Oxygen Partial Pressure In Atmosphere Research Articles

Titania nanorods array homojunction with sub-stoichiometric TiO2 for enhanced methylene blue photodegradation

TiO2 thin films are known to promote photodegradation of dyes and pollutants in water solution via heterogeneous photocatalysis. This ability is guided by the photoexcitation through photons having energies above the band gap. To improve photocatalytic activity, nanostructures with high surface area can be applied, which can ease molecular adsorption/desorption mechanisms, enhance electronic transfer properties and lower excitation energy. For this purpose, square cross-section TiO2 vertically aligned nanorod (TNR) array configuration has been chosen as a semiconductor substrate. On top of it, a thin layer of sub-stoichiometric TiO2−x has been deposited, aiming at inducing a vacancy doped homojunction between two different oxygen rich/deficient TiO2 layers, possibly leading to lower band gap and enhanced photochemical activity. In principle, promotion of electron and holes separation and suppression of charge recombination could occur. Vertically aligned TNRs have been deposited through a hydrothermal growth in acidic conditions on a pre-seeded glass conducting substrate, optimizing the seeding process through spin coating. Sub-stoichiometric TiO2−x layer (50 nm nominal thickness) has been deposited on top of TNRs via radiofrequency magnetron sputtering at three different stoichiometries, tuning the oxygen partial pressure in sputtering argon atmosphere at 10 %, 15 % and 20 %, respectively. Photocatalytic activity has been investigated in the photodegradation of an aqueous solution of methylene blue, both under UV and simulated solar light irradiation at room temperature and atmospheric pressure, resulting in the degradation of methylene blue target molecule up to 99 % under UV and 85 % under simulated solar irradiation after 6 h. These promising achievements unlock new environmental applications for enhanced dye degradation industrial processes.

Round-Robin Measurement of Surface Tension for Liquid Titanium by Electromagnetic Levitation (EML) and Electrostatic Levitation (ESL)

To accurately measure the surface tension of liquid titanium free of contamination from chemical reaction with the supporting materials and dissolution of atmospheric oxygen, the measurement was performed by using electromagnetic levitation (EML) and electrostatic levitation (ESL) in consideration of the influence of oxygen partial pressure of the measurement atmosphere, PO2. When liquid titanium was maintained at 2000 K under Ar–He gas with PO2 of 10 Pa flowing at 2 L·min−1 using EML, the surface tension decreased with time due to the dissolution of atmospheric oxygen into the sample. When the PO2 of the gas was decreased to 10−2 Pa, the oxygen content and the surface tension were confirmed to not vary, even after 120 min. Even though PO2 further decreased to 10−11 Pa under Ar–He–H2 gas, the surface tension slightly increased with time due to gas phase equilibrium between H2 and H2O that allowed for a continuous dissolution of atmospheric oxygen into the liquid titanium. The surface tension of liquid titanium measured by ESL, which prevents contamination of the sample from supporting materials and the high 10−5 Pa vacuum inhibits the dissolution of oxygen, showed almost the same value as that measured under Ar–He gas at PO2 of 10−2 Pa by EML. From the measurement results of EML and ESL, the surface tension of the 99.98 mass % pure liquid titanium, free from any contaminations from chemical reactions, with the supporting material and dissolved oxygen was expressed as σ99.98%=1613−0.2049T−1941 (10−3 N·m−1).

Oxygen Vacancy-Related Cathodoluminescence Quenching and Polarons in CeO2.

We used cathodoluminescence (CL) spectroscopy to characterize the oxygen vacancies (VO) in ceria (CeO2). The effects of the processing atmosphere and thermal quenching temperature on the nature and distribution of the intrinsic defects and on the spectroscopic behavior were investigated. The presence of polarons and associates of the polarons with the oxygen vacancies such as (VO••–CeCe′)• is demonstrated. CL intensity quenching above a critical concentration of VO has been shown. Even though the emission centers in all samples are the same, their concentration changes with the oxygen partial pressure of the processing atmosphere. Deconvolution of the observed CL spectra shows that the emissions originating from the F0 centers prevail over those of F+ centers of VO when the defect concentration is high.

