Oxygen Pressure Range Research Articles

Resistance of Boron Nitride Nanotubes to Radiation-Induced Oxidation.

We present unprecedented results on the damage thresholds and pathways for boron nitride nanotubes (BNNT) under the influence of energetic electrons in an oxidative gas environment, using an environmental aberration-corrected electron microscope over a range of oxygen pressures. We observe a damage cascade process that resists damage until a higher electron dose, compared with carbon nanotubes, initiating at defect-free BNNT sidewalls and proceeding through the conversion from crystalline nanotubes to amorphous boron nitride (BN), resisting oxidation throughout. We compare with prior results on the oxidation of carbon nanotubes and present a model that attributes the onset of damage in both cases to a physisorbed oxygen layer that reduces the threshold for damage onset. Surprisingly, increased temperatures offer protection against damage, as do electron dose rates that significantly exceed the oxygen dose rates, and our model attributes both effects to a physisorbed oxygen population.

Reliable operation of Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes at 600 °C

Beta gallium oxide (β-Ga2O3)-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are, thus, promising for applications in power electronic devices and harsh environment sensors. However, the long-term reliability of these ultra-wideband gap (UWBG) semiconductor devices remains barely addressed and may be strongly influenced by chemical reactions at the p–n heterojunction interface. Here, we experimentally demonstrate operation and evaluate the reliability of Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes during extended operation at 600 °C, as well as after 30 repeated cycles between 25 and 550 °C. The calculated pO2-temperature phase stability diagram of the Ga-Cr-O material system predicts that Ga2O3 and Cr2O3 should remain thermodynamically stable in contact with each other over a wide range of oxygen pressures and operating temperatures. The fabricated Cr2O3:Mg/β-Ga2O3 p–n heterojunction diodes show room-temperature on/off ratios >104 at ±5 V and a breakdown voltage (VBr) of −390 V. The leakage current increases with increasing temperature up to 600 °C, which is attributed to Poole–Frenkel emission with a trap barrier height of 0.19 eV. Over the course of a 140-h thermal soak at 600 °C, both the device turn-on voltage and on-state resistance increase from 1.08 V and 5.34 mΩ cm2 to 1.59 V and 7.1 mΩ cm2, respectively. This increase is attributed to the accumulation of Mg and MgO at the Cr2O3/Ga2O3 interface as observed from the time-of-flight secondary ion mass spectrometry analysis. These findings inform future design strategies of UWBG semiconductor devices for harsh environment operation and underscore the need for further reliability assessments for β-Ga2O3-based devices.

Consistent interpretation of isotope and chemical oxygen exchange relaxation kinetics in SrFe0.85Mo0.15O3-δ ferrite.

This paper is devoted to the study of phase composition and kinetic and thermodynamic characteristics of Mo-doped strontium ferrite SrFe0.85Mo0.15O3-δ (SFM15) under oxygen-conducting membrane working conditions. Single-phase SFM15 with a cubic Pm3̄m structure was synthesized using a ceramic method. It was shown that the molybdenum introduction stabilizes the perovskite cubic structure over a wide range of oxygen pressures and temperatures, preventing the bulk phase transition at high temperatures. Oxygen exchange constants, diffusion coefficients and activation energy of oxygen exchange were obtained using oxygen relaxation and isotopic exchange techniques, and the obtained values are consistent with known literature data. It was shown that the surface reaction rates obtained using chemical and tracer relaxation methods are quantitatively comparable with each other, despite significantly different experimental conditions. This result not only confirms the reliability of the data obtained by independent methods, but also allows one to expand the area of physical conditions for studying the kinetics of oxygen transfer where another method has technical or methodological limitations.

