Pure Oxygen Atmosphere Research Articles

Magnetic properties of amorphous films of Bi/sub 2/O/sub 3/-Fe/sub 2/O/sub 3/-CaO

The magnetic properties of amorphous films with the composition (Bi/sub 2/O/sub 3/)/sub 0.5-x/(Fe/sub 2/O/sub 3/)/sub 0.5/(CaO)/sub x/ are investigated. Samples with x=0-0.05 were prepared in a pure oxygen or mixed oxygen-argon atmosphere by the RF sputtering technique. As-sputtered films are X-ray amorphous and paramagnetic at room temperature. The films show a spontaneous magnetic moment when annealed in air at temperatures T/sub a/ above 400 degrees C. Data on the room temperature saturation induction 4 pi M with T/sub a/, followed by a maximum in 4 pi M for T/sub a/=600-700 degrees C, depending on the x-value. At higher T/sub a/, a decrease in 4 pi M is observed. The room temperature 4 pi M is found to decrease with increase in x, from 970 G for x=0.1 to 300 G for x=0.5. The Curie temperature is lower than the crystallization temperature. Ferromagnetic resonance studies show a uniaxial or planar anisotropy in the films. X-ray diffraction data do not indicate the precipitation of crystalline ferrimagnetic oxides in the annealed films and the observed magnetization is therefore attributed to ordered amorphous clusters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Catalytic destruction of ozone by chlorofluorocarbons and partial restoration by methane in large laboratory experiments

The catalytic destruction of ozone by CFC was quantitatively measured in a large laboratory device which has controlled ultraviolet radiation and gas composition. Experimental results obtained in a pure oxygen atmosphere agreed with theoretical estimates within a factor of two. A destruction ratio of one chloride to 4 × 10 5 ozone molecules was found. The introduction of methane reduced the density of atomic chlorine resulting in a higher equilibrium ozone density. Even with a methane density six times the CFC density, only partial restoration of ozone is possible.

The properties of thiobenzyl esters of the P(III) acids. Part I. The study of the reaction mechanism of 2‐benzylthio‐4,5‐benzo‐1,3,2‐dioxaphospholane with ketones and oxygen by CIDNP 31P

Abstract2‐Benzylthio‐4,5‐benzo‐1,3,2‐dioxaphospholane reacts with oxygen to form the corresponding thiophosphonate, dithiophosphate and pyrophosphite. The reaction was found to be of a radical nature. The mechanism is suggested to involve an initial electron transfer with suitable subsequent transformations. The interaction of the title compound with various ketones in a pure oxygen atmosphere was studied by the CIDNP method.

Use of RBS and Raman spectroscopy to study oxygen mobility in YBaCuO thin films by 18O tracing experiments

The 18O tracer technique is particularly well adapted to study the mobility of oxygen and to evidence the presence of oxygen sites of different energies. The annealing experiments in a pure oxygen atmosphere enriched to > 98% 18O were performed for 19 h at 200 mbar and at three different temperatures: 350, 450 and 650°C. At 350, and at 450°C the orthorhombic phase was totally preserved (Y 1Ba 2Cu 3O 7), whereas at 650°C the nominal oxygen composition was O 6,4 and corresponds to the orthorhombic-te-tragonal transition. All the experiments were performed using YBaCuO samples deposited on MgO substrate by laser ablation or cathodic sputtering. The samples were characterized by X-rays and T c measurements. Their composition and structure were studied by RBS, NRA and ion channeling. The 18O distribution was studied using the resonant nuclear reaction at 629 keV. The localization in the YBaCuO structure of the 18O atoms was studied using Raman spectroscopy. It was found that the 18O uptake at 350°C leads to the formation of a thin surface layer (15 nm) enriched to about 30% whereas the volume enrichment appears to be low (3.5%) and quasi-constant. This result suggests that the 18O exchange at 350°C is controlled by the diffusion. The reoxidation in 18O atmosphere of partially reduced sample is shown to be much quicker than that observed by direct exchange. It was found that the 18O exchange at 450°C leads to a quasi-constant 18O profile (enrichment to 70% was observed after annealing during 19 h). The Raman spectroscopy showed that the annealing at 450°C induced equivalent exchange in CuO 2 planes and of the bridging oxygen. The annealing at 650°C leads to a total exchange between 16O and 18O.

