Investigation of nano-size α- and transition alumina by means of VUV and cathodoluminescence spectroscopy
Abstract
Alumiiniumoksiidi leidub looduses korundina ehk suure kõvadusega mineraalina. Korund kui väärtuslik vääriskivi on tuntud ka punase rubiini ja sinise safiirina. Sellised värvused tekivad lisades alumiiniumoksiidile kroomi, raua või titaani ioone. Peale termodünaamiliselt stabiilse alumiiniumoksiidi, mida tähistatakse α-Al2O3, kristalliseerub alumiiniumoksiid ka mitmete metastabiilsete struktuuridena. Need nn üleminekufaasid on sünteesitavad vaid nanokristallidena ja püsivad vaid teatud temperatuurideni – üle 1100 °C juures on võimalik vaid α-faas. Erinevate kristallstruktuuride rohkus sama keemilise koostise korral ja nanomõõtmelisus teevad alumiiniumoksiidi üleminekufaasid atraktiivseks uurimisobjektiks ja ka oluliseks rakenduslikuks materjaliks.
Antud töö eesmärgiks oli alumiiniumoksiidi üleminekufaaside ja stabiilse α-faasi elektroonsete ja optiliste omaduste võrdlev uurimine ning võimalike kristalliidi suurusest tingitud efektide selgitamine. Peamiste uurimismeetoditena kasutati töös mitmeid luminestsents-spektroskoopilisi tehnikaid, mis on paljudel juhtudel ainete elektronstruktuuri uurimises asendamatud. Töös näidati kuidas muutused kristallstruktuuris mõjutavad 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 näitas, et seda on võimalik kasutada erinevate alumiiniumoksiidi faaside tuvastamiseks. Demonstreeriti ka luminestsentsmeetodite võimekust täpsustada 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 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.
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.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone
Keywords
alumiiniumoksiid, nanokristallid, optilised omadused, elektrilised omadused, luminestsentsuurimismeetodid, spektroskoopia, aluminium oxide, nanocrystals, optical properties, electrical properties, luminescence research methods, spectroscopy