Varying Oxygen Partial Pressure Research Articles

The effect of oxygen partial pressure on physical properties of nano‐crystalline silver oxide thin films deposited by RF magnetron sputtering

AbstractNano‐crystalline silver oxide films were deposited on glass and silicon substrates held at room temperature by RF magnetron sputtering of silver target under different oxygen partial pressures. The influence of oxygen partial pressure on the structural, morphological, electrical and optical properties of deposited films was investigated. Varying oxygen partial pressure during the sputter deposition leads to changes of mixed phase of Ag2O and Ag to a single phase of Ag2O and to AgO. The X‐ray diffraction and X‐ray photoelectron spectroscopy results showed the formation of single phase Ag2O with cubic structure at oxygen partial pressures of 2x10‐2 Pa while the films deposited at higher oxygen partial pressure of 9x10‐2 Pa showed the formation of single phase of AgO with monoclinic structure. Raman spectroscopic studies on the single phase Ag2O showed the stretching vibration of Ag‐O bonds. Single‐phase Ag2O films obtained at oxygen partial pressure of 2x10‐2 Pa were nano‐crystalline with crystallite size of 20 nm and possessed an electrical resistivity of 5.2x10‐3 Ωcm and optical band gap of 2.05 eV. The films deposited at higher oxygen partial pressure of 9x10‐2 Pa were of AgO with electrical resistivity of 1.8x10‐2 Ωcm and optical band gap of 2.13 eV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Proton Conductivity in Acceptor-Doped LaVO4

Electrical characterization of nominally undoped LaVO4, La0.99Ca0.01VO4−δ and La0.95Ca0.05VO4−δ was performed in various partial pressures of oxygen, water vapor and hydrogen isotopes, from 300 to 1100°C by impedance spectroscopy, AC conductivity measurements (10 kHz) and EMF-measurements. XRD, SEM and EDS were used for structural, micro structural and compositional analysis. Acceptor doped LaVO4 is a pure ionic conductor in oxidizing atmospheres in the entire measured temperature range; dominated by proton conductivity at low temperatures (T < 450°C) under wet conditions and oxide ion conductivity at high temperatures. A maximum in the partial proton conductivity of ∼ 6 × 10−5 S/cm was reached at 900°C ( atm). Thermodynamics of hydration and transport parameters for charge carriers in La0.99Ca0.01VO4 were derived from relations between defect chemistry, transport properties and the measured conductivity data and revealed: =− 130 ± 10 J/mol K, ΔS0hydr,corr =− 142 ± 10 J/mol K, =− 110 ± 10 kJ/mol, = 50 ± 6 cm2 K/Vs, = 75 ± 10 kJ/mol, = 120 ± 20 cm2 K/Vs and = 85 ± 10 kJ/mol. The tetrahedron (XO4) volume and migration enthalpy were found to be correlated for the series of monoclinic LaXO4.

Microstructure and Electrical Properties of Amorphous $ \hbox{Bi}_{5}\hbox{Nb}_{3}\hbox{O}_{15}$ Films Grown on Cu/Ti/$ \hbox{SiO}_{2}$/Si Substrates Using RF Magnetron Sputtering

Amorphous Bi5Nb3O15 (BNO) films were grown at room temperature (RT) on a Cu/Ti/SiO2 /Si substrate using radio frequency magnetron sputtering. All the films were well formed on the Cu electrode with a sharp interface between the film and the electrode. The dielectric constant of the amorphous BNO film grown under 25 W was 46, with a low dissipation factor of 2.7% at 100 kHz. This film exhibited a low leakage current density of 5.5 × 10-8 A/cm2 at 4.5 V and a large breakdown voltage of 7.2 V. However, the electrical properties deteriorated as the sputtering power and the growth temperature increased due to the increased surface roughness; this was because a film with a rough surface generally has a larger surface area, and there can be electric field intensification at surface asperity, which degrade the electrical properties of the film. In addition, the electrical properties were not influenced by the oxygen partial pressure (OPP) because the variation of OPP during the growth of the films did not affect their surface roughness. The amorphous BNO film grown on the Cu/Ti/SiO2/Si substrate at RT under 25 W may be a good candidate material for an embedded capacitor.

Effect of substrate temperature and oxygen partial pressure on microstructure and optical properties of pulsed laser deposited yttrium oxide thin films

Yttrium oxide thin films were deposited on Si (1 1 1) and quartz substrates by pulsed laser deposition technique at different substrate temperature and oxygen partial pressure. XRD analysis shows that crystallite size of the yttrium oxide thin films increases as the substrate temperature increases from 300 to 873 K. However the films deposited at constant substrate temperature with variable oxygen partial pressure show opposite effect on the crystallite size. Band gap energies determined from UV–visible spectroscopy indicated higher values than that of the reported bulk value.

