Low Oxygen Activity Research Articles

Decoding the oxidation mechanism of Zircaloy-4 via in situ synchrotron X-ray diffraction and computational elucidation

Comprehensive understanding of the oxidation behavior of Zr alloys, a vital cladding material in nuclear power plants, is essential for developing improved materials and enhancing the safety and performance of nuclear systems. Herein, the bulk oxidation behavior in ambient air atmosphere of Zircaloy-4 were revisited through in situ synchrotron X-ray diffraction and DFT computation. The results reveal the phase transition sequences at a representative 900 °C oxidation temperature: hcp Zr recrystallizes rapidly and then transforms into bcc Zr while reaching the α+β transus, followed by speedy appearance of textured tetragonal (t-)ZrO2, monoclinic (m-)ZrO2 and ZrN that consume oxygen and nitrogen. Continuous air ingress favors t-ZrO2 and m-ZrO2 dominance, accompanied by re-oxidation of ZrN into t-ZrO2 due to its low thermodynamic stability revealed by experiment-informed DFT calculation and low oxygen activity at the oxide-metal interface. Ensemble-averaged lattice volume expansions during phase transitions have been quantified. This expansion-induced compressive stress promotes the presence of a significant fraction of t-ZrO2 at elevated temperature that eventually transforms into m-ZrO2 during cooling.

Alkali desorption from ilmenite oxygen carrier particles used in biomass combustion

Oxygen-carrying fluidized bed materials are increasingly used in novel technologies for carbon capture and storage, and to improve the efficiency of fuel conversion processes. Potassium- and sodium-containing compounds are released during biomass combustion and may have both negative and positive effects on conversion processes. Ilmenite is an important oxygen carrier material with the ability to capture alkali in the form of titanates. This is a desirable property since it may reduce detrimental alkali effects including fouling, corrosion, and fluidized bed agglomeration. This study investigates the interactions of alkali-containing compounds with ilmenite particles previously used in an industrial scale (115 MWth) oxygen carrier aided combustion system. The ilmenite samples were exposed to temperatures up to 1000 °C under inert and oxidizing conditions while the alkali release kinetics were characterized using online alkali monitoring. Alkali desorption occurs between 630 and 800 °C, which is attributed to loosely bound alkali at or near the surface of the particles. Extensive alkali release is observed above 900 °C and proceeds during extended time periods at 1000 °C. The release above 900 °C is more pronounced under oxidizing conditions and approximately 9.1 and 3.2 wt% of the alkali content is emitted from the ilmenite samples in high and low oxygen activity, respectively. Detailed material analyses using scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were conducted before and after temperature treatment, which revealed that the concentrations of potassium, sodium and chlorine decrease at the outermost surface of the ilmenite particles during temperature treatment, and Cl is depleted to a deeper level in oxidizing conditions compared to inert. The implications for ilmenite-ash interactions, oxygen carrier aided combustion and chemical looping systems are discussed.

High-temperature chemical expansion of Pr0.1Ce0.9O2-δ thin films determined by Differential Laser Doppler Vibrometry

The chemical expansion of thin-film praseodymium‑cerium oxide solid solutions (Pr0.1Ce0.9O2−δ, PCO10) is investigated at temperatures ranging from 500 °C to 800 °C and oxygen activities down to about 10−22. Differential Laser Doppler Vibrometry (D-LDV) in combination with periodic electrochemical pumping of oxygen is used to detect very small displacements of the sample surface. With decreasing pumping frequency, the film approaches chemical equilibrium, which is nearly reached at 700 °C below 0.01 Hz. D-LDV enables direct measurement of film thickness changes as well as the bending of the underlying substrates due electrochemically induced lateral mechanical stress. The substrate bending is found to be more than an order of magnitude larger than the film thickness change, increasing the detection limit by the same factor. The thickness change of the thin film is calculated from the measured displacement of the sample surface and compared with directly measured values. Thin-film literature data available for low oxygen activities up to 650 °C agree with our measurements. In addition, the thickness change of the film is determined from the displacement at different sample positions, which validates previous measurements. Finally, thin film chemical expansion is found to be larger by a factor of about 2 than the corresponding bulk value in agreement with predictions from a mechanical model that the out-of-plane expansion of a constrained film is increased.

