Increasing O2 Pressure Research Articles

Accelerated oxidation of epoxy thermosets with increased O2 pressure

Polymer oxidation is usually accelerated with temperature, which is therefore applied in nearly every experimental approach dealing with predictive materials aging. Because of this simple approach, we may tend to neglect that the effective concentration of oxygen also acts as a rate multiplier for oxidation. Increasing the oxygen partial pressure in an aging environment accelerates oxidation, and while there is often a near proportional increase initially, the effect of additional oxygen usually transitions to a saturation level at some elevated pressure. This has been theoretically described in the general autoxidation scheme and is well recognized. However, for many materials the exact rate behavior under moderately increased oxygen concentration remains to be established. We therefore review epoxy oxidation and offer a broader overview of its behavior under increased O2 partial pressure. Experimental data are given for a few thermoset materials demonstrating their rate behavior under O2 partial pressure up to 4 atm, meaning approximately 20 times more than under standard atmospheric conditions. Confirmative evidence suggests that epoxy materials will reach saturation oxidation rates only at significantly higher O2 partial pressure. In such a high-pressure regime it is theoretically possible to not only accelerate oxidation, but to transition into a condition where O2 diffusion can be increased without further accelerating the oxidation rate. This can reduce diffusion limited oxidation effects under specific accelerated aging conditions as a combination of temperature and O2 partial pressure.

Atomistic Modeling of Spinel Oxide Particle Shapes and Reshaping under OER Conditions

The surface configurations of the low-index facets of a set of spinel oxides are investigated using DFT+U calculations to derive surface energies and predict equilibrium nanoparticle shapes via the Wulff construction. Two very different conditions are investigated, corresponding to application either in heterogeneous catalysis or in electrocatalysis. First, the bare stoichiometric surfaces of NiFe2O4, CoFe2O4, NiCo2O4, and ZnCo2O4 spinels are studied to model their use as high-temperature oxidation catalysts. Second, focusing attention on the electrochemical oxygen evolution reaction (OER) and on the CoFe2O4 inverse spinel as the most promising OER catalyst, we generate surface configurations by adsorbing OER intermediates and, in an innovative study, we recalculate surface energies taking into account adsorption and environmental conditions, i.e., applied electrode potential and O2 pressure. We predict that under OER operating conditions, (111) facets are dominant in CoFe2O4 nanoparticle shapes, in fair agreement with microscopy measurements. Importantly, in the OER case, we predict a strong dependence of nanoparticle shape upon O2 pressure. Increasing O2 pressure increases the size of the higher-index (111) and (110) facets at the expense of the (001) more catalytically active facet, whereas the opposite occurs at low O2 pressure. These predictions should be experimentally verifiable and help define the optimal OER operative conditions.

Photophysical oxidation of HCHO produces HO2 radicals.

Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O2 to form HO2. Here we show HCHO has an additional HO2 formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO2 at low pressures by cavity ring-down spectroscopy and indirectly detect HO2 at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO2 to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O2. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O2 pressure.

Electrochemical multi-sensors obtained by applying an electric discharge treatment to 3D-printed poly(lactic acid)

Electrochemical sensors for real-time detection of several bioanalytes have been prepared by additive manufacturing, shaping non-conductive poly(lactic acid) (PLA) filaments, and applying a physical treatment to create excited species. The latter process, which consists of the application of power discharge of 100 W during 2 min in a chamber at a low pressure of O2, converts electrochemically inert PLA into an electrochemically responsive material. The electric discharge caused the oxidation of the PLA surface as evidenced by the increment in the quantity of oxygenated species detected by FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS). Indeed, changes in the surface chemical composition became more pronounced with increasing O2 pressure. After demonstrating the performance of the chemically modified material as individual dopamine and glucose sensors, multiplexed detection has been achieved by measuring simultaneously the two voltammetric signals. This has been performed by collecting the signals in two different regions, a naked chemically modified PLA for dopamine detection and a chemically modified PLA region functionalized with Glucose Oxidase. These outcomes led to define a new paradigm for manufacturing electrodes for electrochemical sensors based on 3D printing without using conducting materials at any stage of the process.

