Ar H2O Research Articles

Effect of Oxygen on Water Vapor Oxidation of Fe by Oxygen Partial Pressure Control Using Gas Sensor

An oxygen pump sensor was installed in front of an oxidation furnace to supply and discharge oxygen to the Ar–H2O atmosphere and accurately control the oxygen partial pressure in the atmosphere. Furthermore, the oxidation rate was set after the oxidation furnace. It was measured by an oxygen sensor and a hydrogen sensor. Pure iron was used as the sample. The purpose of this study was to clarify the oxidation behavior of water vapor and oxygen. As a result, it was clarified that the amount of hydrogen generated by the oxidation of water vapor and Fe decreases when oxygen is supplied. In addition, when the oxygen supply amount increased, oxidation by the water vapor did not occur. On the other hand, it was clarified that when oxygen in the atmosphere is discharged, oxidation by oxygen is suppressed and only oxidation by the water vapor occurs. In the cross-sectional microstructure after oxidation, Fe3O4 was formed in the outer layer in the environment where the oxygen was discharged, but by supplying oxygen, the oxide layer became thicker and Fe2O3 was formed in the outer layer. The mass gain increased as the oxygen was supplied. Considering the ratio of oxidation by water vapor and oxidation by oxygen, it was clarified by this method that the mass gain is greater when the oxidation by oxygen is dominant.

Degradation mechanism of near stoichiometric SiC fibers after air and Ar–H2O–O2corrosion at 1000–1500°C

AbstractThe near stoichiometric SiC fiber has been reported to play significant roles in the application of aeroengine field. An in‐depth understanding on the degradation mechanism of the fiber during its corrosion in air and under a simulated aeroengine environment (PH2O:PO2:PAr = 14:8:78 kPa) will shine a light on the performance evaluations of the near stoichiometric SiC fiber‐based materials as well as the development of their potential applications. In this study, X‐ray diffraction, scanning electron microscope, and FIB‐TEM were utilized to analyze the mechanical properties and microstructure of the fiber. After oxidation in dry air and Ar–H2O–O2for 1 h, respectively, the fiber strength retention rate has been found to decrease with the increased oxidation temperature. The raise in oxidation temperature also led to the increase of the thickness and the crystallization rate of the oxide scale. The most different oxidation behaviors of SiC being treated under the simulated environment than in air are the lower oxidation activation energy and the higher crystallization activation energy for cristobalite. Water vapor can promote the oxidation reaction and inhibit the crystallization of cristobalite in the oxide scale. Few significant differences have been observed otherwise in the oxidation process and oxidation chromatography crystallization mechanism of fibers being treated under different conditions. The increase of oxide layer thickness and the formation of cristobalite impair the structural integrity and compactness of the oxide scale and thus lead to the deterioration of the mechanical properties of SiC fibers. Therefore, it is proposed that oxidation resistance of SiC fiber can be improved by insulating the reaction between the oxidizing agents and the SiC fiber or by increasing the crystallization temperature of cristobalite in the oxidation process and reducing the crystallization rate.

Investigation of gas-liquid discharge characteristics in atmospheric air and argon

A discharge in contact with water in atmospheric air and argon (Ar) was presented. The electrical and optical characteristics were experimentally investigated and compared in different voltage ranges. The waveforms of voltage and current revealed this discharge didn't happen symmetrically in each period due to the asymmetric geometry of the reactor. The OH intensity obtained by Optical Emission Spectrometer (OES), discharge power P and transfer charge Q analyzed by the Lissajous method, and rotational temperature Tr and vibrational temperature Tv fitted by LIFBASE software based on OH spectra were bigger in Ar–H2O dielectric barrier discharge (DBD) than in air-H2O at 11 and 12 kV, except only Tv of Ar–H2O DBD was a little bit smaller than that of air-H2O DBD at 12 kV. Tr and Tv indicated these two gas-H2O DBDs were non thermal-equilibrium discharges. This experimental work is helpful in comprehensively understanding the discharge manner of gas-liquid at electrical, physical, and chemical aspects.

