Water Vapor Oxidation Research Articles

Structure and properties of the modified diffusion nitride-oxide surface layer

To combine the processes of gas nitriding and oxidation in water vapor, the effects of the atmospheric nitrogen potential in dissociated ammonia on the formation of surface diffusion nitride phases are studied.Modification of the surface nitride layer with oxygen was carried out by oxidation of the nitride zone at the second stage of obtaining an oxycarbonitride layer.The corrosion and wear-resistant properties of the nitride-oxide layer t of the lowest nitride layer are investigated, depending on the phase changes in the nitride layer after oxidation. The distributions of the elements in the nitride-oxide layer are determined, and the nature of the formation of the modified surface layer under the combination of gas nitriding with subsequent oxidation in water vapor is established.

Micro structural and X-ray analysis of nitro-oxidated antened steels

The article examines the structural and phase changes of surface diffusion nitride-oxide coatings obtained in the process of nitriding in a gas medium followed by oxidation in water vapor of ferrite-pearlite steels, as well as the effect of phase changes on corrosion properties. The combination of the process of gas nitriding in an ammonia medium followed by oxidation in water vapor (nitro-oxidation) consists in the fact that at the first stage of saturation, nitriding is carried out in a gas atmosphere, and at the second stage, the nitride layer is oxidized in superheated steam.

Design and characterisation of AlCrFeCuNbx alloys for accident-tolerant fuel cladding

High entropy alloys (HEAs) have been considered as candidate materials used in the accident-tolerant fuel (ATF) cladding of reactor due to their promising oxidation and corrosion resistance. AlCrFeCuNbx (x = 0.3, 0.5, 0.7, 1) HEAs were designed to improve the accident tolerance of nuclear fuel in this paper. The microstructure, hardness, quasi-critical water corrosion resistance and high temperature water vapor oxidation of the AlCrFeCuNbx HEAs were systematically investigated. The results show that when x is 1.0, the alloy has optimal hardness, water corrosion and high temperature water vapor oxidation resistance. The X-ray diffraction (XRD) analyses showed that the AlCrFeCuNbx HEAs contained a base centered orthorhombic structure and intermetallic compound Cu9Al4. The AlCrFeCuNb HEA had the best performance of 16.7 mg/dm2 weight gain comparable with traditional Zr–1Nb alloy used for nuclear fuel cladding under operating conditions of nuclear power plant water environment of 360 °C and 18.6 MPa for three days keeping, and θ-Al2O3 and NbO formed on the surface of the HEA after water corrosion. The HEA with the same composition had also excellent oxidation resistance (36.77 mg/dm2 weight gain) better than other compositions, which was two orders of magnitude less than that of the Zr–1Nb alloy in the environment of 1200 °C water vapor. Protective α-Al2O3 and CrNbO4 formed on the surface of the AlCrFeCuNb HEA, which prevented further oxidation of the alloy.

Oxidation of Dibenzothiophene in Water Vapor and Argon at Increased Pressure

This article presents the results of study of dibenzothiophene (DBT, the fuel equivalence ratio $$\varphi = 0.94$$ –1.01) oxidation in a medium of dense water vapor or argon at uniform heating (1 K/min) to 853 K of a tubular reactor made of stainless steel. The oxidation of DBT was found to proceed by the mechanisms of heterogeneous and homogeneous reactions. From the dynamics of the temperature of the reaction mixtures it follows that self-ignition of DBT in an environment of water vapor and argon occurs at 532 K. A Pt-Rh/Pt thermocouple introduced into the center of the reaction volume was found to have a catalytic effect on DBT oxidation. The results of mass spectrometry analysis of gas products show that the degree of carbon burnout exceeds 95% mol. The interaction of stainless steel and H $$_{2}$$ SO $$_{4}$$ formed in the DBT oxidation in water vapor causes steel corrosion, which is suppressed due to neutralization of the acid by calcium carbonate. The composition of the corrosion products was determined by the X-ray diffraction and energy-dispersive analysis methods.

