Diffusion Of Oxygen Molecules Research Articles

Transport of oxygen in silicate glasses

Oxygen can be transported through silicate glasses and melts by water, oxygen and silicon monoxide molecules. These molecules dissolve and diffuse in the open glass structure, and oxygen atoms in them can exchange with the oxygen atoms in the silicate silicon–oxygen network. A large variety of measurements of water diffusion in silicates can be understood with the diffusion-reaction and exchange mechanisms of dissolved water molecules. The permeation measurements by Norton of oxygen through silica glass demonstrate the dissolution and diffusion of oxygen molecules. However, measurements of oxygen diffusion with 18O isotope have given widely different results in different studies. The main reason for these differences is the influence of trace amounts of residual water in the oxygen gas and in the glasses. Thus these isotopic exchange measurements of oxygen diffusion in glasses can be understood and interpreted only when the amount of residual water is measured and controlled. Surface exchange between oxygen atoms in the gas phase and the glass is fast; the reason for a lower concentration ratio of oxygen isotope in the glass surface than in the gas phase is not a slow surface reaction but a slow internal exchange between oxygen atoms in dissolved molecules and network oxygen atoms. Diffusion of oxygen in silicates at temperatures above about 1300°C with high activation energy results from diffusion of dissolved SiO molecules, and is related to viscous flow.

Electron transfer-induced dynamics of oxygen molecules on the TiO2(110) surface.

Diffusion of oxygen molecules on transition metal oxide surfaces plays a vital role for the understanding of catalysis and photocatalysis on these materials. By means of time-resolved scanning tunneling microscopy, we provide evidence for a charge transfer-induced diffusion mechanism for O2 molecules adsorbed on a rutile TiO2(110) surface. The O2 hopping rate depended on the number of surface donors (oxygen vacancies), which determines the density of conduction band electrons. These results may have implications for the understanding of oxidation processes on metal oxides in general.

Surface Dissolution and Diffusion of Oxygen Molecules in SiO2 Glass

Surface dissolution and diffusion of oxygen molecules (O 2 ) in SiO 2 glass were studied by their photoluminescence at 1272 nm excited with a titanium sapphire laser oscillating at 765 nm. The dissolution of O 2 from ambient atmosphere at both surfaces was much faster than the following diffusion of O 2 into SiO 2 glass, indicating that the surface dissolution is not the rate-limiting step for the saturation of SiO 2 glass with O 2 . The time- and temperature-dependent concentration changes of O 2 allow to evaluate the diffusion coefficient and the saturation solubility of O 2 in SiO 2 glass.

Oxidation of Boron Carbide-Silicon Carbide Composite at 1073 to 1773 K

The oxidation behavior of B 4 C-(25-60 mol%)SiC composites prepared by arc-melting was investigated in the temperature range of 1073 to 1773K using a thermogravimetric technique. Liquid borosilicate, solid SiO 2 and carbon were identified as oxidation products by X-ray diffraction and Raman spectroscopy. Mass gain was observed during oxidation at 1073 K, while mass loss due to the vaporization of boron oxide in liquid borosilicate was observed at temperatures of 1273 K and higher. In situ Raman spectra of the surface of B 4 C-SiC composites indicated that the silica concentration in the liquid borosilicate increased with increasing SiC content in the composite. Micro-Raman spectroscopy showed that carbon was enriched in the borosilicate layer close to the oxide/composite interface. The parabolic rate constants for B 4 C-50mol%SiC composites at 1073 K were proportional to ambient oxygen partial pressures ranging between 30 and 100kPa. The diffusion of oxygen molecules through the liquid borosilicate layer could be the rate-controlling process. The increase of SiC content in the B 4 C-SiC composites improved the oxidation resistance in both the mass gain and mass loss regions.

Oxidation Kinetics and Mechanical Property of Stoichiometric SiC Fibers (Hi-Nicalon-S).

The oxidation behavior of stoichiometric SiC fiber (Hi-Nicalon-S) was investigated and compared with those of Nicalon and Hi-Nicalon fibers. Hi-Nicalon-S fiber was oxidized in Ar-O2 gas mixtures with 25-100% O2 at 1273-1773K. The oxidation rates were measured with thermobalance experiments. The thickening in SiO2 film produced a large decrease in the strength of the as-oxidized fiber. The SiO2 film-removed fiber (unoxidized core) retained high strength independent of the SiO2 film thickness. The oxidation rate obeyed the diffusion-controlled contracting-disc formula. The rate constant was proportional to the oxygen partial pressure of Ar-O2 gas mixture. The activation energy was 79kJ/mol. The oxidation rate was considered to be controlled by the diffusion of oxygen molecule through the micropores in SiO2 film. Compared to Nicalon and Hi-Nicalon, Hi-Nicalon-S is resistant to oxidation and has good strength in the oxidized state.

