Oxygen Activity Gradient Research Articles

Effect of Ta on selective oxidation behavior and oxide film adhesion of alumina-forming austenitic alloys

The effects of Ta on the selective oxidation behavior and scale adhesion of Fe-45Ni-25Cr-4Al-1.5Nb-0.4C-xTa (x=0.6, 0.9, 1.2, 1.5 wt%) austenitic alloys at high temperature under low oxygen pressure were investigated. The FCC, MC-(Ta, Nb)C, M23C6 and B2-NiAl phases were found in the annealed alloy. Addition of Ta significantly promoted the formation of an external Al2O3 layer and inhibited growth of Cr2O3 and spinel at 850 °C under oxygen pressures of 10−18 atm and 10−24 atm. Similar oxidation behavior was observed in two-step oxidation process (650 °C for 8 hours and 1000 °C for 16 hours). Ta and Nb preferentially segregated at the interface between the oxide scale-matrix as well as grain boundary within the oxide scale during oxidation, and inhibited outward diffusion of Al and Cr. A large oxygen activity gradient was formed in the oxide film, resulting in the formation of columnar grains. Due to a significant decrease in oxygen activity at the front of the reaction, selective oxidation of alumina was promoted. Furthermore, the scratch test technique was used to quantify the adhesion of oxide scales. Peg-like structures formed by Ta and Nb positively influenced the adhesion properties of the oxide scale.

Oxygen Activity across a YSZ Electrolyte Visualized by Optical Spectroscopic Probes

Solid oxide fuel cells (SOFC) are reversible devices that generate electric energy by oxidation of a fuel, commonly hydrogen. In electrolyser mode (SOEC), a voltage is applied and hydrogen is generated from water. These processes are highly efficient, although one of the main sources of degradation in these devices arises from extreme oxygen activity inside the electrolyte at the interfaces with the electrodes [1, 2], developed because of inappropriate operating conditions or poor electrode performance. In particular, high oxygen partial pressure in the oxygen electrode/electrolyte interface can lead to electrode delamination when working in SOEC mode at high voltages. The purpose of this work is to investigate the oxygen activity profile established across a yttria-stabilized zircona (YSZ) electrolyte upon different polarization conditions.To achieve this goal, several redox ions were studied as spectroscopic probes inside a YSZ matrix and among them terbium was found to be a suitable dopant as optical spectroscopic probe for intermediate and high ranges of oxygen activity [3]. By means of optical absorption and diffuse reflectance, 3+/4+ terbium oxidation states could be identified in samples annealed in different atmospheres. Tb3+ luminescence signals can be used to follow the 3+/4+ dopant redox couple, and using the strong 5D4 → 7F5 Tb3+ luminescence emission [4], a quantitative relation between its intensity and oxygen partial pressure in equilibrium with the doped electrolyte could be established (Fig. 1). Therefore, this signal can be used to quantify the p O 2 in equilibrium at different positions inside the electrolyte.Several experiments were designed in order to obtain different electrochemical parameters from the YSZ-Tb system. First of all, ionic conductivity of the terbium doped YSZ electrolyte was determined with symmetrical cells with platinum electrodes and testing them by impedance spectroscopy under oxygen and hydrogen atmospheres in a wide range of temperatures.Electronic conductivity of YSZ-Tb could be determined using Hebb-Wagner cells with a LSM/YSZ electrode on the oxygen side and an ion-blocking Pt electrode. Similar cells were also polarized under different conditions and cooled down to freeze the oxygen activity gradient. The quenched O2 activity profile was quantified using Tb3+ luminescence signal intensity, and analysed taking into account the actual conductivity of the minority carriers, as determined in the previous experiments.Cells with a LSM/YSZ cermet air electrode and a Ni/YSZ cermet hydrogen electrode were tested in SOEC mode. When in stationary state, cells were quenched, freezing the high-temperature under-voltage terbium oxidation state, as shown in Fig. 2. By analysing the terbium luminescence signal, oxygen activity and oxygen partial pressure across the electrolyte thickness could be determined under different polarization conditions. The results and capabilities of the method will be discussed.[1] A. Virkar, Int. J. Hydrogen Enery 35 (2010) 9527.[2] M.A. Laguna-Bercero et al., Int. J. Hydrogen Energy 36 (2011) 13051.[3] R.I. Merino et al. (2018) https://digital.csic.es/handle/10261/182893.[4] M.R.N. Soares et al., Materials Letters 65 (2011) 1979-1981. Figure 1

