Oxygen-rich Layer Research Articles

Laser-induced fluorescence analysis of plasmas for epitaxial growth of YBiO3 films with pulsed laser deposition

We record the two-dimensional laser-induced fluorescence (LIF) on multiple plasma constituents in a YBiO3 plasma. This allows us to directly link the influence of oxygen present in the background gas during pulsed laser deposition to the oxidation of plasma species as well as the formation of epitaxial YBiO3 films. With spatiotemporal LIF mapping of the plasma species (Y, YO, Bi, and BiO) in different background gas compositions, we find that little direct chemical interaction takes place between the plasma plume constituents and the background gas. However, a strong influence of the background gas composition can be seen on the YBO film growth, as well as a strong correlation between the oxygen fraction in the background gas and the amount of YO in the plasma plume. We assign this correlation to a direct interaction between the background gas and the target in between ablation pulses. In an O2 background, an oxygen-rich surface layer forms in between ablation pulses, which provides additional oxygen for the plasma plume during target ablation. This differs from our previous observations in STO and LAO plasmas, where species oxidation primarily takes place during propagation of the plasma plume towards the substrate.

Submergence, seed germination, and seedling development of the Amazonian floodplain tree Pseudobombax munguba: evidence for root oxytropism

Primary root of seeds germinating while submerged grew upwards towards oxygen-rich surface layers. Height of water column influenced germination and root growth. Seedlings removed from water attached to substrate and grow vertically. Oxygen and light are potentially limiting resources in floodplain forests where plants are subjected to long periods of flooding, particularly in early stages of the life cycle. We experimentally evaluated the effect of flooding and availability of oxygen and light on germination and initial growth of Pseudobombax munguba (Malvaceae), a tree characteristic of the lower portions of the flood-level gradient in Central Amazonian floodplains. Neither flooding nor darkness affected germination (≥93%); however, only seeds that germinated in light developed into seedlings. Germinated seeds floating in water showed positive gravitropic curvature of the primary root and presence of starch-dense amyloplasts (statoliths) in the root cap. Seed germination decreased under 5–7 cm of non-aerated water and the primary root curved upward, extending towards the water surface where oxygen concentration would be higher. Statoliths were not present in the cap cells of these upwardly growing roots suggesting an absence of gravity-directed growth and the involvement, instead, of re-orientation along an oxygen gradient. Although about 50% of seeds germinated under 10 cm of non-aerated water, their primary root did not elongate further after emergence. Seedlings removed from water and positioned horizontally on the surface of a moist, well-aerated substrate attached themselves as roots curved downward, penetrated into the substrate and anchored the plant. The stem bent upright and resumed vertical growth. These features contribute to reducing the time required for establishment in this type of environment where successful colonization is constrained by the short terrestrial phase.

The Role of Nanoscale Seed Layers on the Enhanced Performance of Niobium doped TiO2 Thin Films on Glass.

Transparent conducting oxide (TCO) coatings with decreased cost and greater process or performance versatility are needed for a variety of optoelectronic applications. Among potential new TCO candidates, doped titanium dioxide is receiving particular interest. In this study, niobium-doped titania bilayer structures consisting of a nanoscale seed layer (deposited by atomic layer deposition or RF magnetron sputtering) followed by a thick bulk-like layer were grown directly on glass in order to examine the effects of the seed layer processing on the subsequent crystallization and electrical properties of these heterostructures. Observations from Raman spectroscopy suggest that higher oxygen content in the seed layer suppresses the formation of detrimental titania polymorph phases, found in films produced by annealing directly after synthesis without any exposure to oxygen. Furthermore, our results indicate that the generation of excellent Nb:TiO2 conductors on glass (without breaking vacuum) only occurs within a narrow processing range and that the sequential deposition of oxygen-poor layers on oxygen-rich layers is a critical step towards achieving films with low resistivity.

Laser surface treatment of porous ceramic substrate for application in solid oxide fuel cells

Laser has offered a large number of benefits for surface treatment of ceramics due to possibility of localized heating, very high heating/cooling rates and possibility of growth of structural configurations only produced under non-equilibrium high temperature conditions. The present work investigates oxidation of porous ZrB2-SiC sintered ceramic substrates through treatment by a 1072 ± 10 nm ytterbium fiber laser. A multi-layer structure is hence produced showing successively oxygen rich distinct layers. The porous bulk beneath these layers remained unaffected as this laser-formed oxide scale and protected the substrate from oxidation. A glassy SiO2 structure thus obtained on the surface of the substrate becomes subject of interest for further research, specifically for its utilization as solid protonic conductor in Solid Oxide Fuel Cells (SOFCs).

