Pure Oxygen Atmosphere Research Articles

Effect of low temperature annealing on doping level and superconducting properties for IBAD/PLD–YBCO coated conductor

The effect of low temperature annealing on the superconducting properties of PLD–YBCO tape has been investigated with varying atmosphere. The Tc of the as-prepared sample is 88.5K, for the sample post-annealed at 200°C in Ar atmosphere showed 91.5K, indicating Tc is sensitive to low temperature annealing. In spite of the higher Tc the Jc and μ0Hirr for the Ar-annealed sample were lower than those for the as-prepared sample. This indicates that the as-prepared PLD-YBCO tape is in the carrier overdoped state. It was found that re-annealing in which the sample was slowly cooled from 450°C in pure oxygen atmosphere could restore the Jc-degradation, demonstrating that oxygen absorption-release occurred reversely.

Temperature dependence of metastable alumina formation during thermal oxidation of FeCrAl foils

Industrial FeCrAl foils were isothermally oxidized during 5 hours between 850 and 1000°C in atmosphere of pure oxygen. Characterization of transition and α-alumina phases was performed by XRD and XPS, using reference spectra obtained by various air annealing treatments of pure γ-Al2O3. An original model was proposed to deconvolute XPS spectra to obtain quantification of transition alumina formation and transformation. At 850°C, oxide scales on FeCrAl consisted of transition alumina, whereas higher temperature treatments resulted in decreased amounts of transition aluminas and in increasing α-alumina formation. At 1000°C, the highest temperature studied, the scale could be described by XPS and XRD as pure alpha.

Obtaining of polycrystalline CdTeO 3 by reactive pulse laser deposition

We report the obtaining of as-grown polycrystalline CdTeO 3 thin films grown by the reactive pulsed laser deposition technique in a controlled atmosphere of pure oxygen. X-ray diffraction (XRD) shows no CdTe presence and peaks, corresponding to single-phase of face-centred cubic of polycrystalline CdTeO 3, are well-defined, with a calculated grain size of about 120 nm and a compressive strain of 0.083%. X-ray photoelectron spectroscopy (XPS) shows 11% of CdTe and 89% of CdTeO 3. This CdTe, which was not present in XRD measurements, was related with Te reduction in the surface during the XPS measurement. Atomic force microscopy shows a mainly smooth surface with spheroid-like grains.

Investigation of the role of oxygen induced segregation of Cu during Cu 2O formation on Cu{1 0 0}, Ag/Cu{1 0 0} and Cu(Ag) alloy

In this work we have investigated in situ the nanoscale oxide formation on Cu{1 0 0}, Ag/Cu{1 0 0} surface alloy and polycrystalline Cu(Ag) alloy. The experiments were conducted in pure oxygen atmosphere (∼2 mbar) as a function of surface temperature (320–473 K). The concentration of metallic Cu, Cu 2O and CuO in the surface region was quantified by X-ray photoelectron spectroscopy (XPS). Oxygen induced segregation of the alloy constituents in the near-surface region, as well as the diffusion of oxygen towards the bulk, were investigated using depth-profiles obtained by employing Ar + ion sputtering and Auger electron spectroscopy (AES). We observed only Cu 2O formation on all three surfaces under the oxidation conditions used in these experiments. The formation of Cu 2O was found to be strikingly similar on both Ag/Cu{1 0 0} and Cu{1 0 0}. Our depth-profiling experiments indicate that this is due to a rapid oxygen induced segregation of Cu to the surface and subsequent formation of Cu oxides on top of the Ag overlayer. Evidence of a similar oxidation mechanism was also found on Cu(Ag).

Isothermal degradation and thermooxidative degradation of an epoxy powder coating

Thermogravimetry was used to study the kinetics of isothermal degradation of an epoxy thermoset powder coating in a nitrogen atmosphere and in oxidizing atmospheres of air and pure oxygen. An integral isoconversional procedure was used to analyse how the activation energy varies depending on the degree of conversion and depending on the atmospheres used. In the case of degradation in a nitrogen atmosphere, in addition to the activation energy, the kinetic triplet was completed using an Avrami reaction model and the pre-exponential factor. With this atmosphere, the conclusion was reached that the isothermal and non-isothermal kinetics are equivalent. It was shown that the thermooxidative degradation process is more complex and consists of a two-stage process. The first stage of degradation is similar whether nitrogen, oxygen or air are present. Chain scission occurs and it seems that there is formation of thermally more stable compounds. The second stage of degradation, involving several phenomena, occurs only in the presence of oxygen or air and leads to the total disappearance of the organic material by thermooxidation. These stages are very similar under non-isothermal or isothermal conditions.

