Humid Synthetic Air Research Articles

Unveiling the diffusion pathways under high-temperature oxidation of Cr2AlC MAX phase via nanoscale analysis

The MAX phase chromium aluminium carbide (Cr2AlC) is a potential candidate for high-temperature structural and energy applications due to the unique combination of properties, including excellent oxidation and corrosion resistance in harsh environmental conditions. In this work, the oxidation behaviour of bulk Cr2AlC was studied at temperatures of 1000 °C and 1200 °C in humid synthetic air, mimicking a realistic application environment. An α-Al2O3 scale forms at both temperatures, consisting of both inward and outward-growing layers, with a continuous Cr7C3 layer beneath the oxide. A special focus was put on employing atom probe tomography (APT) to analyse the oxide and carbide layers at the nanoscale. Decomposition of Cr2AlC to Cr7C3 is the main source (>70%) of aluminium needed for the oxide scale formation, while Al depletion in the underlying MAX phase remains marginal (<1 at%), exhibiting a pattern different from the common high-temperature oxidation of alloys. Al diffusion to the oxide scale proceeds predominantly through grain boundaries (GBs) in Cr2AlC and through the bulk in Cr7C3. Ionic diffusion through GBs of the large-grained inner alumina layer seems to be the rate-limiting step for the oxide scale growth, resulting in the apparent parabolic oxidation kinetics. No segregation of Cr, C or any impurities, which could affect ionic transport, was found at these alumina GBs by APT, rendering the results of this study as characteristic of a pure Cr2AlC oxidation.

Enhanced acceptor concentration, proton conductivity, and hydration in multicomponent rare‐earth ortho‐niobates

AbstractThe structural properties of (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)1−xCaxNbO4−δ (x = 0–0.05) series have been studied by X‐ray powder diffraction at room temperature. The thermal properties were investigated using thermogravimetry, from which the concentration of proton defects was determined. Additionally, dilatometry studies were carried out, from which the thermal expansion coefficients and the phase transition temperature between low‐ and high‐temperature polymorphs were determined. In order to determine the electrical properties, the electrochemical impedance spectroscopy measurements under atmospheres of dry synthetic air and saturated with water vapor and heavy water at 300–800°C temperature range were performed. All the studied materials were single‐phase compositions and were stable in monoclinic I2/c crystal structure at room temperature. The water uptake studies revealed a small mass change under the switch from dry to humidified synthetic air atmospheres, suggesting the formation of proton defects. The phase transition from monoclinic to tetragonal crystal structure in the (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)1−xCaxNbO4−δ (x = 0–0.05) series occurs above 650°C. A small amount of calcium dopant increases the total conductivity in all tested atmospheres by two orders of magnitude, for example, from 0.34 μS cm−1 in La0.2Nd0.2Sm0.2Gd0.2Eu0.2NbO4 to 63.7 μS cm−1 in (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)0.95Ca0.05NbO4−δ at 700°C in humid synthetic air atmosphere.

Fabrication of Mn–Co Alloys Electrodeposited on AISI 430 Ferritic Stainless Steel for SOFC Interconnect Applications

Mn–Co alloys were electroplated on AISI 430 stainless steel using an electrodeposition technique with the aim to reduce oxidation and chromium volatilization. The electroplating parameters were designed to improve the coating quality. The increased current density with decreased MnSO4 content resulted in a denser coating layer. A sample coated with 0.10 M CoSO4 and 0.50 MnSO4 at 350 mA cm−2 showed the best oxidation resistance after being oxidized at 800 °C for 90 h. The X-ray diffraction (XRD) result revealed that the oxide growth on the surface of the coated samples mainly formed oxides of MnCo2O4, MnCr2O4, and Cr2O3. The chromium volatilization was evaluated by exposing the coated samples to humidified synthetic air at 800 °C for 96 h. The mass flux of Cr volatilization was on the order of 10−11 g cm−2 s−1. Furthermore, different heat treatments in O2 and CO2 atmospheres were compared. Annealing in CO2 at 800 °C for 4 h helped increase the Mn–Co coating density. The relationship between the porosity and its failure behavior was also discussed.