The Standard Gibbs energies of formation of ACrO 4 (A = Ca, Sr or Ba) from EMF measurements

The thermodynamic stabilities of CaCrO 4, BaCrO 4 and SrCrO 4 are of interest to materials scientists and also in nuclear technology because these are the possible compounds of fission product-cladding chemical interaction under conditions of accident in fast nuclear reactors. Though there are some reports on the thermodynamic stabilities of CaCrO 4 and BaCrO 4 obtained by the fluoride electrolyte EMF method, there is no reliable thermodynamic information on SrCrO 4. Hence the EMF of the galvanic cells Pt,O 2(g)/ CaO,CaF 2/CaF 2/AF 2,ACrO 4,Cr 2O 3/Pt,O 2(g) were studied under an atmosphere of pure oxygen at a pressure of 0.1 MPa over the ranges 788–1070, 851–1116 and 850–1168 K where A is Ca, Sr and Ba respectively. From the cell EMF data, the standard Gibbs energies of formation Δ G f XXX of the chromates, according to the reaction AO(s) + 1 2 Cr 2O 3(s) + 3 4 O 2(g)→ACrO 4(s) were derived to be ΔG XXX f(CaCrO 4) = −1356.6 + 0.31641 T (± 0.63) kJ mol −1, Δ G XXX f(SrCrO 4) = −1359.1 + 0.32998 T (±0.30) kJ mol −1, Δ G XXX f(BaCrO 4 = −1323.7 + 0.31159 T (±0.34) kJ mol −1.

Preparation of YBa2Cu3Ox by Vacuum Calcination and Subsequent Oxidation Method.

Surface carbonate ion observed in YBa2Cu3Ox from a conventional ceramic method was removed during vacuum calcination up to 1200 K. No surface condensation of Y and Ba due to the formation of the Y-CO3 and Ba-CO3 bond was observed in YBa2Cu3Ox prepared by vacuum calcination and subsequent oxidation in a pure oxygen atmosphere. YBa2Cu3Ox prepared by this process had a lower ΔTc value and low temperature coefficient of electrical resistivity than those observed in the sample from the conventional ceramic method. The existence of carbonate ion at the surface and grain boundaries had adverse effects on the superconducting properties of the products.

Study of the ignition of single coal and char particles in a drop tube furnace by a probability method

A drop tube technique coupled with optical particle detection and three color pyrometry has been applied to study the ignition of single particles of a petroleum coke, a high volatile bituminous coal (Freyming-France) and their chars. The objective of the study was to measure the critical ignition temperatures and the ignition delays. This was performed in both cases in a pure oxygen atmosphere using five particle sizes: 100–125 μm, 125–160 μm, 160–200 μm, 200–250 μm and 250–315 μm. Forced ignition delays were measured only for chars. Results show, that critical ignition temperatures decrease with increasing char particle diameter and with decreasing coal or petroleum coke particle diameter. The Semenov's steady state thermal ignition theory was used to predict critical ignition temperatures. A transient heat balance equation was developed to predict ignition delays. The experimental results are correctly accounted for, except in the case of the smallest particles, for which the model over-predicts the ignition delays.

The effect of dissolved oxygen in water on the performance of a high rejection polyamide reverse osmosis membrane

The effect of dissolved oxygen in seawater on the performance of the SW30HR grade of the FILMTEC® FT30 membrane has been examined. Spiral wound elements were soaked in synthetic seawater solutions with pure oxygen or pure nitrogen atmosphere. The performance of the elements was followed during one year of soak time. The element performance was the same whether it had been in contact with 27 mg/L or zero dissolved oxygen. The salt passage of the elements decreased by about 40 percent during the first 0.5 year of soak.

Oxidation resistant MoWCrPd alloys with palladium coatings

Oxidation studies have been carried out on (40–55)wt.%Mo-(35–45)wt.%W-9wt.%Cr-1wt.%Pd alloys with and without palladium coatings in atmospheres of both air and pure oxygen at temperatures between 1000 and 1250 °C. The coated alloys displayed excellent oxidation resistance compared with the uncoated alloys. Palladium electroplated to 7 μm thickness prevented the formation of volatile oxides of molybdenum and tungsten during the initial oxidation stage. Contrary to the behavior of most metals, an increase in oxidation resistance with increase in temperature was observed. The alloys presented are considered to be a major improvement over the existing molybdenum- and tungsten-based alloys.

Evidence for filamentary superconductivity up to 220 K in oriented multiphase Y-Ba-Cu-O thin films

We report on the observation of filamentary superconductivity up to 220 K in multiphase Y-Ba-Cu-O materials that are deposited as highly oriented thin films on (110)-SrTiO 3 substrates by laser ablation from ceramic targets. The high temperature zero resistivity states are reproducible after temperature cycling down to 80 K for samples treated by a special oxygenation and ozonization process at 340 K and measured in a pure oxygen atmosphere. Our results on thin films confirm former experiments of J.T. Chen and co-workers obtained on ceramic samples with preferred crystallite orientation. A close connection between superconductivity and structural instabilities of most likely ferroic nature, which are observed more often for YBa 2Cu 3O 7 in a narrow temperature range near 220 K, is suggested.