Microwave plasma oxidation kinetics of SiC based on fast oxygen exchange

Gate oxide process is one of the most important issues for making high performance SiC power MOSFETs. Previously, it has been demonstrated that plasma oxidation provides a promising way for making advanced SiC power MOSFETs. To further boost the performance of SiC MOSFETs, understanding the plasma oxidation kinetics becomes inevitable. In this contribution, microwave plasma oxidation kinetics of SiC are investigated by labelling the SiO2 with 18O isotopes. Growth of SiO2 chemically comparable to that from 1300 °C thermal oxidation with flat interface is confirmed by XPS and TEM. Moreover, SiO2 with thickness over 50 nm is demonstrated to be obtainable with rapid growth rate (>2 nm/min) at relatively low temperatures (<1000 °C) by properly adjusting the power and pressure. During the plasma oxidation, fast oxygen exchange is proved by experiments, which is significantly different from the oxidation mechanism of thermal oxidation. Furthermore, several possible reasons, such as thermal effect, water vapor oxidation and atomic oxygen diffusion for the fast oxygen exchange are discussed and eliminated, while the presence of defects in SiO2 or SiO2 structural changes are left as possible reasons for the fast oxygen exchange.

Water vapor oxidation behaviors of nuclear graphite IG-110 for a postulated accident scenario in high temperature gas-cooled reactors

Water leakage in accidental conditions of high temperature gas-cooled reactors is one of the most critical problems that can compromise the integrity of different nuclear components. In this study, oxidation behaviors of nuclear graphite IG-110 in water ingress accidental conditions were investigated. Mass loss and oxidation rates were evaluated after oxidation tests at temperatures up to 1400 °C in an Ar-20 vol% H2O mixed atmosphere. The activation energy decreased from 318.6 to 148.9 kJ/mol with temperature, indicating two different oxidation regimes. The cross-sections of the oxidized samples were systematically characterized. The corresponding logarithmic porosity profiles showed a temperature dependency. Pore formation moved toward near-surface regions with increasing temperature and preferential binder oxidation, with filler particle degradation. Furthermore, oxidant concentration profiles and oxidation depths were estimated using a theoretical model and compared with the experimental results. This work provides important benchmark data and safety analysis guidance for the accident scenario in high temperature gas-cooled reactors.

Comparison of the oxidation behavior of a zirconium nitride coating in water vapor and air at high temperature

The oxidation behavior of zirconium nitride coating in high-temperature water vapor and air environments was studied. The parabolic rate constant of ZrN oxidizing in the water vapor environment at 600 °C was approximately 100 times faster than that in air, due to the larger pores and greater number of cracks that were formed across ZrO2 oxide layer formed during the water vapor oxidation process than during the air oxidation process. A bilayer-structure ZrO2 with tetragonal ZrO2 near the ZrN/ZrO2 interface and monoclinic ZrO2 approaching the outer ZrO2 surface were formed in both cases. The lateral cracks across the ZrO2 scale were caused by volume expansion from the tetragonal ZrO2 phase to the monoclinic ZrO2 phase transition.

Effect of water vapor on evolution of a thick Pt-layer modified oxide on the NiCoCrAl alloy at high temperature

To investigate the effect of water vapor on the novel Pt-containing oxide growth behavior, Pt-addition within the oxide layer on the surface of NiCoCrAl coating and furnace cycle tests were carried out at 1050 °C in air and air plus water vapor. The thick Pt-containing oxide layer on NiCoCrAl exhibits a different oxidation growth behavior compared to the conventional Pt-diffusion metallic coatings. The Pt-containing oxide after oxidation in air plus water vapor showed a much thicker oxide layer compare to the ones without Pt addition, and also presented a much better coating adhesion. During the oxidation process in air, Pt promotes the spinel (NiCr2O4) formation. However, the Cr2O3 formed in air with water vapor and fixed Pt within the complex oxide layer. The water vapor promoted the Ni and Co outer-diffusion, and combined with Pt to form CoPt compounds on the surface of the NiCoCrAl coating system.

Resistive switching of Cu/Cu2O junction fabricated using simple thermal oxidation at 423 K for memristor application

Recently, extensive researches have been done on memristor to replace current memory storage technologies. Study on active layer of memristor mostly involving n-type semiconductor oxide such as TiO2 and ZnO. This paper highlight a simple water vapour oxidation method at 423 K to form Cu/Cu2O electronic junction as a new type of memristor. Cu2O is a p-type semiconductor oxide, was used as the active layer of memristor. Cu/Cu2O/Au memristor was fabricated by thermal oxidation of copper foil, followed by sputtering of gold. Structural, morphological and memristive properties were characterized using XRD, FESEM, and current-voltage, I-V measurement respectively. Its memristivity was indentified by pinch hysteresis loop and measurement of high resistance state (HRS) and low resistance state (LRS) of the sample. The Cu/Cu2O/Au memristor demonstrates comparable performances to previous studies using other methods.

