Varying Oxygen Partial Pressure Research Articles

Evolution of the microstructure of sputter deposited TaAlON thin films with increasing oxygen partial pressure

Abstract Recently, quaternary oxynitrides of transition metals and aluminum have attracted increasing interest due to their tunable properties. Within the present work, a series of TaAl(O)N films was sputter deposited using constant nitrogen and varying oxygen partial pressures. The films were grown from single element Ta and Al targets. The deposition parameters were adjusted to obtain a Ta/Al atomic ratio of ~50/50 for the oxygen-free film and were held constant for the following depositions, with the exception of the increasing oxygen partial pressure and compensatory decreasing argon partial pressure. Elastic recoil detection analysis revealed oxygen contents of up to ~26 at.%, while the nitrogen content decreased from ~47 at.% in the oxygen-free film to ~35 at.% in the film with the highest oxygen content, resulting in a significant decrease of the metal/non-metal ratio with increasing oxygen partial pressure. The micro- and bonding structures of the films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and transmission electron microscopy. All films exhibited a dominating face-centered cubic TaN-based structure with indications for additional nanocrystalline and amorphous phase fractions in the oxygen containing films. In addition, the mechanical properties were evaluated by nanoindentation, yielding a decreasing hardness and elastic modulus with increasing oxygen content.

Synthesis and equilibrium oxygen nonstoichiometry of PrBaFe2O5+w

A double-cell perovskite PrBaFe2O5+w is synthesized and its equilibrium nonstoichiometry investigated after quenching from flowing gas of varied oxygen partial pressures and temperatures. Whereas w ​= ​0 is reached at 856 ​°C in wet H2 of pH2O ​= ​0.02 ​bar yielding pO2 ​= ​4×10−21 ​bar, w ​= ​0.75 obtains at 400 ​°C in O2. Below ~1000 ​°C, compositions with w ​≥ ​0.5 order into a Cmmm superstructure that fully converts back upon heating to 1020 ​°C. PrBaFe2O5 of all equivalent Fe2.5+ sites orders into Fe2+ and Fe3+ upon cooling through a first-order transition at 204 ​K, with a 2 ​K hysteresis, as investigated by powder X-ray diffraction and differential scanning calorimetry. The valence-mixed and charge-ordered structures are refined from synchrotron X-ray powder diffraction patterns. The nonstoichiometry w is expressed as a function of T and pO2 of the flowing gas in a formal donor-dopant defect model via least-squares fit of thermodynamic parameters for point-defect equilibria in the solid.

O2-oxidation of individual graphite and graphene nanoparticles in the 1200–2200 K range: Particle-to-particle variations and the evolution of the reaction rates and optical properties

The kinetics for O2 oxidation of individual graphite and graphene platelet nanoparticles (NPs) were studied as a function of temperature (1200–2200 K) at varying oxygen partial pressures, using a single nanoparticle mass spectrometry method. NP temperature (TNP) was measured by measuring the NP thermal emission spectra during the kinetics studies. The initial oxidation efficiency is found to peak in the 1200–1500 K range, dropping by an order of magnitude as TNP was increased above 2000 K. There were large NP-to-NP variations in the oxidation rates, attributed to variations in the NP surface structure. In addition, the oxidation efficiencies evolved, non-monotonically, as the NPs reacted, decreasing by factors of between 10 and 300. This evolution of reactivity is attributed to changes in the NP surface structure due to the combination of oxidation and annealing. The optical properties, including wavelength dependence of the emissivity and the absorption cross section for the 532 nm heating laser, also tended to evolve as the NPs oxidized, but differently for each individual NP, presumably reflecting differences in the initial structures and their evolution.

Tunable Optical Properties of Amorphous-Like Ga2O3 Thin Films Deposited by Electron-Beam Evaporation with Varying Oxygen Partial Pressures.

Ga2O3 thin films were fabricated by the electron-beam evaporation technique at a varying oxygen partial pressure from 0 to 2.0 × 10−2 Pa. The effect of oxygen partial pressure on the crystalline structure and optical properties of the Ga2O3 films was analyzed using sophisticated techniques including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, spectroscopic ellipsometry, ultraviolet-visible spectroscopy and a laser-induced damage test system. The correlation between the oxygen partial pressure and the film’s properties in optics and materials were investigated. XRD and Raman revealed that all films were amorphous in spite of applying a varying oxygen partial pressure. With the change of oxygen partial pressure, XPS data indicated that the content of oxygen in the Ga2O3 films could be broadly modulable. As a result, a changeable refractive index of the Ga2O3 film is realizable and a variable blue-shift of absorption edges in transmittance spectra of the films is achievable. Moreover, the damage threshold value varied from 0.41 to 7.51 J/cm2 according to the rise of oxygen partial pressure. These results demonstrated that the optical properties of Ga2O3 film can be broadly tunable by controlling the oxygen content in the film.