The effects of active layer thickness and annealing conditions on the electrical performance of ZnON thin-film transistors

Thin-film transistors (TFTs) based on ZnON semiconductors were fabricated, and their electrical characteristics were evaluated with respect to the active layer thickness and annealing conditions. At a fixed annealing temperature, the electrical performance decreases with decreasing ZnON thickness. X-ray photoelectron spectroscopy (XPS) studies on the oxygen 1s peak of ZnON suggest that the diminished charge transport properties may be attributed to the formation of an oxygen rich capping layer with fixed thickness on top of the active layer upon heat treatment. Secondary ion mass spectrometry (SIMS) analyses show that indeed the top portion of ZnON films becomes rich in oxygen as the anneal temperature in air increases.The experimental results indicate that optimum field effect mobility and switching ability are achieved at an annealing temperature of 300 °C with a 20 nm-thick ZnON active layer. XPS analyses on the nitrogen 1s peak of ZnON films suggest that the portion of stoichiometric Zn3N2 is highest at this annealing temperature, which is anticipated to provide high conductivity paths to the free electrons. The devices were next subjected to negative bias illumination stress (NBIS), however the amount of ZnON TFT degradation does not exhibit any mobility dependence, unlike what is observed in devices incorporating conventional oxide semiconductors such as In-Ga-Zn-O (IGZO).

Fe-Binding Dissolved Organic Ligands in the Oxic and Suboxic Waters of the Black Sea

In the oxygen-rich layer of the Black Sea, above the permanent halocline, the Fe and nitrate concentrations are low where fluorescence is relatively high , indicating uptake by phytoplankton. In this study we used ligand exchange adsorptive cathodic stripping voltammetry (CLE-aCSV), using 2-(2-Thiazolylazo)-p-cresol (TAC) as measuring ligand, to investigate the role of Fe-binding dissolved organic ligands in keeping Fe in the dissolved phase and potentially biologically available. The conditional stability constant, logK´, was between 21 and 22 in most samples, which is on average lower than in ocean water. The Fe-binding dissolved organic ligand concentrations varied between 0.35 and 4.81 nEq of M Fe, which was higher than the dissolved concentration of Fe (DFe) as found in most samples. At two stations ligands were saturated in the surface. At one station ligands were saturated near the oxycline, where ligand concentrations seemed to increase, indicating that they play a role in keeping Fe in the dissolved phase across the redox gradient. At the fluorescence maximum (between 40 and 50 m), the dissolved organic ligand binding capacity (alphaFeL=K´*[L´]) of Fe was at its highest while the concentration DFe was at its lowest. Here, we find a statistically significant, positive relationship between fluorescence and the logarithm of alphaFeL, along with fluorescence and the ratio of the total ligand concentration over DFe. These relationships are best explained by phytoplankton utilizing Fe from Fe-binding organic ligands, resulting in an increase in free Fe-binding ligands.

Negative bias illumination stress stability of dual‐active layer amorphous indium‐gallium‐zinc‐oxide thin‐film transistor

In the current study, dual‐active layer amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) TFT has been fabricated by sequential deposition of oxygen‐rich layer on a top of oxygen‐poor layer in the active layer through oxygen partial pressure control in order to improve reliability under negative bias illumination stress (NBIS) condition. The method is very simple as it is performed in situ without any post treatment. Reliability of a TFT that had a thickness of oxygen‐poor layer and oxygen‐rich active layer in the ratio of 6:4, quite improved with the threshold voltage shift Δ of a‐IGZO TFT at around 3.2 V under NBIS condition of 1000 s in comparison with a TFT (8.5 V) that comprised only oxygen‐poor active layer. The increased reliability is attributed to decrease interaction with ambient and the reduction in the total neutral oxygen vacancy concentration due to the oxygen rich layer at the back‐channel region, which was confirmed by X‐ray photoelectron spectroscopy (XPS) analysis.

