Oxide Islands Research Articles

High Temperature Oxidation of Additively and Traditionally Manufactured Inconel 718

Abstract Additively manufactured nickel-based superalloys are now being considered as a replacement for traditionally manufactured components on commercial and military aircraft. While the potential of additive manufacturing for gas turbine engines is promising, the quality and performance of additive manufactured components still need extensive study. Coupon-sized test specimens comprised of as-printed additively and traditionally manufactured Inconel 718 with and without yttria-stabilized zirconia thermal barrier coating were tested under simulated isothermal and thermal cycling combustion conditions that were representative of gas turbine environments. Pre-test scanning electron microscopy indicated that traditionally manufactured coupons had a smooth surface finish with minor imperfections while additive manufactured coupons had a rough surface finish. Post-test scanning electron microscopy exhibited differences in oxide scale between the isothermal and thermal cycling conditions. The thermal cycling condition increased the amount of oxide scale for both additively manufactured and traditionally manufactured Inconel 718. The size of the oxide islands on traditionally manufactured coupons was significantly larger than the additively manufactured coupons. The results indicated that differences in surface roughness may affect the growth of the oxidation scale in a high-temperature combustion environment. The benefits of yttria-stabilized zirconia thermal barrier coating were characterized. The substrate of the coupons experienced little to no formation of oxide scales compared to the uncoated additive and traditionally manufactured coupons. The results further suggested that yttria-stabilized zirconia thermal barrier coating can be utilized to provide both insulation and oxidation protection when desired. Post-test energy-dispersive X-ray spectrometry results corresponded well with known elemental compositions and oxidation mechanisms.

Cleaning of tungsten tips for subsequent use as cold field emitters or STM probes

AbstractThis study investigates the crucial process of cleaning cold field emission electron emitters and scanning tunnel microscopy (STM) probes, particularly focusing on tungsten tips. The cleanliness of these tips is essential for maintaining optimal cathode properties, preventing impurities that can significantly affect the emission process. Various cleaning methods, including macroetching, ammonia cleaning, and hydrofluoric acid (HF) cleaning were explored and compared by scanning electron microscopy. The macroetching method, involving a mixture of hydrochloric acid, nitric acid, and hydrogen fluoride, proved to be too reactive, causing significant material removal and altering the tip’s structure. Ammonia cleaning did not significantly improve or harm the samples. However, oxide islands appeared in some areas, suggesting the potential formation of ammonium tungsten oxide. HF cleaning, specifically at 20% and 50% concentrations, demonstrated effectiveness in removing tungsten oxides without damaging the tip. Pre-cleaning with water and ethanol proved beneficial for subsequent HF refinement. Results suggest that HF is the most suitable method for oxide removal but a rinse with water is essential for removing residual sodium hydroxide. To maintain optimal properties, it is crucial to apply a less reactive layer quickly or transfer the tips to a water/ethanol bath to prevent oxidation.

Gold oxide formation on Au(111) under CO oxidation conditions at room temperature.

Although gold-based catalysts are promising candidates for selective low-temperature CO oxidation, the reaction mechanism is not fully understood. On a Au(111) model catalyst, we observe the formation of gold oxide islands under exposure to atmospheric pressures of oxygen or CO oxidation reaction conditions in an in situ scanning tunneling microscope. The gold oxide formation is interpreted in line with the water-enabled dissociation of O2 on the step edges of Au(111). Contaminants on the gold surface can strongly promote the gold oxide formation even on the terraces. On the other hand, TiO2 nanoparticles on the Au(111) do not show any influence on the formation of the gold oxide and are thus not providing a significant amount of atomic oxygen to the gold at room temperature. Overall, the presence of gold oxide is likely under industrial conditions.

Optimizing Single-Mode Power in VCSELs With Antiresonant Oxide Island

Optimizing Single-Mode Power in VCSELs With Antiresonant Oxide Island

Probing Water Dissociation and Oxygen Replacement on Partially Oxygen-Covered Cu(111) by Reflection Absorption Infrared Spectroscopy.