HELIOS3: A stirred bubble column for oxygen addition or reduction in lead-bismuth eutectic

Oxygen control is one of the key technologies to limit corrosion or coolant oxidation in lead bismuth eutectic cooled nuclear reactors. In this paper, we show that a stirred bubble column reactor can be an efficient tool for addition and extraction of dissolved oxygen from lead-bismuth eutectic. The design criteria and the resulting layout of the HELIOS3 reactor are detailed and typical results obtained are described. Dissolved oxygen extraction rates are only given at high oxygen activity, as dissolved corrosion products and oxides can strongly influence the rate of addition or extraction of dissolved oxygen at low oxygen activity.

New insight into the formation and oxygen barrier mechanism of carbonaceous oxide interlayer in a multicomponent carbide

ABSTRACTEarly transition metal carbides are considered to be superior candidate materials for oxidizing environments at temperatures exceeding 2000°C. Generally, the remarkable oxidation resistance is largely attributed to a carbonaceous oxide interlayer (eg, Hf–O–C, Zr–O–C, and Ta–O–C), located at the interface between the external oxide layer and internal carbide (eg, HfC, ZrC, and TaC), acting as the primary oxygen barrier. However, the oxygen barrier mechanism of the carbonaceous oxide interlayer remains unclear. Herein, through studying the oxidation behavior of a novel multicomponent carbide Hf0.5Zr0.3Ti0.2C in oxidizing environments up to 2500°C, the oxygen barrier mechanism of the carbonaceous oxide was recently revealed. We found that the oxygen barrier resulted from the slow oxygen diffusion through the inner grains of Hf‐Zr–Ti–O due to the presence of carbon formed at the grain boundaries because of the existence of compact external oxide layer, beneath which the Hf–Zr–Ti–O–C interlayer possesses much lower oxygen activity and temperature that allow carbon to exist stably. This as‐formed carbon strongly retarded the fast diffusion of oxygen along the grain boundaries of oxides. Additionally, desirable synergisms of the designed multicomponent system, particularly, the outward short‐circuit diffusion of Ti, lead to the self‐healing of the external oxide layer, evidently enhancing integral protection performance against oxidizing environments.

Gd0.2Ce0.8O1.9/Y0.16Zr0.84O1.92 nanocomposite thin films for low temperature ionic conductivity

Gd0.2Ce0.8O1.9/Y0.16Zr0.84O1.92 (GDC/YSZ) nanocomposite is synthesized by a novel hybrid chemical route, where colloidal crystalline GDC nanoparticles from continuous hydrothermal synthesis are dispersed into a metalorganic YSZ matrix precursor. The result is a mixture of metal oxides in which GDC nanoparticles are finely distributed in a continuous metalorganic polymeric matrix to be crystallized after calcination. The GDC nanoparticles reduce the temperature necessary to obtain crystalline YSZ, which is already formed at 400 °C. The nanocomposite reveals structural stability up to 800 °C when treated in both air and reducing atmosphere, showing the onset of diffusion below 1000 °C. The diffusional processes are largely dependent on the nanometric grain size, with Zr4+ diffusing abruptly towards GDC in air at 1000 °C and GDC/YSZ interdiffusion being hindered in reducing environment despite the onset temperature of 900 °C.The nanocomposite precursor is an inkjet-printable reactive water-based material, suitable for the deposition of thin films with a thickness below 100 nm after calcination at 750 °C. The crystal structure of the film reveals no interaction between GDC and YSZ but a microstrain (0.3% tensile strain for YSZ). The thin film microstructure shows a compact layer with 94% density.The nanocomposite shows high oxygen ionic conductivity at low temperatures (>5⋅10-3 S⋅cm-1 at 500 °C), low activation energy (0.55 eV), and dominant oxygen ionic conductivity even in reducing conditions (pO2 < 10–25 atm). We show that these properties arise from the large interface between the components of the composite, due to the embedding of the GDC nanoparticles in the YSZ matrix, while ZrO-CeO intermixing can be avoided and no n-type conductivity is observed even at low oxygen activities and high temperatures.

