Diffusion Of Oxygen Atoms Research Articles

Determination of the Rate Constants of Oxygen Reduction in High‐Temperature Air Electrodes on Solid Oxide Electrolytes

The rate constants of electrochemical oxygen reduction by platinum electrodes on various solid oxide electrolytes have been determined from transient depolarization curves and steady‐state current‐potential curves free of ohmic drop. The rate equations used for the determination were derived on the basis of a mechanism involving dissociation of adsorbed oxygen into atomic species and subsequent diffusion of atomic oxygen through the electrolyte‐electrode interface. The rate constants were determined by comparison of the theoretical curves with the experimental ones. This analytical method has been applied to the air electrodes of doped and doped systems. The latter electrode was found experimentally to be much less polarized than the former. The smaller polarization of the latter is due to a greater reaction area of the electrode and not due to the higher conductivity of the ceria electrolyte used. Addition of ceria to the electrode of the system resulted in a marked decrease in polarization.

Ellipsometric studies of the diffusion of automic oxygen through silicon dioxide thin films

The application of variable angle spectroscopic ellipsometry to the study of diffusion of atomic oxygen through SiO2 is presented. Data are found to be well fit to the universal growth parabola, and the diffusion coefficient is estimated to be 0.00001 sq cm/min.

Theory of internal friction in the high-Tc superconductor YBa2Cu3O7-δ

The internal friction in the high-Tc superconductor YBa2Cu3O7- delta arising from the diffusion of oxygen atoms between the O(1) and O(5) sites under the influence of a periodic mechanical strain is studied microscopically in the linear approximation of the relaxation process. It is analysed for various specific cases in both the tetragonal and the orthorhombic phases as well as in the case of polycrystalline samples.

High-temperature oxidation of titanium-hydride powders

The oxidation kinetics of titanium-hydride powders were studied in the temperature range of 298–1378 K in air at atmospheric pressure. DTA, DSC, X-ray analysis, and scanning electron microscopy were used. Oxidation was found to take place by TiHxOy oxyhydride phases formation. The oxidation reaction rate at temperatures above 870 K was limited by diffusion of oxygen atoms through a rutile scale formed on the surface. The activation energy and preexponential values of the Arrhenius equation for different interaction stages as well as transformation enthalpies were calculated.

Theory of Internal Fraction in the High YBa2Cu3O7-δ

The internal fraction in the high Tc superconductor YBa2Cu3O7-δ arising from diffusion of oxygen atoms between the O(1) and O(5) sites under the influence of a periodic mechanical strain is studied microscopically in the linear approximation of relaxation process. It is analysed for various specific cases both in the tetragonal and the orthorhombic phases as well as in the case of polycrystalline samples.

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO 2 film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO 2 film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.

Kinetics of oxygen absorption by .alpha.-zirconium

The rate of oxygen absorption by polycrystalline {alpha}-zirconium has been measured at oxygen pressures of 6.7 {times} 10{sup {minus}5} and 1.3 {times} 10{sup {minus}4} Pa over the temperature range 973-1098 K by using samples prepared under ultrahigh-vacuum conditions, and the mechanism for the early stage of absorption is discussed. The absorption rate is explained by using a model in which the absorption process comprises three successive steps: the dissociative adsorption of oxygen molecules on the zironium surface with an interaction between oxygen adatoms, the transfer of adatoms at the surface sites to the outermost bulk sites, and the diffusion of oxygen atoms into the bulk. The absorption rate is found to be limited by adsorption under the conditions studied. Kinetic parameters in the model have been evaluated. The activation energy for adsorption is (7.4 {plus minus} 0.6) {times} 10{sup {minus}20} J per O atom, and the energy of the interaction, which is attractive, is {minus}4 {times} 10{sup {minus}21} J per O-O pair. The adsorption site has a potential energy lower by (2.2 {plus minus} 0.1) {times}10{sup {minus}19} J per O atom than the site in the bulk.

Investigation of the SiO2/Si interface. II. Oxidation of an HF-cleaned Si(100) surface using photoemission spectroscopy with synchrotron radiation

The oxidation of a Si(100) surface cleaned using a HF solution and exposed to air, and the composition of the SiO2/Si interface are studied in situ by high resolution (ΔE<0.3 eV) photoemission spectroscopy, using synchrotron radiation. The results are compared with those of a Si(100) surface cleaned by repeated Ar ion sputtering and annealing. The presence of SiC and SiH bonding states at the SiO2/Si interface for the HF-cleaned Si restricts oxidation—the diffusion of oxygen atoms into the Si substrate. In the results, the proportions of Si2+ and Si3+ intermediary states at the interface increase and the interface broadens (0.85–1.0 nm), compared with the oxidation of the clean Si(100) surface.