Epitaxial growth and characterization of magnesium gallate (MgGa2O4) thin films by pulsed laser deposition

In this work, crystalline MgGa2O4 thin films were grown on c-plane sapphire substrates using the pulsed laser deposition (PLD) technique in a broad range of temperatures and oxygen pressures. The parameters for the crystalline growth were within the temperature range of 300 ℃ to 700 ℃ and the pressure range of 1 × 10-1 to 1 × 10-3 Torr. The acquired XRD patterns confirmed the film growth along the [111] preferential crystal orientation in the lattice and the rocking curve measurement of the prominent (222) plane showed an increasing trend of the crystallinity with growth temperature and pressure. Furthermore, the XRD phi (φ) scan demonstrated the six-fold rotational symmetry of the MgGa2O4 films and the epitaxial relationship of 30° between the film and sapphire substrate. The XPS spectra confirmed the presence of +2 and +3 oxidation state for Mg and Ga respectively, in the films and vacancies that increase with decreasing oxygen partial pressure. The direct bandgap of MgGa2O4 films was obtained to be ∼5.27 ± 0.03 eV by analyzing the UV-Vis absorbance spectra. The film surface exhibited a granular morphology with an increasing trend in grain size as the temperature increased. The refractive index and thickness of these films were in the range of ∼1.90 ± 0.02 and ∼70 ± 2.0 nm, respectively determined from the fitted spectroscopic ellipsometry data. The surface roughness was obtained from AFM images and found to be in good agreement with those obtained from the analysis of the ellipsometry data.

The oxidation of an equimolar FeCoNiAl medium-entropy alloy at 900 °C in various oxygen pressures

The oxidation of an equimolar FeCoNiAl medium-entropy alloy was studied at 900 °C in various O2-containing atmospheres over the oxygen pressure range of 10–105 Pa. The alloy oxidized parabolically, having its oxidation rates steadily increasing with oxygen pressure. Duplex scales formed on the oxidized substrate, consisting of an outer CoFe2O4 layer and an inner α-Al2O3 layer. The position of the Pt-marker was located at the outer- and inner-layer boundary, demonstrating that the outward cation diffusion controlled the formation of CoFe2O4, while the growth of Al2O3 was contributed by the inward oxygen diffusion.

Determination of Kinetic Parameters and Identification of the Rate-Determining Steps in the Oxygen Exchange Process for LaNi0.6Fe0.4O3-δ.

The mixed ionic and electronic oxide LaNi0.6Fe0.4O3-δ (LNF) is a promising ceramic cathode material for solid oxide fuel cells. Since the reaction rate of oxygen interaction with the cathode material is extremely important, the present work considers the oxygen exchange mechanism between O2 and LNF oxide. The kinetic dependence of the oxygen/oxide interaction has been determined by two isotopic methods using 18O-labelled oxygen. The application of the isotope exchange with the gas phase equilibrium (IE-GPE) and the pulsed isotope exchange (PIE) has provided information over a wide range of temperatures (350-800 °C) and oxygen pressures (10-200 mbar), as each method has different applicability limits. Applying mathematical models to treat the kinetic relationships, the oxygen exchange rate (rH, atom × cm-2 × s-1) and the diffusion coefficient (D, cm2/s) were calculated. The values of rH and D depend on both temperature and oxygen pressure. The activation energy of the surface exchange rate is 0.73 ± 0.05 eV for the PIE method at 200 mbar, and 0.48 ± 0.02 eV for the IE-GPE method at 10-20 mbar; for the diffusion coefficient, the activation energy equals 0.62 ± 0.01 eV at 10-20 mbar for the IE-GPE method. Differences in the mechanism of oxygen exchange and diffusion on dense and powder samples are observed due to the different microstructure and surface morphology of the samples. The influence of oxygen pressure on the ratio of contributions of different exchange types to the total oxygen exchange rate is demonstrated. For the first time, the rate-determining step in the oxygen exchange process for LNF material has been identified. This paper discusses the reasons for the difference in the mechanisms of oxygen exchange and diffusion.