Understanding Corrosion of Ni-Cr Alloys in Molten Chloride Salts

Molten chloride salts possess many attractive properties for use as a coolant and storage medium in advanced nuclear reactors and in concentrating solar power systems. Among the many challenges is the corrosion of salt-facing structural components. In molten halide salts, the passive films that typically form on the surface of corrosion resistant alloys either do not form, or are unstable due to the very low oxygen activities. Therefore, corrosion of materials is dependent largely on interfacial dissolution reactions between the molten salt and the bare metal surface in which alloy constituents with the most stable chlorides tend to be the most likely to dissolve. For most common structural alloys, chromium has the most stable chloride, and thus selective dissolution of chromium from the alloy into the salt is the dominant form of material degradation in molten halides. While this fact is well known and backed-up by experimental data, the mechanism of chromium depletion from alloys is not yet known. To truly understand corrosion in molten salts, the mechanism of chromium depletion from alloys must be understood and quantified. This talk will discuss results of a combined experimental and computational strategy aimed at fundamentally understanding corrosion of alloys in molten chloride salts. Controlled capsule tests are conducted in which model Ni-Cr alloys are exposed to molten KCl-MgCl2 at temperatures in excess of 600°C to measure degradation in molten salt. In an effort to maintain greater control of the molten salt environment, new processes to sufficiently purify salts are discussed. Quantification of salt impurities is matched to corrosion results in an effort to quantify the effect of salt impurity concentrations on corrosion rates. Traditional experimentation is coupled with x-ray spectroscopy and thermodynamic modeling to identify relevant reactions and to develop a thermodynamic description of the alloy-salt system. This work is funded by the Laboratory Directed Research and Development Fund at Oak Ridge National Laboratory.

Enzymatic fuel cells with an oxygen resistant variant of pyranose-2-oxidase as anode biocatalyst

In enzymatic fuel cells (EnFCs), hydrogen peroxide formation is one of the main problems when enzymes, such as, glucose oxidase (GOx) is used due to the conversion of oxygen to hydrogen peroxide in the catalytic reaction. To address this problem, we here report the first demonstration of an EnFC using a variant of pyranose-2-oxidase (P2O-T169G) which has been shown to have low activity towards oxygen. A simple and biocompatible immobilisation approach incorporating multi-walled-carbon nanotubes within ferrocene (Fc)-Nafion film was implemented to construct EnFCs. Successful immobilisation of the enzymes was demonstrated showing 3.2 and 1.7-fold higher current than when P2O-T169G and GOx were used in solution, respectively. P2O-T169G showed 25% higher power output (maximum power density value of 8.45 ± 1.6 μW cm−2) and better stability than GOx in aerated glucose solutions. P2O-T169G maintained > 70% of its initial current whereas GOx lost activity > 90% during the first hour of 12 h operation at 0.15 V (vs Ag/Ag+). A different fuel cell configuration using gas-diffusion cathode and carbon paper electrodes were used to improve the power output of the fuel cell to 29.8 ± 6.1 µW cm−2. This study suggests that P2O-T169G with low oxygen activity could be a promising anode biocatalyst for EnFC applications.