Ba0.5Gd0.8La0.7Co2O6−δ Infiltrated BaZr0.8Y0.2O3−δ Composite Oxygen Electrodes for Protonic Ceramic Cells

In this study, a composite oxygen electrode is prepared by infiltrating a protonic-electronic conducting material, Ba0.5Gd0.8La0.7Co2O6−δ (BGLC) into a proton-conducting BaZr0.8Y0.2O3−δ (BZY20) backbone. The composite oxygen electrode is studied in a symmetrical cell configuration (BGLC-BZY20//BZY20//BGLC-BZY20). The electrode and cell performance are characterized via electrochemical impedance spectroscopy (EIS) with varying the operating conditions, including temperatures, oxygen, and steam partial pressures, with the purpose to identify and characterize the different electrochemical processes taking place in the oxygen electrode. Three electrode reaction processes are observed in the impedance spectra, which are tentatively assigned to i) diffusion of adsorbed oxygen/proton migration/hydroxyl formation, ii) oxygen reduction, and iii) charge transfer, going from the low- to high-frequency range. The BGLC-BZY20 electrode developed in this work shows a low polarization resistance of 0.22, 0.58, and 1.43 Ω cm2 per single electrode in 3% humidified synthetic air (21% O2/79% N2) at 600 °C, 550 °C, and 500 °C, respectively. During long-term measurement, the cell shows no degradation in the first 350 h but degrades afterward possibly due to insufficient material stability.

Ba0.5Gd0.8La0.7Co2O6−δ Infiltrated BaZr0.8Y0.2O3-δ Composite Oxygen Electrodes for Protonic Ceramic Electrolysis Cells

Protonic ceramic electrolysis cells (PCECs) are drawing attention for hydrogen production due to their high efficiency and flexibility, as well as high purity of the produced hydrogen at intermediate temperatures (400-600 ℃). However, one of the major challenges for further improvement of PCECs performance has been the low reaction kinetics at the oxygen electrodes which can lead to high polarization resistances (Rp). In this study, a mixed proton-electronic conducting (MPEC) Ba0.5Gd0.8La0.7Co2O6− δ (BGLC) and proton-conducting BaZr0.8Y0.2O3- δ (BZY20) composite oxygen electrode material was prepared by infiltration and studied as symmetric cells (BGLC-BZY20//BZY20//BGLC-BZY20). The oxygen electrode operating conditions (temperature, oxygen and steam partial pressures) were systematically varied in order to identify and characterize the different electrochemical processes that take place in the oxygen electrode under realistic operation conditions. Two obvious electrode reaction processes were observed in the impedance spectra; the middle frequency one is tentatively ascribed to a charge transfer process and the low frequency one clearly involves oxygen surface exchange reactions. In addition, the results confirm the infiltrated cells exhibit significantly decreased polarization resistance (Rp) and enhanced long-term durability. The polarization resistances increase with pH2O and decrease with pO2. The Rp for infiltrated BZY20 electrodes are 0.44, 1.17 and 2.87 Ω cm2 in 3 % humidified synthetic air (21% O2/79% N2) at 600, 550 and 500 ℃ respectively. In addition, the electrode demonstrated good stability with no detectable degradation within 400 h durability test in 10% humidified synthetic air at 600 ℃. Overall, we conclude that the BGLC infiltrated BZY20 is a promising oxygen electrode material for PCECs operated at intermediate temperatures.

Ba0.5Gd0.8La0.7Co2O6−δ Infiltrated BaZr0.8Y0.2O3-δ Composite Oxygen Electrodes for Protonic Ceramic Electrolysis Cells

In this study, a composite oxygen electrode is prepared by infiltrating a protonic-electronic conducting material, Ba0.5Gd0.8La0.7Co2O6−δ (BGLC) into a proton-conducting BaZr0.8Y0.2O3-δ (BZY20) backbone. The composite oxygen electrode is studied in a symmetric cell configuration (BGLC-BZY20//BZY20//BGLC-BZY20). The electrochemical performance is characterized by systematically varying the operating conditions, including temperature, oxygen, and steam partial pressures, with the purpose to identify and characterize the different electrochemical processes taking place in the oxygen electrode. Two electrode reaction processes are observed in the impedance spectra: one in the middle-frequency range tentatively ascribed to the process of proton transfer from the electrode to the electrolyte, and the other in the low-frequency range corresponding to diffusion of O− species. The BGLC-BZY20 electrode developed in this work shows a low polarization resistance of 0.44, 1.17, and 2.87 Ω cm2 in 3 % humidified synthetic air (21% O2/79% N2) at 600, 550, and 500 ℃, respectively.