Dynamics of Silicon Oxidation

ABSTRACTThe rate-determining process in dry oxidation of silicon for thicker oxides (say above 10 nm) is probably the interstitial diffusion of oxygen molecules. For thinner oxides (a few nm), this simple picture is inadequate. The Deal-Grove model of oxidation kinetics fails. Oxide usually grows by an essentially layer-by-layer process, with growth at terraces, not steps, and perhaps oscillatory roughening. Isotope experiments show that interstitial and network oxygens exchange close to the Si/oxide interface and close to the oxide/gas interface. These results imply limiting mechanisms other than diffusion, probably involving charged oxygen species. We have made Density Functional (DFT) Generalised Gradient Approximation (GGA) calculations for a range of neutral and charged oxygen species to assess relative stabilities, diffusion mechanisms, and propensity for isotope exchange. These results identify acceptable mechanisms. We have also used Monte-Carlo methods to examine the consequences of the image interaction bias, and also the effects of the charge transfer processes (like tunnelling) by which charged species form. We comment on implications for oxide quality, especially on the relationship between growth processes and the charge and energy localisation components of breakdown.

Oxygen Regeneration of Discharged Manganese Dioxide Electrode: II . General Phenomena Observed on Electrodeposited Layer Electrodes and Membrane Electrodes

The diffusion of oxygen molecules in both external and interstitial electrolyte solutions and the low solubility of this gas in the concentrated solutions are the processes that limit the kinetics of oxygen regeneration of discharged electrode. Consequently, to increase the kinetics of regeneration it is necessary: (i) to shorten the length of oxygen diffusion pathway. In practice, this involves placing the oxygen source near the surface of the discharged manganese dioxide particles. The most favorable arrangement is surrounding the particles with a thin electrolyte film allowing quick oxygen dissolution up to saturation; (ii) to increase the interface between particles and electrolyte so as to get the largest area where oxidation reactions by oxygen can occur, (iii) to decrease the diffusion path of the oxygen in the interstitial and external electrolyte. To improve the oxygen regeneration, it is better to optimize the electrode device rather than to look for new materials having improved oxygen reoxidation constants. Particularly, the most important improvements would be the formation of a large area thin electrode device. With a membrane electrode device the oxygen‐regeneration capacity reaches 2.9 e/Mn, i.e., an increase of about 450%, compared to the case without regeneration. By comparison a thick electrode device shows an increase of 250%.

E.s.r. studies on oxidation processes in irradiated polyethylene: 2. Diffusion of oxygen into crystalline regions

The oxidation process in irradiated polyethylene was investigated by means of the e.s.r. method, and diffusion of oxygen molecules into the crystalline region of polyethylene was studied in detail. Computer simulation was carried out in order to determine various parameters of the process including diffusion constant. In the course of the simulation, the crystallite was assumed to have a plate-shaped form and the diffusion constant was considered to be larger at the region near the surface of the crystallite, D f , and to be smaller at the inner region of the crystallite, D s . Making these assumptions in the simulation gave a much better result than the assumption based on a sphere-shaped crystallite. D f D s was found to be about 2 for thicker crystallite and it was 4 ∼ 5 for thinner crystallite. Diffusion constants at various temperatures were determined, and the order of magnitude of diffusion constants in the crystalline region was found to be 10 −16 cm 2/sec at about 320K both for thicker and thinner crystallites. The activation energy of diffusion of oxygen in the crystallites was found to be 32 kcal/mol.

Formation of carbonaceous deposits upon contact of petroleum products with heated metal surface

1. The formation of deposits on a heated metal surface is a consequence of the oxidation of hydrocarbon molecules adsorbed on the surface. Anything that increases the adsorptivity of hydrocarbons on the metal surface (high molecular weights, straight-chain structure, the presence of polar impurities) will also increase the intensity of deposit formation. 2. When the surface temperature is increased above a certain limit (Tm), the average residence time of the adsorbed molecules on the surface becomes less than the average time required for diffusion of oxygen molecules from the bulk product to the sorbed molecules. Simultaneously, a decrease is noted in the adsorption activity of the deposits formed on the surface As a result of the action of these factors, the intensity of deposit formation shows an extremal variation with increasing initial surface temperature.