High-Performance Titanium Oxynitride Thin Films for Electrocatalytic Water Oxidation

TiNxOy (TiNO) thin films with superior electrochemical properties have been synthesized in situ using a pulsed laser deposition method and a varied oxygen partial pressure from 5 to 25 mTorr. The e...

CO2‐resistant SDC‐SSAF oxygen selective dual‐phase hollow fiber membranes

AbstractCe0.85Sm0.15O1.925–Sm0.6Sr0.4Al0.3Fe0.7O3‐δ (SDC‐SSAF) hollow fiber membranes aimed for oxyfuel application are fabricated via a phase inversion‐sintering technique using powders synthesized from EDTA–citrate complex method. Oxygen ions transport is revealed to be the limiting step that determines the overall oxygen transport rate in SDC‐SSAF. SDC‐SSAF hollow fiber membrane attained a maximum flux of 2.6 ml min−1 cm−2 at 950°C under an oxygen partial pressure difference of 1/0.02 atm (feed/permeate). Variation in oxygen partial pressure on the permeate side exerts larger influence on the oxygen permeation flux relative to the change on the feed side. Stability tests over 100 h demonstrate stable oxygen permeation flux under both He and CO2 sweep conditions.

The role of doping and microstructure on hydrogen solubility in monoclinic ZrO2: Experimental validations of simulated defect chemistry

The effect of chemical doping and ceramic microstructure on H solubility was measured systematically for the first time in monoclinic ZrO2. Excitingly, the influence of chemical doping by Fe, Cr and Ta cations on bulk H solubility was in qualitative agreement with defect chemistry simulations based on density functional theory and statistical thermodynamics. This is the first experimental validation of the recently-developed modeling framework, which is capable of high-throughput defect chemistry computations of light elements in metal oxides. H solubility was quantified using temperature-programmed desorption and was measured for various microstructures and under varied oxygen partial pressure, enabling us to identify the microstructural origin (i.e. bulk, surfaces and grain boundaries) of sorbed H defects and infer their defect type. Doping the oxide resulted in a significant increase in the H-uptake time, indicating that solute cations slowed H defect diffusion. Ta doping offered the lowest bulk H solubility at all measurement temperatures (410 °C–60 °C), though Fe doping also lowers bulk solubility at lower temperatures and yielded the most tightly bound H defects of all oxides. This could be significant in applications where one wishes to minimize H permeation via limiting diffusion kinetics. Based on this work we emphasize the role of both doping and microstructure on the overall H uptake and highlight the importance of elucidating the relative nuances of each oxide system; we thus reiterate the value of a high-throughput computational analysis.

Significant ion conduction in Cu acceptor-substituted bismuth titanate polycrystalline ceramics

This paper reports that Cu acceptor substitution at the Ti site has significant effects on structure and electrical properties of Aurivillius-phase Bi4Ti3O12 polycrystalline ceramics. Phase purity, microstructure and defect chemistry of Bi4Ti3−xCuxO12−x (BiT–100xCu, x = 0–0.12) are characterized by XRD, SEM, EDS and EPR measurements. Neutron diffraction refinement indicates that BiT–8Cu has an orthorhombic symmetry [space group: B2cb; lattice parameters: a = 5.4116(1) A, b = 32.833(1) A, c = 5.4479(1) A and V = 967.98(5) A3]. Impedance spectra of BiT–100xCu were measured under variable oxygen partial pressure (pO2). Bi4Ti3O12 shows mixed electronic (hole) and ionic conduction owing to the existence of oxygen vacancies ($$ {\text{V}}_{\text{O}}^{ \cdot \cdot } $$) arising from the loss of Bi2O3 during sintering. Interestingly, bulk conductivity in Cu-substituted Bi4Ti3O12 ceramics is predominately ionic due to the fact that there are considerably additional oxygen vacancies introduced into the perovskite lattices. The optimum composition of BiT–8Cu shows high ionic conductivity in the bulk with a value of ~ 0.007 S/cm at 650 °C.

