Abstracts Category : Other

Investigation of nano-size - and transition alumina by means of VUV and cathodoluminescence spectroscopy

by Marek Oja

Alumiiniumoksiidi leidub looduses korundina ehk suure kvadusega mineraalina. Korund kui vrtuslik vriskivi on tuntud ka punase rubiini ja sinise safiirina. Sellised vrvused tekivad lisades alumiiniumoksiidile kroomi, raua vi titaani ioone. Peale termodnaamiliselt stabiilse alumiiniumoksiidi, mida thistatakse -Al2O3, kristalliseerub alumiiniumoksiid ka mitmete metastabiilsete struktuuridena. Need nn leminekufaasid on snteesitavad vaid nanokristallidena ja psivad vaid teatud temperatuurideni le 1100 C juures on vimalik vaid -faas. Erinevate kristallstruktuuride rohkus sama keemilise koostise korral ja nanomtmelisus teevad alumiiniumoksiidi leminekufaasid atraktiivseks uurimisobjektiks ja ka oluliseks rakenduslikuks materjaliks. Antud t eesmrgiks oli alumiiniumoksiidi leminekufaaside ja stabiilse -faasi elektroonsete ja optiliste omaduste vrdlev uurimine ning vimalike kristalliidi suurusest tingitud efektide selgitamine. Peamiste uurimismeetoditena kasutati ts mitmeid luminestsents-spektroskoopilisi tehnikaid, mis on paljudel juhtudel ainete elektronstruktuuri uurimises asendamatud. Ts nidati kuidas muutused kristallstruktuuris mjutavad erinevate alumiiniumoksiidi faaside elektronstruktuuri ning omaergastustega seotud kiirguslikke protsesse. Leiti samuti, et hoolimata erinevustest kristallstruktuurides on punktdefektide luminestsentsomadused erinevates alumiiniumoksiidi faasides sarnased. Cr3+ lisanditsentri kiirguse uurimine nitas, et seda on vimalik kasutada erinevate alumiiniumoksiidi faaside tuvastamiseks. Demonstreeriti ka luminestsentsmeetodite vimekust tpsustada rakenduslikult oluliste oksiid-dispersiooniga tugevdatud teraste koostist.; 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 -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 -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 -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 differentAdvisors/Committee Members: Kirm, Marco, juhendaja (advisor), Feldbach, Eduard, juhendaja (advisor).