Oxygen non-stoichiometry, high-temperature properties, and phase diagram of CaMnO3–δ

The oxygen content in CaMnO3–δ is studied by a coulometric titration technique depending on temperature and oxygen partial pressure variations in the ambient atmosphere. The δ–T phase diagram is derived from the obtained data where single-phase fields are outlined for orthorhombic, tetragonal, and cubic structures. The thermal expansion coefficient considerably larger in the cubic phase than in the tetragonal one is related with formation of large Mn3+ cations at depletion of oxygen from the crystalline lattice. Negative thermopower is explained by concomitant reactions of oxygen loss and charge disproportionation, \( {\hbox{2M}}{{\hbox{n}}^{{{4 + }}}}{\hbox{ = M}}{{\hbox{n}}^{{{3 + }}}}{\hbox{ + M}}{{\hbox{n}}^{{{5 + }}}}{ } \). The forbidden energy gap in CaMnO3–δ is evaluated to be about 0.5 eV.

Influence of oxygen partial pressure on properties of N-doped ZnO films deposited by magnetron sputtering

N-doped ZnO films were radio frequency (RF) sputtered on glass substrates and studied as a function of oxygen partial pressure (OPP) ranging from 3.0×10 −4 to 9.5×10 −3 Pa. X-ray diffraction patters confirmed the polycrystalline nature of the deposited films. The crystalline structure is influenced by the variation of OPP. Atomic force microscopy analysis confirmed the agglomeration of the neighboring spherical grains with a sharp increase of root mean square (RMS) roughness when the OPP is increased above 1.4×10 −3 Pa. X-ray photoelectron spectroscopy analysis revealed that the incorporation of N content into the film is decreased with the increase of OPP, noticeably N 1s XPS peaks are hardly identified at 9.5×10 −3 Pa. The average visible transmittance (380-700 nm) is increased with the increase of OPP (from ∼17% to 70%), and the optical absorption edge shifts towards the shorter wavelength. The films deposited with low OPP (≤ 3.0×10 −4 Pa) show n-type conductivity and those deposited with high OPP (≥ 9.0×10 −4 Pa) are highly resistive (>10 5 Ω·cm)

Kinetic analysis and modelling of PET macromolecular changes during its mechanical recycling by extrusion

The rheometer cavity has been chosen to analyze, in carefully controlled exposure conditions, the PET macromolecular changes generated during its mechanical recycling by extrusion. Isothermal ageing experiments at 280 °C, under constant or variable oxygen partial pressures (between 0% and 21% of the atmospheric pressure), have allowed us to establish that two types of oxidative macromolecular changes take place successively in an extruder reactor. Chain scissions predominate in the “strongly oxygenated” zones (at the feeder and die), whereas chain couplings (mainly chain branching) predominate in “poorly oxygenated” zones (in the middle of the reactor). Thus, it appears that the relative predominance of both types of modifications is closely related to the extruder geometry and size (in particular, the feeder and die sections and the screw length). A kinetic model of thermal ageing of molten PET has been built to check these assumptions. It describes satisfyingly all the rheometric results obtained in the present study.

Thermodynamic studies of MgO saturated EAF slag

Foamy slags are widely practiced in electric arc furnace (EAF) steel production, reducing energy consumption, improving yield, reducing furnace noise, and increasing refractory service life. Control of slag chemistry within a specific range is necessary to achieve maximum benefits from slag foaming. The 'optimum' slag during EAF steel production is MgO saturated, containing a suspension of magnesium wüstite (MgO.FeO) particles. A thermodynamic program, FACTSAGE, was utilised to study a simplified steel slag system containing four slag oxides (MgO–CaO–FeO–SiO2), to predict the dual saturation level of CaO and MgO phases and to predict the MgO slag saturation level with variations in oxygen partial pressure, temperature and slag basicity. Results from thermodynamic calculations indicate that there are linear relationships among slag oxides and slag basicity which can be used to predict and control slag chemistry for foaming. Model data were compared with experimental data and models developed by other researchers.