Rovibrational jet-cooled spectroscopy of the Kr–H2O van der Waals complex in the [formula omitted] bending mode region of H2O

Five rovibrational bands of the Kr–H2O complex have been recorded in the ν2 bending region of H2O using a quantum cascade laser coupled to a pulsed slit supersonic jet. Four of them have been unambiguously assigned to the Σ and Π states in v2=1,111,110, and 212 rotational levels of the monomer based on ground state combination differences and similarities with the vibration–rotation tunneling states of Ar–H2O. One of the bands is tentatively assigned to a combination band with two quanta of intermolecular van der Waals stretching mode (νs). Due to the efficient rovibrational cooling in our pulsed supersonic expansion, all observed bands originate from the lowest lying states. Four of them are from the lowest ortho ground state Σe(101) and one of them is from the lowest para ground state Σe(000). The jet-cooled spectra have been analyzed in terms of a nearly free internal rotor model taking into account Coriolis couplings between close lying Σ and Π levels. Molecular parameters for the five upper vibrational states, band origin, rotational and centrifugal distortion constants have been accurately determined. The β parameter describing the Coriolis coupling between the Σ and Π states originating from the (212) state has also been obtained.

Electrochemical Behavior of (Fe,Ni)Ox-Based Anodes for Solid-Oxide Fuel Cells in Methane-Containing Atmospheres

The work is devoted to investigation of the electrochemical behavior of (Fe,Ni)Ox-based composite anodes in the hydrogen- and methane-containing fuel. Among the studied composites, the optimum electrochemical characteristics were observed for anodes with Fe : Ni ratio approaching 2. In particular, for the electrodes with initial composition 50 vol % Fe0.67Ni0.33Oх–50 vol % Zr0.85Y0.15O1.93 the anode overpotential equals 20–30 mV at a current density of 50–80 mA/cm2 in 10% Н2–Ar–H2O at relatively low temperatures (873–923 K). Increasing current leads to further activation, presumably due to a partial oxidation of metallic particles located at the anode surface. However, the microstructure degradation of the anode layers still represents a significant problem for their utilization. Testing of the electrocatalytic activity of the anodes fabricated from Ni, Zr0.83Sc0.17O1.92 (ScSZ) and Ce0.9Gd0.1 O2 – δ (GDC) revealed a high activity toward catalytic partial methane oxidation with the subsequent electrochemical oxidation of the conversion products, as well as formation of carbonaceous deposits at the nickel surface. The methane conversion degree on Ni-anode comes to 60–90% and decreases with time and on cooling. The doping of nickel oxide with iron lowers the conversion degree and promotes the carbon poisoning, supposedly, because of the lowering of the anodic current density resulting from the worsening of the electrochemical activity.

Influence of Different Annealing Atmospheres on the Mechanical Properties of Freestanding MCrAlY Bond Coats Investigated by Micro-Tensile Creep Tests

The mechanical properties of low-pressure plasma sprayed (LPPS) MCrAlY (M = Ni, Co) bond coats, Amdry 386, Amdry 9954 and oxide dispersion strengthened (ODS) Amdry 9954 (named Amdry 9954 + ODS) were investigated after annealing in three atmospheres: Ar–O2, Ar–H2O, and Ar–H2–H2O. Freestanding bond coats were investigated to avoid any influence from the substrate. Miniaturized cylindrical tensile specimens were produced by a special grinding process and then tested in a thermomechanical analyzer (TMA) within a temperature range of 900–950 °C. Grain size and phase fraction of all bond coats were investigated by EBSD before testing and no difference in microstructure was revealed due to annealing in various atmospheres. The influence of annealing in different atmospheres on the creep strength was not very pronounced for the Co-based bond coats Amdry 9954 and Amdry 9954 + ODS in the tested conditions. The ODS bond coats revealed significantly higher creep strength but a lower strain to failure than the ODS-free Amdry 9954. The Ni-based bond coat Amdry 386 showed higher creep strength than Amdry 9954 due to the higher fraction of the β-NiAl phase. Additionally, its creep properties at 900 °C were much more affected by annealing in different atmospheres. The bond coat Amdry 386 annealed in an Ar–H2O atmosphere showed a significantly lower creep rate than the bond coat annealed in Ar–O2 and Ar–H2–H2O atmospheres.

