Marek Oja will defend his doctoral thesis titled "Investigation of nano-size α- and transition alumina by means of VUV and cathodoluminescence spectroscopy" on 22. May 2017 at 16:15 at W. Ostwaldi 1, room B103.
dr Marco Kirm, Institute of Physics, University of Tartu
dr Eduard Feldbach, Institute of Physics, University of Tartu
dr Stephane Guizard, Ecole Polytechnique, France
Aluminium oxide or alumina is found in nature as a hard mineral called corundum. Corundum as a gemstone is known as a red ruby or blue sapphire. These colours are achieved by substituting chromium, iron or titanium ions into alumina. Besides the thermodynamically stable phase of alumina called α-Al2O3 there exist several metastable alumina phases with different crystal structures. These so-called transition phases can be synthesized only as nanocrystals and are stable up to certain temperatures – over 1100 °C only α-phase exists. Highly variable crystal structure of chemically the same composition and nanocrystallinity makes transition phases of alumina an attractive objects of investigation and relevant material for applications.
The aim of this work was the investigation of electronic and optical properties of transition aluminas in comparison with that of α-phase and of possible effects caused by the crystallite size. Various luminescence spectroscopy techniques were used as the main investigation methods, which are in many cases indispensable for investigation of electronic structure. It was demonstrated how electronic structure in different transition aluminas is influenced by changes of crystal structure. It was found also that despite differences in crystal structures the luminescence properties of point defects of different transition aluminas are similar. The study of Cr3+ impurity centre emission showed the possibility to use it for identifying of different alumina phases. The capability of luminescence methods was demonstrated as well to clarify the composition of oxide dispersion strengthened steels, important material for many applications.