Effects of water vapor on protectiveness of Cr2O3 scale at 1073 K

Fe-Cr alloy is commonly being used as boiler tube’s material. It is subjected to prolonged exposure to water vapor oxidation. The ability to withstand high temperature corrosion can normally be attributed to the formation of a dense and slow growing Cr-rich-oxide scale known as chromia, Cr2O3 scale. However, oxidation may limit the alloy’s service lifetime due to decreasing of its protectiveness capability. This paper is to presents an experimental study of thermo gravimetric and Fourier transform infrared analysis of Cr2O3 at 1073 K in dry and humid environment. Samples were used from commercially available Cr2O3 powder. It was cold-pressed into pellet shape of 12 mm diameter and 3 mm thick with hydraulic press for 40 min at 48 MPa. It then sintered at 1173 K in inert gas environment for 8 h. The samples are cooled and placed in 5 mm diameter platinum pan. It is subjected to reaction in dry and wet environment at 1073 K by applying 100%-Ar and Ar-5%H2 gas. Each reaction period is 48 h utilizing Thermo Gravimetric Analyzer, TGA to quantify the mass changes. After the reaction, the samples then characterized with Fourier Transform Infrared Spectroscopy, FT-IR and Field Emission Electron Scanning Microscopy, FE-SEM. The TGA result shows mass decreasing ratio of Cr2O3 in wet (PH2O = 9.5x105Pa) and dry environment is at a factor of 1.2 while parabolic rate at 1.4. FT-IR results confirmed that water vapor significantly broaden the peaks, thus promotes the volatilization of Cr2O3 in wet sample. FESEM shows mostly packed and intact in dry while in wet sample, slightly porous particle arrangement compare to dry. It is concluded that water vapor species decreased Cr2O3 protectiveness capability.

Analysis of Water Vapor Oxidation of Fe and Fe–Cr Alloys by Measuring the Partial Pressures of Hydrogen and Oxygen

The oxidation behaviors of the Fe, Fe–15mass%Cr and Fe–30mass%Cr alloys were investigated at 1173 K in Ar–H2O and Ar–O2–H2O atmospheres using an oxygen sensor and a hydrogen sensor installed at the outlet of an experimental reactor. In Ar–H2O, the changes in the hydrogen partial pressure were continuously measured using the hydrogen sensor, and the amounts of generated hydrogen and resultant consumed oxygen were calculated. The calculated gross mass gain was in close agreement with that gravimetrically obtained after the experiment. In Ar–O2–H2O, the amount of oxygen consumed from the ambient oxygen gas in the atmosphere was calculated from the change in the oxygen partial pressure measured using the oxygen sensor. In this case, the gross mass gain was analyzed as the sum of the amount of oxygen consumed from the ambient oxygen and that supplied from the steam dissolution. The oxidation patterns were classified into three types: (1) water vapor oxidation (in Ar–H2O), (2) water vapor oxidation and oxygen oxidation (in Ar–500 ppmO2–H2O except for Fe–30Cr), and (3) oxygen oxidation (in Ar–1%O2–H2O). It was demonstrated that a gas monitoring method using a combination of hydrogen and oxygen sensors is useful for investigating the water vapor oxidation in an atmosphere containing water vapor and oxygen.

Special Issue on Oxidation in Water Vapor

Special Issue on Oxidation in Water Vapor

Kinetically driven switching and memory phenomena at the interface between a proton-conductive electrolyte and a titanium electrode.