Characterization of sputtered NiO thin films

Nickel oxide (NiO) thin films were deposited by RF magnetron sputtering process at different RF powers and substrate temperatures in a pure oxygen atmosphere. The structural, optical and electrical properties of NiO films were investigated using X-ray diffraction (XRD), visible spectrum and Hall effect measurements. The dependences of film properties on substrate temperature, crystalline structure and natural aging effect were studied. The results show that the resistivity increases as sputtering power increases from 100 to 200 W at constant temperature. The lowest resistivity and Hall coefficient obtained are 16.7 Ω cm and 1.99 cm 3/C, respectively, as the sputtering power is 100 W and substrate temperature is 350°. The highest carrier concentration obtained is 3.13×10 18 cm −3 as the sputtering power is 100 W and substrate temperature is 350°. The crystal structure was analyzed by X-ray diffraction method. The preferred orientation of NiO film changes from (111) to (200) when the substrate temperature varies from unheated condition to 350°. Electrical properties of NiO films were unstable and show a natural aging effect. Resistivity of NiO films increases as the time of natural aging increases. Under the substrate-unheated condition, the transmittance of as-deposited samples is lower compared to the film prepared at substrate temperature of 350 °C. The change in transmittance may be due to the microstructural change in the material. It is suggested that the sputtering power affects the preferred orientation of NiO film. Higher substrate temperature induces larger grain size and more perfect crystalline structure, which lead to low resistivity of NiO film.

Effect of Film Thickness on Structural and Electrical Properties of Sputter-Deposited Nickel Oxide Films

This work studies dependences of resistivity, carrier concentration, mobility and structural properties on the thickness of nickel oxide (NiO) films deposited onto glass substrates by RF magnetron sputtering in a pure oxygen atmosphere at an RF power 200 W. The electrical properties were measured by Hall Effect measurements. The X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses of nickel oxide films indicates that these films are polycrystalline when the samples are prepared with an unheated substrate (Ts ¼ 303 K) and using a substrate at a higher substrate temperature (Ts ¼ 673 K). The thickness of the films varied in the range from 50 to 300 nm. The variations of the microstructural parameters, such as crystallite size (L), dislocation density (� ), stacking fault probability (� ), strain () and density (D), with film thickness and substrate temperature were investigated. The results show the crystallite sizes increaser as the thickness of the film increases. The variation of the dislocation density and the stacking fault probabilities and strain decrease as the thickness increases. The resistivity of NiO film is increased with an increase in film thickness, which is related to the decrease of carrier concentration with film thickness. The NiO film with a thickness of 200 nm has a minimum resistivity of 0:69 � 10 � 2 � m when deposited at substrate temperature of 303 K.

Chemical properties and GMR improvement of specular spin valves with nano-oxide layers, formed in ambient mixed gases

Specular spin valves (SVs) containing nano-oxide layers (NOLs) structured as substrate/seed/AF/P1/NOL/P2/Cu/F/NOL, have been fabricated. The NOLs were formed by natural oxidation in different ambient atmospheres of pure oxygen, oxygen/nitrogen and oxygen/argon gas mixtures. The fabrication conditions were optimized to enhance the magnetoresistance (MR) ratio, to suppress the interlayer coupling fields (Hf) between the free and pinned layers, to suppress the high interface density of the NOL, to ease the control of the NOL thickness and to form a smooth NOL/P2 interface for promoting specular electron scattering. The characteristics of our specular SVs are the MR ratio of 14.1%, the exchange bias field of 44–45 mT, and Hf weaker than 1.0 mT. The optimal conditions for oxidation time, total oxidation pressure and the annealing temperature were found to be 300 s, 0.14 Pa (oxygen/argon = 80/20) and 250°C, respectively. Also, the origin of thermal stability of MMn-based (M = Fe, Pt, Ir, etc) specular SVs has been explained in detail by chemical properties of NOL using secondary-ion mass spectroscopy and x-ray photoelectron spectroscopy depth profile analyses. Thermal stability turns out to be caused by a decrease in MR ratios at high temperatures (>250°C), which is a serious problem for device applications using the SV structure as a high density read head device.