High temperature electrical conduction in nanoscale hafnia films under varying oxygen partial pressure

Point defect equilibration in nanocrystalline hafnium oxide thin films in the monoclinic (m-HfO2) phase was studied by electrochemical measurements performed under varying temperature and oxygen partial pressure (PO2) on films of 35–63 nm thickness on single crystal MgO and Al2O3 substrates. The conductance varied as (PO2)n, where n is the in the range ∼+1/11 to ∼+1/14, at high PO2. The increasing conductance with PO2 suggests that the electronic conduction in the HfO2 films is p-type and oxygen interstitials or hafnium vacancies, rather than oxygen vacancies, could be dominant charged point defects in nanocrystalline, undoped m-HfO2 films.

Low-Field Magnetic Resonance Imaging to Visualize Chronic and Cycling Hypoxia in Tumor-Bearing Mice

Tumors exhibit fluctuations in blood flow that influence oxygen concentrations and therapeutic resistance. To assist therapeutic planning and improve prognosis, noninvasive dynamic imaging of spatial and temporal variations in oxygen partial pressure (pO(2)) would be useful. Here, we illustrate the use of pulsed electron paramagnetic resonance imaging (EPRI) as a novel imaging method to directly monitor fluctuations in oxygen concentrations in mouse models. A common resonator platform for both EPRI and magnetic resonance imaging (MRI) provided pO(2) maps with anatomic guidance and microvessel density. Oxygen images acquired every 3 minutes for a total of 30 minutes in two different tumor types revealed that fluctuation patterns in pO(2) are dependent on tumor size and tumor type. The magnitude of fluctuations in pO(2) in SCCVII tumors ranged between 2- to 18-fold, whereas the fluctuations in HT29 xenografts were of lower magnitude. Alternating breathing cycles with air or carbogen (95% O(2) plus 5% CO(2)) distinguished higher and lower sensitivity regions, which responded to carbogen, corresponding to cycling hypoxia and chronic hypoxia, respectively. Immunohistochemical analysis suggests that the fluctuation in pO(2) correlated with pericyte density rather than vascular density in the tumor. This EPRI technique, combined with MRI, may offer a powerful clinical tool to noninvasively detect variable oxygenation in tumors.

Fabricating TiO2 Photocatalysts by rf Reactive Magnetron Sputtering at Varied Oxygen Partial Pressures

Titanium dioxide (TiO2) thin films were fabricated onto non-alkali glass substrates by rf reactive magnetron sputtering at room temperature using Ti-metal target at varied oxygen partial pressure [O2/(Ar + O2)]. The sputtering deposition was performed under an rf power of 200 W. The target to substrate distance was kept at 80 mm, and the total gas pressure was 10 mTorr after 2 h of deposition. It was found that the crystalline structure, surface morphology, and photocatalytic activities of the TiO2 thin films were affected by the oxygen partial pressure during deposition. The XRD patterns exhibited a broad-hump shape indicating the amorphous structure of TiO2 thin films. The thin films deposited at a relatively high value of oxygen partial pressure (70%) had a good photo-induced decomposition of methylene blue (MB), photo-induced hydrophilicity, and had a small grain size.

Study of bactericidal efficiency of magnetron sputtered TiO 2 films deposited at varying oxygen partial pressure

TiO 2 thin films have been deposited at different Ar:O 2 gas ratios (20:80,70:30,50:50,and 40:60 in sccm) by rf reactive magnetron sputtering at a constant power of 200 W. The formation of TiO 2 was confirmed by X-ray photoelectron spectroscopy (XPS). The oxygen percentage in the films was found to increase with an increase in oxygen partial pressure during deposition. The oxygen content in the film was estimated from XPS measurement. Band gap of the films was calculated from the UV–Visible transmittance spectra. Increase in oxygen content in the films showed substantial increase in optical band gap from 2.8 eV to 3.78 eV. The Ar:O 2 gas ratio was found to affect the particle size of the films determined by a transmission electron microscope (TEM). The particle size was found to be varying between 10 and 25 nm. The bactericidal efficiency of the deposited films was investigated using Escherichia coli ( E. coli) cells under 1 h UV irradiation. The growth of E. coli cells was estimated through the Optical Density measurement by UV–Visible absorbance spectra. The qualitative analysis of the bactericidal efficiency of the deposited films after UV irradiation was observed through SEM. A correlation between the optical band gap, particle size and bactericidal efficiency of the TiO 2 films at different argon:oxygen gas ratio has been studied.