Oxidation behavior of SiC-AlN ceramic materials

Abstract This paper describes the oxidation behavior of monophased 2H solid solution SiC-AlN materials. Oxidation behavior of three compositions, 30, 50 and 80 wt% SiC, is investigated. Oxidation experiments are carried out in the temperature range of 1450 to 1600°C under pure oxygen atmosphere. The starting oxidation temperature of monophased SiC-AlN compounds is higher than 1450°C, whereas the oxidation temperatures of SiC and AlN start at a temperature of 800–1000°C. A thermogravimetric study of the oxidation kinetics does not allow the determination of reaction mechanisms, gaseous species being trapped beneath oxides scale. The discontinuous escape of gas disrupts this layer, thus disturbing the recording of the weight measurement.

Crystallization in amorphous Fe78B13Si9

Isothermal crystallization in amorphous Fe78B13Si9 with annealing temperature Ta ≥ 500°C has been measured by a differential scanning calorimeter in pure oxygen and nitrogen atmospheres. Our results showed that oxygen has more pronounced effect on the 2nd crystallization peak than on the 1st crystallization peak. The heat released from the 1st crystallization is enhanced, while that of the 2nd crystallization is reduced by oxygen. X-ray results indicates that the two crystallization peaks are mainly due to the formation of α-Fe crystallites. The Avrami exponent ni for each peak of crystallization was examined and analyzed. Selective oxidation and out-diffusion of silicon have been observed. Although the phenomena were not very significant, due to shorter annealing time, they were more apparent for the oxygen case than for the corresponding nitrogen case.

Effect of vacuum calcination and subsequent oxidation on the superconducting properties of YBa 2Cu 3O x

The formation of Y-CO 3 and Ba-CO 3 bonds observed in YBa 2Cu 3O x from a conventional ceramic method in an air or oxygen atmosphere was eliminated only after vacuum calcination and subsequent oxidation in a pure oxygen atmosphere. YBa 2Cu 3O x prepared by this heat treatment showed a sharp superconducting transition at around 92 K, which was the same as that observed in the sample made by using a conventional ceramic method. The temperature coefficient of electrical resistivity, R 100 R 300 = 0.27 , for this sample became smaller than those of 0.47 and 0.34 for YBa 2Cu 3O x prepared by the conventional ceramic method in air or oxygen atmosphere, respectively.

Oxidation of molybdenum-chromium-palladium alloys

Isothermal and cyclic oxidation studies have been carried out on (80–90) wt.% Mo-(8–17) wt.% Cr-(0.5–3) wt.% Pd alloys in atmospheres of both air and pure oxygen at temperatures between 1000 and 1250°C. The Pd additions decreased the oxidation rate with the most pronounced effect being observed for an alloy of 80 wt.% Mo-17 wt.% Cr-3 wt.% Pd. Palladium played a major role in providing the necessary oxidation protection by accelerating the formation of Cr2O3 layers at low Cr concentrations. Contrary to the behavior of most metals, an increase in oxidation resistance with increase in temperature was observed. Although the alloy systems were not truly oxidation resistant, definite improvement in oxidation resistance was achieved. The oxidation mechanism of Mo-Cr-Pd alloys is described.

Oxygen‐Regeneration of Discharged Manganese Dioxide Electrode: I . General Phenomena Observed on Thick Powder Electrodes

The results are obtained in the following conditions: The rate of discharge of the manganese dioxide has been chosen by analogy with the I.E.C. standard for the tape recorder discharge test of LR6 (AA size) alkaline cells. A reduction drain of 25 mA/g of was passed through the electrode for 4000s. The circuit was open for 23h and the electrode is exposed to a pure oxygen atmosphere. The electrode device contains 40 mg of and 160 mg of graphite. Nine industrial samples are studied. Thick manganese dioxide electrode discharged above 0 V/ENH (>1.2 V/Zn/Zn++) can be regenerated by oxygen. The best results are obtained for EMD , with high compacting pressure of pellet, small size of manganese dioxide particles, short diffusion path of oxygen, and large contact surface with gaseous oxygen. When such conditions are fulfilled, the total capacity reaches 1.2 e/Mn. Without oxygen‐regeneration, the capacity measured at 0 V/NHE under argon is about 0.48–0.50 e/Mn. This corresponds to a relative increase of about 250% of its initial capacity. Indeed, these data are related to the described experimental conditions. They must be improved by the use of optimized conditions such as a lower ratio. Above 0.09 V/NHE, there is no significant oxygen‐reoxidation kinetics. This potential is identical to the potential of manganese dioxide during the catalyst decomposition of hydrogen peroxide.