Morphological and physiological changes in Saccharomyces cerevisiae by oxidative stress from hyperbaric air

Increase in air or oxygen pressure in microbial cell cultures can cause oxidative stress and consequently affect cell physiology and morphology. The behaviour of Saccharomyces cerevisiae grown under hyperbaric atmospheres of air and pure oxygen was studied. A limit of 1.0 MPa for the air pressure increase (i.e. 0.21 MPa of oxygen partial pressure) in a fed-batch culture of S. cerevisiae was established. Values of 1.5 MPa air pressure and 0.32 MPa pure oxygen pressure strongly inhibited the metabolic activity and the viability of the cells. Also, morphological changes were observed, especially cell-size distribution and the genealogical age profile. Pressure caused cell compression and an increase in number of aged cells. These effects were attributed to oxygen toxicity since similar results were obtained using air or oxygen, if oxygen partial pressure was equal to or higher than 0.32 MPa. The activity of the antioxidant enzymes, catalase and superoxide dismutase (SOD) (cytosolic and mitochondrial isoformes) indicated that the enzymes have different roles in oxidative stress cell protection, depending on other factors that affect the cell physiological state.

Effects of mixed gases on characteristics of specular spin-valves containing oxide layers

Specular spin valves containing oxide layers (OXLs), structured as substrate/seed/AF/P1/OXL/P2/Cu/ F/OXL, have been fabricated. The OXLs were formed by natural oxidation in different ambient atmospheres of pure oxygen, of oxygen/nitrogen, and of oxygen/argon gas mixtures. The fabrication conditions were optimized to enhance the magnetoresistance (MR) ratio, to suppress the interlayer coupling fields (Hf) between the free and pinned layers, to suppress the high interface density of the OXL, to ease the control of the OXL thickness and to form a smooth OXL/P2 interface for promoting specular electron scattering. The characteristics of our specular spin valves are as follows: the MR ratio of 14.1%, the exchange bias field of 44–45 mT and Hf weaker than 1.0 mT. The optimal conditions for oxidation time, total oxidation pressure and the annealing temperature were found to be of 300 s, 0.13 Pa (oxygen/ argon = 8/2), and 250 °C, respectively. The results suggest that specular spin-valves containing OXLs satisfy the requirements of read head performances in high density (100 Gbits/in2) recording devices. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of nanophase CeO 2-Pt/C cathode catalyst for direct methanol fuel cell

Incorporation of nanophase ceria (CeO 2) into the cathode catalyst Pt/C increased the local oxygen concentration in an air atmosphere, leading to enhanced single-cell performance of direct methanol fuel cell (DMFC). Ceria doped catalysts were effective at low oxygen partial pressure (≤0.6 atm) conditions and 1 wt.% CeO 2 doped Pt/C exhibited the highest performance. The effect of ceria was more prominent with air as the cathode reactant and the ceria acted as a mere impurity in a pure oxygen atmosphere, decreasing the DMFC performance. Impedance spectra showed a decrease in polarization resistance with the ceria addition to the cathode catalyst in low-potential regions confirming the facile mass transfer of the reactant oxygen molecules to catalytic sites. Transmission electron microscopy (TEM) pictures showed a uniform distribution of CeO 2 around platinum sites.