Sulphur removal in ironmaking and oxygen steelmaking

Sulphur removal in the ironmaking and oxygen steelmaking process is reviewed. A sulphur balance is made for the steelmaking process of Tata Steel IJmuiden, the Netherlands. There are four stages where sulphur can be removed: in the blast furnace (BF), during hot metal (HM) pretreatment, in the converter and during the secondary metallurgy (SM) treatment. For sulphur removal a low oxygen activity and a basic slag are required. In the BF typically 90% of the sulphur is removed; still, the HM contains about 0.03% of sulphur. Different HM desulphurisation processes are used worldwide. With co-injection or the Kanbara reactor, sulphur concentrations below 0.001% are reached. Basic slag helps desulphurisation in the converter. However, sulphur increase is not uncommon in the converter due to high oxygen activity and sulphur input via scrap and additions. For low sulphur concentrations SM desulphurisation, with a decreased oxygen activity and a basic slag, is always required.

High-Temperature Oxidation of FeCr(Ni) Alloys: The Behaviour After Breakaway

The oxidation of an austenitic FeCr(Ni) steel at 600 °C in H2O + KCl + O2/H2 + H2O + Ar environments is studied up to 168 h. The oxidation behaviour after “breakaway” is investigated by microstructural investigations, and thermodynamic calculations (Thermo-Calc) on the FeCr(Ni)–O system are used to interpret the results. The characteristic “breakaway” scale morphology exhibits an outward-growing iron oxide and a bottom layer consisting of inward-growing spinel oxide and reaction zones containing Cr-rich oxide precipitates and Cr-depleted metal. The equilibrium calculations show that a miscibility gap appears in the FeCr spinel oxide below 665 °C influencing the microstructure of the inward-growing scale. Equilibrium calculations on the FeCrNi–O system show that Ni is not expected to enter the spinel oxide in low oxygen activity conditions, in accordance with observations.

Properties of Alumina/Chromia Scales in N2-Containing Low Oxygen Activity Environment Investigated by Experiment and Theory

The FeCrAl alloy Kanthal APMT™ was exposed to N2–5%H2 at 900 °C. Trace oxygen in the gases supplied an oxygen activity which was sufficient to render alumina and chromia thermodynamically stable. The exposures revealed that the oxide scale was penetrated by nitrogen exclusively at chromia domains. Microscopic analyses of the oxide scale did not reveal micro-cracks that could serve as points-of-entry for nitrogen. Instead it is suggested that nitrogen is transported through a dense chromia layer. Density functional theory was employed to investigate decisive nitrogen surface chemistry and transport properties in chromia and alumina. The study was used to validate that the complex redox chemistry of Cr3+ as opposed to Al3+ is a sufficient discriminating factor between alumina and chromia, facilitating N2 dissociation and mobility of N in chromia.

Nitridation of a Fecral Alloy in H2+N2 Environment at 900oc: A Microstructural Study

When exposed to O2 and/or H2O at high temperature, iron-chromium-aluminum (FeCrAl) alloys form an adherent a-Al2O3 scale that grows very slowly. Because of the excellent protective properties of this scale, the FeCrAls resist high temperature corrosion and oxidation very well and they are used in a wide range of high temperature applications up to 1300oC. However, when exposed to nitrogen-containing environments with low oxygen activity, e.g. H2+N2 mixtures, the FeCrAls become sensitive to nitridation. Such environments are often used in heat treating and sintering operations. This paper investigates the nitridation behavior of the FeCrAl alloy Kanthal APMT at 900°C. In order to provoke nitridation, the material was exposed to a N2+H2 mixture with very low water vapour content. Microstructural analyses after 24h exposure revealed that a continuous Al2O3 scale covered almost all of the alloy surface. However, the oxide scale contained a distribution of Cr2O3 patches with diameters of 10-40 mm. Cross-sectional studies using Focused Ion Beam (FIB) milling showed internal nitridation of the alloy, displaying a variety of precipitates below the Cr2O3 patches, including needle shaped AlN, equiaxed Cr2N and CrN particles. In contrast, the alloy beneath the Al2O3 scale was not nitrided. To investigate possible reasons for the formation of chromia patches we kept track of specific regions before and after exposure. Hence, several analytical techniques were employed to fully characterize the alloy microstructure prior to the exposures, using Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and Energy Dispersive X-ray (EDX) analysis. Some markers were used to enable identification of specific areas during post-exposure analysis and to enable correlation of chromia formation and internal nitridation with the alloy microstructure.

