Browsing by Author "Bakradze, Georgijs"

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Initial oxidation of zirconium: oxide-film growth kinetics and mechanisms
(2011) Bakradze, Georgijs; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
The present thesis addresses the growth kinetics, chemical constitution, morphology and atomic transport mechanisms of zirconium-oxide films, as grown by the dry, thermal oxidation of single-crystalline Zr surfaces at low oxidation temperatures. To this end, bare (i.e. without a native oxide) well-defined single-crystalline Zr(001) and Zr(100) surfaces were exposed to oxygen gas in the temperature range of T = 300-450 K. The current study, for the first time, presents a direct comparison of the initial oxidation of single-crystalline Zr surfaces with basal and prism orientations. The relationships between the oxidation kinetics, the developing microstructure and the crystallographic orientation of the parent metal substrate have been established by application of various (surface-)analytical techniques. On the basis of the thus-obtained results, a Zr oxidation mechanism in the low-temperature regime (< 500 K) has been proposed. As evidenced by ellipsometry, after a short initial stage of fast oxide-film growth, a near-limiting thickness of the oxide film is attained at T < 375 K on both surfaces. Distinct differences in the oxidation kinetics for the two Zr substrate orientations become apparent at T > 375 K: the Zr(100) plane oxidizes more readily than the more densely-packed Zr(001) plane under the same experimental conditions. At T > 375 K, the oxidation rate of Zr becomes governed by thermally-activated dissolution and diffusion of oxygen into the Zr substrate. The changes in the local chemical states of Zr and O in the thin zirconium-oxide films have been investigated with increasing oxidation temperature. To this end, the oxide-film valence-bands (VB) and the Auger-parameters of Zr and O in the grown oxide films were resolved from measured XPS spectra of the oxidized Zr surfaces in the oxidation-temperature range of T = 300-450 K. The changes in the shape of the oxide-film VB spectra and the local chemical states of Zr and O in the oxide films evidence that the oxide films grown at T ≤ 400 K are predominantly amorphous, whereas a tetragonal ZrO2-like phase develops at T > 400 K. The formation of the tetragonal zirconia modification in the oxide films developing at 450 K is supported by HR-TEM analysis. The exposure of the bare Zr surfaces to pure oxygen gas at low leads to the initial, very fast formation of a dense arrangement of small oxide nuclei clusters, which completely cover the bare Zr surface after 300 s of exposure. With increasing temperature the mobility of adsorbed O species and/or Zr species on the oxidizing surface and in the developing oxide becomes promoted, thereby promoting the restructuring/reorientation of the oxide clusters into bigger agglomerates, e.g. with increasing oxidation time at constant temperature, as driven by the Gibbs-Thomson effect. In this thesis for the first time, two-stage tracer oxidation experiments have been applied to study the atomic transport mechanisms in thin (< 10 nm) oxide films, as formed during the initial stages of the low-temperature oxidation of the bare, single-crystalline Zr(001) and Zr(100) surfaces at 450 K. The observed differences in shape of the measured tracer profiles for different stages of oxidation indicate a change in the oxide-film growth mechanism during oxidation: i.e. a change of the predominant transport mechanism from inward oxygen transport by a combination of lattice and short-circuit mechanisms during the initial oxidation stage to inward oxygen transport by only the lattice mechanism during later oxidation stages. A low-temperature oxidation mechanism for zirconium at T = 450 K was proposed. Oxygen transport through the developing oxide film requires coupled fluxes of inwardly migrating O anions and outwardly migrating O vacancies, as supplied by the slow continuous O dissolution into Zr. The resulting Zr(O) solid-solution phases formed at the metal/oxide interface are evidenced by an interfacial suboxide layer in the in-situ AR-XPS and ellipsometry analysis. O vacancies, as generated in the interfacial suboixde layer by the slow, but continuous, dissolution of O into the Zr substrate, diffuse outwardly through the O sublattice of the crystalline zirconia overlayer towards the oxide/gas interface to be filled by chemisorbed O surface species at the gas/oxide interface. The thus-established outward flux of O vacancies is accompanied by a net inward lattice flux of oxygen anions. At the early oxidation stage, oxygen is transported inwardly via both the lattice and short-circuit transport mechanism. At later oxidation stages, the contribution of O short-circuit transport becomes negligible, as attributed to a reduction of grain-boundary (GB) density in the oxide-film in association with an equilibration of the GB structure, in possible combination with the accumulation of space charge in the vicinity of the GB in the oxide-film. The overall oxide-film growth rate at 450 K is limited by the O dissolution rate (i.e. supply of oxygen vacancies into the growing oxide film) at the suboxide/metal interface.