Relation between oxygen activity gradient in the internal oxidation zone of Mn alloyed steel and the composition of oxide precipitates

An internal oxidation zone with (Mn1−x,Fex)O mixed oxide precipitates occurs after annealing a Fe – 1.7at.% Mn steel at 950°C in N2 plus 5vol% H2 gas mixture with dew point of 10°C. Local thermodynamic equilibrium in the internal oxidation zone is established during annealing of the Mn alloyed steel. As a result, the composition of (Mn1−x,Fex)O precipitates depends on the local oxygen activity. The oxygen activity decreases as a function of depth below steel surface, and consequently the concentration of Fe decreases in the (Mn1−x,Fex)O precipitates.

Wafer-scale growth of VO2 thin films using a combinatorial approach.

Transition metal oxides offer functional properties beyond conventional semiconductors. Bridging the gap between the fundamental research frontier in oxide electronics and their realization in commercial devices demands a wafer-scale growth approach for high-quality transition metal oxide thin films. Such a method requires excellent control over the transition metal valence state to avoid performance deterioration, which has been proved challenging. Here we present a scalable growth approach that enables a precise valence state control. By creating an oxygen activity gradient across the wafer, a continuous valence state library is established to directly identify the optimal growth condition. Single-crystalline VO2 thin films have been grown on wafer scale, exhibiting more than four orders of magnitude change in resistivity across the metal-to-insulator transition. It is demonstrated that ‘electronic grade' transition metal oxide films can be realized on a large scale using a combinatorial growth approach, which can be extended to other multivalent oxide systems.

The Linear Expansion of Mayenite (Ca12Al14O32)O under the Change in Oxygen Activity

The paper studies the chemical expansion of mayenite induced by variation in oxygen activity. The value of the former is measured by dilatometry. Peculiar structural features of mayenite allow one to predict its high stability under the oxygen activity gradient. The change in the mayenite size is shown to be remarkably smaller compared to that in conventional materials.

Stress analysis and fail-safe design of bilayered tubular supported ceramic membranes

Supported ceramic membranes based on mixed ionic and electronic conductors are a promising technology for oxygen separation applications. In addition to chemically induced stress under oxygen activity gradients in the materials, strain mismatch between membrane and support gives rise to considerable stress that may compromise mechanical reliability. This paper presents an analysis of stress generated in tubular supported membranes during operation. Closed-form analytical solutions for stresses due to external pressures, strain gradients, and mismatch in materials properties are derived. Stress distributions in two membrane systems have been analyzed and routes to minimize stress are proposed. For a Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane supported on a porous substrate of the same material under pressure-vacuum operation, the optimal configuration in terms of minimizing the risk of fracture by ensuring only compressive stresses in the component is achieved by placing the support on the feed side of the membrane. For a Ce0.9Gd0.1O1.95−δ membrane on a MgO support, stress due to thermal strain mismatch is as large as that due to oxygen activity gradient. Tailoring the thermal expansion coefficient of the support is an effective method to alleviate the total stress. Failure criteria for membrane fracture under compression are thereafter presented. It is found that the tolerable flaw size for fracture in compression is in the millimeter range for both membrane systems at operating conditions in the range of practical interest.

Evaluating oxygen diffusion, surface exchange and oxygen semi-permeation in Ln2NiO4+δ membranes (Ln=La, Pr and Nd)

To clearly identify the rate determining step (r.d.s.) in the oxygen semi-permeation of dense Ln2NiO4+δ membranes (with Ln=La, Pr and Nd), a specific apparatus was designed to measure the oxygen semi-permeation flux and activity gradient at both membrane surfaces. From these measurements, the coefficients for the oxygen diffusion and surface exchange at both membrane surfaces were determined. Their values were compared to the values reported in the literature for similar membrane materials. We concluded that the oxygen flux through these membranes was limited by the oxygen surface exchange at the oxygen-lean face and not oxygen volume diffusion, as previously claimed.