Investigation of the effect of oxygen partial pressure on the phase composition of copper oxide nanoparticles by vacuum arc synthesis

Copper oxide nanoparticles were obtained in the plasma of a low-pressure arc discharge. The effect of the partial pressure of oxygen (10-40%) on the physical properties of the deposited nanoparticles has been studied. X-ray diffraction analysis shows that the cubic structure of Cu2O changes to monoclinic CuO with increasing O2 pressure. The results of Raman spectroscopy further confirmed the phase variations of copper-based oxide nanoparticles. X-ray photoelectron spectroscopy confirmed the change in the binding energy in the oxidation state of nanoparticles. The optical band gap of the deposited Cu2O is 2.12 eV, while that of CuO is 1.79-1.82 eV. Keywords: vacuum arc, oxides, nanoparticles, plasma-chemical reactions.

Исследование влияния парциального давления кислорода на фазовый состав наночастиц оксида меди вакуумно-дугового синтеза

Copper oxide nanoparticles were obtained in the plasma of a low-pressure arc discharge. The effect of the partial pressure of oxygen (10-40%) on the physical properties of the deposited nanoparticles has been studied. X-ray diffraction analysis shows that the cubic structure of Cu2O changes to monoclinic CuO with increasing O2 pressure. The results of Raman spectroscopy further confirmed the phase variations of copper-based oxide nanoparticles. X-ray photoelectron spectroscopy confirmed the change in the binding energy in the oxidation state of nanoparticles. The optical band gap of the deposited Cu2O is 2.12 eV, while that of CuO is 1.79-1.82 eV.

Effect of the thickness of single-walled carbon nanotube electrodes on the discharge properties of Li–air batteries

We prepared electrodes of a variety of thicknesses with single-walled carbon nanotubes (SWCNTs) to investigate the effects of electrode thickness on various properties of LABs, including capacity and discharge voltage. The electrode thickness was easily controlled between 29 and 138 μm by adjusting the weight of the SWCNTs dispersed in methanol. From the results of discharge tests under the flow conditions of O2 at atmospheric pressure, it was revealed that the discharge capacity and voltage of the LABs varied depending on the electrode thickness. Notably, in the case of atmospheric pressure, the thinnest 30-μm electrode demonstrated three times higher capacity per unit weight compared with the 125-μm electrode. Moreover, the thinner electrodes also provided top performance with respect to discharge voltage. We also performed discharge tests under increased O2 pressure at 3 atm. In this case, the capacity also increased in relation to decreasing thickness of the electrode, though the trend was slightly different compared with that of the discharge tests carried out at atmospheric pressure. Although thickness is generally an essential parameter for electrochemical devices, it was further revealed here that thickness plays an important role in LABs with respect to the O2 concentration in the electrolyte.

Effects of reoxidation on the dielectric and energy storage properties of Ce-doped (Ba,Sr)TiO3 ceramics prepared by hot-pressed sintering

Fully densified Ba0.6Sr0.34Ce0.04TiO3 ceramics with small grain microstructure (0.3−0.8 μm) and high insulation (∼1014Ω cm) could be successfully obtained by hot-pressed sintering (HPS) coupled with subsequent thermal annealing in oxygen atmosphere. The effects of annealing on the microstructure, dielectric, electric and energy storage properties were investigated with scanning electron microscope, X-ray photoelectron spectroscope, impedance and ferroelectric analyzers. The preferential oxidation of grain boundaries to gains resulted in the formation of electrically inhomogeneous microstructure, which could be much improved by increasing O2 pressure during annealing process. The reduction of Ti4+ in the as-sintered HPS sample could be decreased with increasing the annealing temperature up to 1200 °C in flowing O2 gas, leading to the decrease in dielectric relaxation and corresponding permittivity/loss above room temperature. Maximum energy density of 1.75 J/cm3 could be obtained under the field of 235 kV/cm with the efficiency of 85 %.

The Role of Hyperbaric Oxygen Treatment for COVID-19: A Review.