Effect of oxygen pressure during the growth of ZnSnO3 epitaxial thin films on LiNbO3 substrates

High-quality ZnSnO3 epitaxial films are grown on LiNbO3(0001) substrates by pulsed laser deposition in an oxygen pressure range of 0.1–10 Pa. The crystalline quality, composition, surface topography, structure, defect formation energy and photovoltaic properties of the epitaxial films are found to be highly sensitive to change in the oxygen pressure by experiments and first-principles density functional theory (DFT). The epitaxial relationship between the film and the substrate is identified as ZnSnO3(0001) || LiNbO3(0001) with ZnSnO3 [2¯ 110] || LiNbO3[2¯ 110]. The film grown atPO2 = 10 Pa has the highest crystal quality, but the photodetector based on the film obtained atPO2 = 0.1 Pa presents the highest responsivity (2.24 A/W at 1 V) and light/dark current ratio (∼105 at 1 V) under the 254-nm-light irradiation. Our results indicate that the as-grown ZnSnO3 epitaxial film should be a potential material for fabricating high-performance UV photodetectors.

Gold Extraction from a Refractory Sulfide Concentrate by Simultaneous Pressure Leaching/Oxidation

Most gold deposits occur associated with sulphides like pyrite and arsenopyrite; thus, precious metal dissolution is possible by oxidizing auriferous sulfide concentrate using simultaneous pressure oxidation and cyanidation. The effectiveness of this process of extraction can be influenced by the temperature, cyanide (NaCN) concentration, and oxygen pressure. In this study, we conducted experiments to characterize the effects on gold extraction of ores using a range of sodium cyanide concentrations (1–8 g/L), temperatures (40–75 °C), and oxygen pressures (0.5–1.1 MPa). Characterization of the ores showed that pyrite and quartz were the main minerals present in the concentrate. The best results in terms of the highest extraction of Au were obtained with an oxygen pressure of 0.5 MPa, 6 g/L sodium cyanide, and a temperature of 75 °C, along with a constant stirring speed of 600 rpm. These conditions allowed for approximately 95% gold extraction in 90 min.

Controlling the Size of Au Nanoparticles on Reducible Oxides with the Electrochemical Potential.

Controlling the size of Au nanoparticles (NPs) and their interaction with the oxide support is important for their catalytic performance in chemical reactions, such as CO oxidation and water-gas shift. It is known that the oxygen vacancies at the surface of support oxides form strong chemical bonding with the Au NPs and inhibit their coarsening and deactivation. The resulting Au/oxygen vacancy interface also acts as an active site for oxidation reactions. Hence, small Au NPs are needed to increase the density of the Au/oxide interface. A dynamic way to control the size of the Au NPs on an oxide support is desirable but has been missing in the field. Here, we demonstrate an electrochemical method to control the size of the Au NPs by controlling the surface oxygen vacancy concentration of the support oxide. Oxides with different reducibilities, La0.8Ca0.2MnO3±δ and Pr0.1Ce0.9O2-δ, are used as model support oxides. By applying the electrochemical potential, we achieve a wide range of effective oxygen pressures, pO2 (10-37-1014 atm), in the support oxides. Applying the cathodic potential creates a high concentration of oxygen vacancies and forms finely distributed Au NPs with sizes of 7-13 nm at 700-770 °C in 10 min, while the anodic potential oxidizes the surface and increases the size of the Au NPs. The onset cathodic potential required to create small Au NPs depends strongly on the reducibility of the support oxide. The Au NPs did not undergo sintering even at 700-770 °C under the cathodic potential and also were stable in catalytically relevant conditions without potential.