Effect of Elemental Substitution in (Ba,Sr)6RE2Co4O15−BaCe0.5Pr0.3Y0.2O3-δ for Cathodes of Proton-Conducting Ceramic Fuel Cells

Solid oxide fuel cells are promising energy conversion devices because of high efficiency and high fuel flexibility, but their high-operating temperature (700-900 ºC) causes problems such as fast degradation of components and long start-up/shut-down time. To overcome these problems, low- temperature operation is preferable. Proton-conducting ceramic fuel cells (PCFCs) are promising energy conversion devices for intermediate-temperature operation, especially below 600 ºC. This is because the activation energy of proton migration in proton-conducting oxides, such as rare earth-doped barium cerate, is lower than that of the oxide ion. At the current state, however, the performance of PCFCs is mainly limited by the cathode materials.Recently, we have reported that the composite of Ba4Sr2Sm2Co4O15 (BSSC4224)–BaCe0.5Pr0.3Y0.2O3 (BCPY) showed higher activity below 600 ºC in 3% humidified synthetic air, as compared with high-performance cathodes reported1). In this composite, BSSC4224 mainly served as an active site for the dissociative absorption of oxygen. In this study, then, the effect of the elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)6 RE 2Co4O15, on the ASR was studied. Symmetric cells consisting of the BaCe0.8Y0.2O3 (BCY) disk electrolyte and Ba5SrRE 2Co4O15 (RE =Nd, La, Eu, Sm, Pr, and Gd), Ba4Sr2Gd2Co4O15 or Ba5SrGd2Co4O15 (BSGC5124)–BCPY electrodes were fabricated. The electrode performance was measured in 3% humidified synthetic air (21% O2–79% N2) at 450–700 ºC by impedance spectroscopy.In impedance analysis, BSGC5124 had the lowest ASR below 550 ºC among Ba5SrRE 2Co4O15 electrodes. Then, the composite of BSGC5124–BCPY was prepared and its performance was compared with that of BSSC4224–BCPY composite. We found that BSGC5124–BCPY has comparable area-specific resistance and activation energy to BSSC4224–BCPY although BSGC5124 itself had much lower area-specific resistance and activation energy than BSSC4224. Therefore, we concluded that the dissociative adsorption of oxygen on the BSGC5124 phase is not the rate limiting step for the oxygen reduction reaction in this composite system. [1] T. Matsui, K. Manriki, K. Miyazaki, H. Muroyama, K. Eguchi, J. Mat er. Chem. A, 6, 14188 (2018).

Effect of Cation Ordering on the Performance and Chemical Stability of Layered Double Perovskite Cathodes.

The effect of A-site cation ordering on the cathode performance and chemical stability of A-site cation ordered LaBaCo2O5+δ and disordered La0.5Ba0.5CoO3−δ materials are reported. Symmetric half-cells with a proton-conducting BaZr0.9Y0.1O3−δ electrolyte were prepared by ceramic processing, and good chemical compatibility of the materials was demonstrated. Both A-site ordered LaBaCo2O5+δ and A-site disordered La0.5Ba0.5CoO3−δ yield excellent cathode performance with Area Specific Resistances as low as 7.4 and 11.5 Ω·cm2 at 400 °C and 0.16 and 0.32 Ω·cm2 at 600 °C in 3% humidified synthetic air respectively. The oxygen vacancy concentration, electrical conductivity, basicity of cations and crystal structure were evaluated to rationalize the electrochemical performance of the two materials. The combination of high-basicity elements and high electrical conductivity as well as sufficient oxygen vacancy concentration explains the excellent performance of both LaBaCo2O5+δ and La0.5Ba0.5CoO3−δ materials at high temperatures. At lower temperatures, oxygen-deficiency in both materials is greatly reduced, leading to decreased performance despite the high basicity and electrical conductivity. A-site cation ordering leads to a higher oxygen vacancy concentration, which explains the better performance of LaBaCo2O5+δ. Finally, the more pronounced oxygen deficiency of the cation ordered polymorph and the lower chemical stability at reducing conditions were confirmed by coulometric titration.