Synthesis Conditions and Structure of Layered Manganites Ln2BaMn2O7 – δ(Ln = Pr, Nd)

Conditions (temperature, oxygen partial pressure in the gas atmosphere, and heating time) for the synthesis of polycrystalline layered manganites Ln2BaMn2O7 – δ (Ln = Pr, Nd) with an orthorhombic structure (Fmmm) and a specific oxygen non-stoichiometry were proposed for the first time. The temperature boundaries for the orthorhombic (Fmmm) to tetragonal (I4/mmm) structural phase transition were determined using high-temperature X-ray diffraction and differential scanning calorimetry. The temperature of the structural transition increases with increasing atomic number of the rare earth metal present in the manganite.

The effect of electrodes on microstructures and switching behaviors of ZnO-based resistive memory

In this study, ITO and Pt layers were deposited on (100) Si substrate respectively as bottom electrode for ZnO-based resistive memory. The ITO layer was textured and its preferred orientation is highly dependent on the oxygen partial pressure of growth atmosphere. As the oxygen content increases in the range of 0%–15%, the preferential growth orientation tends to change from (400) to (222). (002)-textured ZnO thin films with different preferred orientation degree sputtered on Pt and two kinds of oriented ITO layers were comparatively investigated. The device with (400)-ITO bottom electrode was found to have the best resistive switching performance. Meanwhile, the differences in switching behaviors were also found to be closely related to the thickness-dependent crystalline structure and top contact electrode/interface. This study provides an in-depth understanding of the structural design and structure-activity relationship of high-performance ZnO-based resistive memory.

Influence of calcination atmosphere on the phase evolution mechanism of Bi-2223 high temperature superconductor

Precursor powders of Bi2Sr2Ca2Cu3O10+δ (Bi-2223) high temperature superconducting tapes were prepared with spray pyrolysis technique. By tuning the oxygen partial pressure in calcination atmosphere as 1.0%, 7.5%, and 10.0%, respectively, the influences of calcination atmosphere on the phase evolution dynamics during the precursor powders calcination process have been discussed. Then the optimal calcination temperatures have been obtained correspondingly. Moreover, Bi-2223 multi-filament tapes have been fabricated with the precursor powders calcinated under different atmosphere. The effects of precursor powder calcination parameters on the phase composition, microstructures as well as the current capacity of final tapes have been systematically studied. Due to the proper secondary phase content and enhanced Bi-2223 texture structures, the maximum critical current of 109 A at 77 K, self-field, corresponding to the critical current density of 23.3 kA cm−2 has been obtained with the oxygen partial pressure of 7.5% in calcination atmosphere and the calcination temperature of 790 °C.

Pressure dependence of the laser-metal interaction under laser powder bed fusion conditions probed by in situ X-ray imaging

Laser powder bed fusion (LPBF) additive manufacturing and laser welding are powerful metal processing techniques with broad applications in advanced sectors such as the biomedical and aerospace industries. One common process variable that can tune laser-material interaction dynamics in these two techniques is adjustment of the composition and pressure of the atmosphere in which the process is conducted. While some of the physical mechanisms that are governed by the ambient pressure are well known from the welding literature, it remains unclear how these mechanisms extend to the distinct process conditions of LPBF. In situ studies of the differences in subsurface structure and behavior are essential for understanding the effects of gas pressure and composition on the LPBF processes. This article reports the use of in situ X-ray imaging to directly probe the morphological evolution of the liquid-vapor interface during laser melting as a function of ambient pressure and oxygen partial pressure under LPBF conditions in 316 L steel, Ti-64, aluminum 6061, and Nickel 400. We observe significant changes in melt pool morphology as a function of pressure. Furthermore, similar changes in morphology occur due to an increase in oxygen partial pressure in the process atmosphere. Temperature- and composition-dependent changes in surface tension of the liquid metal drive this change in behavior, which has the potential to influence defect creation and final morphology in LPBF parts.