New Electrode Materials for Symmetrical Solid Oxide Fuel Cells Based on Perovskites (La,Ca)(Fe,Co,Mg,Mo)O3 – δ

The conductivity of perovskites La0.3Ca0.7Fe0.6Mg0.175Mo0.225O3 – δ (LCF6) and La0.3Ca0.7Fe0.5Mg0.25Mo0.25O3 – δ (LCF5) is studied in the temperature interval of 573–1173 К at varied oxygen partial pressure. It is found that when going from air to the reductive atmosphere (8% Ar/H2), the conductivity of both samples at 1173 K increases from 4.6 to 25 S/cm for LCF6 and from 0.5 to 10 S/cm for LCF5. For LCF5, the conductivity is shown to be virtually independent of the oxygen partial pressure throughout the studied intervals of temperature and partial pressure, whereas for LCF6 two regions are observed. The conductivity of LCF5 remains constant at the cyclic change of the atmosphere from air to the reductive atmosphere and back. The method of impedance spectroscopy is used for studying the electrochemical behavior in air of the porous electrode based on (La,Ca)(Fe,Mg,Mo)O3 – δ deposited on the solid electrolyte of Ce0.8Gd0.2O1.9 (GDC). The polarization resistance of the electrode/electrolyte interface (Rη) is found to be 2.7 and 3.6 Ω cm2 for electrodes of LCF5 and LCF6, respectively, at 1173 K. The partial substitution of Co for Fe allows the Rη value of La0.3Ca0.7Fe0.45Co0.05Mg0.25Mo0.25O3 – δ and La0.3Ca0.7Fe0.55Co0.05Mg0.175Mo0.225O3 – δ to be decreased to ∼1.2 Ω cm2. Moreover, these materials demonstrate the CTE values close to that of GDC and also exhibit the acceptable conductivity in both air and reductive atmosphere and, hence, can be recommended for using as the electrode material in symmetrical solid-oxide fuel cells.

Evaluation of stability of Pr2−xNdxNiO4+δ by thermogravimetry under various oxygen partial pressures

The thermodynamic and kinetic stability of Pr2−xNdxNiO4+δ was evaluated by thermogravimetry (TG) under a varying oxygen partial pressure (P(O2)). This study was performed because Pr2−xNdxNiO4+δ has been anticipated as the new cathode material for solid oxide fuel cells. It was found that Pr2NiO4+δ decomposed to a mixture of PrO1.83 and Pr4Ni3O10+δ at a P(O2) of 1 atm above 750 °C. For specimens with x ≤ 0.5, similar decomposition was observed above 750 °C; however, this decomposition was not observed for specimens with x ≥ 0.75. Regeneration of Pr2−xNdxNiO4+δ from the decomposed species was observed as an abrupt mass loss in TG curves for x = 0.0–0.5 at a higher temperature; this could be regarded as the approximate equilibrium temperature for the decomposition reaction. Using the approximate equilibrium temperature under the examined P(O2), thermodynamically stable P(O2) and temperature region of Pr2NiO4+δ was clarified and represented using a van’t Hoff plot. It was revealed that the thermodynamically stable region expanded only slightly with the partial substitution of Nd at the Pr site. Reaction kinetics of the decomposition was evaluated by isothermal TG at various P(O2). With decreasing P(O2) or increasing Nd content, kinetic stability of Pr2−xNdxNiO4+δ significantly improved; this was in good agreement with the observation of no decomposition for specimens with x ≥ 0.75. A higher activation energy was observed for decomposition kinetics in the Nd substituted specimen than that obtained by solely decreasing P(O2); this indicated that Nd substitution was more effective in preventing oxidation of Pr, which induced decomposition.

Phase growth analysis of sputtered TiO2 thin films at low oxygen partial pressures using XANES and XRR

This paper reveals the impact of oxygen in the phase growth of titanium oxide thin films. Films were developed using dc magnetron sputtering with Ti metal target at very low oxygen partial pressures (0–0.04) for 30 min. X-ray reflectivity (XRR) of the films was done to ascertain the variations in the structural properties of the films. Film roughness continually increases even for slight variations in oxygen partial pressure while the film density stabilizes within 3.6–3.8 g cm−3 with increasing oxygen content. L2,3 and O K-edge x-ray absorption near-edge structure (XANES) spectroscopy was done to identify the phase structures and oxidation states of the films. As-deposited films are all amorphous in nature, with no major change in their properties apart from increased thickness (280–910 Å). After post heating the films at 400 °C for 5 h in vacuum (10−5 torr) various crystalline phases evolve. A slight increase in oxygen partial pressure leads to a gradual transformation in the evolved phase of TiO2 films. Ti2O3 phase with Ti [Varghese et al 2001 Sensors Actuators B Chem. 81 32–41] coordination environment and Ti3+ oxidation state was detected with O2 partial pressure of 0.015 whereas a metastable anatase and stable rutile phase with Ti [Mosquera et al 2014 J. Anal. At. Spectrom. 29 736] coordination environment and Ti4+ oxidation state was observed with 0.02 and 0.04 O2 partial pressures, respectively.