STUDIES ON THE PHOTODEGRADATION OF CELLULOSE

Reaction mechanisms of photodegradation of cellulose have been proposed under consideration of the experimental facts obtained in report (1) to (VI). The degradation mechanisms are different according to wavelength of light used in irradiation experiments, i.e., (1) on irradiation by λ 1850A, the photochemically primary dissociation occurs in cellulose by its strong absorption of light of this wavelength, the reaction being independent of the presence of oxygen. At the comparatively early period of exposure, direct scission of glucosidic linkage predominates but on prolonged exposure, the formation of 2, 3- dialdehyde by splitting off H-atoms from secondary carbon atoms of the molecule occurs, the latter being easily degradated by alkali medium such as cuam solution. (2) On the other hand, coexistence of oxygen is indispensable to the photodegradation by λ 2537 A irradiation. In this case, absorption matter of λ 2537A is oxygen molecule (Herzberg band), because cellulose is transparent against visible and ultraviolet light having a wavelength longer than λ 2000 A. The main initial step of this reaction is as follows: the excited oxygen molecule thus formed abstracts a hydrogen atom from 1-carbon atom of pyranose ring to form cellulose radical and this reacts with oxygen and other cellulose molecule to produce cellulose peroxide as can be seen in the case of autoxidation of ordinary hydrocarbons. This peroxide is then decomposed photochemically and at the same time, the chain is broken into two chain fragments, accompanying the formation of δ-lactone as the end group of one fragment. The end group of the other fragment has the same configuration as ordinary cellulose molecule. The lactone end group is further hydrolyzed photochemically under the trace of water to form hydroxy-carboxilic acid end group.The quantum yields of each wavelength are calculated and the following values are obtained: γ (1850A)=1×10-2, γ(2537A)=0.5×10-2. Both values are far smaller than unity. The cause of these small values may be attributed mainly to the cristalinity of cellulose fine structure, i.e., in the case of λ 1850A irradiation, recombination of two dissociated chain fragments occurs in crystal region according to Franck-Rabinowitch theory, while in the case of λ 2537A irradiation, diffusion of oxygen molecule, necessary for photoxidation, into this region are difficult and in consequence, photoxidative degradation is inhibited. In both cases, role of crystal region reduces the quantum yield.

STUDIES ON THE PHOTODEGRADATION OF CELLULOSE

Reaction mechanisms of photodegradation of cellulose have been proposed under consideration of the experimental facts obtained in report (1) to (VI). The degradation mechanisms are different according to wavelength of light used in irradiation experiments, i.e., (1) on irradiation by λ 1850A, the photochemically primary dissociation occurs in cellulose by its strong absorption of light of this wavelength, the reaction being independent of the presence of oxygen. At the comparatively early period of exposure, direct scission of glucosidic linkage predominates but on prolonged exposure, the formation of 2, 3- dialdehyde by splitting off H-atoms from secondary carbon atoms of the molecule occurs, the latter being easily degradated by alkali medium such as cuam solution. (2) On the other hand, coexistence of oxygen is indispensable to the photodegradation by λ 2537 A irradiation. In this case, absorption matter of λ 2537A is oxygen molecule (Herzberg band), because cellulose is transparent against visible and ultraviolet light having a wavelength longer than λ 2000 A. The main initial step of this reaction is as follows: the excited oxygen molecule thus formed abstracts a hydrogen atom from 1-carbon atom of pyranose ring to form cellulose radical and this reacts with oxygen and other cellulose molecule to produce cellulose peroxide as can be seen in the case of autoxidation of ordinary hydrocarbons. This peroxide is then decomposed photochemically and at the same time, the chain is broken into two chain fragments, accompanying the formation of δ-lactone as the end group of one fragment. The end group of the other fragment has the same configuration as ordinary cellulose molecule. The lactone end group is further hydrolyzed photochemically under the trace of water to form hydroxy-carboxilic acid end group.The quantum yields of each wavelength are calculated and the following values are obtained: γ (1850A)=1×10-2, γ(2537A)=0.5×10-2. Both values are far smaller than unity. The cause of these small values may be attributed mainly to the cristalinity of cellulose fine structure, i.e., in the case of λ 1850A irradiation, recombination of two dissociated chain fragments occurs in crystal region according to Franck-Rabinowitch theory, while in the case of λ 2537A irradiation, diffusion of oxygen molecule, necessary for photoxidation, into this region are difficult and in consequence, photoxidative degradation is inhibited. In both cases, role of crystal region reduces the quantum yield.