Laser synthesis of volatile memristors based on niobium oxide thin films

Amorphous NbOx (x ≤ 2.5) thin films with the presence of nanocrystallites, which are promising for use as an active region of volatile memristors were fabricated on c-Al2O3 substrates by the laser synthesis. The films were obtained from targets of niobium metallic during ablation of laser with λ = 248 nm or λ = 532 nm in oxygen pressure range from 5 to 50 mTorr. The effect of the oxygen partial pressure and the wavelength of ablating laser on the chemical, structural, electrical, and optical properties of the films was studied. The optimal conditions for the formation of Nb2O5 and NbO2 stable phases in the films were determined. It was found that the amorphous NbOx phase, which was a source of defects associated with oxygen deficiency, required for the creation of memristors, appeared only at low oxygen pressures in the chamber (∼ 5 ⋅ 10−3 − 10−1 Ohm ⋅ cm) during the film synthesis by the laser with both λ = 248 nm and λ = 532 nm. Volatile memristors of Nb/NbОх/Nb2O5/Pt/с-Аl2O3 (x ≤ 2.5) in cross-bar geometry were created demonstrating long-term stability of operation for 60 DC cycles with ΔRHRS/RHRS < 8% at an operating switching voltage of ∼ 0.5 V.

Adsorption of Oxygen to High Entropy Alloy Surfaces for up to 2 ML Coverage Using Density Functional Theory and Monte Carlo Calculations

High-entropy alloys (HEAs) manufactured with refractory elements are candidates for high-temperature structural applications. To enhance our understanding of oxidation in these complex systems, the early stages of oxidation on the surface of Al10Nb15Ta5Ti30Zr40 were studied using density functional theory and thermodynamic modeling. Surface slabs were generated from bulk configurations sampled from equilibrium using a multicell Monte Carlo method for phase prediction. The bulk structure was found to be a single BCC phase in good agreement with experimental observations. The oxygen adsorbed with a strong preference for sites with Ti and Zr and avoided sites with Nb-Al and Nb-Ta. The surface was shown to be highly reactive to oxygen, yielding a dominating oxygen coverage of two monolayers over the temperature range of 100 to 2600 K and an oxygen pressure range of 10-30 to 105 bar. Recovering a clean surface slab was not achieved until pressures approached vacuum conditions and temperature exceeded 1900 K, demonstrating the difficulty of oxygen removal from the surface. Grand canonical Monte Carlo simulations showed that high Nb content in the top surface layer reduced the surface reactivity to incoming oxygen. Inward oxygen diffusion at low coverage was preferred in regions rich with Zr but slowed with the addition of Ti and Al. Diffusion rates drastically decreased at 1 ML, especially in the region rich with Ti and Zr, where strong metal-oxygen bonds were reported. Our results indicated that a high content of Ti and Zr increased the reactivity of the HEA surface to oxygen. The presence of Nb also enhanced the resistance to oxygen adsorption, especially when partnered with Al and Ta. Inward oxygen diffusion was likely to occur at low coverage in regions rich with Zr but could be protected against with the addition of Al and Ti. The limitations of the present work are discussed. This study may provide insights that assist with devising short- and long-term mitigation strategies against material degradation related to high-temperature oxidation.

Photolysis of C2H2F2Br2 Mixture with O2 in the Oxygen Pressure Range 1–3.5 Torr

Photolysis of C2H2F2Br2 Mixture with O2 in the Oxygen Pressure Range 1–3.5 Torr

Moderating oxygen deficiency induced better thermochromic properties of monoclinic vanadium dioxide thin films

Monoclinic vanadium dioxide (M-VO2) is a kind of typical thermal phase change materials, but its relatively high phase transition temperature (68 °C) and low solar modulation efficiency greatly hinder the applications as energy materials. In this work, well-crystalline M-VO2 were prepared by pulsed laser deposition (PLD) under the adjustable oxygen pressure range. By fine adjusting the low oxygen pressure (0.1–0.9 Pa), VO2 maintain the monoclinic crystal and excellent photoelectrical properties, even if oxygen deficiency and other defects exist. Results indicate that the phase-transition temperature (Tc) decreases with the decrease of oxygen pressures while the solar modulation efficiency (ΔTsol) increases with the decrease of oxygen pressures. Moreover, the M-VO2 thin films prepared with 0.3 Pa oxygen pressure showed the best photoelectric performance, in which the sheet resistance changes up to three orders of magnitude, the phase-transition temperature is 61 °C and the ΔTsol is as high as 8.3%. This work has demonstrated the role of moderate oxygen defects in optimizing the photoelectric properties of M-VO2, also provided a feasibility study on how to deposit pure M-VO2 by PLD.