P-type SnO thin films prepared by reactive sputtering at high deposition rates

SnO is an ideally suitable p-type conductive material, with large hole mobility, and has attracted great interest in connection with next-generation electronic applications. In the present work, tin oxide (SnOx) thin films were deposited on unheated soda lime glass substrates by reactive DC sputtering from a pure Sn target. The structural, optical and electrical properties of the films were analysed as a function of the oxygen partial pressure in the sputtering atmosphere and of the post-deposition annealing temperature in air. A structural analysis was carried out using Raman spectroscopy and X-ray diffraction. Optical and electrical characterizations were performed using photo-spectrometry and Hall effect measurements, respectively. The films grown at room temperature and low oxygen pressures reached high deposition rates of above 45 nm/min, showing poorly crystalline SnO and low transparency. Subsequent heating to 350 °C allowed to achieve a more crystalline tetragonal SnO with an average visible transmittance of 65%, a p-type conductivity of 0.8 S/cm, and a hole mobility of 3.5 cm2/(V s).

Effects of sintering atmosphere on microstructure and dielectric properties of (Yb + Nb) co-doped rutile TiO2 ceramics

(Yb1/2Nb1/2)0.05Ti0.95O2 ceramics are fabricated by the solid state reaction method. The effects of oxygen partial pressure in the sintering atmosphere (10% ≤ x ≤ 100%) on crystal structures, microstructure and dielectric properties of (Yb1/2Nb1/2)0.05Ti0.95O2 are investigated. All samples have rutile structure, and average grain size rises from 5 μm to 28 μm with x increasing from 10% to 100%. All samples show colossal permittivity (above 104). The dielectric properties of x ≥ 50% samples show well temperature (120 K–370 K) and frequency stability (100 Hz–100 kHz). The grain resistivity of x = 100% sample is three orders of magnitude higher than that of x = 10%. The breakdown voltage improves from 246 to 716 V/cm with increasing oxygen partial pressure. Both the Ti valence and oxygen concentration have been influenced by sintering atmosphere.

Oxidation behaviors of ZrB2–SiC binary composites above 2000 °C

Rapid oxidation testing for monolithic ZrB2 and ZrB2–SiC binary composites with different SiC contents (0–30vol%) was performed using an electric heating system above 2000°C. The system used in this study achieved the high heating rate of 250°C/s. The experimental results showed that the morphologies of the oxidized specimens depended strongly on the SiC content. The formation mechanism of SiC-depleted layers beneath the surface scale above 2000°C differed completely from that below 2000°C. Although the holding time was below 10s, SiC-depleted layers were formed because the oxygen partial pressure of the air atmosphere was not enough to form SiO2 by the oxidation of SiC. It was determined that ZrB2–20vol% SiC showed the best oxidation resistance above 2000°C at high heating rates.

Contacting Mechanically Exfoliated β-Ga2O3 Nanobelts for (Opto)electronic Device Applications

We systematically investigated the properties of metal contacts deposited on exfoliated β-Ga2O3 nanobelts. Unintentionally doped β-Ga2O3 was mechanically exfoliated from bulk β-Ga2O3 crystal and transferred onto SiO2/Si substrate having a back gate configuration. Electrodes were formed by depositing Ti/Au or Ni/Au onto the transferred β-Ga2O3 nanobelts, followed by rapid thermal annealing (RTA) with different ambient gases and temperatures. Using scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS), it was shown that titanium reacts with oxygen to form titanium oxide, which has ohmic behavior. In contrast, nickel does not form an ohmic contact despite 600°C thermal treatment. The lower oxygen partial pressure in a nitrogen atmosphere as compared to air results in more oxygen vacancies within the Ga2O3 nanobelts during RTA and a negative threshold voltage shift. A decreased current level after high temperature annealing can be ascribed to significant outward diffusion of oxygen and gallium atoms and to oxidation of the metal electrode. Our results can pave a route to demonstrating high performance β-Ga2O3 nanobelt-based (opto)electronic devices.