Effect of Refractory on Nonmetallic Inclusions in Al-Killed Steel

To understand the effect of refractory on the evolution of inclusions in the secondary steelmaking, steel samples were taken at different stages of the process train. Laboratory experiments were carried out using different refractories (alumina, spinel, and MgO). In the laboratory study, the types of inclusions considered were alumina, spinel, and calcium aluminate. The focus was given to Al-killed steel. The results showed that alumina refractory and spinel refractory had little effect on all the three types of inclusions, while the effect of MgO refractory depended on the activity of dissolved oxygen in liquid steel. With lower oxygen activity, alumina inclusions could transform into spinel inclusions. No evident change could be found for spinel and calcium aluminate inclusions. When the oxygen activity was high enough, spinel inclusions could not be generated from alumina inclusions. The laboratory results helped in understanding the evolution of the inclusions in the industrial process.

The Effects of Low Oxygen Activity Conditions on the Phase Equilibria and Cation Occupancy of Strontium Barium Niobate

Strontium barium niobate (SBN) is a tungsten bronze family ferroelectric which shows promising thermoelectric properties under reducing conditions. It is found here that the enhanced electrical conductivity of oxygen‐deficient SBN correlates with the formation of a NbO2 secondary phase. The effects of the reducing environment and the NbO2 phase formation are studied via a detailed defect chemistry analysis. Increasing amounts of the NbO2 phase are accompanied by an interesting mechanism where the A‐site occupancy of the SBN matrix increases. The resulting donor defects source the large carrier concentrations which cause the enhanced electrical conductivity necessary for thermoelectric performance. In investigation of the phase equilibria, it is found that a solid solution between (Sr0.6,Ba0.4)Nb2O6 and (Sr0.6,Ba0.4)1.2Nb2O6 exists and that the A‐site filling is found to occur at more modest reduction conditions in Sr‐ and Ba‐rich compositions. Finally, thermogravimetric analysis of the reoxidation process is performed, and the results suggest that the A‐site filling is compensated ionically. Not only do the presented results explain the enhanced electrical conductivity of oxygen‐deficient strontium barium niobate but also modification of the site occupancies by reduction and reoxidation may widen the design space for property modification in tungsten bronze‐structured materials in general.

Accelerated ceria–zirconia solubilization by cationic diffusion inversion at low oxygen activity

Fast elemental diffusion at the Gd-doped ceria/Y-stabilized zirconia interface occurs under reducing conditions at low oxygen activity and high temperatures.

Direct In-Situ Investigation of Selective Surface Oxidation During Recrystallization Annealing of a Binary Model Alloy

The study of high temperature oxidation processes depends mainly on ex situ investigations after the high temperature process is completed. In this study Ambient Pressure Photoelectron Spectroscopy was used to investigate its potential for an in situ analysis of the underlying mechanisms of selective oxidation occurring during high temperature annealing. As an example, the evolution of the surface oxide states of an iron-manganese model alloy (with 2 wt% Mn) during an annealing step simulating recrystallization annealing was studied. It was shown that an investigation of the surface after the annealing step does not provide reliable information about the oxidation state during the annealing step. At low partial pressures of water in a forming gas atmosphere the kinetics of water dissociation on the surface are not high enough to provide enough oxygen for reaching the thermodynamically expected activities. This unexpected low oxygen activity on the surface is also shown to affect the uptake of hydrogen into the Fe–2Mn sample.