Experimental and theoretical aspects of the formation of radical cations from tripyrrolidinobenzenes and their follow-up reactions

Tripyrrolidinobenzene radical cations (I*+), obtained from the corresponding arenes by oxidation with silver nitrate, are specially stabilized and thus allow specific reaction pathways of arene radical cations to be investigated separately and individually. Radical cations l*+, for instance, generated under exclusion of oxygen, undergo dimerization to 2, or they abstract hydrogen from the solvent to form 3. In a pure oxygen atmosphere, the O2 reaction products 6 and 7 are formed, respectively, either exclusively or together with 2 and 3. Kinetic measurements give the following order of reactivity for these individual processes: reaction with O2 > dimerization (initial (l) being ca. lo4 M under flash photolysis conditions) > H-abstraction from solvent. The changes in the product spectrum upon modification of the reaction conditions are in accord with the kinetic results. The dimeric u complexes 2 show surprisingly facile dissociation into two radical cations, two (l*+), with a much higher dissociation rate for the alkyl derivatives 2H than for 2a. Dissociation is enhanced substantially by light or in the presence of T donors. Individual product formation, rate of reactions of the radical cations l, and photochemical cleavage of the dimeric u complexes 2 can be rationalized, by qualitative and quantitative MO considerations, in terms of their relative frontier orbital energies.

Theoretical consideration on the ignition of hot iron in high pressure oxygen.

Ignition of iron blocks in a pure oxygen atmosphere is studied using the following model. A heated iron block is exposed suddenly to 100% oxygen atmosphere. Oxidation occurs at the iron block surface, and an iron oxide layer is formed on the surface. Beneath the oxide layer, a melted iron layer exists. Ignition occurs when the melted iron layer flows.Critical conditions for ignition were calculated. When the initial oxide layer thickness was constant, the maximum surface temperature increased as the initial temperature increased. Ignition occurred when the maximum surface temperature exceeded a critical value of about 1540 K. Ignition occurred also at lower initial temperature, as the initial oxide layer thickness decreased. The conditions were compared with experimental data for ignition of iron cylinders. It was found that the model proposed in this study can accurately describe iron block ignition.

Mechanisms of crystallization of amorphous Y 1 Ba 2 Cu 3 O 7-x thin films during annealing in oxygen and argon atmospheres: “in situ” measurements.

Abstract Superconducting Y1Ba2Cu3O7-x (YBC) thin films have been grown on Yttria Stabilized Zirconia (YSZ) substrates by r-f magnetron sputtering and subsequent annealing either under argon followed by oxygen or under pure oxygen atmosphere. The crystallization of the amorphous YBC films and the 1-2-3 phase formation appear strongly dependent on the annealing atmosphere. The YBC crystallization process has been compared to the one of CuO films and related to the CuO → Cu2O transition. Similarities have been found between “in situ” resistivity curves observed for YBC and pure CuO as a function of temperature and pressure of oxygen. Best Tc (R = 0 at 87 K, △t = 2 K) has been observed for YBC films heated at 880°C under argon then cooled down under oxygen.

X-ray diffraction analysis on 2223 Tl-oxide single crystals

Single crystals of superconducting oxide Tl 2−xBa 2+yCu 3O 10−z with x∼0.4, y∼0.4 and z∼0 were synthesized by the flux growth method. Mixtures of oxide powders having a nominal metal ratio Tl/Ba/Ca/Cu at 2/2/2/3 or 2/2/3/4 were heated at 1193 K in a pure oxygen atmosphere at 1 atm for 1 h and then cooled slowly. Crystal structures at room temperature, 180 K and 130 K were investigated by means of 4-circle X-ray diffraction measurement. Splitting of the oxygen site at 4e (0 0 z) with z∼0.28 to 16n( x 0 z) or 16m( x x z) sites with x∼0.05 and z∼ 0.28 in the Tl-O layer is observed in some crystals. At 130 K and 180 K, the O 4-site seems to split vertically into “dumbbell” sites. At the same temperatures, CuO 5 pyramids are compressed vertically, suggestive of a charge transfer from the Tl-O double layer to the pyramid layer.

Oxidation of molybdenum-tungsten-chromium-silicon alloys

The oxidation kinetics of (50–60) wt.% Mo-(35–47) wt.% Cr-(2–5) wt.% Si and (30–40) wt.% Mo-(30–40) wt.% W-(27–37) wt.% Cr-(0–3) wt.% Si alloys were studied between 1000 and 1200°C in a pure oxygen atmosphere. The oxidation of Mo-W-Cr-Si alloys resembled that of Mo-Cr-Si alloys but was much more oxidation resistant. In general, oxidation resistance increased with increasing Cr and Si content. Alloys with good oxidation behavior had a thin outer Cr2O3 layer and an internal oxidation zone (in both Mo and Mo-W alloys). Alloys displaying poor oxidation behavior had a porous Cr2O3 layer (in Mo alloys) or layers of oxides of W and Cr (in Mo-W alloys). Although the alloy systems were not truly oxidation resistant, definite improvement in oxidation resistance was achieved.