Functionalization of hexagonal boron nitride in large scale by a low-temperature oxidation route

A simple and effective method to functionalize hexagonal boron nitride (hBN) was demonstrated by low temperature (600°C) oxidation of hBN powders in air. An oxygen-rich amorphous layer was observed on the surface of oxidized hBN particles. Compared to raw hBN, the oxidized hBN particles showed better affinity with ethanol. In addition, the oxidation process and mechanism were also investigated in detail. The results showed that the longer oxidation time, the thicker oxidation coating would be formed. In the beginning of oxidation, oxygen could insert into the lattice ascribe to the surface defects of hBN. With oxidation time increasing, more oxygen would embed in the inner of h-BN particles, causing the intense lattice distortion in local zone. With the oxidation process proceeded, the amorphization behavior would appear, and form a coating of amorphous boron oxides on the surfaces of h-BN particles. This method holds great promise for the application of hBN materials.

Effect of microstructure on oxygen rich layer evolution and its impact on fatigue life during high-temperature application of α/β titanium

The near alpha titanium alloy, Ti-6424S, is utilized in many critical high-temperature aerospace components due to its unique properties. However, oxygen ingress during elevated-temperature exposure induces formation of a subsurface brittle oxygen-rich layer (ORL), resulting in a deterioration of mechanical performance. This paper, for the first time, establishes the effect of the underlying microstructure on the formation and evolution of the ORL in α/β titanium alloys. In addition, models were developed to predict (i) the evolution of ORL as a function of the material microstructure, (ii) the effect of ORL on the critical strain for in-service crack initiation, and (iii) estimates of fatigue life of components made from a specific microstructure during in-service high temperature exposure and formation of ORL. In particular, five different microstructures were produced by tailored heat-treatments and thermally exposed at 650 °C up to 420 h. The base metal and the ORL were quantified using microhardness indentations, optical microscopy, and scanning electron microscopy (electron backscatter diffraction (EBSD), backscattered electron (BSE), and secondary electron (SE) imaging). The effective diffusion coefficients (Deff) for each microstructure were calculated and then integrated into a critical strain model to predict crack initiation strain as a function of exposure time. The predicted ORL thickness was used to estimate fatigue life using experimentally measured crack growth data. The largest Deff coefficient was observed in a colony microstructure, while a basketweave microstructure showed the smallest Deff. For several bimodal microstructures, Deff was noted to increase with increasing area fraction of secondary alpha colonies.

Refilled friction stir spot welding of aluminum alloy to galvanized steel sheets

Dissimilar materials lap joining of aluminum alloy to galvanized steel sheets was investigated with refilled friction stir spot welding (Refilled FSSW). For the lap joint between 1.0mm thick Novelist AC 170 PX aluminum alloy and 1.2mm thick ST06 Z galvanized steel sheets, the maximum tensile/shear fracture load is 3044N with cross-tension fracture load of 296N. For that between 1.5mm thick Aleris Superlite 200 ST aluminum alloy and 1.2mm thick ST06 Z galvanized steel sheets, the maximum tensile/shear fracture load reaches 4500N and the maximum cross-tension fracture load is 359N. All samples failed through the Al/steel interface during tensile/shear and cross-tension tests. The microstructure, composition, fractogragh and fracture mode of Al/steel joints were analyzed with electron probe microanalysis (EPMA), scanning electron microscope (SEM) and X-ray diffraction (XRD). Zinc and oxygen rich layer were observed in the interface on aluminum alloy side, and ZnO phase was detected on the fractured surface of both aluminum alloy and steel sides. The samples during tensile/shear test failed mainly through shear brittle fracture with features of cleavage and intergranular fracture feature. The spot weld can be distinguished as pin-refilled zone and sleeve-plunging zone due to the distribution of zinc coating.