The influence of oxygen partial pressure in the growth atmosphere on the coloration of SrTiO3single crystal fibers

A variety of SrTiO3 (STO) fibers have been grown by the laser-heated pedestal growth method to determine the influence of the oxygen partial pressure in the growth atmosphere and the composition of the feed material on the coloration of SrTiO3. Crystals grown in oxygen rich environments are typically brown and show high Sr/Ti-ratios. Decreasing oxygen partial pressure leads to less coloration and decreasing Sr/Ti-ratios. Growth under very low oxygen partial pressure produces crystals with blueish and greyish tints. By using a feed rod with Sr-excess in an atmosphere that contains about one vol% oxygen, colorless STO fibers can be produced. The main reason for the low transmittance in crystals grown in high oxygen partial pressures is probably off-stoichiometry and the formation of scattering centers from clustered vacancies. The formation energy of the metal vacancies is considerably higher in oxygen poor environments which hinders the formation of the scattering centers. This phenomenon is further prevented by Sr-excess in the feed material that compensates SrO(g) evaporating from the melt.

Phase Field Modeling Applied to Reactive Air Brazing: Investigating Reaction Kinetics with Focus on Oxygen Exchange

The role of oxygen exchange for the microstructure formation and reaction kinetics in reactive air brazing (RAB) using Ag–Cu brazing fillers is investigated by means of phase‐field modeling. In addition to the microstructure evolution, the simulations allow computing differential scanning calorimetry (DSC) curves from the overall enthalpy change as a good representation for the reaction kinetics. The reaction kinetics are investigated with a focus on the interplay between the phase formation and oxygen exchange between the brazing filler and the ambient atmosphere. For the oxygen exchange, a simple permeability model is introduced, which is based on the interface mobility of the gas‐melt phase boundary and which describes the oxygen flux j proportional to the equilibrium partial pressure difference Δp. It is observed that the L1 liquid phase can serve as a “buffer,” providing or absorbing oxygen for phase transformation reactions. This “buffering” changes the oxygen activity, which in turn affects reaction temperatures and thus allows controlling reaction temperatures by adjusting the oxygen partial pressure in the ambient atmosphere: Higher oxygen levels increase the temperature of CuO formation, which requires oxygen, and lower the transformation temperatures for fcc‐Ag solidification, which itself releases oxygen possibly leading to gas pore formation.

Plasma Polymerization Inside Tubes in Hexamethyldisiloxanes and Ethyne Glow Discharges: Effects of Deposition Atmosphere on Wetting and Ageing in Solvents

The coating deposition inside tubes becomes increasingly important for fluidic applications, in which inner surfaces are chemically and mechanically strained by the flowing liquid and by scratching of particles. The developed process for tube coating, presented in this work, is based on the discharge in the precursor gas atmosphere between two mesh electrodes at the ends of the tube. The gas mixture is introduced on one end and pumped through the electrode on the other end. Igniting plasma inside the tube, the tube walls are the barrier to the atmosphere. Especially pulsed DC discharges for plasma polymerization in this alignment lead to good coating results, which is shown in this work focusing on deposition in pure and mixed hexamethyldisiloxane, ethyne, and oxygen atmospheres. Chemical binding, wetting, and ageing are strongly influenced by the choice of the gas mixtures. Sufficient oxygen partial pressure in the deposition atmosphere leads to hydrophilic behavior of the SiO2-like polymer-like carbon coatings, all other applied atmospheres to generally hydrophobic behavior of pure and Si-doped plasma polymers, respectively.

Growth of the acidophilic iron–sulfur bacterium Acidithiobacillus ferrooxidans under Mars-like geochemical conditions