Synthesis, Sintering, Transport Properties, and Surface Exchange of La2Ni0.9Cu0.1O4+δ

Dense La2Ni0.9Cu0.1O4+δ ceramics were sintered from powders synthesized through a wet‐chemical citrate nitrate route with optimized ratios of citrate to nitrate. Less citrate decreases the required sintering temperature and improves the oxygen permeativity. The oxygen permeation was measured as a function of oxygen activity gradient, membrane thickness (0.4–2.6 mm) and temperature (800°C–950°C). The oxygen self diffusion coefficient DO and the surface exchange coefficient k show Arrhenius‐type behaviors with activation energies of ~50 and ~100 kJ/mol, respectively. The oxygen chemical diffusion coefficient Dchem and surface exchange coefficient kchem, measured by conductivity relaxation, exhibit Arrhenius‐type behaviors with activation energies of 62 and 104 kJ/mol, respectively. Dchem and kchem are related to DO and k through the thermodynamic factor ωO.

Partial electronic conductivity and electrolytic domain of bilayer electrolyte Zr 0.84Y 0.16O 1.92/Ce 0.9Gd 0.1O 1.95

Partial electronic conductivity and total conductivity have been determined by Hebb–Wagner polarization method and a.c. impedance spectroscopy, respectively, on bilayer electrolyte Zr 0.84Y 0.16O 1.92(YSZ)/Ce 0.9Gd 0.1O 1.95(GDC) with thickness ratios 10 − 3 /1 and 10 − 4 /1 at 800°, 900° and 1000 °C, respectively. While their ionic conductivities remain close to that of GDC, the electronic conductivities are suppressed the more from that of GDC towards that of YSZ the higher the thickness ratio, as expected. Even when the GDC-side is exposed to reducing atmosphere, the electronic conductivity is also suppressed, but to a less extent. It is suggested that oxygen activity distribution is discontinuous across the YSZ/GDC interface under ion-blocking condition, refuting the “continuity hypothesis” that has been usually adopted in calculating the oxygen activity distribution across a multilayer of mixed conductor oxides. The electrolytic domain widths of the bilayer electrolyte are reported depending on temperature, thickness ratio and direction of oxygen activity gradient imposed.

Hydrogen Permeability through n-type Cr2O3 Scale at 1273 K under the Oxygen Activities of 1.6 × 10–18 – 1.0 × 10–16

The effect of dissolution of hydrogen on the defect structure of Cr2O3 was studied. The hydrogen flux and permeability through Cr2O3 scale were measured at 1273 K in the oxygen activities of 1.6 × 10–18 – 1.0 × 10–16. The oxygen activities on the both sides of the sample were controlled by fixing the H2/CO2 and CO/CO2 ratios to measure the hydrogen permeability without oxygen activity gradient through the sample. The measured hydrogen flux and permeability was proportional to aH21/2 and aO2–1/8 respectively. The activity dependences indicate that dissolved hydrogen is the neutral hydrogen at oxygen site as a minor defect and does not affect the oxidation behavior of chromia-forming alloys.

Simulation of tin penetration in the float glass process (float glass tin penetration)

Abstract The flat glass produced by the float glass process has a tin-rich surface due to the contact with molten tin. The penetration of tin into the glass surface is assumed to involve coupled diffusion of stannous (Sn 2+ ) and stannic (Sn 4+ ) ions. The diffusion coefficients of these ions were calculated using the modified Stocks–Einstein relation with the oxidation velocity of stannous ions depending on the oxygen activity in the glass. The ion diffusion was analyzed using a coupled diffusion simulation with a modified diffusion coefficient to compensate for the negative effect of the glass ribbon’s stretching or compressing in the glass forming process. Tin penetration simulations for both green glass and clear glass show an internal local tin concentration maximum in green glass which is quite different from that in clear glass. The local maximum in the profile is associated with the accumulation of stannic ions where the greatest oxygen activity gradient occurs. Since more float time is needed in the manufacture of thicker glass plate, the tin penetrates to a greater depth with the maximum deeper in the glass and the size of the maximum larger for thicker glass.