Layer-by-Layer Self-Assembled Osmium Polymer-Mediated Laccase Oxygen Cathodes for Biofuel Cells: The Role of Hydrogen Peroxide

High potential purified Trametes trogii laccase has been studied as a biocatalyst for oxygen cathodes composed of layer-by-layer self-assembled thin films by sequential immersion of mercaptopropane sulfonate-modified Au electrode surfaces in solutions containing laccase and osmium-complex bound to poly(allylamine), (PAH-Os). The polycation backbone carries the Os redox relay, and the polyanion is the enzyme adsorbed from a solution of a suitable pH so that the protein carries a net negative charge. Enzyme thin films were characterized by quartz crystal microbalance, ellipsometry, cyclic voltammetry, and oxygen reduction electrocatalysis under variable oxygen partial pressures with a rotating disk electrode. New kinetic evidence relevant to biofuel cells is presented on the detection of traces of H(2)O(2), intermediate in the O(2) reduction, with scanning electrochemical microscopy (SECM). Furthermore the inhibitory effect of peroxide on the biocatalytic current resulted in abnormal current dependence on the O(2) partial pressure and peak shape with hysteresis in the polarization curves under stagnant conditions, which is offset upon stirring with the RDE. The new kinetic evidence reported in the present work is very relevant for the operation of biofuel cells under stagnant conditions of O(2) mass transport.

Micro-scale modeling of flow and oxygen transfer in hollow-fiber membrane bundle

The aim of this work was to develop a modeling approach to solve the flow and oxygen transfer when the blood passes through the hollow-fiber membrane bundle. For this purpose, a “two-region” modeling approach was developed regarding the hollow-fiber and blood regions. The oxygen transfer in these regions was defined with separate diffusion processes. Two-dimensional single and multi-fiber geometries were created and flow solutions were obtained for a non-Newtonian fluid. The convection–diffusion–reaction equation was solved to produce the oxygen partial pressure distributions. As a benefit of coupling the interstitial flow field into the oxygen transfer through the hollow-fiber membrane bundle, the membrane resistance was taken into consideration. Thus, varying oxygen partial pressures were observed on the outer fiber surface, which is contrary to the common simplifying assumptions of negligible membrane resistance and uniform oxygen content on the fiber surface (traditional approach). It was illustrated that, the current approach can be utilized to predict the mass transfer efficiencies without overestimating as compared to the predictions obtained with the traditional approach. Utilization of the current approach was found to be beneficial for the geometries with lower packing density which allows significant P O 2 variations on the fiber surfaces. For the geometries with dense packings, the above simplifying assumptions could be applicable. The model predictions were validated with the experimental measurements taken from a benchmark device.

Preparation and structural characterization of SnO 2 and GeO 2 methanol steam reforming thin film model catalysts by (HR)TEM

Structure, morphology and composition of different tin oxide and germanium oxide thin film catalysts for the methanol steam reforming (MSR) reaction have been studied by a combination of (high-resolution) transmission electron microscopy, selected area electron diffraction, dark-field imaging and electron energy-loss spectroscopy. Deposition of the thin films on NaCl(0 0 1) cleavage faces has been carried out by thermal evaporation of the respective SnO 2 and GeO 2 powders in varying oxygen partial pressures and at different substrate temperatures. Preparation of tin oxide films in high oxygen pressures (10 −1 Pa) exclusively resulted in SnO phases, at and above 473 K substrate temperature epitaxial growth of SnO on NaCl(0 0 1) leads to well-ordered films. For lower oxygen partial pressures (10 −3 to 10 −2 Pa), mixtures of SnO and β-Sn are obtained. Well-ordered SnO 2 films, as verified by electron diffraction patterns and energy-loss spectra, are only obtained after post-oxidation of SnO films at temperatures T ≥ 673 K in 10 5 Pa O 2. Preparation of GeO x films inevitably results in amorphous films with a composition close to GeO 2, which cannot be crystallized by annealing treatments in oxygen or hydrogen at temperatures comparable to SnO/SnO 2. Similarities and differences to neighbouring oxides relevant for selective MSR in the third group of the periodic system (In 2O 3 and Ga 2O 3) are also discussed with the aim of cross-correlation in formation of nanomaterials, and ultimately, also catalytic properties.

Ab initio simulations of the Ag(111)/Al2O3 interface at intermediate oxygen partial pressures

The relative stability of different realizations of the Ag(111)/Alumina interfaces with varying oxygen partial pressures is investigated by means of ab initio density functional theory (DFT) simulations. Previous theoretical studies of similar systems always involve oversimplified geometries like stoichiometric Al-terminated, Al-rich, or O-terminated alumina interfaces. Such framework cannot explain the experimental behavior observed at intermediate oxygen partial pressure. Our approach, instead, suggests that the oxygen at the interface can play an important role at intermediate concentrations, leading to a more realistic interpretation of the experimental data.

