Low Oxygen Potential Research Articles

Lead-bismuth eutectic corrosion behavior of NbMoVCr refractory multi-principal element alloys coating synthesized by magnetron sputtering

Nanocrystalline NbMoVCr refractory multi-principal element alloys coatings are fabricated by magnetron sputtering technology to improve lead-bismuth eutectic (LBE) corrosion resistance of structural materials, and their corrosion behavior in oxygen-saturated liquid LBE at 600 and 650 °C has been investigated. The coatings exhibit good phase structural stability even at 650 °C exposure. Moreover, the coatings show promising LBE corrosion resistance with parabolic oxidation rate constant 3–5 orders of magnitude lower than other candidate steels (such as 316 L, HT9, T91, SIMP, CLAM, etc.) which is due to the formation of protective (CrV)2O3 solid solution layer. The oxidation process of the coating is mainly dominated by kinetic factors, including diffusion driving force (related to LBE solubility) and diffusion energy barrier (related to atomic radius). By the way, at the grain boundaries with much lower oxygen potential, thermodynamic factors dominate the oxidation process, and V with the highest oxygen activity shows evidence of selective oxidation.

Microbiome-liver crosstalk: A multihit therapeutic target for liver disease

Liver disease has become a leading cause of death, particularly in the West, where it is attributed to more than two million deaths annually. The correlation between gut microbiota and liver disease is still not fully understood. However, it is well known that gut dysbiosis accompanied by a leaky gut causes an increase in lipopolysaccharides in circulation, which in turn evoke massive hepatic inflammation promoting liver cirrhosis. Microbial dysbiosis also leads to poor bile acid metabolism and low short-chain fatty acids, all of which exacerbate the inflammatory response of liver cells. Gut microbial homeostasis is maintained through intricate processes that ensure that commensal microbes adapt to the low oxygen potential of the gut and that they rapidly occupy all the intestinal niches, thus outcompeting any potential pathogens for available nutrients. The crosstalk between the gut microbiota and its metabolites also guarantee an intact gut barrier. These processes that protect against destabilization of gut microbes by potential entry of pathogenic bacteria are collectively called colonization resistance and are equally essential for liver health. In this review, we shall investigate how the mechanisms of colonization resistance influence the liver in health and disease and the microbial-liver crosstalk potential as therapeutic target areas.

Incompatibility between serpentinization and epidote formation in the lower oceanic crust: Evidence from the Oman Drilling Project

AbstractIt is a general tendency that epidote, which is a typical greenschist facies mineral, is scarce in the lower oceanic crust, in spite of the widespread occurrence of the other minerals indicative of similar temperature conditions such as chlorite, actinolite, prehnite and serpentine. To find the cause of this, we carried out petrological analyses of lower crustal rocks of the Oman ophiolite sampled by the Oman Drilling Project of the International Continental Scientific Drilling Program (ICDP). Petrographic observations revealed the tendency, as expected, that the amount of epidote formed by static alteration of plagioclase decreases with depth. Because mineral assemblages indicative of a wide range of temperature conditions from amphibolite to subgreenschist facies occur throughout the cores without systematic variations of abundance, the decrease of epidote amount cannot be explained by the difference of temperature condition of alteration. Petrographic observations also revealed that epidote is absent or rare in rocks containing serpentinized olivine in contrast to prehnite showing a close association with serpentinization of olivine. In an exceptional sample containing both epidote and serpentinized olivine, epidote occurs with chlorite that cuts or replaces plagioclase, mantles adjacent olivine and is connected with chlorite + lizardite veins cutting mesh‐forming serpentine veins. The distribution and mode of occurrence of epidote suggest decoupling of its formation with the main stage of serpentinization. Serpentine veins cutting olivine to form mesh texture are typically lizardite with magnetite ribbons at vein centres and have compositions of lizardite–cronstedtite solid solution at vein margins or in magnetite‐free veins, suggesting a chemical condition with low silica and low oxygen potentials at an early stage of serpentinization. Thermodynamic modelling for olivine and plagioclase alteration at greenschist facies conditions indicates that silica potential for plagioclase alteration to form prehnite + chlorite and epidote + chlorite could be higher than for olivine serpentinization. On the other hand, oxygen potential for the prehnite + chlorite formation is lower than for the epidote + chlorite formation and is comparable with that for olivine serpentinization. From the observations and analyses, it is concluded that epidote formation is inhibited by olivine serpentinization, which maintains a reducing condition for alteration in the lower oceanic crust.