I–V relations and enhanced mobility of metal vacancies in acceptor doped polycrystalline BaTiO3 under oxygen activity gradient

Acceptor doped barium titanate ceramic samples were prepared and exposed to oxygen activity gradients at elevated temperatures. When an oxygen activity gradient is imposed on such a pellet, a pin junction is formed. In situ measurements of the I–V relations were performed under various oxygen activity gradients. These measurements show very strong memory effects. An explanation of this behaviour is suggested based on the assumption of enhanced mobility of metal vacancies due to the oxygen activity gradient.

Transport properties and defect analysis of La 1.9Sr 0.1NiO 4 + δ

The oxygen flux through La 1.9Sr 0.1NiO 4 + δ has been measured as a function of oxygen activity gradient and temperature (750–1000 °C). The oxygen nonstoichiometry was determined by thermogravimetry in the temperature range of 400–1000 °C and oxygen partial pressures of 0.0002–1 atm. The total conductivity was measured over a similar range of conditions. The oxide ion partial conductivity derived from the oxygen flux data is approximately 4 orders of magnitude lower than the total, mainly p-type electronic conductivity. The defect structure was derived based on the data. Combining the oxygen flux and oxygen nonstoichiometry, the self diffusion coefficient of oxygen interstitials was evaluated.

Steady-State Permeation of Oxygen Through La1.9Sr0.1NiO4 + δ

The steady-state oxygen permeation through was measured as a function of oxygen activity gradient, membrane thickness (0.37–2.62 mm), and temperature . The self-diffusion coefficient of oxygen and the surface-exchange coefficient were extracted from flux data based on a simple transport model using average bulk transport parameters. The activation energies of and are 65 and 100 kJ/mol, respectively, and agree well with literature secondary-ion mass spectrometry data obtained at lower temperatures. The oxygen transport through the membrane was also simulated through a numerical analysis, with results comparable to those derived from the simple transport model.

An oxygen permeable membrane La 0.8Sr 0.2(Ga 0.8Mg 0.2) 1 − xCr xO 3 − δ for a reduced atmosphere

Mixed electron hole and oxide ion conducting perovskite-type oxides, La 0.8Sr 0.2(Ga 0.8Mg 0.2) 1 − x Cr x O 3 − δ (0 ≤ x ≤ 1.0) , were prepared by solid state reaction. The phase stability and the oxygen permeation properties of the oxides were examined as a function of the content of Cr. La 0.8Sr 0.2(Ga 0.8Mg 0.2) 1 − x Cr x O 3 − δ has a perovskite related tetragonal phase with x = 0.1 to 0.8. The total electrical conductivity of La 0.8Sr 0.2(Ga 0.8Mg 0.2) 1 − x Cr x O 3 − δ increases with increasing x. The oxygen permeation flux across the La 0.8Sr 0.2(Ga 0.8Mg 0.2) 1 − x Cr x O 3 − δ membranes at higher temperatures increases with x up to x = 04. The maximum oxygen permeation flux of 1.6 × 10 − 7 mol − 1 cm − 2 at 1100 °C in a oxygen activity gradient of air/10 − 2 Pa is observed in La 0.8Sr 0.2(Ga 0.8Mg 0.2) 0.6Cr 0.4O 3 − δ . This perovskite-type oxide is stable under an oxygen partial pressure of 7 × 10 − 10 Pa at 1000 °C.

Loading path effect on the chemical expansion in substoichiometric LSF based perovskite

Permeation induced expansion in substoichiometric perovskites is a key point of their mechanical reliability under oxygen activity gradient. This paper proposes a macroscopic modelling of this phenomenon based on the choice of oxygen activity as state variable. This approach helps to underline the asymmetry of the chemical expansion in its dependence on the difference between the actual surrounding atmosphere and the sintering one. It leads to the development of the notion of loading path in terms of surrounding atmosphere change. The investigations show that the intrinsic chemical expansion behaviour is independent of the sintering atmosphere. The apparent chemical expansion behaviour is driven by the loading path. A study of the sensitivity of each parameters of the model helps to better estimate the possibilities and the limits of the proposed approach.

