Defects in high lattice energy metal oxides by magnetic resonance and X-ray diffraction

The properties of all crystalline solids are affected by structural defects and disorder. Some crystal structures tolerate or even favor various forms of defects while others restrict imperfections. How does the structure of high-valence metal oxides accommodate and keep impurities? This PhD was undertaken to experimentally examine some relationships between defects in high-valence metal oxides and their properties.

As an ideal model system and technologically important material, a large part of the work is focused on alumina in two forms: anodic alumina (AAO) and fumed alumina. Some other high-valence oxides such as TiO2 and CeO2 were studied to a lesser extent. AAO films anodized in H2C2O4 were subjected to heat treatments up to 1 050 ◦C mostly in air and investigated by MAS NMR, MQMAS, XRD, ESR, FESEM and nanoindentation. A series of fumed aluminas with BET surface areas from 220 to 30m2 g−1 were also investigated by similar techniques. Deemed an essential technique to the research, the MQMAS experiment was practised on the 23Na isotope in a heavily disordered metal amide material. Additionally, it was necessary to carry out a systematic study of Guinier X-ray diffraction cameras in order to identify sources of instrumental and procedural errors in the XRD data. With the implementation of a rotatable, circular sample holder, the IPreader digitization software, and a data averaging script, the quality of the data was suitable for a reliable Rietveld refinement as quantitatively tested.

With the heat treatments of AAO films and ESR, correlation between color and concentration of paramagnetic impurities was observed. The microstructural transformations in AAO, including those involving disorder, were monitored up to the highest temperature, and correlated with the mechanical properties. Similarly, a clear and consistent correlation between the microstructure of fumed alumina and its surface area, or equivalently, flame residence time, was observed. Several H species in the form of isolated OH groups were detected.

Overall, the degree of defects and disorder in these oxides depended on the temperature and duration of heat treatment. The higher the temperature, the lower the level of impurities and disorder after cooling. In the case of AAO, a thermal hardening occurred above 1 000 ◦C. A similar trend is expected in other high-valence metal oxides if the atmosphere and cooling rate are chosen right.