An attempt for a unified description of mechanical testing on Zircaloy-4 cladding subjected to simulated LOCA transient

During a Loss Of Coolant Accident (LOCA), an important safety requirement is that the reflooding of the core by the emergency core cooling system should not lead to a complete rupture of the fuel rods. Several types of mechanical tests are usually performed in the industry to determine the degree of cladding embrittlement, such as ring compression tests or four-point bending of rodlets. Many other tests can be found in the open literature. However, there is presently no real intrinsic understanding of the failure conditions in these tests which would allow translation of the results from one kind of mechanical testing to another. The present study is an attempt to provide a unified description of the failure not directly depending on the tested geometry. This effort aims at providing a better understanding of the link between several existing safety criteria relying on very different mechanical testing. To achieve this objective, the failure mechanisms of pre-oxidized and pre-hydrided cladding samples are characterized by comparing the behavior of two different mechanical tests: Axial Tensile (AT) test and “C”-shaped Ring Compression Test (CCT). The failure of samples in both cases can be described by usual linear elastic fracture mechanics theory. Using interrupted mechanical tests, metallographic examinations have evidenced that a set of parallel cracks are nucleated at the inner and outer surface of the samples just before failure, crossing both the oxide layer and the oxygen rich alpha layer. The stress intensity factors for multiple crack geometry are determined for both AT and CCT samples using finite element calculations. After each mechanical test performed on high temperature steam oxidized samples, metallography is then used to individually determine the crack depth and crack spacing. Using these two important parameters and considering the applied load at fracture, the stress intensity factor at failure is derived for each tested sample. This procedure provides an assessment scheme to determine experimentally the fracture toughness of the prior-β region in the mid-wall of the oxidized samples. The obtained fracture toughness for CCT and AT samples are thus compared, confirming that the linear elastic fracture mechanics is a relevant tool to describe the strength of LOCA embrittled cladding alloys.

High temperature hydrogenation of Ti–V alloys: The effect of cycling and carbon monoxide on the bulk and surface properties

In the present work, the high temperature (425–575 °C) hydrogen storage properties of Ti–V alloys have been studied, both at static conditions and in a flow of hydrogen gas. The selected isothermal temperature range is considered as an optimal condition for hydrogen sorption enhanced steam reforming. When hydrogenation and dehydrogenation were performed in pure hydrogen gas and in vacuum, a large reversible hydrogen capacity of 3.95 wt. % H was obtained, demonstrating completeness of the formation and decomposition of (Ti,V)-dihydrides. However, when cycling was performed in a flow of hydrogen gas, the reversible hydrogen capacity decreased to ∼2 wt. % H caused by the formation of stable lower (Ti,V)-hydrides. A further decrease in the reversible hydrogen storage capacity took place when pure hydrogen gas was replaced by a mixture of hydrogen and carbon monoxide CO. This decrease was caused by the formation of an oxygen rich layer on the surface of the alloy, which was partially blocking the hydrogen exchange between the surface and the bulk of the sample.

High temperature tribological behavior of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C coatings against aluminum alloys

Abstract Friction and wear behavior of tetrahedral amorphous carbon (ta-C) and fluorinated ta-C (ta-C–F) coatings were studied in the temperature range between 25 °C and 500 °C. Pin-on-disk type tests conducted on ta-C against Al–6.5% Si (319 Al) showed that the steady-state coefficient of friction (μ s ) decreased from 0.36 at 25 °C to 0.11 at 400 °C. The reduction in friction was attributed to sliding-induced sp 3 to sp 2 transformation (graphitization) of the coating and formation of oxygen rich transfer layer as revealed by X-ray photoelectron and micro-Raman spectroscopy. The coefficient of friction of ta-C–F also decreased with an increase in the test temperature. A low μ s of 0.13 was observed at 400 °C compared to 0.39 at 25 °C. The maximum value of running-in coefficient of friction (μ R ) for ta-C–F tested at 400 °C was 0.38, lower than the μ R of ta-C (0.65). Both coatings maintained low wear at elevated temperatures but at 500 °C they showed high wear and high coefficient of friction and did not reach a steady state friction regime. The atomic % of F transferred to the Al surface increased with an increase in the test temperature up to 400 °C. It was suggested that a sp 2 carbonaceous transfer layer passivated by F atoms was responsible for the further decrease in μ s and μ R of ta-C–F at elevated temperatures (200 °C–400 °C) compared to ta-C.

