the sample had been subjected to a localised heating effect.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:664174 |
| Date | January 2002 |
| Creators | Youngson, Innes Douglas |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11662 |
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