Thin polycrystalline zinc oxide films obtained by oxidation of metallic zinc films

Thin polycrystalline ZnO films were obtained by thermal oxidation of metallic Zn films, thermally deposited on various substrates, such as silica, sapphire and glass, in both air and pure oxygen atmospheres. The quality of the ZnO layers was asserted by Hall effect, cathodoluminescence and atomic force microscopy measurements. Electron concentration of 7.32×10 12 cm −3 and mobility of 14.2 cm 2/V s with root mean square roughness of 30 nm were obtained for the 900 °C annealed ZnO films in oxygen. Room temperature cathodoluminescence spectra consisted of a narrow near band edge luminescence band and a broad defect-related green band with peak positions at 380 and 500 nm, respectively. ZnO film luminescence properties improved dramatically with the increase of annealing temperature and decrease of O 2 pressure.

Thermally grown ruthenium oxide thin films

Thin ruthenium films were sputtered on oxidized silicon wafers and subsequently thermally oxidized in oxygen∕nitrogen or oxygen ambient in the temperature range from 400 to 700 °C. The morphological and structural properties of grown ruthenium oxide films were analyzed by atomic force microscopy, Raman spectroscopy, and x-ray diffraction, with electrical properties analyzed by van der Pauw technique and metal–oxide semiconductor (MOS) structures. Significant roughening of RuO2 was observed for prolonged oxidation in oxygen∕nitrogen atmosphere, or for short times in pure oxygen atmosphere. Raman spectroscopy revealed superior structural properties of thermally grown RuO2, in comparison to sputtered or metalorganic chemical vapor deposition RuO2 films. Thermally grown RuO2 is shown to have a work function of 5.1 eV, making it a suitable choice of metals, together with Ru (4.6 eV work function) for electrodes in complementary MOS n-channel and p-channel transistors.

Impact of the Atmosphere on the Reactivity of Peroxidic Species upon Photooxidation of Polymers

AbstractSummary: This paper gives an account of preliminary results about the impact of an oxidative atmosphere on peroxidation. Attention is focused on the level, nature, and reactivity of (hydro)peroxides with respect to the oxidative component concentration. Our main result is surprising: a lower peroxidation rate occurs, but a higher content of hydroperoxides are formed in air than in pure oxygen atmospheres. The results may lead to an approach to understand the influence of atmospheric factors on the outdoor weathering of polymers.The device used to measure the oxygen uptake of the systems studied here.imageThe device used to measure the oxygen uptake of the systems studied here.

Effects of environment and temperature on the mechanical behavior of the Ni–19Si–3Nb intermetallic alloy doped with boron and carbon

The effect of small additions of boron and carbon on the environment embrittlement at different temperature of Ni–19Si–3Nb based alloy was investigated by atmosphere-controlled tensile test in various conditions. The results revealed that the Ni–19Si–3Nb base alloy exhibits ductile mechanical behavior (UTS∼1250 MPa, ε∼14%) at room temperature in vacuum (2×10 −4 torr) as well as in pure oxygen atmosphere. This indicates that environmental embrittlement due to water vapor is the major factor to the deterioration of the ductility of Ni–19Si–3Nb base alloy. In parallel, 300 ppm boron addition significantly improves the ultimate tensile strength and the ductility (UTS≥1200 MPa, ε≥12%) in the Ni–19Si–3Nb base alloy at room temperature in various atmosphere of pure oxygen, air (contains 14000 ppm water vapor), and pure water vapor (contains 850 ppm water vapor). In addition, the yield strength of the Ni–19Si–3Nb–0.15B–0.1C alloy increases with temperature, and the maximum yield strength occurs at 873 K. However, the ductility of the Ni–19Si–3Nb–0.15B–0.1C alloy drops significantly when the temperature increases to 973 K.

Injection moulded low cost bipolar plates for PEM fuel cells

The development of bipolar plates that can be produced by standard mass production techniques is a main issue for the commercialization of PEM fuel cells, as bipolar plates contribute significantly to the cost structure of PEM stacks. In recent years, the University of Duisburg-Essen together with the Zentrum für BrennstoffzellenTechnik GmbH (ZBT) has identified a number of carbon–polymer composites with densities of 1.6 g/cm3, specific bulk conductivities between 5 and 150 S/cm and material prices between 2 and 10 €/kg. Standard composite mixtures consist of a thermoplast and a carbon compound mixture with additional additives to increase the conductivity of the compound material. The composites generally show high corrosion resistance in the PEM fuel cell environment. Composite material samples proved to be absolutely stable in immersion tests in sulphuric acid and deionized water under pure oxygen atmosphere for several thousand hours. ZBT has successfully demonstrated the production of bipolar plates by injection moulding with cycle times of 30–60 s. With the help of tailored moulds injection moulding of bipolar plates becomes price competitive even for comparatively small series in the range of several thousand plates. PEM stacks with injection moulded bipolar plates of 2.5–4 mm thickness and an electrical power of up to 200 W have been constructed and successfully operated.