The recent coronavirus disease 2019 (COVID-19) pandemic produced high and excessive demands for hospitalizations and equipment with depletion of critical care resources. The results of these extreme therapeutic efforts have been sobering. Further, we are months away from a robust vaccination effort, and current therapies provide limited clinical relief. Therefore, several empirical oxygenation support initiatives have been initiated with intermittent hyperbaric oxygen (HBO) therapy to overcome the unrelenting and progressive hypoxemia during maximum ventilator support in intubated patients, despite high FiO2. Overall, few patients have been successfully treated in different locations across the globe. More recently, less severe patients at the edge of impending hypoxemia were exposed to HBO preventing intubation and obtaining the rapid resolution of symptoms. The few case descriptions indicate large variability in protocols and exposure frequency. This summary illustrates the biological mechanisms of action of increased O2 pressure, hoping to clarify more appropriate protocols and more useful application of HBO in COVID-19 treatment.

Simultaneous High Conversion and Selectivity in Olefin Oxidation with Oxygen Through Solid/Liquid/Gas Three‐Phase Interface Design

AbstractIn oxidation reactions with oxygen as oxidant, low O2 concentration on catalyst surface under ambient pressure is a major kinetic limitation for selective oxidation of olefins; while increasing O2 pressure to enhance the reaction rate suffers lower selectivity due to thermodynamic factors. In this study, we demonstrate that simultaneous enhancement of conversion and selectivity can be achieved under ambient pressure by solid/liquid/gas three‐phase interface design. We produced Au nanoparticles supported on graphene aerogel with superaerophilic surface (denoted as Au/GA) as the catalyst for demonstration. The results show that O2 gas shows “bursting” behavior on the surface of Au/GA under water, resulting in rapid adsorbing of O2 in 98 ms and therefore high O2 concentration on the surface of catalyst. Thus, solid/liquid/gas three‐phase interface is formed under this condition, leading to high conversion as well as high selectivity at ambient O2 pressure. Such strategy is expected to find wide applications in many other selective oxidation reactions that traditionally needed high pressure.

Effect of oxygen partial pressure on the tuning of copper oxide thin films by reactive sputtering for solar light driven photocatalysis

Nanostructured copper oxide thin films were deposited on soda lime glass substrates by reactive radio frequency (RF) magnetron sputtering using pure Cu target. The effect of oxygen (O2) partial pressure (5–50%) on physical properties of deposited films were investigated. The XRD analysis indicates the cubic structure of Cu2O changes to monoclinic CuO as a function of increasing O2 pressure. The Raman spectroscopy results further confirmed the phase variations of Cu based oxide films. Morphological analysis showed that the deposited films are highly uniform with nano gravels and agglomerated nano flake like structures for Cu2O and CuO thin films respectively. The surface roughness of the films decreases from 2.05 to 1.07 nm by increasing O2 pressure. X-ray photoelectron spectroscopy confirmed the binding energy variation in the oxidation state of the films. The optical band gap of deposited Cu2O is 2.12 eV, while that of CuO varies in the range of 1.79–1.82 eV. Hall effect measurement revealed that all the films exhibit p-type conductivity. The photocatalytic activity studies of as-deposited films have been carried out with different organic pollutants such as methylene blue, methyl orange and ciprofloxacin under solar irradiation. The Cu2O thin films exhibited higher degradation efficiency than CuO films under identical conditions. The mechanism of photocatalytic activity of copper oxide thin films is explained.

Effect of sputtering pressure on structural and optical properties of silver oxide thin films; Kramers–Kronig method

Silver oxide nano layers were prepared by RF magnetron sputtering on amorphous SiO2 substrates. O2 pressure in chamber was varied from 1 to 4 and 7 mTorr during growth process. The effects of different O2 pressure on structural, morphological and optical properties of the films were investigated by means of X-ray diffraction, scanning electron microscopy, atomic force microscopy and UV–Vis spectroscopy analyses. Optical reflectance measured in the wavelength of 350–950 nm by spectroscopy. Other optical properties and optical band gaps were calculated using Kramers–Kronig relations. The X-ray diffraction measurements showed change in crystalline structure with increasing O2 pressure. Preferred orientation has been changed to another growth orientation at 4 mTorr O2 pressure. The Atomic force microscope images showed increasing in roughness consistently by increasing oxygen pressure. The thickness of the thin films decreases (from 217 to 180 nm) with increasing O2 pressure. Optical results revealed that the highest optical band gap of 3.1 eV and the highest transmittance of ~ 80% were achieved at lower O2 pressure (1 mTorr).