X-ray microanalysis and high-resolution transmission electron microscopy of the reduced titanium-niobium oxides

Phase relationships in TiNb 2O 7 and Ti 2Nb 10O 29 reductions at 1400°C were investigated by means of X-ray microanalytical electron microscopy and high-resolution transmission electron microscopy (TEM). Compositions of phases present in equilibrium were obtained by applying thin-crystal approximation by which Nb Ti ratios in different phases were determined; their oxygen content was inferred from structural considerations. In this manner, phase relationships in that portion of the TiO 2NbO 2NbO 2.5 equilibrium diagram with 2.417 ≥ x (in MeO x ) ≥ 2 were defined. Data obtained, in combination with high-resolution electron microscopy observations, confirmed that the reduction reaction, in part, is a heterogeneous process controlled by outward diffusion of both metal and oxygen atoms. Recombination of the diffused particles leads to the formation of separate crystals. The original block structure phase undergoes transformation in a quasihomogeneous manner either to an isomorphous phase in the binary NbO system or to a structurally related lower composition oxide. A new superstructure Me 25O 60(Ti 7.16Nb 42.84O 120) has been detected as an intermediate metastable phase, generated in the reduction of TiNb 2O 7 to stable Me 12O 29(Ti 1.53Nb 10.47O 29) and MeO 2(Ti 0.52Nb 0.48O 2) phases. Consideration of phase relationships among Me 25O 60, Me 12O 29, and MeO 2 suggests a chemical mechanism for the reaction concerned. The Me 25O 60 superstructure has a monoclinic symmetry with cell parameters a = 19.0 Å, b = 3.8 Å, c = 26.6 Å, α = 90°, β = 90°, γ = 78.5°, as determined from the structure image calculations.

Interaction of oxygen with a clean Ir(111) surface

HREELS and XPS methods were used for the investigation of the interaction of oxygen with Ir(111). At temperatures of 180 and 300 K oxygen is adsorbed dissociatively, 0.52±0.08 oxygen atoms per one Ir atom corresponding to a saturated coverage. Adsorbed oxygen atoms are characterized by a v (Ir-O) mode at 550 cm −1 . With rising temperature (above 300 K) diffusion of oxygen atoms into the subsurface Ir layer begins. At 600 K and an oxygen pressure of 1×10 −5 Pa, formation of a surface oxide begins which characterizes the initial stage of Ir(111) oxidation. A v (Ir-O) peak at about 800 cm −1 in the HREEL spectrum corresponds to the surface oxide. More intensive oxidation of Ir(111) proceeds at 850 K and an oxygen pressure of 0.1 MPa, the surface oxide formed differing in composition from IrO 2 .

High temperature (900 – 1300°C) mechanical behaviour of dendritic web grown silicon ribbons: - strain rate and temperature dependence of the yield stress

The study indicates that the deformation characteristics of silicon ribbons (dendritic web grown) at high temperature do not represent a simple extension of the low temperature behaviour. Below 1200°C, where the deformation is mostly thermally activated, normal trends are observed. Above 1200°C, a strengthening effect is observed. This phenomena is a strong function of the strain rate, being the most significant for the slowest strain rate. The increase in the strength levels can be attributed to the diffusion of oxygen atoms (excess interstitial oxygen) to the dislocation cores which effectively locks the dislocations.

Surface diffusion of oxygen atoms individually observed by STM

We report on the first dynamical studies with scanning tunneling microscopy (STM). Single excess oxygen atoms migrating on the oxygen-induced (2 × 1) and c(2 × 2) reconstructions on Ni(110) and Ni(100), respectively, are detected space and time resolved. In principle, for varying temperature, the data reveal the local diffusion coefficient, the activation energy and the density of adatoms on terraces and at steps. For the Ni(110) (2 × 1), even the adsorption sites are identified.

Surface diffusion of oxygen atoms individually observed by STM

We report on the first dynamical studies with scanning tunneling microscopy (STM). Single excess oxygen atoms migrating on the oxygen-induced (2×1) and c(2×2) reconstructions on Ni(110) and Ni(100), respectively, are detected space and time resolved. In principle, for varying temperature, the data reveal the local diffusion coefficient, the activation energy and the density of adatoms on terraces and at steps. For the Ni(110) (2×1), even the adsorption sites are identified.