Probing the correlation between phase evolution and growth kinetics in the oxide layers of tungsten using Raman spectroscopy and EBSD

Tungsten, a plasma-facing material for future fusion reactors, may be exposed to air during abnormal operation or accidents. Only limited information is available on the evolution of related oxide phases. This work addresses the effect of substrate orientation on structural variations of tungsten oxides. Annealing experiments in an argon-oxygen atmosphere have been conducted at T = 400 °C under varying oxygen partial pressure and oxidation time. A combination of EBSD, Raman spectroscopy and confocal microscopy shows preferential oxidation initially on {111} base material planes. The oxide scale changes its phase composition dynamically, influencing the kinetics of its growth.

Scale up of Biofilm Electrodes

Bioelectrochemical systems (BESs) utilize the ability of microbes to exchange electrons with solid inert electrodes to convert chemical energy to electrical energy, and have potential applications in wastewater treatment, chemical synthesis, and resource recovery. Optimization of large scale systems is required in order to implement BESs at industrial scale. Several reports documented the loss of current density in large scale biofilm electrodes. We hypothesize that the loss of current density is attributed to substrate transport limitations. The heterogeneity of local conditions on biofilm electrodes at low substrate environments are expected to decrease the current density of large electrodes. In this work, we tested the effect of substrate concentration on current density of anodes and cathodes. Anodic and cathodic biofilms are enriched on electrodes of different electrode sizes (15-460 cm2). Following enrichment, the anodes are tested in synthetic wastewater medium with varying electron donor concentrations, while the cathodes are tested in synthetic wastewater with varying oxygen partial pressures. Our results demonstrate the relationship between substrate concentration and the scaleup of anodic and cathodic biofilms.

Thermodynamic of intrinsic defects in β-Ga2O3

The stability of intrinsic point defects in β-Ga2O3 under varied temperature, Fermi energy and oxygen partial pressure has been studied using the hybrid functional (HSE06) and thermodynamic method. The results indicate that the contribution of vibrational entropy to formation energy should not be ignored, especially under high temperature. Gallium interstitials, oxygen vacancies, and gallium vacancies are the most stable intrinsic point defects in crystal when Fermi level varies from the valence-band maximum to the conduction-band minimum. Donors (oxygen vacancies and gallium interstitials) and acceptors (gallium vacancies and oxygen interstitials) have deep-level transitions (more than 1eV), so they don't contribute to the unintentional n-type or p-type conductivity. We propose that the high temperature and high oxygen partial pressure are in favor of p-type doping in β-Ga2O3 crystal by weakening the self-compensation effect.

Effect of Oxygen Partial Pressure on the Phase Stability of Copper⁻Iron Delafossites at Elevated Temperatures.

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO2), a material class with promising thermoelectric properties at high temperatures, were investigated. This work focuses on the phase stability of CuFeO2 with respect to the temperature and the oxygen partial pressure. For this reason, classical material characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction, were combined in order to elucidate the phase composition of delafossites at 900 °C at various oxygen partial pressures. The experimentally obtained results are supported by the theoretical calculation of the Ellingham diagram of the copper–oxygen system. In addition, hot-stage X-ray diffraction and long-term annealing tests of CuFeO2 were performed in order to obtain a holistic review of the phase stability of delafossites at high temperatures and varying oxygen partial pressure. The results support the thermoelectric measurements in previous publications and provide a process window for the use of CuFeO2 in thermoelectric generators.

Synthesis of phase pure vanadium dioxide (VO2) thin film by reactive pulsed laser deposition

Thin films of phase pure VO2 (M1 phase) are deposited on thermally grown oxide (Si/SiO2) substrate by reactive pulsed laser deposition of vanadium metal target. The influence of deposition parameters is studied by varying oxygen partial pressure while keeping other parameters constant. A thin film obtained at 50 mTorr is found to be phase pure by XRD and Raman spectral studies. SEM and AFM studies show smooth morphology with a surface roughness of 3–5 nm. The quality of the thin film was further established by characterizing the 68 °C (TSMT) transition by 2–3 orders of jump in resistance and a 60% change in reflectivity with minimum hysteresis. This single step process of deposition would be a suitable method for fabricating devices for smart window and metamaterial applications.

Superoxide Formation in Cardiac Mitochondria and Effect of Phenolic Antioxidants.

