Controlled Oxygen Partial Pressure Research Articles

Oxygen partial pressure control for microgravity experiments

A system consisting of a high-temperature yttrium-stabilized zirconia (YSZ) based oxygen ion pump and potentiometric sensor enables precise measurement and control of oxygen partial pressure, pO2, at elevated temperatures within 0.2 to 10−20 bar. The principle of operation as well as the influence of temperature and gas buffers like H2/H2O on the pO2 is discussed. The ion pump is controlled by a microcontroller system and adjusts the oxygen partial pressure with an uncertainty of Δlog(pO2/bar)<0.02 and response times between 5 and 90s over the entire pO2 range.The oxygen ion pump is tested in combination with the electromagnetic levitation. Here, the surface tension of molten Ni at 1720°C as a function of oxygen partial pressure is determined. A good agreement of this measurement with calculated value confirms the applicability of the system for high-temperature measurement and control of pO2. The developed hardware is suitable for the electromagnetic levitation facility onboard the international space station (ISS).

Processing conditions for (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors

Abstract (Nd, Eu, Gd)-Ba-Cu-O ternary bulk superconductors have high potential for practical applications since they exhibit very high critical current densities and thus high field trapping capabilities. (Nd, Eu, Gd)-Ba-Cu-O superconductors are synthesized in a reduced oxygen atmosphere, which requires a control of oxygen partial pressure and needs a special device for hot seeding. In the present study, for simplicity, we employed Ar gas flow into the furnace to control oxygen partial pressure instead of flowing oxygen-controlled gas. Hence, it was necessary to modify the melt processing conditions to produce a single domain. Through the optimization of seeding temperature and cooling rate, we obtained the processing conditions, in which a single domain bulk (Nd, Eu, Gd)-Ba-Cu-O of 20 mm diameter could be synthesized.

Thermodynamic stability, structural and electrical characterization of mixed ionic and electronic conductor La2Mo2O8.96

Thermogravimetric analysis (TGA) technique in controlled oxygen partial pressure (pO(2)) atmospheres has been used to obtain equilibrium oxygen content data as a function of pO(2) on the La(2)Mo(2)O(9-δ) system resulting from the partial reduction of fast oxide-ion conductor La(2)Mo(2)O(9) (LM). Thermodynamic conditions for stabilization of crystalline La(7)Mo(7)O(30) and amorphous La(2)Mo(2)O(7-y) at 718 °C have been determined and discussed. At 608 °C, the compound reported for the first time La(2)Mo(2)O(8.96) (LM896) has been found. The crystalline form and transition temperature in LM896 have been identified by X-ray diffraction at room temperature (XRD) and at controlled temperature. Conductivity curves obtained by electrochemical impedance spectroscopy (EIS) as a function of temperature for both LM and LM896 have been compared. The results indicate that LM896 is a mixed ionic and electronic conductor (MIEC).

Factors for Improvements of Catalytic Activity for Zirconium Oxide-Based Oxygen-Reduction Electrocatalysts

Zirconium oxide based catalysts were evaluated as a non-precious cathode for polymer electrolyte fuel cells. Samples were prepared from zirconium carbonitrides under controlled oxygen partial pressure from 10-2 to 10-22 atm at 1000oC. Significant decrease in lattice parameter and Zr and O occupancy at around specific mass fraction revealed from high resolution XRD with Rietveld analysis, indicating formation of oxygen vacancies in monoclinic ZrO2. It is extrapolated from XRD, XPS, TOF-SIMS analyses that the oxide might be formed with the defects such as oxygen vacancy at both surface and bulk. Additionally, formation of the oxide with high concentration of the defects might be caused by behavior of the carbon on the surface. The oxygen vacancies could be active sites for the ORR, and increasing such oxygen-vacancy sites is one of the factors to improve the catalytic activity for the ORR in Zr-CNOs.