Stoichiometric dependent optical limiting in PLD SiOx thin films

In this letter, optical limiting property of the insufficiently oxidized silicon oxide (SiOx) thin films is reported. Films were deposited by Pulsed Laser Deposition technique using Q-switched Nd: YAG laser (532 nm) onto fused silica substrate at a substrate temperature of 400 °C by varying the O2 pressure in the range of 5×10 -5 to 0.5 mbar. Energy Dispersive X-Ray spectra showed the increase in oxygen content with increasing O2 pressure. Raman spectra of SiOx films depicted the presence of micron sized clusters composed of nanocrystalline Silicon embedded in uniform matrix of oxidized amorphous Silicon. The open Z-scan of the thin films, under cw He-Ne laser irradiation, showed strong reverse saturation absorption (RSA) features and non linear absorption (NLA) coefficient, β, was found to be decreasing from 23.5 cm/W to 1.64 cm/W, with increase in O2 pressure from 5×10 -5 to 10 -1 mbar, respectively. Also, the SiOx films except that with maximum oxygen content showed optical limiting, where limiting threshold increases with increasing transparency controlled by oxygen content. The key feature of the present work is the tunability in linear absorption, nonlinear RSA and optical limiting in the SiOx films which can be used as novel material for optical switching application.

Charge competition with oxygen molecules determines the growth of gold particles on doped CaO films

The influence of gas-phase oxygen on the growth of Au nanoparticles on Mo-doped CaO films has been investigated by means of low temperature scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Whereas at ideal vacuum conditions, only 2D Au islands develop on the oxide surface, the fraction of 3D deposits increases with increasing O2 pressure until they become the dominant species in 106 mbar oxygen. The morphology crossover arises from changes in the interfacial electron flow between Mo donors in the CaO lattice and different ad-species on the oxide surface. In the absence of 02 molecules, the donor electrons are predominately transferred to the Au ad-atoms, which consequently experience enhanced binding to the oxide surface and agglomerate into 2D islands. In an oxygen atmosphere, on the other hand, a substantial fraction of the excess electrons is trapped by adsorbed O2 molecules, while the Au atoms remain neutral and assemble into tall 3D particles that are typical for non-doped oxides. Our experiments demonstrate how the competition for charge between different adsorbates governs the physical and chemical properties of doped oxides, so widely used in heterogeneous catalysis.

Structural, electrical, and optical characterizations of laser deposited nanometric iron oxide films

The structural, electrical, and optical characteristics of nanometric Fe2O3−x films fabricated on ⟨100⟩ Si and SiO2 substrates by ablating an iron target with a KrF excimer laser in low pressure (0.05–1.0 Pa) O2 atmosphere are reported. The thickness of films fabricated with 4000 laser pulses is ∼80 nm for samples deposited at the lowest pressure (0.05 Pa) and decreases gradually to 50 nm at the highest used pressure (1.0 Pa). The film mean composition results close to FeO at lower pressures (0.05–0.10 Pa) and to Fe2O3 at higher pressures (0.5–1.0 Pa). From glazing incidence x-ray diffraction spectra, it was inferred that deposits are poorly crystallized, especially the ones prepared at relatively high oxygen pressures (0.5–1.0 Pa). Scanning electron microscopy inspections show that surfaces are plane and uniform, with very rare (<0.1 μm−2) submicron droplets. From x-ray reflectivity spectra, surface roughness results very low (0.1–0.5 nm, increasing with O2 pressure). The films present a semiconductor behavior, with electrical energy gaps increasing from 0.43 to 0.93 eV with increasing O2 pressure, while the optical energy gap varies from 1.60 eV (0.05 Pa) to 1.74 eV (1.0 Pa). Very high thermoelectromotive force coefficient values were measured (up to 1.65 mV/K). It is also shown that films present promising gas sensing properties.

Ethylene combustion on unsupported and supported Pd: a comparative study

Ethylene combustion is studied at elevated pressures on Pd(100) and compared with supported Pd catalysts. Specific rates of CO2 formation were determined on Pd(100), 1 wt% (dPd = 2.5 nm) and 5 wt% (dPd = 4.2 nm) Pd/SiO2 catalysts as a function of the O2/C2H4 ratio over the temperature range 428–473 K. Reaction rates and apparent activation energies on these catalysts are particle size and reactant ratio dependent. The surface carbon concentration on Pd(100) depends on the ratio of the reactants and decreases with increasing O2 pressure. The combustion kinetics is related to the corresponding surface carbon coverage; the highest reaction rate was achieved for relatively low coverage of surface carbon.