Oxidation kinetics of LaB6 in oxygen rich conditions

This paper investigated the oxidation behavior of LaB6 in the temperature range of 700–1185 °C. The results obtained by thermogravimetric analysis (TGA) showed that the initial oxidation has parabolic kinetics in the temperatures between 800–945 °C. The oxidation process exhibited temperature dependence with an activation energy of 195±15 kJ/mol in air and in pure oxygen atmosphere. Two possible reactions, either the evaporation of B2O3 species (occurring ⩾945 °C) or the oxidation of surface LaB6 (⩽800 °C), influence the oxidation mechanism to be interfacial reaction control, which results in mass loss deviating from parabolic behavior. The details of the microstructure evolution of the formation of glass and crystalline La(BO2)3 grains are investigated. The effects of porosity, the melting of La(BO2)3, coarsening of La(BO2)3 grains are reported and discussed.

Effect of oxygen post-annealing on the magnetoresistance of highly epitaxial La0.7Ca0.3MnO3 thin films

Abstract Epitaxial La 0.7 Ca 0.3 MnO 3 (LCMO) thin films have been deposited by high pressure DC sputtering technique on SrTiO 3 substrates. As-grown films exhibit metal–insulator transition temperature ( T MI ) with values ranging from 131 to 220 K. The maximum value of the magnetoresistance is of about −77% in a magnetic field of 0.5 T for T MI =131 K. The same films, annealed in a pure oxygen atmosphere, exhibit transitions to a metallic state at temperatures ranging from 210 to 284 K. Transmission electron microscopy analyses show a coherent growth between film and substrate.

Influence of Lime/Limestone Addition on the SO2 and NO Formation during the Combustion of Coke Pellet

Isothermal combustion experiments of coke pellet mixed with lime/limestone had been performed to investigate the effect of lime/limestone addition on the reduction of NO and SO2 emission. The coke pellets were combusted between 700°C and 1300°C under pure oxygen atmosphere. Combustion kinetics and conversion data of SO2 and NO were obtained from the thermogravimetry of coke pellet and gas analysis data. Adsorption of NO by lime mixed with coke was not occurred in the experimental ranges. It was possible to reduce most of SO2 emission below 900°C by adding lime/limestone to coke. However, the reduction efficiency of SO2 by the addition of lime/limestone decreased with increasing temperature due to the decomposition reaction of CaSO4. The variation of the conversion to NO and SO2 with the combustion conditions during the combustion of coke pellet was compared with the results of thermodynamic calculation.

Effects of a pure oxygen atmosphere on enzymatic browning of harvested litchi fruit

SummaryPericarp browning and decline in fruit quality are major problems of harvested litchi fruit. The effects of a pure O2 atmosphere on enzymatic browning and quality of harvested litchi fruit were investigated. Litchi fruit were kept in air or pure O2 for 6 d at 28°C. Changes in browning, polyphenol oxidase (PPO), anthocyanase, total soluble solids, titratable acidity and ascorbic acid were monitored. Effects of pure O2 on in vitro activities of PPO and anthocyanase were also evaluated. Exposure of litchi fruit to pure O2 significantly (P≤0.05) reduced skin browning during storage. Pure O2 inhibited the activities of PPO and anthocyanase which may account for browning inhibition. Furthermore, pure O2-treatment inhibited the in vitro activities of PPO and anthocyanase during incubation for 4 h. Litchi fruit exposed to pure O2 for 6 d had higher total soluble solids and titratable acidity than those kept in air, but lower ascorbic acid. Thus, application of pure O2 effectively reduced skin browning and maintained high total soluble solids and titratable acidity levels of pulp in harvested litchi fruit.