Batch fabrication of nanopatterned graphene devices via nanoimprint lithography

Previous attempts to tune the electrical properties of large-scale graphene via nanopatterning have led to serious degradation of the key electrical parameters that make graphene a desirable material for electronic devices. We use thermal nanoimprint lithography to pattern wafer-scale graphene on a 4-in. wafer with prefabricated 25 mm2 devices. The nanopatterning process introduces a modest decrease in carrier mobility and only a minor change in residual doping. Due to the rapid fabrication time of approximately 90 min per wafer, this method has potential for large-scale industrial production. The chemiresistive gas sensing response towards NO2 was assessed in humid synthetic air and dry air, with devices showing a response to 50 ppb of NO2 only when nanopatterned.

Room-temperature dosimeter-type gas sensors with periodic reset

NO2 sensing tests have been performed on room-temperature-operated SnO2 gas sensors. Due to elongated response and recovery time constants these sensors exhibited a dosimeter rather than a conventional concentration-dependent NO2 response. In order to enable continuous monitoring such sensors need to be subjected to periodically repeated sense & reset operations. Reset procedures which proved to be effective in restoring baseline conductance again are low-temperature annealing (T∼120°C), ultraviolet (UV) light exposure, and exposure to humidified synthetic air (T∼300K, r.h.∼80%). Unlike high-temperature gas sensors, room-temperature operated sensors exhibit very little intrinsic sensor drift. Severe levels of sensor drift, however, can arise from contamination-induced effects in the sensor periphery. It is demonstrated that such contamination-induced sensor drifts can be effectively compensated by exploiting the knowledge about the timing of the sense & reset operations.

Influence of humidified synthetic air feeding conditions on the stoichiometry of (La1-xSrx)yCoO3\u2212\u03b4 and La0.6Sr0.4Co0.2Fe0.8O3\u2212\u03b4 cathodes under applied potential measured by electrochemical in situ high-temperature XRD method

The solid oxide fuel cell symmetrical cells with porous (La1-xSrx)yCoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ electrodes for intermediate temperature applications have been studied under electrochemical polarization and synthetic air + H2O vapor (so called moisturized cathode gas) feeding conditions using high-temperature electrochemical in situ X-ray diffraction method, chronoamperometry, cyclic voltammetry, and impedance spectroscopy methods. Changes in the lattice parameters and electrochemical activity of La0.6Sr0.4CoO3−δ, (La0.6Sr0.4)1.01CoO3−δ, (La0.6Sr0.4)0.99CoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ were calculated depending on the temperature (T), electrode potential (E), and oxygen partial pressure (pO2) applied. Influence of H2O vapor in synthetic air on (La1-xSrx)yCoO3−δ parameters was irreversible and continued expansion of (La1-xSrx)yCoO3−δ lattice observed in H2O vapor + synthetic air feeding conditions has been observed continuously after changing wet cathode gas to dry oxygen. Nearly, reversible behavior of La0.6Sr0.4Co0.2Fe0.8O3−δ lattice has been established and for La0.6Sr0.4Co0.2Fe0.8O3−δ, the cell volume and polarization resistance started to decrease after changing humidified synthetic air to dry synthetic air in the cathode compartment. For the slightly cationic deficient (La0.6Sr0.4)0.99CoO3−δ, the cathode structure is more stable and the electroreduction of the oxygen was faster. Detailed comparison of experimental data demonstrates that the dependence of the crystallographic parameters on the electrode potential and temperature applied is in a good agreement with the electrochemical impedance spectroscopy data, indicating that the electrocatalytic activity of the cathode decreases with the rise of Fe ion concentration in B-position of LSCF cathode.