Plasma kinetics in a nanosecond pulsed filamentary discharge sustained in Ar–H2O and H2O

The plasma kinetics of Ar–H2O and H2O at atmospheric pressure are of interest for applications in biotechnology where rare-gas plasma jets treat liquid surfaces and in water treatment where discharges are generated in bubbles or directly in liquid water. Due to evaporation resulting from heat transfer to the liquid, for many conditions the mole fraction of water in the plasma can be large—approaching nearly pure water. In this paper, results are discussed from a combined experimental and computational investigation of the chemical kinetics in a high electron density plasma filament sustained in Ar–H2O at atmospheric pressure. The chemical kinetics were simulated using a 0D global model, validated by measurements of the absolute OH and H densities by laser induced fluorescence (LIF) and two-photon absorption LIF. The primary sources of H and OH during the discharge pulse are dissociative excitation transfer from metastable Ar atoms and Ar dimer excimers at low water concentration and electron impact dissociation of H2O at high water concentration. In spite of their similar sources, the density of OH was measured to be two orders of magnitude smaller than that of H at power densities on the order of 105 Jm−3. This disparity is due to electron impact dissociation of OH during the discharge pulse and rapid reactions of OH in the presence of high H and O densities in the afterglow. It is often assumed that OH is the dominant non-selective reactive species in water-containing plasmas. These results reinforce the importance of atomic species such as H and O in water containing high energy density plasmas. A numerical parametric study revealed that the lowest energy cost for H2O2 production is achieved at low energy densities in pure water. The high concentration of atomic radicals, which rapidly recombine, results in an overall lower energy efficiency of reactive species production. In particular, the selectivity of H2O2 production decreases with increasing power density which instead favors H2 and O2 production.

Redox Behavior and Transport Properties of Composites Based on (Fe,Ni)3O4 ± δ for Anodes of Solid Oxide Fuel Cells

The Fe–Ni–O system designed for producing bimetal-containing composite anodes of solid oxide fuel cells (SOFCs) was studied. The solubility of nickel in the structure of spinel (Fe,Ni)3O4 ± δ at atmospheric oxygen pressure is ~1/3. Moderate reduction at 1023 K and p(O2) ≈ 10–20 atm leads to partial decomposition of spinels, forming an electron-conducting phase (Fe,Ni)1–yO and submicron bimetallic Fe–Ni particles on the oxide surface, which have potentially high catalytic activity. The electron conductivity has a thermally activated character and increases substantially during the reduction. In the anode conditions of SOFCs, the electric conductivity reaches 30–100 S/cm, while the thermal expansion coefficients are ~12 × 10–6 K–1, which ensures compatibility with solid electrolytes. At the same time, significant volume changes during the redox cycling (up to ~1% on the linear scale) necessitate the introduction of additional components such as yttria-stabilized zirconia (YSZ). The polarization resistance of the model composite anode of reduced Fe2NiO4 ± δ and YSZ deposited on the YSZ solid electrolyte membrane was ~1.8 Ohm cm2 at 923 K in a 4% H2–Ar–H2O atmosphere.

On the Pre-oxidation Treatments of Four Commercial Ni-Based Superalloys in Air and in Ar–H2O at 950 °C

The short-term oxidation behaviour of RA 602CA, Inconel 693, Manaurite 40XO and Sumitomo 696, which are four alloys recommended for hydrocarbon processing, was studied both in air and in Ar–H2O to determine the conditions of pre-oxidation treatments. Regardless of the considered material, the oxidation rate at 950 °C was systematically higher in Ar–H2O than in dry air. Surface examination of the Al-containing alloys indicated that they were not uniformly oxidized all over the surface. All Al-containing alloys (from 1.6 to 3.2% wt) formed an external protective alumina scale and behaved as alumina-forming alloys in dry air at 950 °C. In contrast, these alloys developed a rate-controlling chromia scale and severe internal oxidation in a H2O-containing atmosphere. Compared with their oxidation behaviour in air + H2O, the phenomenon was significantly enhanced in the atmosphere that coupled water vapour with a low oxygen pressure. Consequently, the Al-containing alloys should not be pre-oxidized in a water vapour atmosphere prior to long exposure to a corrosive atmosphere. In contrast, the chromia-forming Al-free 696 alloy exhibited identical oxidation behaviour in both atmospheres, demonstrating that the degradation of this alloy was not significantly affected by water vapour.

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.