Chlorination of silver dosed with potassium and barium in presence of oxygen: An X-ray photoelectron spectroscopy study

XPS studies show that the presence of chemisorbed chlorine stabilizes and also enhances molecular dioxygen species on Ag surfaces dosed with either K or Ba. The surface atomic oxygen is found to become depleted on chlorination. The variation in the nature of surface species with respect to temperature shows chlorine-induced diffusion of atomic oxygen into the subsurface region at 300 K. For coverages of potassium up to 8 × 10 14 atoms/cm 2, preferential chloridation of Ag occurs while at higher potassium coverages, KCl formation is distinctly observed on the surface. In the case of barium, two types of adsorbed chlorine species, Cl(α) and Cl(β), associated with Ag and Ba, respectively, are clearly seen even at low barium coverages. This is believed to be due to the higher valence occupation of barium compared to potassium. The Cl(α) species associated with Ag is found to occupy a preferred site on both K- and Ba-dosed surfaces, involving chemisorptive replacement of O(α) to the subsurface region.

Kinetics and mechanism of the solid-state high-temperature transformation of andalusite (Al 2SiO 5) into [formula omitted] (3Al 2O 3·2SiO 2) and silica (SiO 2)

The kinetics of the high-temperature transformation of andalusite (<40 μm) were investigated by means of X-ray techniques. The transformation interval was determined to lie between about 1250 and 1500°C. The best fitting of the kinetic data was achieved with an exponential law of the type: 1 − α = exp (−kt + ct/(t + 1)). The kinetic curves are associated with a topotactic transformation mode with preservation of the aluminium-oxygen octahedral chains and diffusion of aluminium, silicon and oxygen atoms. Below 1380°C, mullite (+SiO2) formation is essentially restricted to an initial rapid period of the transformation, because nucleation and product growth occur only at energetically favored lattice sites (e.g. grain boundaries). Above 1380°C, mullite and SiO2 nucleation and growth may take place throughout the volume of the low-temperature phase (andalusite) over the whole range investigated.

Impedance spectroscopy of the Pt/Yttria doped ceria interface

Complex impedance measurements have been made on sputtered Pt/Yttria doped ceria interfaces. A single arc substantially depressed below the real axis was observed for the electrode reaction. The activation energy for this process was independent of oxygen partial pressure and electrode morphology (150 ± 20 kJ mol−1) and is consistent with the diffusion of atomic oxygen on the surface of the Pt metal.

Dynamics of the disturbed ionosphere

The ionospheric storm process at F layer heights is reviewed and an explanation in terms of wind-induced diffusion of atomic oxygen is given.

Oxygen Transfer on Substituted ZrO2, Bi2 O 3, and CeO2 Electrolytes with Platinum Electrodes: II . A‐C Impedance Study

An equivalent electrical circuit that describes the electrode processes on different electrolytes, using porous Pt electrodes, is given. Diffusional processes are important and have to be presented by Warburg components in the circuit. The overall electrode process is rate limited by diffusion of atomic oxygen on the electrode surface for stabilized zirconia and substituted ceria (low ). On stabilized bismuth sesquioxide diffusion of atomic oxygen on the electrolyte surface is rate limiting at high while at low another process, probably diffusion of electronic species in the electrolyte, is dominant. One of these processes plays a role too on substituted ceria at high , where a charge transfer process is dominant. These results are consistent with the mechanisms developed in part I of this paper.

Theory of ignition of metallic particles

This article proposes a model of the ignition of metallic particles in which it is assumed that the metal particles enter into the oxidized medium in a reduced state and the particles are free of the oxide film on the surface. The influence of temperature on the rate of the oxidation reaction in the presence of the oxide film on the metal surface is determined primarily by the dependence of the coefficients of diffusion of metal and oxygen atoms in the solid oxide. The high activity of metals in reactions with oxidants can be explained by the low activation energy of the process in the absence of an oxide film on the surface. For low activation energy, as well as for high temperatures (of the order of the melting temperature of the oxide), at which the oxidation process proceeds, the temperature dependence of the oxidation reaction rate is weak, since the exponent in the exponential of the Arrhenius dependence is small. It is concluded that the proposed theoretical model of the process of ignition of deoxidized single metal particles describes satisfactorily the basic quantitative characteristics of ignition of aluminium particles and, in particular, the dependence of the ignition temperature ofmore » aluminium particles on the particle size and oxidant concentration in the medium.« less