The question of life on Mars has been in focus of astrobiological research for several decades, and recent missions in orbit or on the surface of the planet are constantly expanding our knowledge on Martian geochemistry. For example, massive stratified deposits have been identified on Mars containing sulfate minerals and iron oxides, which suggest the existence of acidic aqueous conditions in the past, similar to acidic iron- and sulfur-rich environments on Earth. Acidophilic organisms thriving in such habitats could have been an integral part of a possibly widely extinct Martian ecosystem, but remains might possibly even exist today in protected subsurface niches. The chemolithoautotrophic strain Acidithiobacillus ferrooxidans was selected as a model organism to study the metabolic capacities of acidophilic iron–sulfur bacteria, especially regarding their ability to grow with in situ resources that could be expected on Mars. The experiments were not designed to accurately simulate Martian physical conditions (except when certain single parameters such as oxygen partial pressure were considered), but rather the geochemical environment that can be found on Mars. A. ferrooxidans could grow solely on the minerals contained in synthetic Mars regolith mixtures with no added nutrients, using either O2 as an external electron acceptor for iron oxidation, or H2 as an external electron donor for iron reduction, and thus might play important roles in the redox cycling of iron on Mars. Though the oxygen partial pressure of the Martian atmosphere at the surface was not sufficient for detectable iron oxidation and growth of A. ferrooxidans during short-term incubation (7 days), alternative chemical O2-generating processes in the subsurface might yield microhabitats enriched in oxygen, which principally are possible under such conditions. The bacteria might also contribute to the reductive dissolution of Fe3+-containing minerals like goethite and hematite, which are characterized by a high thermodynamic stability. Even in a desiccated environment, A. ferrooxidans survived for one week under simulated Martian shallow subsurface conditions (6hPa, −20°C, 0.13% O2) in the form of dried biofilms without loss of viability. Low temperature and low oxygen pressure were favorable to survival. Thus, the acidophilic iron–sulfur bacterium A. ferrooxidans may be considered a plausible candidate of a potential Martian food web based on its metabolic capacities. As an autotroph it would be located at the base of such a food web, providing organic carbon.

Electro-degradation and resistive switching of Fe-doped SrTiO3 single crystal

In this work, the results of the electrocoloration of strontium titanate single crystals with different iron concentrations are presented. The samples of SrTiO3(100) doped with 0.06 at. % and 0.13 at. % of iron were electroreduced at low pressure (10−8 mbar) and elevated temperature (250 °C) using a DC voltage of 200 V. This led to the migration of oxygen vacancies and subsequent electrocoloration of the samples, which was confirmed by optical analysis and electrical measurements. Evolution of the color front was compared with finite element calculations of electric potential indicating good agreement. Both macroscopic and nanoscopic measurements showed insulator-metal transition at several hundreds of seconds (0.06%Fe) and resistive switching behavior. We found that the resistive switching is clearly modified by the oxygen partial pressure of the ambient atmosphere. Moreover, after electroreduction, in the region between the electrodes, stripes can be found following simple crystallographic directions connected with the extended defects and easy diffusion paths also observed in the single crystals of undoped strontium titanate. Furthermore, migration of negatively charged oxygen ions towards the anode led to the formation of oxygen bubbles trapped between the surface of the crystal and the electrode. Using atomic force microscopy, we were able to measure the geometry of a bubble and calculate the oxygen pressure necessary for the formation of such bubbles and the total amount of oxygen ions trapped within.

용융(熔融) PbO-SiO2계(系) 슬래그와 Bi 사이의 Bi와 Pb의 분배거동(分配擧動)

용융 PbO-<TEX>$SiO_2$</TEX>계 슬래그와 Bi를 <TEX>$775{\sim}850^{\circ}C$</TEX>의 마그네시아 도가니 중에서 평형시켜 Pb와 Bi의 평형분배에 대하여 조사하였다. 분위기 중의 산소분압은 <TEX>$P_{CO2}/P_{CO}$</TEX>의 비율을 조정하여 제어하였다. Pb의 분배비인 (%PbO)/[%Pb]는 슬래그 중의 <TEX>$SiO_2$</TEX> 농도가 증가함에 따라 증가하였으며, Bi의 분배비인 (<TEX>$%Bi_2O_3$</TEX>)/[%Bi]는 슬래그 중의 <TEX>$SiO_2$</TEX> 농도가 증가함에 따라 감소하였다. 그리고 반응 온도가 높을수록 금속 상중의 Pb 농도는 증가하였다. 이러한 결과는 열역학적으로 예측한 결과와 잘 일치하였다. The equilibrium distribution of bismuth and lead between molten PbO-<TEX>$SiO_2$</TEX> slag and bismuth phase was studied in the temperature range of <TEX>$775^{\circ}C$</TEX> to <TEX>$850^{\circ}C$</TEX> in a MgO crucible. The oxygen partial pressure of atmosphere was controlled by <TEX>$P_{CO2}/P_{CO}$</TEX> ratio. The value of <TEX>$(%PbO)_{slag}/[%Pb]_{metal}$</TEX> increased with increasing <TEX>$SiO_2$</TEX> content of slag, and the value of <TEX>$(%Bi_2O_3)_{slag}/[%Bi]_{metal}$</TEX> decreased with increasing <TEX>$SiO_2$</TEX> content of slag. The concentration of Pb in metal increased with increasing temperature. These experimental results agreed well with the thermodynamic prediction.