Stabilization of the (1 1 1) surface of NiO and CoO by segregation of point defects

Vacancies are ubiquitous point defects in transition metal oxides. Surprisingly, the effect of vacancies on the structural stability of oxide surfaces has been neglected in the last decades. On the other hand, experimentally observed surface structures of oxides are frequently in contrast to theoretical predictions. A typical example is the widely observed (111) surface of the rock-salt-type NiO and CoO, which is predicted to be polar and unstable according to electrostatic arguments. Previous theoretical models proposed to compensate the surface polarity assume a perfect bulk, despite the high density of cation vacancies in these materials. Through first-principles calculations, we show in this work that the cation vacancies have a strong tendency to segregate to the surface, leading to the formation of surfaces with high-proportion of cation vacancies even at low oxygen potentials. Energetically, the segregated surfaces are more stable than the well-known octopolar reconstruction. The role of segregation of point defects in surface behaviors of transition metal oxides may be of significance for understanding their catalysis and electronic properties.

Effect of the electrolyte composition on the electrochemical behavior of Nd fluoride complex in a LiF-NdF3-Nd2O3 molten salt

Abstract The electrochemical behavior of Nd fluoride complexes for Nd electrowinning has been investigated according to the electrolyte composition in the LiF-NdF3 system. The electrochemical characteristics of the LiF-NdF3 electrolyte at low oxygen potential under acidic conditions and the LiF-NdF3-Nd2O3 electrolyte at high oxygen potential under basic conditions were also studied. The Nd fluoride complex behavior in the electrolyte was investigated by cyclic voltammetry for a concentration change of NdF3 with LiF at 1100 °C from 10 wt% to 64.6 wt%. The experimental results indicated that NdF3 is reduced to Nd(0) via a multi-step process. NdF2 could be produced by the comproportionation reaction between NdF3 and Nd metal. Additionally, the rate equation for NdF2 generation was derived through a reactivity test between the metal and the Nd2O3-free electrolyte. However, NdF2 production was suppressed in LiF-NdF3-Nd2O3 electrolyte.

Enhancing the Oxygen Barrier Properties of Nanocellulose at High Humidity: Numerical and Experimental Assessment

Films formed from cellulose nanofibrils (CNFs) are known to be good barrier materials against oxygen, but they lose this feature once placed in humid conditions. To tackle this issue, we applied an optimized pressing condition under elevated temperature to increase the films’ density and improve their barrier performance. Furthermore, a water barrier coating was employed on the surfaces to control the moisture uptake at high relative humidity (RH). Neat self-standing films of CNF with the basis weight of 70 g/m2 were made through a filtration technique and pressed for 1 hour at 130 °C. The resulting nanostructures were covered on both sides using a water-borne barrier layer. Hot-pressing resulted in a significant reduction in oxygen transmission rate (OTR) values, from 516.7 to 3.6 (cm3/(m2·day)) and to some degree, helped preserve the reduced oxygen transmission at high relative humidity. Introducing 35 g/m2 of latex coating layer on both sides limited the films’ swelling at 90% RH for about 4 h and maintained the OTR at the same level. A finite element model was used to predict the dynamic uptake of water into the systems. The model was found to over-predict the rate of water uptake for uncoated samples but gave the correct order of magnitude results for samples that were coated. The obtained data confirmed the positive effect of hot-pressing combined with coating to produce a film with low oxygen transmission rate and potential to maintain its oxygen barrier feature at high relative humidity.

Thermodynamic and Dynamic Study on the Carbon Deposition on an Iron Surface in a C–H–O System

The C–H–O ternary phase diagrams in equilibrium with graphite were developed to determine the gas compositions and boundaries for graphite formation in 500–900 °C. Thermogravimetric calculation results showed that carbon deposition was favored under the conditions of low temperature, high carbon, and low oxygen potential. The carbon deposition with reduced iron was carried out in a syngas of H2 and CO by thermal gravimetric experiments. The results showed that carbon deposition started at 400 °C, accelerated at 600–800 °C, and stopped at 1000 °C. Carbon deposition was accelerated by the presence of H2, which was increased with the CO ratio in the gas mixture, and depressed by the addition of CO2. The reduced iron with large surface area fabricated the carbon deposition. Cementite (Fe3C) was formed as intermediate that accelerated the carbon deposition rate. The C–H–O ternary phase diagrams in equilibrium with Fe3C were also provided.