Numerous studies have examined the switching properties of semi- or ion-conductors and isolators; however, most of these have focused on the ohmic resistance characteristics. Here, we report a new type of polarity-dependent switching phenomenon obtained for electrical devices with the configuration: metal working electrode│Si0.97Al0.03H0.03P2O7-polytetrafluoroethylene composite electrolyte│Pt/C counter electrode. The counter electrode is reversibly active for the water vapor oxidation and evolution reactions. The composite electrolyte exhibits high withstanding voltage capability in the bias voltage range of ±7 V. When titanium was employed as the working electrode, the anodic polarization resistance was approximately two orders of magnitude greater than the cathodic polarization resistance. The ohmic resistance of the device was almost unchanged, regardless of the bias voltage polarity. Moreover, kinetically induced high-resistance/low-resistance states could be cyclically switched through positive/negative bias voltage pulses, and these states were also confirmed to be memorized at open circuit.

Atomic-scale Studies of Uranium Oxidation and Corrosion by Water Vapour.

Understanding the corrosion of uranium is important for its safe, long-term storage. Uranium metal corrodes rapidly in air, but the exact mechanism remains subject to debate. Atom Probe Tomography was used to investigate the surface microstructure of metallic depleted uranium specimens following polishing and exposure to moist air. A complex, corrugated metal-oxide interface was observed, with approximately 60 at.% oxygen content within the oxide. Interestingly, a very thin (~5 nm) interfacial layer of uranium hydride was observed at the oxide-metal interface. Exposure to deuterated water vapour produced an equivalent deuteride signal at the metal-oxide interface, confirming the hydride as originating via the water vapour oxidation mechanism. Hydroxide ions were detected uniformly throughout the oxide, yet showed reduced prominence at the metal interface. These results support a proposed mechanism for the oxidation of uranium in water vapour environments where the transport of hydroxyl species and the formation of hydride are key to understanding the observed behaviour.

Intermediate Temperature Internal Oxidation of a SiC/SiCN Composite with a Polymer‐Derived Matrix

SiC‐based composites exhibit oxidative embrittlement at intermediate temperatures. Although the mechanisms of internal oxidation in composites with initially hermetic matrices have been studied extensively, comparable studies on composites with semipermeable matrices, such as those produced by polymer infiltration and pyrolysis, have not been reported. The present article focuses on the latter class of composites, specifically a SiCf /SiCNm with a dual BN/Si3N4 fiber coating. It describes detailed SEM and TEM analyses of the microstructure before and after oxidation in dry air or water vapor at 800°C. The results show that internal oxidation is more aggressive in water vapor and occurs appreciably even in the absence of an applied stress. The sequence of oxidation of the constituent phases appears to be consistent with the underlying thermodynamic hierarchy for the respective oxidation reactions. Notably, contrary to existing models based on preferential oxidation of BN coatings, oxidation occurs first on the SiC fiber surfaces and the Si3N4 overcoat; crystalline BN remains even after significant fiber and matrix oxidation has occurred. The results are discussed in terms of rate‐controlling kinetic processes, the effect of oxidant type, and applied stress.

Enhanced catalytic activity of Ni3Al foils towards methane steam reforming by water vapor and hydrogen pretreatments

A pretreatment procedure consisting of water vapor oxidation followed by H2 reduction has been developed for enhancing the catalytic activity of as-rolled Ni3Al foils for steam reforming of methane. The catalytic activities of the as-rolled Ni3Al foils in the temperature range of 873–1023 K at a steam to carbon ratio of 1.0 were found to be significantly improved by the pretreatment procedure consisting of a water vapor oxidation at 873–923 K for 1 h followed by a H2 reduction at 873 K for 1 h. Surface characterization of the foils before and after the reactions revealed that many fine NiO particles were formed on the surface by the water vapor oxidation at 873–923 K, whereas much less NiO particles were formed by the water vapor oxidation at low temperatures (<873 K) or high temperatures (>973 K). During the subsequent H2 reduction treatment at 873 K, These NiO particles were reduced to metallic Ni. It is supposed that the enhancement in catalytic activity is attributed to the fine metallic Ni particles formed on the foil surface by the pretreatment procedure.