Equilibrium and stability of polarization in ultrathin ferroelectric films with ionic surface compensation

Thermodynamic theory is developed for the ferroelectric phase transition of an ultrathin film in equilibrium with a chemical environment that supplies ionic species to compensate its surface. Equations of state and free energy expressions are developed based on Landau-Ginzburg-Devonshire theory, using electrochemical equilibria to provide ionic compensation boundary conditions. Calculations are presented for a monodomain PbTiO$_3$ (001) film coherently strained to SrTiO$_3$ with its exposed surface and its electronically conducting bottom electrode in equilibrium with a controlled oxygen partial pressure. The stability and metastability boundaries of phases of different polarization are determined as a function of temperature, oxygen partial pressure, and film thickness. Phase diagrams showing polarization and internal electric field are presented. At temperatures below a thickness-dependent Curie point, high or low oxygen partial pressure stabilizes positive or negative polarization, respectively. Results are compared to the standard cases of electronic compensation controlled by either an applied voltage or charge across two electrodes. Ionic surface compensation through chemical equilibrium with an environment introduces new features into the phase diagram. In ultrathin films, a stable non-polar phase can occur between the positive and negative polar phases when varying the external chemical potential at fixed temperature, under conditions where charged surface species are not present in sufficient concentration to stabilize a polar phase.

Effect of oxygen partial pressure on the crystallization of CeO 2 and La 2Zr 2O 7 bi-axially textured thin films

Films of CeO 2 and La 2Zr 2O 7 have been formed by metal-organic decomposition and their crystallization studied under different oxygen partial pressures. Single crystalline substrates were use to model the phenomenon: (0 0 1)SrTiO 3 (STO) for CeO 2 and (0 0 1)LaAlO 3 (LAO) for La 2Zr 2O 7. A chemical interaction has been noticed in the case of CeO 2/STO above 900 °C, which also depends on the oxygen partial pressure, such an effect was not observed for LZO/LAO. The presence of oxygen, even in traces (10–100 ppm), favors the growth of randomly oriented grains while low oxygen partial pressure provides conditions where the heterogeneous nucleation on the substrate dominates. Once nucleated under low oxygen partial pressure, annealing can be performed under increasing controlled oxygen partial pressure without modification of the texture but with an improved crystallization. Application of this to coated conductors must be restricted in the field where oxidation of the substrate can be avoided.

TUNABLE MAGNETIC AND ELECTRICAL PROPERTIES OF Co-DOPED ZnO FILMS BY VARYING OXYGEN PARTIAL PRESSURE

High quality Co -doped ZnO films with good reproducibility have been prepared under different oxygen partial pressure by radio-frequency magnetron sputtering. These films were characterized using numerous characterization techniques including X-ray diffraction, electrical transport, and magnetization measurements. The effect of oxygen partial pressure on the structural, magnetic, and electrical properties of Co -doped ZnO films has been systematically studied. It was found that the structural, magnetic, and electrical properties of Co -doped ZnO films are dependent on oxygen partial pressure. The saturated magnetization of Co -doped ZnO films rapidly increases with decreasing oxygen partial pressure, whereas the resistivity decreases with decreasing oxygen partial pressure. Our findings indicate that the magnetic and electrical properties of Co -doped ZnO films can be tuned by careful control of oxygen partial pressure. In addition, it was further demonstrated that the oxygen vacancy defect is absolutely necessary to induce ferromagnetic couplings in Co -doped ZnO films.

Analysis of structural phase transition of Nd 2NiO 4 + δ by scanning thermal measurement under controlled oxygen partial pressure

The dependence of the structural phase transition behavior of Nd 2NiO 4 + δ between the tetragonal and orthorhombic phases on the oxygen partial pressure, P(O 2), has been investigated by differential scanning calorimetry, DSC, and thermogravimetry and differential thermal analysis, TG-DTA. The structural phase transition temperature, T P, decreased with decreasing P(O 2), while the variations of enthalpy, Δ H, entropy, Δ S, estimated from the peak area of DSC signals and oxygen content at the phase transition, Δ δ, determined from TG curves, exhibited little dependence on P(O 2). From the relationship between T p and P(O 2), the variations of standard enthalpy, Δ H°, and entropy, Δ S°, at the phase transition have been evaluated. By considering Δ δ, Δ H° and Δ S° exhibited fair agreement with Δ H and Δ S evaluated by DSC.