Surface segregation effect for prevention of oxidation in Ni‐X (X=Sn, Sb) alloy by in situ photoelectron spectroscopy

Ni‐base alloys has been widely used for chemical plants because of their high strength and excellent oxidation resistance. In particular, the addition of Sn and Sb in Ni‐base alloys significantly improves the metal dusting resistance. It is indicated that Sn and Sb are segregated on the alloy surface in the metal dusting environment; however, the details have not been clarified yet. The behavior of the Ni‐Sn and Ni‐Sb alloys under a high‐temperature oxidation environment was investigated by in situ X‐ray photoelectron spectroscopy (XPS). It was confirmed that Sn and Sb have been segregated at the surface of their alloys during oxidation in low oxygen potential, PO2, environment. These results indicate that Sn and Sb segregation improves the metal dusting resistance.

Phase Transformation of Thermally Grown FeO Formed on High-Purity Fe at Low Oxygen Potential

The isothermal phase transformation behavior of thermally grown oxide scale of FeO on high-purity Fe was investigated under conditions of low oxygen potential. The phase transformation comprised three reactions, namely Fe3O4 precipitation, magnetite seam formation, and the eutectoid decomposition. The nucleation rate of Fe3O4 precipitation decreased with decreasing oxygen potential, whereas the incubation period for the eutectoid decomposition became longer. In contrast, magnetite seam formation was independent of the oxygen potential. The overall phase transformation was found to reach completion faster in an atmosphere with a lower oxygen potential. The effect of the oxygen potential on the phase transformation of FeO scale was rationalized by the different degree of supersaturation for Fe3O4 precipitation, which plays a crucial role in the eutectoid reaction.

Aluminium self-diffusion in high-purity α-Al2O3: Comparison of Ti-doped and undoped single crystals

Reliable data on self-diffusion of aluminium in α-Al2O3 are scarce and do not yet enable deriving quantitative conclusions about the impact of deliberate aliovalent doping. Therefore, the present study (1280 ≤ T/ °C ≤ 1500, pO2 = 200 mbar) was based on carefully grown high-purity single crystals doped with Ti. The experimentally determined 26Al tracer diffusivity increased with the Ti concentration and confirmed the incorporation of Ti on Al sites, TiAl•, with aluminium vacancies, VAl″′, formed for charge compensation. The observed relation between these two point defect concentrations as a function of the Ti concentration and the temperature can be quantitatively modelled by taking into account probable TiAl•:VAl″′ clusters. Using binding energy starting values from published computer simulations for the fit procedure cluster binding energies are obtained. The surprisingly high Al diffusivity in undoped samples can be tentatively rationalised by injection of Al interstitials at the liquid/solid interface because of the low oxygen potential during the crystal growth process.

Study of stand-support sintering to achieve high oxygen potential in iron ore sintering to enhance productivity and reduce CO content in exhaust gas

Abstract Coarse coke particles, thick combustion zone, and poor thermal permeability during sintering have been widely reported to lead to incomplete solid fuel combustion and low oxygen potential, which induces the formation of harmful gas and decreases the energy efficiency. To solve these problems, stand-support sintering with an ultrathick 1000 mm bed was first proposed in this paper. The influences of the stand height and coke size on the process parameters and sinter quality were studied through a series of experiments. The results showed that the average gas flow velocity increased from 0.24 m/s to 0.32 m/s and the maximum combustion zone thickness decreased from 202 mm to 140 mm, which indicated that stand-support sintering could substantially improve the thermal permeability and enhance the oxidizing atmosphere. The fuel consumption was reduced by 1.28 kg/t (2.25%), the generated CO in exhaust gas was reduced, and the production efficiency increased by 17.4% when the stand height increased from 0 to 500 mm. Moreover, a 3D high-energy X-ray computerized tomography (CT) scanner was first used to quantitatively confirm the effect of the stand on the distribution and size of the pores in the whole sinter cake. Research findings showed that the porosity increased by 7.1% when the height of the stand increased from 0 to 500 mm, which further illustrated the effect of stand-support sintering. The present study provided an effective way to optimize the energy efficiency and reduce the CO content in exhaust gas.