Direct measurement of additional Ar–H2O vibration–rotation-tunneling bands in the millimeter–submillimeter range

Three new weak bands of the Ar–H2O vibration–rotation-tunneling spectrum have been measured in the millimeter wavelength range. These bands were predicted from combination differences based on previously measured bands in the submillimeter region. Two previously reported submillimeter bands were also remeasured with higher frequency resolution. These new measurements allow us to obtain accurate information on the Coriolis interaction between the 101 and 110 states. Here we report these results and the associated improved molecular constants.

Discharge plasma in an argon–water vapor mixture as a gas laser medium

The possibility of obtaining the inverse population of the excited states of hydroxyl molecules generated in low-pressure discharge plasma in an Ar–H2O mixture was studied. The possibility of inversion of both the A 2Σ+ state relative to the X 2Π ground state and inside the vibrational-rotational structure of the levels of the A 2Σ+ excited state of the OH molecule was analyzed.

Analysis of a perpendicular band in Ar–H2O with origin close to the ν1 + ν3, R(0) line in H2O

We have used continuous-wave cavity ring-down spectroscopy to record a band with origin close to 7275.1cm−1 in an Ar supersonic expansion seeded with H2O. It is assigned to ν1+ν3 ←GS, Π(101)←Σ(000) in Ar–H2O. The rotational structure (Trot=12K) is analyzed and the lines fitted (σ=0.0008cm−1) together with literature microwave data. The fit includes strongly perturbed Q lines, from interaction with a Π state with origin determined to be close to 7274.5cm−1. The mean upper state predissociation lifetime is determined to be 3ns for all reported e and f upper levels.

Effect of in-situ gas changes on thermally grown chromia scales formed on Ni–25Cr alloy at 1000°C in atmospheres with and without water vapour

Regarding oxidation resistance, most high temperature alloys rely on the formation of a protective chromia surface scale during service. In the present study, the oxidation behaviour of a Ni–25%Cr model alloy was investigated during single- and two-stage oxidation in Ar–O2 and Ar–H2O gas mixtures at 1000°C. In the two-stage experiments, the test gas was changed after a predefined oxidation time from dry to wet gas, and vice versa, without intermediate specimen cooling. Oxidation kinetics were measured using thermogravimetry and post exposure characterisation was accomplished using scanning and transmission electron microscopy, focused ion beam and energy dispersive X-ray spectroscopy techniques. The single stage exposure to Ar–O2 resulted in the formation of a voided, poorly adhering chromia scale exhibiting a coarse, equi-axed morphology. In Ar–7%H2O a fine, columnar grained oxide scale was formed which was free of microvoidage within the scale and at the scale/metal interface and therefore exhibited excellent adherence to the metallic substrate. During two-stage exposure to Ar–O2 and subsequently to Ar–H2O, a fine grained, columnar shaped oxide developed on top of the coarse oxide scale formed in the first oxidation stage. The opposite effect occurred when the exposure started in the wet environment. The mechanisms which are responsible for the observed oxidation features are discussed on the basis of classical oxidation theory in combination with previous results on oxidation of chromia forming alloys in oxygen rich gases and water vapour containing, low p(O2) environments.

New rovibrational bands of the Ar–H2O complex at the ν2 bend region of H2O

Several new rovibrational bands of the Ar–H2O complex at the ν2 bend region of H2O have been recorded using a multipass infrared absorption spectrometer with a quantum cascade laser. For five of these bands, the upper states have been unambiguously assigned to the Σ and Π levels of the (212) and ns=1,(101) states of the complex. These new internal rotation states have similar energy patterns as those detected previously in the ground vibrational state of water, indicating that the ν2 excitation of the H2O subunit exerts only minor perturbation on these states. Another five new bands belonging to the Ar–H2O complex have also been observed with their upper states tentatively labeled as the “ns=1,(110)” and “ns=1,(212)” states.