Influence of Oxygen Partial Pressure on Synthetic Coal Slag Infiltration into Porous Al2O3 Refractory

The infiltration characteristics of synthetic coal slag into Al2s3 refractory material with a temperature gradient induced along the slag's penetration direction were investigated with respect to time and oxygen partial pressure of the experimental atmosphere. Synthetic slag, which is representative of an average of the ash contents from United States coal feedstock, was melted in either an oxidizing air atmosphere or a reducing CO/CO2 gas mixture with a ratio of 1.8. The experiments were conducted with a hot‐face temperature of 1450°C, and the slags were deposited onto refractory samples in the same atmospheres as they were originally melted. A comparison between the infiltrations in the CO/CO2 and air atmospheres revealed that differences in oxygen partial pressure changed the mode in which the slag interacted with the refractory. While infiltrations in CO/CO2 atmosphere demonstrated elevated Al2O3 concentrations in the slag owing to refractory dissolution, infiltrations in air atmosphere showed enrichment of SiO2 and Al2O3 in the slag because iron‐oxide from the slag incorporated into the corundum refractory. For both cases, the reactions led to increases in viscosity, but the effect was more profound in the air atmosphere, where penetrations were found to be shallower. The oxygen partial pressure's influence on the slag's composition, primarily with iron‐oxide species, and on viscosity played a pivotal role in governing the effective penetration into the refractory.

Hot spot in GdBa2Cu3O7−δ-based composite ceramics rods and their applications for oxygen sensors

A hot spot, which is a local area glowing orange, appears in a LnBa2Cu3O7−δ (Ln: rare earth element) ceramic rod when a voltage exceeding a certain value is applied to the rod at room temperature. After the appearance of the hot spot, the current changes according to the oxygen partial pressure in ambient atmosphere, which acts as an oxygen sensor without the need for any heating system. The GdBa2Cu3O7−δ rod tended to be melted and broken by a sustained presence of the hot spot in a high oxygen partial pressure Po2 (∼100 kPa). The composite rod containing high melting point materials, such as BaAl2O4, BaZrO3 and Gd2BaCuO5, showed a remarkable high durability in O2 atmosphere. In a low Po2 (< 0.02 kPa), the current through the GdBa2Cu3O7−δ rod decreases to almost zero and the hot spot disappeared, resulting in an insensitive rod to oxygen. The composite rod containing CuO detected oxygen even in Po2 < 0.002 kPa.

Oxygen partial pressure dependence of surface tension and its temperature coefficient for metallic melts: a discussion from the viewpoint of solubility and adsorption of oxygen

Surface tension of molten Si, Ag, and Fe–18Cr–8Ni alloy (Type-304 stainless steel) were measured as a function of the temperature and oxygen partial pressure of an ambient atmosphere by an oscillating drop method using electromagnetic levitation, which assures measurement over a wide range of temperatures below and above the melting point and of oxygen partial pressure. For molten silicon, the lower limit of measured temperature was determined by equilibrium oxygen partial pressure for SiO2 formation; as oxygen partial pressure increases, undercooled conditions necessary for taking measurements become smaller. This is attributed to the low solubility of oxygen in molten silicon. For Ag, however, surface tension can be measured in a wide range of temperature and oxygen partial pressures due to the high solubility of oxygen. A boomerang-shaped behavior of surface tension was observed for Ag in an atmosphere with high oxygen partial pressure; surface tension has a maximum value when plotted against temperature. This boomerang-shaped behavior is attributed to desorption of oxygen at high temperature; surface tension increases with increasing temperature. The boomerang-shaped behavior was also observed for Fe–18Cr–8Ni alloy.