Study on Sintering of Artificially Oxidized Steel Compacts

Abstract Sintering of Cr-prealloyed PM steels requires atmospheres with good quality – low oxygen potential – to achieve satisfactory sintering results. But during heating even the best atmospheres may be oxidizing, the system turns to reducing conditions only at high temperatures, which can be monitored by thermal analysis. During the dewaxing process, oxidizing conditions are favourable for effective dewaxing without sooting and blistering. However, this may result in some oxygen pickup during heating, and then the final properties of the produced parts may be strongly influenced by this intermediate oxidation. This study demonstrates the behaviour of artificially oxidized steels (Fe-C and Fe3Cr-0.5Mo-C) during the sintering process by stepwise sintering. Iron and steel powder were slightly oxidized and then pressed and sintered at different temperatures. In parallel, as a second approach, pressed samples were oxidized and then sintered. Density, hardness and impact energy were measured and dilatometry/MS was used for online monitoring of the sintering process. The starting oxygen content of 0.20 to 0.30 wt% is high enough to change the sintering behaviour of the materials, but still leads to rather good properties. Thermal analysis showed that most of the oxygen picked up was present as iron oxides on the surface which were reduced by hydrogen at rather low temperatures, confirming that these were iron oxides, which also holds for the Cr-prealloyed variant. The biggest influence on the final performance was exerted by the final carbon content and the microstructural development of the material.

Study on reaction of 15-15Ti with fission products under various oxygen potential

ABSTRACT The effect of oxygen potential on the Fuel-Cladding Chemical Interaction (FCCI) of 15-15Ti cladding material was studied in this paper. In the test, the materials have been exposed to simulated fission products at 600◦C for 120 h under the oxygen potential in equilibrium with Mo/MoO2 and Cr/Cr2O3.The general corroded depth was roughly 30 µm. Furthermore, the results also show that corroded depth on the cladding surface increase with the increase of oxygen potential. No intergranular attacks were observed, which might be caused by the absence of the high-temperature gradient. A single layer of CrTe is assumed to be formed under low oxygen potential, whereas cesium and oxygen may play an important role under high oxygen potential. Moreover, FCCI may impose a great threat to the integrity of China designed 15-15Ti material.

Influence of Nickel on Biomass Pyro-Gasification: Coupled Thermodynamic and Experimental Investigations

The present research falls within the scope of the chemical influence of heavy metals on thermochemical conversion of biomass. Specifically, the impact of nickel during pyrolysis and CO2-gasification of willow samples was studied by experimental and thermodynamic approaches. For this purpose, willow wood samples were impregnated with various contents of nickel. The pyro-gasification tests were performed in a laboratory fixed bed reactor, providing the chemical composition of solid residue and major gases. Our thermodynamic simulations, based on a specific calculation procedure, were found to corroborate most of the experimental data. In addition to these methodological findings, the main results of the study are (i) Ni has a noticeable catalytic activity even at low contents (between 0.016 and 0.086 mol per kg of wood), (ii) due to the low oxygen potential of the system C + CO2, Ni stays in an active metal form as long as some carbon is left in the reactor, and (iii) Ni prevents the volatilization of sulfur during the gasification, due to the formation of a stable Ni3S2 solid phase.

Corrosion and Carburization Behaviour of Ni-xCr Binary Alloys in a High-Temperature Supercritical-Carbon Dioxide Environment

Pure Ni and Ni-xCr (x = 7, 14, 22 and 27 wt%) binary alloys were exposed to supercritical-carbon dioxide environment at 600 °C and 20 MPa for 200 h. For pure Ni, a thick NiO layer was formed on the surface. Meanwhile, for Ni-7Cr alloy, an inner oxide layer consisted of rather irregular chromia and NiO was formed below the outer NiO layer. When Cr content was greater than 14%, a continuous chromia layer was formed, resulting in much lower weight gain and oxide thickness. However, amorphous carbon layers had developed along the oxide–matrix interface when chromia was formed. The presence of the carbon layer was explained in view of the high C activity corresponding to the low equilibrium oxygen potential of chromia.

Degradation of Alloy 800H Used in Ethylene Furnace Tube Application by the Combined Effects of Carburization and Thermal Fatigue

An ethylene furnace tube made of the wrought alloy 800H has developed through the thickness crack. The cracking has been correlated with massive precipitation of embrittling carbide phases due to carburization attack and the thermally induced stresses during decoking cycles. Changes in microstructure and state of stress are reflected on the crack propagation mode, which is observed to change from fatigue near the outer surface to intergranular near the inner surface. It is concluded that the tube has been subjected to an environment of low oxygen potential and high carbon activity precluding the formation of protective oxide by the Cr2O3-forming alloy 800H. Process modification to reduce the decoking frequency and/or replacing the tube material with one capable of developing more protective oxide such as the Al2O3-forming alloy 214 may be considered to combat the problem.