Effect of SO2 on oxidation of metallic materials in CO2/H2O‐rich gases relevant to oxyfuel environments

In an oxyfuel plant, heat exchanging metallic components will be exposed to a flue gas that contains substantially higher contents of CO2, water vapor, and SO2 than conventional flue gases. In the present study, the oxidation behavior of the martensitic steel P92 was studied in CO2‐ and/or H2O‐rich gas mixtures with and without addition of SO2. For this purpose, the corrosion of P92 at 550 °C up to 1000 h in Ar–H2O–SO2, Ar–CO2–SO2, Ar–CO2–O2–SO2 and simulated oxyfuel gas (Ar–CO2–H2O–O2–SO2) was compared with the behavior in selected SO2‐free gases. The oxidation kinetics were estimated by a number of methods such as optical microscopy, scanning electron microscopy with energy and wave length dispersive X‐ray analysis, glow discharge optical emission spectroscopy, X‐ray diffraction as well as transmission electron microscopy.The experimental results revealed that the effect of SO2 addition on the materials behavior substantially differed, depending on the prevailing base gas atmosphere. The various types of corrosion attack affected by SO2 could not be explained by solely comparing equilibrium activities of the gas atmospheres with thermodynamic stabilities of possible corrosion products. The results were found to be strongly affected by relative rates of reactions of the various gas species occurring within the frequently porous corrosion scales as well as at the scale/gas‐ and scale/alloy interfaces. Whereas SO2 addition to Ar–CO2 resulted in formation of an external mixed oxide/sulfide layer, the presence of SO2 in oxyfuel gas and in Ar–H2O–SO2 resulted in Fe‐sulfide formation near the interface between inner and outer oxide layer as well as Cr‐sulfide formation in the alloy. In the latter gases, the presence of SO2 seemed to have no dramatic effect on oxide scale growth rates.

Absolute OH density measurements in the effluent of a cold atmospheric-pressure Ar–H2O RF plasma jet in air

Absolute OH densities are obtained in a radio-frequency-driven Ar–H2O atmospheric-pressure plasma jet by laser-induced fluorescence (LIF), calibrated by Rayleigh scattering and by UV broadband absorption. The measurements are carried out in ambient air and the effect of air entrainment into the Ar jet is measured by analyzing the time-resolved fluorescence signals. The OH densities are obtained for different water vapor concentrations admixed to the Ar and as a function of the axial distance from the nozzle. A sensitivity analysis to deduce the accuracy of the model-calculated OH density from the LIF measurement is reported. It is found that the UV absorption and the LIF results correspond within experimental accuracy close to the nozzle and deviate in the far effluent. The possible reasons are discussed. The OH densities found in the plasma jet are in the range (0.1–2.5) × 1021 m−3 depending on the water concentration and plasma conditions.

Synergistic sterilization effect of microwave-excited nonthermal Ar plasma, H2O2, H2O and TiO2, and a global modeling of the interactions

A microwave-excited atmospheric-pressure plasma jet (uAPPJ) exhibited a synergistic sterilization effect when combined with hydrogen peroxide (H2O2), distilled water (DW) and titanium dioxide (TiO2) photocatalysis. The sterilization efficacy of H2O2–uAPPJ increased as the H2O2 concentration increased. The addition of TiO2 also remarkably increased the sterilization efficacy. To find the main factor for the sterilization effect, optical emission spectra and the degradation rate of a methylene blue solution were measured. Numerical analysis, a newly developed global modeling, was also conducted to discover the mechanisms. Both experimental measurements and global modeling results suggested that combinations of H2O2, DW and TiO2 increased the generation of hydroxyl radicals (·OH), which are known to be strong bactericidal agents. It was revealed that charged species, especially electrons, have a dominant role in the increase of ·OH.

Spectral characteristics of the emission from a barrier discharge plasma based on an Ar–H2O mixture in the UV spectral region

Spectral characteristics of the emission from a barrier discharge plasma based on an Ar–H2O mixture in the UV spectral region

OH density measurement by time-resolved broad band absorption spectroscopy in an Ar–H2O dielectric barrier discharge

We report results of a novel time-resolved broad-band absorption spectroscopy experiment for OH density measurement applied to a pulsed dielectric barrier discharge in Ar/H2O mixtures. The measurement is aimed at the calibration of our previous OH LIF measurements in the same discharge. The apparatus is simple and cheap, being based on a UV LED light source and a non-intensified, non-cooled, gateable linear CCD array as a detector. The set-up is capable of ruling out both medium/long-term drifts of the UV source and of the discharge, and discharge emission from the measurement. Performances of the set-up are discussed, together with possible improvements for its use as a standalone technique.