Stress response of Escherichia coli induced by surface streamer discharge in humid air

Inactivation of Escherichia coli by means of surface streamer discharge has been investigated to obtain new insights into the key mechanisms involved, with a particular emphasis placed on the microbial response to plasma-induced stress. The surface streamer discharge was produced in coplanar dielectric barrier discharge electrode geometry, and was driven by an amplitude-modulated ac high voltage in humid synthetic air at atmospheric pressure. The response to plasma-induced stress was evaluated by using conventional cultivation, sublethal injury and resazurin assay and the LIVE/DEAD® BacLight™ Bacterial Viability kit. Compared to conventional cultivation, the LIVE/DEAD® test labels bacteria with damaged membranes, while resazurin assay tracks their metabolic activity. Our results clearly demonstrate that the treated bacteria partly lost their ability to grow properly, i.e. they became injured and culturable, or even viable but nonculturable (VBNC). The ability to develop colonies could have been lost due to damage of the bacterial membrane. Damage of the membranes was mainly caused by the lipid peroxidation, evidencing the key role of oxygen reactive species, in particular ozone. We conclude that the conventional cultivation method overestimates the decontamination efficiency of various plasma sources, and must therefore be complemented by alternative techniques capable of resolving viable but nonculturable bacteria.

Low-frequency noise characterization of single CuO nanowire gas sensor devices

Low-frequency noise properties of single CuO nanowire devices were investigated under gas sensor operation conditions in dry and humid synthetic air at 350 °C. A 1/f noise spectrum was found with the normalized power spectral density of current fluctuations typically a factor of 2 higher for humid compared to dry atmosphere. A core-shell nanowire model is proposed to treat the noise as parallel combination of gas-independent bulk and gas-dependent surface noise components. The observed increase in 1/f noise in the presence of water vapor is explained in terms of Hooge's mobility fluctuation model, where the increased surface noise component is attributed to carrier scattering at potential fluctuations due to hydroxyl groups at the nanowire surface.

Single CuO nanowires decorated with size-selected Pd nanoparticles for CO sensing in humid atmosphere

We report on conductometric gas sensors based on single CuO nanowires and compare the carbon monoxide (CO) sensing properties of pristine as well as Pd nanoparticle decorated devices in humid atmosphere. Magnetron sputter inert gas aggregation combined with a quadrupole mass filter for cluster size selection was used for single-step Pd nanoparticle deposition in the soft landing regime. Uniformly dispersed, crystalline Pd nanoparticles with size-selected diameters around 5 nm were deposited on single CuO nanowire devices in a four point configuration. During gas sensing experiments in humid synthetic air, significantly enhanced CO response for CuO nanowires decorated with Pd nanoparticles was observed, which validates that magnetron sputter gas aggregation is very well suited for the realization of nanoparticle-functionalized sensors with improved performance.

Analysis of the Responses of Metal-Oxide Semiconductor Nanostructures to Nitrogen Dioxide

The paper concerns analyses of the nanostructures based on metal oxide semiconductors, such as titanium dioxide (TiO2) and zinc oxide (ZnO) exposed to nitrogen dioxide (NO2) in an atmosphere of dry and humid synthetic air. The investigations were performed at a rather low temperature amounting to 120 ◦C. The responses of sensors di ering in the morphology of their structures have been compared. Also the time of their responses were compared with each other. The paper describes theoretically changes in the e ective resistance of these structures due to the interaction of the gas. The presented results are being discussed based on the analysis of physical and chemical phenomena.