High temperature carburization behaviour of Mn–Cr–O spinel oxides with varied concentrations of manganese

Mn–Cr–O spinel often formed on austenitic alloys is an oxide phase that could be protective against high temperature carbonaceous attack. In this research, various Mn–Cr–O samples were tested in carburization environments with controlled oxygen partial pressures. The stoichiometric MnCr 2O 4 shows better resistance to carburization and coke formation than the Mn-rich Mn–Cr–O and the Cr-rich Mn–Cr–O samples because of its highest thermodynamic stability as compared with MnO and Cr 2O 3. (Mn,Cr) 7C 3 formed after carburization is catalytic to coke formation, and was found instable at higher levels of H 2O/oxygen and may form volatile phases in the presence of H 2O, leading to a continuous reduction in sample weight.

Retention, Recovery and Recycling of Metal Values from High Alloyed Steel Slags

Retention, Recovery and Recycling of Metal Values from High Alloyed Steel SlagsThe work was carried out in four parallel directions. The thermodynamic activities of oxides of Cr in steel slags were determined by slag-gas equilibration technique. The ratio of Cr2+/Cr3+in CaO-MgO(-FeO)-AlO3-SiO2-CrO×system slags was measured by X-ray absorption near edge spectra (XANES). High-temperature mass spectrometry method was also used to obtain the distribution of chromium oxides. A mathematical correlation was established for estimating the ratio of Cr2+/Cr3+as a function of temperature, partial pressure of oxygen and slag basicity. Laboratory investigations of the decarburization of high alloy steels under controlled oxygen potentials have been carried out to retain Cr in the steel phase. A mathematical model has been developed for the decarburization process with controlled oxygen partial pressure. Experimental and theoretical investigations have been carried out in optimizing the Mo-additions to steel in the EAF practice in Uddeholm Tooling AB. Substantial saving of Mo as well as less emissions of Mo-bearing dust are indicated in the study. A salt extraction process was developed to extract the metal values from steel slags. Successful extractions, followed by electrolysis indicate that this could be a viable route towards recovery of metals from metallurgical slags.

Dilatometry and high-temperature X-ray diffractometry study of La 0.6Sr 0.4Ti 0.1Fe 0.9O 3 − δ and La 0.6Sr 0.4Ti 0.3Fe 0.7O 3 − δ oxygen-permeable membranes

The thermal expansion behavior of La 0.6Sr 0.4Ti 1 − x Fe xO 3 − δ (x = 0.7 and 0.9) was investigated by dilatometry and in situ powder X-ray diffractometry at 25 and 1000 °C under controlled oxygen partial pressures in air and at reducing atmosphere, P O2 = 0.21 and 1 × 10 − 20 atm, respectively. The sintered samples of x = 0.7 and 0.9 showed positive thermal expansions, i.e., 12.4 and 14.4 × 10 − 6 K − 1 , respectively, in air up to 1000 °C, and also larger expansions at lower partial oxygen pressures, with a linear thermal expansion ratio factor of 0.10 and 0.40%. The change in the crystal lattice parameters analyzed by XRD Rietveld refinement showed that the crystal lattice volume increased with an increase in temperature and a decrease P O2. The linear thermal expansion rates estimated from volume change of the XRD data for the oxides of x = 0.7 and 0.9, were 0.13 and 0.27%, which corresponds to those of estimated from the dilatometry.

Phase Equilibria of “Cu2O”-“FeO”-CaO-MgO-Al2O3 Slags at PO2 of 10-8.5 atm in Equilibrium with Metallic Copper for a Copper Slag Cleaning Production

Limited data are available on phase equilibria of the multicomponent slag system at the oxygen partial pressures used in the copper smelting, converting, and slag-cleaning processes. Recently, experimental procedures have been developed and have been applied successfully to characterize several complex industrial slags. The experimental procedures involve high-temperature equilibration on a substrate and quenching followed by electron probe X-ray microanalysis. This technique has been used to construct the liquidus for the “Cu2O”-“FeO”-SiO2-based slags with 2 wt pct of CaO, 0.5 wt pct of MgO, and 4.0 wt pct of Al2O3 at controlled oxygen partial pressures in equilibrium with metallic copper. The selected ranges of compositions and temperatures are directly relevant to the copper slag-cleaning processes. The new experimental equilibrium results are presented in the form of ternary sections and as a liquidus temperature vs Fe/SiO2 weight ratio diagram. The experimental results are compared with the FactSage thermodynamic model calculations.