Laser synthesis of non-volatile memristor structures based on tantalum oxide thin films

The amorphous thin TaOx films (x ≤ 5) of different thicknesses with different levels of oxygen vacancy content, which are promising for use as an active region of non-volatile memristors, were fabricated by the pulsed laser deposition in a drop-free mode on the c-sapphire substrates. The films were obtained from tantalum metal targets at substrate temperatures of 25 °C and 350 °C in the oxygen pressure range from 0.5 to 50 mTorr. The relationship between the partial oxygen pressure and the optical, electrical, and chemical properties of the films was studied. It was found that the amorphous TaOx phase, which is a source of defects associated with oxygen deficiency, required for the creation of memristors, appears only at low oxygen pressures in the chamber (~ 5 × 10−4 Torr), this leads to low values resistivity in the films (~ 10−3 Ohm · cm). Memristor structures Ag(Pt)/TaOx/TaOy/Ta/c-Al2O3 (x > y) in cross-bar geometry have been created that do not require electroforming, due to the choice of design and deposition conditions. Memristor using two mechanisms of resistive switching based on oxygen vacancies and silver cations showed very reliable RS performance with memory window performance RON/ROFF ~ 103 and RESET operating voltage ~ 1 V.

Plasmonic effect of diffused Ag nanoparticles in EB evaporated Ag/TiO2 bilayer thin films and role of oxygen pressure

Oxygen pressure mediated interfacial diffusion of silver up to top surface of thick (∼120 nm) TiO2 layer in e-beam evaporated Ag/TiO2 bilayer thin films has been observed. This effect has been investigated in oxygen pressure range of 5.9 × 10−5-5.8 × 10−4 mbar during evaporation. The amount of Ag diffusion (5–24%) has been found to be correlated with the induced porosity of TiO2 layer. Both compositional and structural characterizations confirm Ag0 chemical state of diffused silver. Scanning Electron Microscopy revealed Ag aggregates having an approximate average diameter of ∼4 nm and ∼24 nm on top surface. Spectroscopic ellipsometry analysis demonstrates that plasmonic absorption caused by these Ag nanoparticles manifests significant modulation of effective complex refractive indices (ECRI) of TiO2 layer. Specifically, the imaginary part of ECRI of TiO2 exhibits substantial non-zero value (∼6.5 × 10−2) in the visible wavelength range which is otherwise close to zero (<10−5) for intrinsic TiO2 thin film. Finally, the optical role of diffused silver on wavelength and peak of interference absorption in Ag/TiO2 bilayer thin film system has been demonstrated. This investigation gives insight on plasmonic contribution of diffused Ag nanoparticles and its consequence on ECRI of dielectric thin film due to in-situ e-beam evaporation without any post deposition treatment.

Electrochemical Polarization Dependence of the Elastic and Electrostatic Driving Forces to Aliovalent Dopant Segregation on LaMnO3.

Segregation of aliovalent dopant cations is a common degradation pathway on perovskite oxide surfaces in energy conversion and catalysis applications. Here we focus on resolving quantitatively how dopant segregation is affected by oxygen chemical potential, which varies over a wide range in electrochemical and thermochemical energy conversion reactions. We employ electrochemical polarization to tune the oxygen chemical potential over many orders of magnitude. Altering the effective oxygen chemical potential causes the oxygen nonstoichiometry to change in the electrode. This then influences the mechanisms underlying the segregation of aliovalent dopants. These mechanisms are (i) the formation of oxygen vacancies that couples to the electrostatic energy of the dopant in the perovskite lattice and (ii) the elastic energy of the dopant due to cation size mismatch, which also promotes the reaction of the dopant with O2 from the gas phase. The present study resolves these two contributions over a wide range of effective oxygen pressures. Ca-, Sr-, and Ba-doped LaMnO3 are selected as model systems, where the dopants have the same charge but different ionic sizes. We found that there is a transition between the electrostatically and elastically dominated segregation regimes, and the transition shifted to a lower oxygen pressure with increasing cation size. This behavior is consistent with the results of our ab initio thermodynamics calculations. The present study provides quantitative insights into how the elastic energy and the electrostatic energy determine the extent of segregation for a given overpotential and atmosphere relevant to the operating conditions of perovskite oxides in energy conversion applications.

