Browsing by Author "Lyapin, Andrey"

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The initial stages of the oxidation of zirconium
(2005) Lyapin, Andrey; Mittemeijer, Eric J. (Prof. Dr. Ir.)
The initial, thermal oxidation of bare polycrystalline Zr substrates has been investigated using angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and in-situ, spectroscopic ellipsometry (SE). Oxidation experiments were performed by exposure of a bare Zr substrate for various times in pure O2 gas at various temperatures (T) and partial oxygen pressures (pO2) within the ranges of 300 – 773 K and 1.3×10-7 – 1.3×10-4 Pa, respectively. Detailed analysis of measured Zr 3d XPS spectra of the oxidized Zr substrates showed the presence of three different chemical species of Zr in the oxide film: one oxidic and two weaker suboxidic components (designated as ZrO2, and lower and higher binding energy interface-oxide component, respectively). As evidenced from the construction of a so-called relative depth plot, the developing oxide films are constituted of stoichiometric ZrO2 (ZrO2 component) in combination with gradients of Zr enrichment and O deficiency in the region of the oxide-film adjacent to the metal/oxide interface. The degree of Zr enrichment and O deficiency decrease from the metal/oxide interface towards the oxide surface. The effective-depth distributions and individual sublayer thicknesses of the non-stoichiometric and stoichiometric oxide species within the developing oxide film, as established with ellipsometry, are in good agreement with the corresponding results as determined independently with AR-XPS. Two different oxide-film growth regimes have been recognised: a short, initial regime of very fast oxide-film growth, which is followed by a second stage of much slower, but continued oxide-film growth for oxidation temperatures higher than 423 K. As evidenced by both AR-XPS and ellipsometry, initial oxide formation on the bare Zr substrate starts with the nucleation and rapid growth of a Zr-enriched and O-deficient Zr-oxide with an overall O/Zr-ratio of the oxide film lower than 2 (the initial O/Zr-ratio decreases with increasing temperature). At the onset of the second, slow oxidation stage, the growth rate of the non-stoichiometric oxide sublayer levels off, attaining a constant ‘limiting’ thickness that increases with increasing temperature. Subsequent, continued (for 423 K < T < 523 K) growth occurs by the approximately linear overgrowth of stoichiometric ZrO2 (the constant growth rate increases with increasing temperature). At T > 573 K, the formation and continued growth of stoichiometric ZrO2 competes with the dissolution and subsequent inward diffusion of oxygen into the Zr substrate that is accompanied by a partial decomposition of the developing oxide film. The enhanced dissolution of O into the Zr substrate for T > 573 K is due to the associated increase of the maximum amount of oxygen that can be dissolved in α-Zr in combination with a significant increase of the oxygen dissolution rate. The mechanism of initial oxide-film growth on the bare Zr substrate was investigated by modelling the kinetics of oxide-film growth as a function of the oxidation time, temperature and pO2. To this end, a coupled currents description for the fluxes of Zr cations and electrons (by both tunnelling and thermionic emission) in a homogeneous surface-charge field comprising the growing oxide film were considered, while adopting the oxide-oxygen and metal-oxide work functions (relative to the conduction band in the oxide), as well as the rate-determining energy barrier for cation motion, as fit parameters. It follows that, for all pO2 and temperatures studied, the oxide-film growth is limited by the electric-field enhanced migration of Zr cations into or through the developing oxide film. For oxide film thicknesses  2 nm, electron tunnelling transport occurs at a much faster rate than the intrinsic ionic transport (with large, nearly equal forward and reverse electron tunnelling currents). Above this critical oxide-film thickness (and only for relatively elevated temperatures of T ≥ 523 K), the net electronic current by tunnelling drops to zero, and the much slower electron transport by thermionic emission remains. Then, this electron transport by thermionic emission across the developing oxide film co-determines the growth rate, while the cation transport remains rate-limiting. The increase of the rate-determining activation energy for cation motion, as well as the increase of the metal-oxide work function in combination with a decrease of the oxide-oxygen work function, with increasing temperature are attributed to the gradual transformation of the initially amorphous, overall non-stoichiometric oxide film into a crystalline oxide film mainly constituted of crystalline ZrO2.