Since mitochondria are the main cellular source of reactive oxygen species, it is important to study the effect of dietary phenolic compounds on the level of ROS in these organelles. Using the EPR spectroscopy and TIRON probe, the ability of the investigated phenols (quercetin, rutin, caffeic acid, curcumin, and resveratrol) to scavenge superoxide anion radicals generated by isolated heart mitochondria of Wistar rats under variable oxygen partial pressure was studied. It was shown that during a 10 min incubation, caffeic acid in concentrations of 10-500 μM most effectively scavenged superoxide radicals formed in the complex III of the mitochondrial respiratory chain. A comparable antioxidant effect of rutin under these experimental conditions was observed at higher concentrations of 1-10 mM. The antioxidant activity of quercetin in the concentration range of 10-500 μM during the first minutes of incubation was higher than that of caffeic acid. Of the phenolic compounds studied, curcumin had the least effect on the superoxide radicals.

Photodegradation of Stearic Acid Adsorbed on Copper Oxide Heterojunction Thin Films Prepared by Magnetron Sputtering

Nanocrystalline copper oxide thin films were fabricated by reactive DC magnetron sputtering. The structure and optical properties of the films were measured with X-ray diffraction, scanning electron microscopy, and spectrophotometry. Variations of oxygen partial pressure resulted in oxide composition ranging from Cu, Cu-Cu2O, Cu2O-CuO1−x, and CuO. Optical band transitions at 2.06 eV and 2.55 eV were found for Cu2O corresponding to the direct forbidden and direct allowed interband transitions. For CuO an indirect allowed interband transition was found at 1.28 eV. The photocatalytic activity was determined by quantifying the rate constant and quantum yield (destroyed molecules/photons absorbed) under stearic acid degradation. Photocatalytic activity was found to be highest in mixed-phase films with Cu-Cu2O films the highest. Results from post-annealed Cu-Cu2O and CuO films show similar results. We interpret our results as being due to efficient electron-hole charge separation in the heterojunction films. The obtained quantum yields were generally about ten times lower than for comparable dense TiO2 and WO3 binary oxides, which calls for further studies of the spectral dependence of the quantum yield and electron-hole pair life times for oxides with different purity levels.

Rhodamine (B) photocatalysis under solar light on high crystalline ZnO films grown by home-made DC sputtering

ZnO thin films were deposited by home-made DC sputtering of zinc target under mixed gases (Argon, Oxygen) plasma on glass substrates. Films were deposited by varying oxygen partial pressure (PO2) from 0.09 to 1.3 mbar in the deposition chamber, at a fixed substrate temperature of 100 °C. The samples were characterized by photoluminescence (PL), X-ray diffraction (XRD), optical transmissions (UV–vis), scanning electron microscopy (SEM) and electrical (Hall effect) measurements. The results indicate that by varying the oxygen pressure in the deposition chamber, the films show a precise and well defined photoluminescence emissions for each range of pressure covering almost the entire visible domain (UV, UV-Violet, Violet, Blue, and Red) with high intensities. Moreover, the deposited films have different defects levels. The XRD analysis indicates that the films are well grown along the c-axis peak, but with different crystalline quality. Optical measurements reveal a high transmission, up to 90%, in the spectral region between 400 and 2500 nm and a large variation of the optical band gap (3.16–4.34 eV). As an application of the deposited ZnO films, the photo-catalytic degradation of a synthetic solution of Rhodamine B (RhB) poured on a ZnO thin film was successfully achieved and an elimination rate of 38% was obtained after exposing the film to solar light for 3 h.

Influence of Oxygen Partial Pressure Variation on Oxidation Resistance of Ti-46.7Al-1.9W-0.5Si at 750, 850 and 950 °C

γ-TiAl intermetallic alloys can be potentially employed as structural materials for many industries where high temperature and access of atmosphere with variable oxygen partial pressures are often met. Different partial pressures of oxygen may influence oxidation behavior and induce different oxide scale phase constitutes. This study investigated the influences of oxygen partial pressures on the oxidation resistance of a γ-TiAl alloy, Ti-46.7Al-1.9W-0.5Si (at.%), in Ar-O2 environments (5%O2, 20%O2 and 80%O2) at 750, 850 and 950 °C, respectively. The oxidation products were characterized using scanning electron microscopy, energy-dispersive x-ray analysis, x-ray diffraction and transmission electron microscopy. The oxidation kinetic results achieved by a discontinuous gravimetric method revealed that the alloy oxidized following a parabolic rate law. The oxidation resistance of the alloy was enhanced with increasing the oxygen partial pressures at all three experimental temperatures. Multilayered oxide scales of TiO2/Al2O3 were developed during the oxidation process. The enhancement of oxidation resistance was resulted from preferential generation of Al2O3 and the reduction in defects in TiO2 with increasing the oxygen partials pressures.