Oxidation of veratryl alcohol by molecular oxygen in aqueous solution catalyzed by cobalt salen-type complexes: the effect of reaction conditions

A series of Schiff-base salen-type cobalt complexes Co(salen) (1) [salen=N,N′-bis(salicylidene)ethylenediamine], Co(α-CH3salen) (2) [α-CH3salen=N,N′-bis(α-methylsalicylidene)ethylenediamine], Co(4-OHsalen) (3) [4-OHsalen=N,N′-bis(4-hydroxosalicylidene)ethylenediamine], Co(sulfosalen) (4) [sulfosalen=N,N′-bis(5-sulfonatosalicylidene)ethylenediamine], Co(acacen) (5) [acacen=N,N′-bis(acetylacetone)ethylenediamine)] and Co(N-Me-salpr) (6) [N-Me-salpr=bis(salicylideniminato-3-propyl)methylamine] were studied as catalysts for dioxygen activation in the oxidation of 3,4-dimethoxy benzylalcohol (veratryl alcohol) in basic aqueous conditions. The oxidation occurs selectively at benzylic position as in all experiments veratraldehyde was the only observed product. The highest catalytic activity was reached with the unsubstituted complex (1), which was used for further studies on the effect of temperature, pressure, time and pH. The effect of pH was noted to be critical and the reaction proceeds up to 72 h if the pH is kept at about 12.5. The need for higher temperatures (over 60 °C) was also confirmed. Reactivity of the catalyst increase linearly with increased O2 pressure, the best turnover numbers (TON=moles of veratryl aldehyde produced per mole of complex) reached in similar conditions were 58 for ambient pressure and 103 for oxygen pressure of 10 bar. The catalytic activity can be further enhanced if the catalyst:substrate ratio is increased. Turnover numbers as high as 330 were detected with catalyst:substrate ratio 1:5950 at ambient pressure.

Structural and morphological properties of evaporated SiOx films

Abstract SiO x films with thickness ranging from 50 nm to several micrometres were evaporated on crystalline Si at difTerent O2 pressures. The stress of these films, evaluated by profilometer measurements, is tensile and increases with increasing O2 pressure. Atomie force microscopy data show a quite smooth surface of the SiO x films, with a rms roughness less than 1 nm for all film deposited at low O2 pressures. The growth in a reactive atmosphere increases the roughness to 1.5 nm and changes strongly the film morphology. Surface morphology also evolves on the thickness, following a dynamie growth model with negligible surface diffusion. The chemical composition and the inner structure were investigated by Raman spectroscopy.

Preparation and properties of vanadium dioxide thermochromic thin films

Deposition of vanadium dioxide and the study of its electrical and optical properties at varying deposition conditions have been presented. The materials have been deposited by reactive r.f. magnetron sputtering technique in Ar, O2 ambient followed by annealing post-treatment. Electrical conductivity measurements indicate that oxygen pressure plays an important role in obtaining VO2 and atP O 2=2.4% stoichiometric VO2 can be obtained. The deposition rate of oxides decreases with increasing O2 pressure and the rate of VO2 was about 130A/min. Optical studies show that VO2 films exhibit thermochromism and it has the potential application for energy efficient solar energy utilization.

Relaxation Phenomena of the Concentration Polarization in High Temperature Air Cathodes

Abstract The transient depolarization curves of Pt/ZrO2 high temperature air cathodes have been measured and compared with theoretical curves calculated by assuming a diffusion mechanism of atomic oxygen. The decay equation used involves the limiting current density, time constant for the decay rate and effective reaction area, as parameters to be determined. These parameter values were obtained by curve fitting. The polarization loss was found to decrease with increasing O2 pressure at the rate expected from the diffusion mechanism. The performance of the electrodes was enhanced as the sintering temperature rose. This trend was reflected on an increase in the parameter of effective reaction area. The performance was improved by loading an excess current. This effect was explained as being due to an increase in the diffusivity of Oa.