Mechanism of Iron Oxide Scale Reduction in 5%H2–N2 Gas at 650–900 °C

The reduction behaviour of the oxide scale on hot-rolled, low-carbon steel strip in 5%H2–N2 gas at 650–900 °C was studied. In general, the reduction rate of the oxide scale at the centre location was more rapid than that at the near-edge location. In both cases, the reduction rates at 650 °C were extremely low and the rates increased with increased temperature, reaching their maxima at 850 °C. Arrhenius plot of the rate constant derived from the early parabolic stage revealed that the reduction mechanism at 650–750 °C differed from that at 750–850 °C, with the former being oxygen diffusion in α-Fe and the latter most likely iron diffusion in wustite. In all cases, a thin iron layer formed on the scale surface within a very short time and then the thickness of this layer remained essentially unchanged, while the scale layer was gradually reduced via outward migration of the inner wustite–steel interface, as a result of inward iron diffusion through the wustite layer to that interface. More rapid oxygen diffusion through the thin surface iron layer than the oxygen supply rate through interface reaction was believed to result in a lower oxygen potential at the outer iron–wustite interface, thus providing a driving force for iron to diffuse through the wustite layer. The inner wustite–iron interface became undulating initially; then with the rapid advance of some protruding sections, some parts of the wustite layer were reduced through first, and finally the remaining wustite islands were reduced to complete the reduction process. Porosities were generated when wustite islands were reduced due to localized volume shrinkage. Higher oxygen concentrations in the scales of the near-edge samples were believed to be responsible for their slower reduction rates than those of the centre location samples.

The structure of liquid UO2−x in reducing gas atmospheres

High energy X-ray diffraction experiments performed on hypostoichiometric UO2−x liquids in reducing gas mixtures of 95%Ar:5%CO and 95%Ar:5%H2 are compared to that conducted in a pure Ar atmosphere [Skinner et al., Science 346, 984 (2014)]. The measurements are pertinent to severe accident scenarios at nuclear reactors, where core melts can encounter reducing conditions and further shed light on the oxide chemistry of the low valence states of uranium, particularly U(III), which become stable only at very high temperatures and low oxygen potentials. The radioactive samples were melted by floating small spheres of material using an aerodynamic levitator and heating with a laser beam. In the more reducing environments, a 1.7% shift to lower Q-values is observed in the position of the principal peak of the measured X-ray structure factors, compared to the more oxidizing Ar environment. This corresponds to an equivalent elongation in the U-U nearest neighbor distances and the U-U periodicity. The U-O peak (modal) bond-length, as measured from the real-space total correlation functions, is also observed to expand by 0.9–1.6% under reducing conditions, consistent with the presence of 15–27% U3+ cations, assuming constant U-O coordination number. The slightly larger U-U elongation, as compared to the U-O elongation, is interpreted as a slight increase in U-O-U bond angles. Difficulties concerning the determination of the hypostoichiometry, x, are discussed, along with the future directions for related research.

Effects of Temperature and Oxygen Potential on Removal of Sulfur from Desulfurization Slag

The effects of temperature (1373–1673 K) and oxygen potential on removal of sulfur from hot metal desulfurization slag were investigated in laboratory-scale experiments. Both CaS and CaSO4 exist as sulfur compounds in hot metal desulfurization slag. CaS can be removed under the condition of higher oxygen potential, whereas CaSO4 can be removed at a lower oxygen potential. Thus, in order to remove both CaS and CaSO4 from desulfurization slag, it is important to control the temperature and oxygen potential to the optimum values. In this study, a higher sulfur removal ratio exceeding 90% was obtained under the conditions of a temperature range of 1473–1673 K and oxygen potential range of 10−3−10−8 atm. These results were in good agreement with thermodynamic calculations.

Numerical simulation of iron whisker growth with changing oxygen content in iron oxide using phase-field method

Phase-field method is used to simulate the growth of iron whiskers on a gas–solid interface by changing the oxygen content in iron oxide. Results show that oxygen content significantly influences not only the crystal lattice but also the diffusion direction of iron ion vacancies and iron ions, wherein iron ions diffuse to low-oxygen-content areas and iron ion vacancies diffuse to high-oxygen-content areas. The catastrophic swelling of the micro-area volume and the growth of iron whiskers are caused by the enrichment of iron ions in low-oxygen-content areas. The simulation revealed the effects of the reduction rate and the oxygen potential of gas. Iron whiskers are longer and stronger when the oxygen potential of reducing gas decreases from −133.96kJ·mol−1 to −160.01kJ·mol−1 or when the reduction rate increases from 6.3×10−3mol·s−1 to 7.3×10−3mol·s−1. Iron whiskers can appear earlier with a low oxygen potential or a high reduction rate. Both oxygen potential and reduction rate promote the nucleation and growth of iron whiskers.