A novel approach to the characterization of thin oxide layers

A novel approach to the characterization of thin coatings or surface layers by means of the ASTAR hardware attached to the transmission electron microscope (TEM) and the dedicated software, used for orientation and phase map analyses, was applied to examine the Sanicro 25 steel after 500h of oxidation in water vapor at 700°C. Phase composition and grain orientation maps as well as texture analysis of thin (i.e. with a thickness of below 1µm) oxide layers were performed using a new TEM technique similar to electron backscatter diffraction in the scanning electron microscope. The spatial resolution of this technique was sufficient for the imaging of nm-sized grains and a full characterization of the thin oxide scale. The acquired results were verified using data obtained by means of conventional selected area electron diffraction (TEM-SAED), energy-dispersive X-ray spectroscopy and texture analyses using X-ray diffraction techniques. It should be noted that high-quality TEM samples are required for such investigations.

Surface‐oxide‐controlled InAlN/GaN MOS‐HEMTs with water vapor

We have investigated the effect of oxidant sources on the performance of InAlN/GaN metal–oxide–semiconductor high electron mobility transistors (MOS‐HEMTs). We clarified that the surface‐oxide of InAlN comprises aluminum oxide (Al2O3) and indium oxide (In2O3) when using conventional oxygen (O2) plasma, and In2O3 causes the gate leakage and current collapse of InAlN/GaN MOS‐HEMTs due to its narrow bandgap and electron traps, such as oxygen vacancies. Then, we proposed water (H2O) vapor oxidation to decrease In2O3 for the surface‐oxide by focusing on the thermal stability of intermediate hydroxide. Aluminum hydroxide [Al(OH)x] is thermally stable because it changes into Al2O3 at 300 °C. On the other hand, indium hydroxide [In(OH)x] is thermally unstable because it desorbs at 150 °C. Therefore, Al2O3 was selectively increased when using 300 °C H2O vapor because of In(OH)x desorption, and the gate leakage and current collapse were successfully reduced due to the decrease in In2O3. We concluded from these results that a key role in improving the performance of InAlN/GaN MOS‐HEMTs is decreasing In2O3 for the surface‐oxide.

The scaling behavior and mechanism of Ti2AlC MAX phase coatings in air and pure water vapor

The scale behavior of Ti2AlC coating at 750°C in air and pure water vapor was investigated. A four-layered scale, a thick TiO2 and Al2O3 mix oxide outer layer, followed by a thin α-(Al, Cr)2O3 sublayer, a thick Fe2O3 and TiO2 mix oxide mid-layer and a thin Al2O3-rich oxide inner layer in sequence, developed on the Ti2AlC coatings in air. Whereas internal oxidation occurred, no distinct oxide scale formed on the Ti2AlC coating in the case of the oxidation in pure water vapor. The Ti2AlC coating improved the oxidation resistance of 316LSS in air, especially in wet air.

Water Vapor Oxidation of Ferritic 441 and Austenitic 316L Stainless Steels at 1100 °C for Short Duration

A ferritic 441 and an austenitic 316L steels have been exposed to wet argon at 1100°C. This study focus on the characterization of the oxide scales formed after different exposure times in the range of 2.5-20min. Raman spectroscopy, XRD, SEM and XPS have been used. For all exposure times, 316L forms a breakaway type thick oxide scale (rupture of the pre-existing passivating film) with iron oxides on its outer part and a mix of spinels with Fe, Cr and Ni for its inner part. For lower water vapor partial pressure, iron oxides are constituted of wüstite. For higher water vapor partial pressure, iron oxides are constituted of a layer of hematite over a layer of magnetite slightly enriched in chromium. Due to strong oxidation condition, oxide scale is not always homogeneous and iron oxides spallation may occur. For 2.5min of oxidation on 441, a very thin layer of protective chromium oxide is formed. For longer exposure time, an almost homogeneous and much thicker layer mainly consisting of Cr2O3 is produced. The thickness varies slightly and gradually from 4 to 20min of oxidation. There are Mn-Cr spinels mixed with the chromium oxide. The most external part is strongly enriched in Mn and Fe in a spinel structure. The diffusivity of chromium is regarded as the main cause of the difference of oxidation behavior. In both cases, the first step is a very thin chromium oxide layer. When the oxidation conditions becomes too strong in terms of exposure time or water vapor partial pressure, this oxide layer breaks. The ferritic steel is able to heal and thicken its protective chromium oxide, preventing the breakaway. The healing would be due to the high diffusivity of chromium. The thickening would be caused by the presence of the Mn-Cr spinels which are a less effective diffusion barrier. The lower diffusivity of chromium in austenite promotes the breakaway.