Carbon nanotube induced microstructural characteristics in powder metallurgy Al matrix composites and their effects on mechanical and conductive properties

In this study, Al matrix composites (AMCs) containing different contents (0–1 vol.%) of carbon nanotubes (CNTs) were fabricated by powder metallurgy (PM). A mechanical mixing process was used to disperse CNTs in Al powders. The powder mixtures were consolidated by spark plasma sintering and subsequent hot-extrusion. The microstructures, tensile properties, electrical conductivity (E.C.) and thermal conductivity (T.C.) of CNT/Al composites were examined. Within 0.75 vol.%, CNTs could be well dispersed and aligned in Al matrix. However, 1 vol.% CNTs led to agglomerations and resultant degraded tensile properties. With CNT additions, Al grains were refined under the pinning effect of CNTs at grain boundaries. An oxygen-rich layer was observed at PM CNT-Al interfaces by combination of transmission electronic microscopy and energy dispersive spectrometer analysis. The oxygen-rich layer was favorable to improve the bonding conditions between CNTs and Al matrix, leading to CNT fracture during composite failure. The refined grains and oxygen-rich interfaces were responsible for the improvement of tensile strength, but decreases of E.C. and T.C. of CNT/Al composites. Considering the CNT induced microstructure characteristics, grain & interface modified models were further set up. They showed good predictions on both the tensile strength and conductivity of CNT/Al composites. The results might provide new insights for designing simultaneously strong and high-conductivity metal matrix composites.

Boosting Thermoelectric Performance by Controlled Defect Chemistry Engineering in Ta-Substituted Strontium Titanate

Inspired by recent research results that have demonstrated appealing thermoelectric performance of A-site cation-deficient titanates, this work focuses on detailed analysis of the changes in performance promoted by altering the defect chemistry mechanisms. The series of cation-stoichiometric SrTi1–xTaxO3±δ and A-site deficient Sr1-x/2Ti1–xTaxO3-δ compositions (0.05 ≤ x ≤ 0.30) with cubic perovskite-like structure were selected to demonstrate the defect chemistry engineering approaches, which result in promising electric and thermal properties. High power factors were observed in compositions where appropriate concentration of the charge carriers and their mobility were attained by the presence of strontium- and oxygen vacancies and suppressed formation of the oxygen-rich layers. Noticeable deviations from stoichiometric oxygen content were found to decrease the lattice thermal conductivity, suggesting good phonon scattering ability for oxygen vacancies, vacant A-sites, and oxygen-excessive defects, while ...

Internal oxidation of laminated ternary Ru–Ta–Zr coatings

Researchers have observed the internal oxidation phenomenon in binary alloy coatings when developing refractory alloy coatings for protective purposes by conducting annealing at high temperatures and in oxygen-containing atmospheres. The coatings were assembled using cyclical gradient concentration deposition during cosputtering by employing a substrate holder rotating at a slow speed. The internally oxidized zone demonstrated a laminated structure, comprising alternating oxygen-rich and oxygen-deficient layers stacked in a general orientation. In the current study, Ru–Ta–Zr coatings were prepared with various stacking sequences during cosputtering. The Ru–Ta–Zr coatings were annealed at 600°C in an atmosphere continuously purged with 1% O2–99% Ar mixed gas for 30min. A transmission electron microscope was used to examine the periods of the laminated layers and crystallinity of the annealed coatings. Depth profiles produced using an Auger electron spectroscope and X-ray photoelectron spectroscope were used to certify the periodic variation of the related constituents and chemical states of the elements, respectively. The results indicate that the internally oxidized ternary coatings are stacked of Ru-, Ta2O5-, and ZrO2-dominant sublayers and that the stacking sequences of the sublayers affect the crystalline structure of the coatings. Zr is oxidized preferentially in the Ru–Ta–Zr coatings, increasing the surface hardness of the oxidized coatings.

Effect of environmental sulfur on the structure of alumina scales formed on Ni-base alloys

Short-term oxidation exposures of an alumina-scale forming γ′-Ni3Al-based model alloy in air and O2+0.1%SO2 at 900°C revealed that the presence of sulfur can affect the kinetic competition between the θ and α structural isomorphs of Al2O3. After 2h exposure, metastable θ-Al2O3 growth predominated in air alone; whereas, a much larger percentage of stable α-Al2O3 formed during oxidation in O2+0.1%SO2. This promotion of α-Al2O3 establishment was due to sulfur enrichment on the alloy surface, which occurred even when samples were exposed to O2+0.1%SO2 in a low-temperature, pre-test position (∼150°C), i.e., prior to insertion into the hot zone. It was determined from XPS measurements that the sulfur was mainly in the S6+ valence state and, correspondingly, in the form of NiSO4. Cross-sectional scanning transmission electron microscopy (STEM) using energy-dispersive X-ray spectroscopy (EDS) corroborated the XPS results by detecting that a ∼20nm zone of sulfur enrichment within the surface region of a ∼90nm oxygen-rich layer formed during the pre-test exposure. A systematic explanation for this intriguing observation of sulfur promoting α-Al2O3 establishment is provided from the perspective of kinetics competition between θ and α. This explanation was supported by kinetic calculations and complementary tests in a low pO2 atmosphere.