Characterizations of Fe Doping on B‐Site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ Interconnect Materials for SOFCs

The microstructure, lattice parameters, and electrical conductivity mechanisms for Fe doping on B‐site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ were systematically investigated. The oxygen nonstoichiometry was measured by means of thermogravimetry as a function of oxygen partial pressure. In this study, the concept of defect chemistry is used to explain the relationship between the concentration of electron hole with the electrical conductivity. Based on the result of electrical conductivity in air, it is concluded that the concentration of electron hole at high oxygen activity is larger than that at low oxygen activity. This is due to the fact that (La0.8Ca0.2)CrO3−δ‐based ceramics are p‐type conductors, the electrical conductivity is dominated by the concentration of hole. At higher Fe‐doping level, the compensation mechanism at high oxygen activity is significantly dominated by the formation of oxygen vacancy, that is, ionic compensation. The compensation mechanism at low oxygen activity is significantly dominated by the formation of the formation of Cr4+, that is, electrical compensation at lower Fe‐doping level. Based on oxygen nonstoichiometry data, it is found that with increasing Fe‐doping amount on B‐site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ specimens, the initial weight‐stable temperature shifted to lower temperature which might be highly related with the change in compensation mechanism at the temperature.

Modelling and experiment of selective oxidation and nitridation of binary model alloys at 700 °C – The systems Fe, 1 wt.%{Al, Cr, Mn, Si}

Selective oxidation experiments of binary iron alloys, containing 1wt.%Al, Cr, Mn or Si were performed. Using hydrogen/water vapour mixtures adjusts low oxygen activities at the surface that hinder the formation of an outer oxide scale. This keeps the surface unchanged and generates a worst-case scenario for oxide and nitride formation inside the material. Corrosion depth and precipitate composition were observed by SEM/EDX and compared to theoretical simulations. Variations of the gas atmosphere were performed to investigate the presence of nitrides under experimental conditions. Results help to broaden knowledge of single-element effects in corrosion and the understanding of complex alloy compositions.

Densification of Highly Defective Ceria by High Temperature Controlled Re-Oxidation

Highly enhanced densification and grain growth of Ce0.9Gd0.1O1.95-δ (CGO, gadolinium-doped ceria, with 10 mol% Gd) is achieved in low oxygen activity atmospheres. However, the material can suffer mechanical failures during cooling when the re-oxidation process is not controlled due to the large volume changes. In this work, the redox process of CGO is investigated using dilatometry, microscopy, electrochemical impedance spectroscopy and thermodynamic analysis. In addition, the conditions allowing controlled re-oxidation and cooling in order to preserve the mechanical integrity of the CGO component are defined: this can be achieved over a wide temperature range (800−1200°C) by gradually increasing the oxygen content of the atmosphere. It is found that the electrical conductivity of the CGO, particularly at low temperature (<450°C) is influenced by the sintering and controlled re-oxidation conditions. An increase in activation energy for conduction at low temperature is observed as the re-oxidation temperature decreases. Moreover it was observed that the ionic conductivity blocking effect, usually associated with grain boundary contributions, is not influenced by the grain size but rather by the chemical history of the material.

Influences of non-stoichiometry on thermodynamics and kinetics of iron oxides reduction processes

Non-stoichiometry influences both the thermodynamic and kinetic analyses of the iron oxide redox processes. The thermochemical data of iron oxide redox reactions in various textbooks are not consistent, and the kinetic characteristics are not well understood because of the non-stoichiometry. To clarify such confusions, some famous thermodynamic data are compared, and highly precise experimental work conducted for verification. It is shown that the thermodynamic data for the pure iron oxide reduction reactions from JANAF agree well with the experimental results; the eutectoid temperature of iron oxides was proven to be 576°C; Dieckmann’s defect model of magnetite was proven in good agreement with the experimental results only at high oxygen activities but not low oxygen activities; and the dependences of iron deficiency on Δwt-% (weight loss ratio) and Fe2+% (ferrous ratio) were calculated and experimentally verified in pure iron oxides reduction processes.