Room-temperature growth of oxide layers on aluminum in highly diluted mixtures of oxygen with argon (O2 molar fractions 20 ppm ≤ $$x_{O_2}$$ x O 2 ≤ 500 ppm, partial pressures 2 Pa ≤ $$p_{O_2}$$ p O 2 ≤ 50 Pa) flowing through a dielectric-barrier discharge (DBD) reactor is studied, including oxidation in the pre- and post-discharge regions (PrD, PoD) adjacent to the main DBD. Three different mechanisms of plasma-enhanced oxidation were found to prevail, depending on the location of the sample: (1) In the close PrD region, up to 1 cm upstream from the discharge, accelerated growth of Al2O3 is due to the irradiation of the sample surface by highly energetic (9.8 eV) argon excimer radiation in the presence of O2. (2) In the remote PoD, a few cm downstream from the DBD, oxidation can largely be attributed to oxygen atoms, with number densities typically between 1 and 5 × 1014 cm−3. Here, analysis in terms of Cabrera–Mott (CM) theory results in CM potentials between − 1.5 and − 2.1 V. (3) In the DBD itself both O atoms and VUV photons generally play an important role but, under special conditions, an additional oxidation mode can be identified, characterized by a much larger limiting thickness: While, in general, oxide growth by O atoms and/or VUV photons virtually stops at thicknesses X between 5 and 6 nm, much thicker oxide films can be achieved in the downstream region of the main DBD, with thicknesses growing with the length of the DBD zone. Tentatively, we attribute this observation to negative oxygen ions Om− (1 ≤ m ≤ 3) accumulating in the gas while passing the reactor. Any direct electrical effects of the discharge process on the oxidation can probably be neglected.
Number densities of oxygen atoms, nO, in Ar-O2 mixtures with small initial O2 fractions, $${x}_{{O}_{2}}$$ x O 2 < 1%, flowing through a dielectric-barrier discharge (DBD), are calculated using a plug-flow reactor model, presuming that dissociation and excitation of oxygen species are solely driven by energy-transfer from long-lived excited Ar species, collectively denoted as Ar*. The rate by which Ar* species are generated is calculated from the volume density of power dissipated in the DBD. To obtain extended post-discharge (PD) regions with large nO, experiments were performed with $${x}_{{O}_{2}}$$ x O 2 = 100 ppm. For such low O2 fractions, the time-dependence of nO in the DBD and the early PD can be calculated by a closed equation. Calculations are compared with optical emission spectroscopic (OES) results, utilizing the proportionality of O-atom emission intensity at 777.4 nm to nO. O-atom densities in the PD are made accessible to OES using a tandem setup with a second DBD as sensing discharge. Model testing by experiment is based on the functional dependence of nO on DBD-residence time and PD-delay time, respectively. Wall losses of O atoms in asymmetrical DBD reactors are calculated by an alternative to Chantry’s equation. The agreement between O-atom densities attained at the DBD exit and experimental results is generally good while the speed of rise of nO in the discharge is overestimated, due to the assumption of a constant wall-loss frequency, kW. Compared with literature data, kW is orders of magnitude higher in the DBD and at least one order of magnitude lower in the PD.
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