The oxidation behavior of a Ni2FeCoCrAl0.5 high-entropy superalloy in O2-containing environments

The oxidation behavior of a Ni2FeCoCrAl0.5 high-entropy superalloy was studied at 900 °C in O2-containing environments over the oxygen pressure range of 10 - 1.0 × 105 Pa. The superalloy oxidized parabolically at PO2< 2.1 × 104 Pa, having its oxidation rates increasing with increasing oxygen pressure. An exclusive α-Al2O3 layer formed on the alloy with its thickness increased with oxygen pressure. On the contrast, a mass-lost kinetics was observed and heterophasic scales of intermixed α-Al2O3, Cr2O3, and M-Cr spinels (M = Ni, Fe, or Co) were detected at PO2 = 1.0 × 105 Pa.

Equilibrium of Defects and Electrical Conductivity of Cation-Deficient Double Cobaltites

The crystal structure, oxygen nonstoichiometry, and electrical conductivity in PrBa1 – xCo2O6 – δ cobaltites with double perovskite structure are studied. It is shown that defective cobaltites within region of homogeneity 0 < х ≤ 0.1 have a tetragonal structure (space group P4/mmm) with parameters ap × аp × 2аp, where аp ~ 3.8 A. Coulometric titration is used to measure the content of oxygen in oxides over a wide range of temperatures and oxygen pressures in the gas phase, to calculate the concentrations of electronic defects, and to determine the limits of the thermodynamic stability of composition х = 0.1. It is established that increasing the concentration of cationic vacancies in the barium sublattice reduces the mobility of the hole charge carriers and contributes to a drop in the thermal expansion coefficient.

Thermal oxidation of Poly(dicyclopentadiene)– kinetic modeling of double bond consumption

This paper reports chemical changes that occur in polydicyclopentadiene during thermal oxidation at several temperatures and oxygen pressures. A particular attention was paid to the double bond consumption since these latter are associated with crosslinking and subsequent changes in mechanical properties. A kinetic model was derived from the experimental results. Its rate constants were assessed from the “inverse approach” based on their selective identification under specific ageing conditions (for example under oxygen excess). The resulting model satisfactorily describes both carbonyl formation and double bond consumption for a wide range of temperature and oxygen pressure for the first time. This can be exploited to predict the changes of local mechanical properties.

Structural and luminescence properties of laser assisted Eu3+ doped BaZrO3 thin films

Eu3+ doped BaZrO3 thin films were deposited on quartz substrates in a wide range of oxygen pressure (1–300 mTorr), using Pulsed laser deposition technique. X-ray diffraction pattern showed the different growth orientation of the films with oxygen pressure (PO2). The different growth mechanisms were drawn to explanin the thin films growth mechanism at the different oxygen pressure. Time of flight secondary ion mass spectroscopy analysis exhibited homogeneous distribution of all the elements throughout the depth of the films. The depth profile anaylsis revealed sharp interface at the low PO2 and broad intermixing region of the films with the substrate at low PO2. Transmittance spectra revealed the different optical bandgap with different PO2. Excitation spectra showed a broad absorption band centered at 260 nm due to the absorption in the defect states. The emission spectra showed intense emission at 597 nm corresponding to the 5D0→7F1 transition of Eu3+. Apart from this, a broad blue-green emission was observed centered at 440 nm due to the trap levels associated with the oxygen vacancy below the conduction band. The energy transfer mechanism between the host to Eu3+ ion was described via a defect state process. Colour coordintes of the emission spectra showed that colour emission of thin films can be tuned by varying the oxygen pressure. The short-excited state lifetime and high value of internal quantum efficiency make these films potential candidates for light emitting diode application.