Laser filament-induced aerosol formation

Abstract. Using the aerosol and cloud simulation chamber AIDA, we investigated the laser filament induced particle formation in ambient air, humid synthetic air, humid nitrogen, argon–oxygen mixture, and pure argon in order to simulate the particle formation under realistic atmospheric conditions as well as to investigate the influence of typical gas-phase atmospheric constituents on the particle formation. Terawatt laser plasma filaments generated new particles in the size range 3 to 130 nm with particle production rates ranging from 1 × 107 to 5 × 109 cm−3 plasma s−1 for the given experimental conditions. In all cases the particle formation rates increased exponentially with the water content of the gas mixture. Furthermore, the presence of a few ppb of trace gases like SO2 and α-pinene clearly enhanced the particle yield by number, the latter also by mass. Our findings suggest that new particle formation is efficiently supported by oxidized species like acids generated by the photoionization of both major and minor components of the air, including N2, NH3, SO2 and organics.

Investigations for the Validation of the Defect Based Scale Failure Diagrams—Part I: Nickel Oxide

Mechanical stresses imposed on protective oxide scales can lead to cracking and failure of the scale and consequently to a loss of the protective properties. Therefore, an assessment of the mechanical stability limits is of great interest. In this work, a new concept using defect based oxide scale stability diagrams to assess the mechanical stability limits is discussed. In contrast to mechanical failure diagrams proposed earlier, the presented model is based on physical defect size instead of scale thickness. Nickel oxide scales on high purity (99.99 %) nickel were thermally grown in dry and humidified synthetic air to provide a model oxide system. SEM investigations were carried out to examine the physical defect structure in the oxide scales and mechanical 4-point bend testing was used to measure the critical strain for through scale cracking. The data served for establishing a data base for the validation of the defect based scale failure model.

Using a cell-based gas biosensor for investigation of adverse effects of acetone vapors in vitro

In this study, a cell-based gas biosensor is presented used for the detection and investigation of gaseous organic compounds in air. The response of living human nasal cells (RPMI 2650) and human lung cells (A549) towards the direct exposure of gaseous substances for 10min is monitored with a multi-parametric sensor system. Changes in the cellular impedance, oxygen consumption rate and acidification rate can be recorded after the exposure and represent the cytotoxicity of the present gas. The sensor is able to notify the presence of acetone in aqueous solution (2%) but in notably lower concentrations in the gas phase (100–333ppm) within 30–60min after the end of the gas exposure. Cell viability is not affected by a sequential exposure to humidified synthetic air (60%r.h.) with a flow rate of 300ml/min and therefore offers the possibility for a continuous air monitoring. In addition, exposure to synthetic air has no influence on the signals of consecutive acetone exposure. The system might be used in the future for the monitoring of ambient air in work spaces.

Gas-sensing studies on SnO2 or NASICON-type composite materials

NASICON \(\left( {Na_{1 + x} Zr_2 Si_x P_{x - 3} O_{12} ;0 \leqslant x \leqslant 3} \right)\) powders were prepared by solid-state synthesis, a fraction was ion exchanged by Li or K, verified by X-ray diffraction and energy dispersive X-ray analysis, blended with tin oxide powder, transferred to thick film pastes, and dispensed on a commercial sensor substrate (Heraeus, Germany). Simultaneous gas sensitivity measurements on nine SnO2/NASICON-type (\(M_{1 + x} Zr_2 Si_x P_{x - 3} O_{12} \), x = 0, 2.2, 3, M = Li, Na, K) composites in thermocyclic sensor operation mode by exposure to different concentrations of Ethanol, Toluene, Propylene, CO, and H2 in humidified synthetic air show a strong correlation of the gas-sensing properties with the mobile ion concentration and type of the solid electrolyte additive.

System for in situ studies of atmospheric corrosion of metal films using soft x-ray spectroscopy and quartz crystal microbalance

We present a versatile chamber ("atmospheric corrosion cell") for soft x-ray absorption/emission spectroscopy of metal surfaces in a corrosive atmosphere allowing novel in situ electronic structure studies. Synchrotron x rays passing through a thin window separating the corrosion cell interior from a beamline vacuum chamber probe a metal film deposited on a quartz crystal microbalance (QCM) or on the inside of the window. We present some initial results on chloride induced corrosion of iron surfaces in humidified synthetic air. By simultaneous recording of QCM signal and soft x-ray emission from the corroding sample, correlation between mass changes and variations in spectral features is facilitated.