Stability of MnCr2O4 spinel and Cr2O3 in high temperature carbonaceous environments with varied oxygen partial pressures

Abstract In this investigation, Cr 2 O 3 and MnCr 2 O 4 were comparatively tested at 1050 °C in carbonaceous environment with varied oxygen partial pressures. MnCr 2 O 4 exhibits much better resistance to carbonaceous attack than Cr 2 O 3 . The carburization rate of MnCr 2 O 4 decreases sharply with increasing oxygen partial pressures. The oxygen partial pressures have less effect on the carburization resistance of Cr 2 O 3 . The increased resistance of MnCr 2 O 4 to carburization is attributed to the dissolution of MnO into Mn–Cr–O spinel lattices with elevated oxygen partial pressures, which retards the decomposition and carburization of Mn–Cr–O spinel. The thermodynamic equations defining the carburization stability of MnCr 2 O 4 and Cr 2 O 3 are modified.

Reversible change in crystal structure of Ba(or Sr)FeO3-.DELTA. associated with oxygen sorption/desorption by pressure variation

Oxygen-enriched air has been expected to be used in several industrial processes for the purpose of less energy consumption as well as less CO 2 emission. Thus, an innovative and economical oxygen-separation method is eagerly needed for a large-scale production of oxygen-enriched air. Pressure-swing adsorption (PSA) method is considered to be effective for this purpose, if the high-performance oxygen adsorbents can be developed. It has been lately reported that La-Sr-Co-Fe-based perovskite-type oxide is a good candidate as a potential oxygen adsorbent for PSA. However, its cost is still relatively high, because the oxide is composed of relatively expensive elements of La and Co. Here, we compared the oxygen sorption behavior of the BaFeO 3-δ and SrFeO 3-δ samples which are composed of much inexpensive elements. These oxide samples were synthesized and then evaluated for oxygen sorption and desorption characteristics by the variation of oxygen partial pressure, with the simultaneous observation of change in crystal structure. As a result, it was found that the prepared BaFeO 3-δ and SrFeO 3-δ samples gave a relatively high amount of sorbed oxygen at lower (ca. 300°C) and higher (ca. 700°C) temperature, respectively. In addition, the reversible change in crystal structure associated with oxygen sorption/desorption was observed for both oxides at each best operating temperature.

Structural and optical properties of γ-alumina thin films prepared by pulsed laser deposition

Preparation of γ-alumina thin films by pulsed laser deposition from a sintered α-alumina target is investigated. The films were deposited on (100) silicon substrates at 973 K with varying oxygen partial pressures in the range 2.0 × 10 − 5–3.5 × 10 − 1 mbar. X-ray diffraction results indicated that the films were polycrystalline γ-Al 2O 3 with cubic structure. The films prepared in the oxygen partial pressure range 2.0 × 10 − 5 –3.5 × 10 − 2 mbar contained nanocrystals of sizes in the range 10–16 nm, and became amorphous at pressures > 3.5 × 10 − 1 mbar. Topography of the films was examined by atomic force microscopy using contact mode and it showed the formation of nanostructures. The root-mean square surface roughness of the film prepared at 2.0 × 10 − 5 mbar and 3.5 × 10 − 1 mbar were 1.4 nm and 3.5 nm, respectively. The thickness and optical properties were studied using ellipsometry in the energy range 1.5–5.5 eV for three different angles of incidence. The refractive index was found to decrease from 1.81 to 1.73 with the increase of oxygen partial pressures from 2.0 × 10 − 5 to 3.5 × 10 − 2 mbar. The variation in the refractive index has been found to be influenced by the microstructure of the films obtained as a function of oxygen partial pressure.

Application of Nano-Sized CeO&lt;sub&gt;2&lt;/sub&gt; Powder for Fast Response Oxygen Sensors

For resistive oxygen sensor elements of Ce1-xZrxO2 (x = 0.0, 0.05, 0.1, 0.2), grain diameter was varied in the range of 86 – 300 nm by changing sintering temperature or changing Zr content. The grain diameter decreased with increasing Zr content. The response time was approximately proportional to the square of the grain diameter. In the relationship between the amplitude of sensor output, An and the frequency, f of sine wave of variation in oxygen partial pressure, the gradient in the high-frequency region of a plot of log An vs. log f in was approximately –0.5. From these results, it was concluded that the sensor response was determined by the oxygen vacancy diffusion rate. The grain diameter of Ce0.8Zr0.2O2 element was 86 nm and the response time at 1073 K was 9 ms, which result opens the door to the technological development of independent control of engine cylinders.