Catalytic Activity of Zirconium Based Cathode without Platinum for Oxygen Reduction Reaction

Partially oxidized zirconium carbonitrides (Zr-CNO) powders were evaluated as a non-precious cathode for polymer electrolyte fuel cells. Zr-CNO powders were prepared from zirconium carbonitrides (ZrC0.5N0.5) with heat-treatment under controlled oxygen partial pressure from 10-3 to 10-22 atm at 1000oC. The voltammetry was performed to evaluate the catalytic activity for the oxygen reduction reaction (ORR) under nitrogen and oxygen in 0.1 mol dm-3 H2SO4 at 30oC. The catalytic activity for the ORR significantly improved with partial oxidation. In particular, the onset potential increased drastically with slight increase of ratio of cubic ZrO2 phase, and became constant over 0.90 V vs. RHE. XRD spectrum and the color of specimen indicated that the growing oxide phase had oxygen vacancies by substituting some of the oxygen atoms with nitrogen and carbon atoms. It is predicted that carbon and nitrogen atoms in starting material, ZrC0.5N0.5, may affect the production of oxygen vacancies according to TOF-SIMS measurements. Therefore, the defects such as oxygen vacancies in the Zr-CNO might affect the catalytic activity for the ORR.

Formation of Orthorhombic and Multiferroic Hexagonal Phases from an Undercooled RMnO3 (R=Rare-Earth Element) Melt Using a Containerless Technique

Containerless solidification of the undercooled RMnO 3 melt was carried out to study the formation of orthorhombic and hexagonal phases under controlled oxygen partial pressure. The o-RMnO 3 (R = La to Gd) perovskite with a space group of Pbnm and h-RMnO 3 (R = Dy to Lu) with a space group of P6 3 cm were solidified from an undercooled melt at P O2 , = 10 5 Pa. The RMnO 3 samples showed the undercooling of 230-380 K at P O2 , = 10 5 Pa. The surface morphologies changed from rough spherical to a smooth faceted planes with a decreasing ionic radius from La (0.1032 nm) to Lu (0.0861 nm). X-ray diffractometry and scanning electron microscopy results showed the existence of o-RMnO 3 and h-RMnO 3 . Thermodynamic stabilities of the as-solidified phases were studied at 10 5 Pa; as a result, the hexagonal phases changed to orthorhombic for the DyMnO 3 and HoMn0 3 systems after annealing at 1673 K. Based on the thermogravimetric analysis results, oxygen deficiency, bulk composition, and tolerance factor were calculated.

A novel ultra-high temperature oxidation technique in flowing gas with controlled oxygen partial pressure

For the purpose of investigating ultra-high temperature oxidation, a novel induction heating facility has been established. The oxidation kinetics of several typical ultra-high temperature materials (UHTMs), including two graphite-based composites (C/C and ZrB2/C) and two ternary Zr-Al-C ceramics (Zr2Al3C4 and Zr2[Al(Si)]4C5), were tested by utilizing this facility. It has been identified that the tested cylindrical samples with dimensions of Φ 20 mm × 20 mm can be oxidized uniformly. The maximum temperature of 2450°C can be achieved on graphite-based composites, and the oxygen partial pressure can be controlled in the range of 102–105 Pa. This novel technique exhibits many advantages, such as an extremely high heating rate of about 20°C/s, easy controlling of temperature and gas pressure, low energy consumption, low cost, and high efficiency. Therefore, it provides a potential way for profoundly investigating the ultra-high temperature oxidation behaviors of UHTMs.

Microstructure design by mechanical alloying

Mixing and coprecipitation processes are, often, not enough in order to reach materials holding several functional components, like selective catalysts that must work simultaneously. Even though when a homogeneous and fine distribution of the constituents is obtained, the affinity between equal phase particles leads to coarsening during the consolidation (sintering) process, as well as on application, such as the material can loose high reactivity. The present work proposes a new consolidation route – sintering by activated surface (SAS) – that employs sacrificial metal layers to avoid coarsening and to increase the diffusion profiles during sintering, once high activity surfaces are exposed during the first sintering step. Regarding limited oxygen potential is established in the sintering atmosphere, the SAS effect is engaged when a specific projected powder microstructure obtained by mechanical alloying (MA) processing is provided. The MA is driven in such a way that yields cermet powders particles with lamellar pod-like like structures, as shown in the SEM image. This projected morphology comprises the ceramic round particles plated by thin metal layers or embedded on them. Porous nickel–zirconia based cermets are studied with Cu and some selected refractory metal additives. The refractory metals are expected to repeal Cu, which remains in pure state at the cermet. By its turn, Cu addition is postulated to prevent coking when fuel-reforming reactions are involved at the application (e.g. in solid oxide fuel cells). Furthermore, Cu is desired since it promotes shrinkage and lower the sintering temperatures. The SAS process running under argon atmospheres with controlled oxygen partial pressure is found to further reduce the sintering temperature by 100–300 °C, for cermets final densities above 60%TD. The sintering behaviour depends on the chosen additive, being Ag, Cu and Mo the most effective ones. The resulted sintered parts attain a suitable density and phase dispersion for catalysis applications.