Improvement of Energy Capacity with Vitamin C Treated Dual-Layered Graphene-Sulfur Cathodes in Lithium-Sulfur Batteries.

A graphene-based cathode design for lithium-sulfur batteries (LSB) that shows excellent electrochemical performance is proposed. The dual-layered cathode is composed of a sulfur active layer and a polysulfide absorption layer, and both layers are based on vitamin C treated graphene oxide at various degrees of reduction. By controlling the degree of reduction of graphene, the dual-layered cathode can increase sulfur utilization dramatically owing to the uniform formation of nanosized sulfur particles, the chemical bonding of dissolved polysulfides on the oxygen-rich sulfur active layer, and the physisorption of free polysulfides on the absorption layer. This approach enables a LSB with a high specific capacity of over 600 mAh gsulfur (-1) after 100 cycles even under a high current rate of 1C (1675 mA gsulfur (-1) ). An intriguing aspect of our work is the synthesis of a high-performance dual-layered cathode by a green chemistry method, which could be a promising approach to LSBs with high energy and power densities.

Removal of heat-formed coating from a titanium alloy using high pressure waterjet: Influence of machining parameters on surface texture and residual stress

Abstract Titanium alloys are widely used in the aerospace and medical industries owing to their high strength to weight ratio and outstanding corrosion resistance. A problem for titanium or titanium alloys is the existence of a hard, brittle and oxygen-enriched layer on the surface (so called alpha case). This is usually formed during hot forming processes or after long-term service at elevated temperatures in an open-air environment. With the development of waterjet systems, high pressure waterjet has shown its capability for the removal of such hard and difficult-to-machine coatings. Waterjet machining is usually associated with a surface roughening, which is unwanted for most of aerospace applications, but is beneficial for medical application where fixation is required (e.g. metal orthopedic implants). A potential benefit of waterjet material removal is that the process may introduce compressive residual stress to the machined surface and subsurface layers. In this study, Ti–6Al–4V with an alpha case layer was subjected to plain waterjet impact over a range of parametric conditions, to fully remove the alpha case layer. The resulting surfaces were then analyzed to demonstrate the influence of process parameters on both surface roughness and residual stress measured using X-ray diffraction (XRD).

Structural investigations of iron oxynitride multilayered films obtained by reactive gas pulsing process

Multilayered Fe–O–N films were deposited on glass and silicon substrates using reactive magnetron sputtering with a reactive gas pulsing process. The argon and nitrogen flow rates were kept constant during sputtering of a pure iron target while the oxygen flow rate was pulsed during deposition. To determine the film composition, Rutherford backscattering spectrometry was used. X-ray diffraction revealed the formation of a face-centered cubic (fcc) structure with a lattice parameter close to that of γ″′-FeN. In addition, traces of iron oxide Fe1−xO (wüstite) have been detected in some films. Transmission electron microscopy provides clear evidence for the formation of a multilayer structure for all the coatings. Electron energy loss spectroscopy was used to check that each layer contains both nitrogen and oxygen. The local environment of iron atoms studied by 57Fe Mössbauer spectrometry at 300K indicated that the samples consist mainly of an iron oxynitride phase with a NaCl-type structure where oxygen atoms partially substitute nitrogen ones. However, in accordance with the X-ray diffraction results, a small amount of iron nitride (FeN) and oxide (Fe1−xO) is also present in the nitrogen-rich and oxygen-rich layers, corresponding to the sputtering time (toff) and (ton). Thus the hypothesis of a multilayer structure, type (γ″′-FeN/Fe1−xO) can be excluded because the spectra Mössbauer always reveal the presence of iron oxynitride.