Ba/Ti ratio effect on oxygen re-equilibration kinetics of donor-doped BaTiO 3

In an oxygen activity gradient, BaTiO 3 ceramic generally relaxes twofold: a fast global relaxation in the oxygen sublattice and a sluggish in-grain relaxation in the cation sublattice(s), thus allowing one to simultaneously determine the two chemical diffusivities. [Yoo et al., J. Am. Ceram. Soc., 88, 617–623 (2005)]. Upon this ground, the chemical diffusivities have been measured, by a conductivity relaxation technique, on donor(La Ba •)-doped BaTiO 3 ceramics with different Ba/Ti ratios, Ba 1 − x La x Ti 1 − x / 4 O 3, Ba 1 − 3 x / 2 La x TiO 3 and Ba 1 − x La x TiO 3 all with x = 0.01, against oxygen activity in the range of − 15 ≤ log(Po 2/atm) ≤ 0 at 1200 °C. It has been found that the chemical diffusivities for the cation sublattice(s), and those for oxygen as well, are essentially the same irrespective of the Ba/Ti ratios. This fact indicates either that only either of Ba or Ti is mobile or both have about the same mobility. Circumstantial evidences support the latter.

Hydration and oxidation kinetics of a proton conductor oxide, SrCe0.95Yb0.05O2.975

The oxidation and hydration kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975), were examined via conductivity relaxation upon a sudden change of oxygen activity in a fixed water-activity atmosphere, and vice versa, in the ranges of -4.0 < log a(O(2)) < or = 0.01 and -5.0 < log a(H(2)O) < -2.0 at 800 degrees C. It was found that under an oxygen-activity gradient in a fixed water-vapor-activity atmosphere, the conductivity relaxation with time is monotonic with a single relaxation time (as usual), yielding a chemical diffusivity that is unequivocally that of the component oxygen. In a water-activity gradient in a fixed oxygen activity atmosphere, on the other hand, the conductivity relaxation appears quite unusual, exhibiting an extremum after an initial transient. The conductivity relaxation upon hydration or oxidation, in general, is quantitatively analyzed in terms of two apparent chemical diffusivities for component oxygen and hydrogen, respectively. The inner workings of hydration is discussed, and the as-evaluated chemical diffusivities are reported and compared with the conventional chemical diffusivity of water.

The Influence of Vacuum Treatment on Inclusion Composition - Theoretical Study

Two industrial studies and one set of lab scale trials have been done. In addition, a theoretical study has been done. The main focus has been on non metallic inclusion composition during the ladle refining operation in industrial steel production. Sampling has been done together with careful inclusion determination. The inclusion composition is related to different variables. In the industrial trials samples have been taken at different steps during the ladle refining period. Steel and slag composition as well as temperature and oxygen activity have been determined. The thesis is based on five supplements with different major objectives, all related to the inclusion composition. The equilibrium top slag-steel bulk and inclusions-steel bulk were investigated by comparison between calculated and measured oxygen activity values. The oxygen activity and relation to temperature has also been discussed as well as oxygen activity and temperature gradients. The effect of vacuum pressure on inclusion composition has been evaluated in a theoretical study as well as lab scale trials. The inclusion composition has been studied during the industrial ladle treatment process. The inclusion composition was related to top slag composition and other parameters during ladle treatment. The major findings in the thesis are the lack of equilibrium conditions with respect to top-slag and steel bulk before vacuum treatment. The inclusions have been found to be closer to equilibrium with the steel bulk. Al/Al2O3 equilibrium has been found to control the oxygen activity after Al-deoxidation. Evaluation of inclusion composition during the ladle refining has revealed that the majority of the inclusions showed a continuous composition change throughout the ladle refining process, from high Al2O3, via MgO-spinel to finally complex types rich in CaO and Al2O3. The final inclusion composition after vacuum treatment was found to be close to the top slag composition. Vacuum pressure has been found to have a theoretical effect on inclusion composition at very low pressures.

Thermomechanical modelling of ion-conducting membrane for oxygen separation

Dense mixed ionic conducting membranes are subjected to stresses induced by the oxygen activity gradient under operating conditions. This works proposed a thermomechanical model, taking into account such chemical induced strain. This model permits predictive simulation by the finite element method in transient and steady state to give guidelines for reactor operating conditions optimization. Modelling has shown that the maximum stress on the membrane depends on thermal gradient along the membrane and transient phase's management.