On the thermodynamic stability of La 2Mo 2O 9− δ oxide-ion conductor

The stability of La 2Mo 2O 9− δ oxide-ion conductor was studied under Ar–H 2 and controlled oxygen partial pressure ( pO 2) atmospheres within the range 608 ≤ T ≤ 1000 °C, by thermogravimetry (TG), X-ray Diffraction (XRD), Temperature Controlled X-ray Diffraction (TCXRD) and Scanning Electron Microscopy (SEM). At 608 °C under Ar–H 2 flowing atmosphere, La 2Mo 2O 9 phase was found to be unstable and reduction leads to an amorphous phase with composition close to La 2Mo 2O 6.88. The stability of this amorphous phase under Ar–H 2 was analyzed by TCXRD and found to be stable below 900 °C. Isothermal TG under controlled pO 2 at 1000 °C indicated that La 2Mo 2O 9− δ is not stable below 10 −7 Pa and decomposition took place without the formation of La 7Mo 7O 30 or amorphization.

Control of reactive high power impulse magnetron sputtering processes

High power impulse magnetron sputtering (HIPIMS) is a technologically important physical vapour deposition (PVD) process that is able to provide a highly ionised flux of sputtered species. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor applications). HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to better coating structure–property relationship control through self-species (sputtered metal) plasma/ion assistance. Many of the technologically important thin films (e.g. transparent conductive oxides, permeation barrier coatings, etc.) are sputtered from metal targets in a reactive gas atmosphere, usually Ar + O 2 or N 2, to ensure industrially relevant coating deposition rates. Enhanced structure–property relationship control of these thin film materials is highly desirable; hence, it also is desirable to use HIPIMS in a reactive deposition mode. Preliminary trials of reactive HIPIMS however have indicated that the control of this process using conventional means, such as conventional plasma emission monitoring (PEM) is difficult. Thus, the application of reactive HIPIMS is rather limited and the potential benefits are not realised, especially in the areas where precise process control and long term stability in a reactive environment are required. In this paper reactive HIPIMS process (Ti in Ar/O 2 atmosphere) is investigated and various control options are evaluated. The application of a recently developed PEM based reactive HIPIMS control method is reported. Performance of the developed technique is compared to that of the conventional PEM, Penning-PEM and λ-sensor based methods. It is shown that conventional PEM is impractical to control reactive HIPIMS, while the constant reactive gas flow method does not lead to a stable deposition process. The new PEM based process control technology was shown to provide precise control and stable operation of reactive HIPIMS discharges anywhere within the hysteresis loop. It was also found to be superior when compared to oxygen partial pressure control based techniques.

Control and Measurement of Oxygen Partial Pressure, and Thermodynamic Properties

Control and Measurement of Oxygen Partial Pressure, and Thermodynamic Properties

The synthesis under controlled oxygen partial pressure and the characterization of a layered perovskitesystem Sr2V1−xMoxO4

We have successfully synthesizedSr2V1−xMoxO4 fromx = 0.3 to 1.0 by an improved process. The layered compoundSr2VO4 is an antiferromagneticinsulator with S = 1/2, whilethe isostructural Sr2MoO4 is a Pauli paramagnetic metal. The solid solutionSr2V1−xMoxO4 of these materials is expected to show a metal–insulator transition. In this paper wereport the synthetic process and the results of the magnetic susceptibility and theelectrical resistivity. The selection of the starting materials and the adjustment ofO2 partial pressureof less than 10−12 atm were indispensable for the synthesis. The reducing conditions during the sintering hadbeen determined rationally, according to thermodynamical analysis. Kondo-like behavior inboth resistivity and magnetic susceptibility were observed in a limited range ofV-containing samples.