The presence of chemisorbed oxygen on the Cu(111) surface is known to strongly reduce the activation barrier for water dissociation as compared to bare Cu(111). Here, we present direct experimental evidence for the hydrogen abstraction mechanism responsible for the facile H2O dissociation on an O/Cu(111) surface using reflection absorption infrared spectroscopy (RAIRS) in combination with isotopically labeled reactants. We also observe that chemisorbed hydroxyl species produced by water dissociation on the O/Cu(111) surface undergo an efficient hydrogen atom transfer from trapped water molecules, leading to the rapid replacement of the initial oxygen isotope coverage and the detection of only a single hydroxyl isotopologue on the surface, in apparent contradiction with the hydrogen abstraction mechanism. In the presence of Cu2O oxide islands on the O/Cu(111) surface, water dissociation occurs selectively at the edges of those islands, leading to the self-assembly of isotopically ordered structures.

Unraveling surface structures of gallium promoted transition metal catalysts in CO2 hydrogenation

Gallium-containing alloys have recently been reported to hydrogenate CO2 to methanol at ambient pressures. However, a full understanding of the Ga-promoted catalysts is still missing due to the lack of information about the surface structures formed under reaction conditions. Here, we employed near ambient pressure scanning tunneling microscopy and x-ray photoelectron spectroscopy to monitor the evolution of well-defined Cu-Ga surfaces during CO2 hydrogenation. We show the formation of two-dimensional Ga(III) oxide islands embedded into the Cu surface in the reaction atmosphere. The islands are a few atomic layers in thickness and considerably differ from bulk Ga2O3 polymorphs. Such a complex structure, which could not be determined with conventional characterization methods on powder catalysts, should be used for elucidating the reaction mechanism on the Ga-promoted metal catalysts.

Short-range positional order in phase separated indium gallium oxide islands deposited in a reducing atmosphere

We report a spontaneous phase separation in indium gallium oxide thin films with a nominal composition of (In0.006Ga0.994)2O3 grown on sapphire (0001) substrates using a powder sputtering method in a reducing atmosphere. The In-rich (In0.24Ga0.76)2O3-x domains are non-stoichiometric and located underneath the surface islands, whereas the In-poor domains are nearly stoichiometric Ga2O3, forming a continuous flat surface, though both domains have a monoclinic crystal structure. Furthermore, the In-rich domains exhibit a short-range positional order with a correlation length of approximately 1 μm because the phase separation occurs along the binodal decomposition accompanying periodic concentration profiling of In atoms. The analysis of X-ray photoelectron spectroscopy confirmed that the non-stoichiometry of the thin films originated from the formation of Ga sub-oxide (Ga2O) and metallic Ga, whereas the chemical state of the In atoms was close to In2O3 rather than metallic In. Our results indicate that the spontaneous phase separation occurs in (InxGa1-x)2O3 thin films even at an In content of 0.6 at%. Also, the non-stoichiometry associated with oxygen vacancies is an important parameter for causing spontaneous phase separation via the segregation of In atoms.

A Platform for Addressing Individual Magnetite Islands Grown Epitaxially on Ru(0001) and Manipulating Their Magnetic Domains.

We have grown high-quality magnetite micrometric islands on ruthenium stripes on sapphire through a combination of magnetron sputtering (Ru film), high-temperature molecular beam epitaxy (oxide islands), and optical lithography. The samples have been characterized by atomic force microscopy, Raman spectroscopy, X-ray absorption and magnetic circular dichroism in a photoemission microscope. The magnetic domains on the magnetite islands can be modified by the application of current pulses through the Ru stripes in combination with magnetic fields. The modification of the magnetic domains is explained by the Oersted field generated by the electrical current flowing through the stripes underneath the magnetite nanostructures. The fabrication method is applicable to a wide variety of rock salt and spinel oxides.

Corrosion Performance of Commercial Alloys and Refractory Metals in Conditions for Electrorefining of Spent Nuclear Fuels

Molten LiCl-KCl salt and liquid cadmium are proposed as the electrolyte and the reactive cathode for the electrorefining of spent nuclear fuels, but they can be corrosive to the structural alloys. The down-selection of existing materials through corrosion testing is necessary to ensure the longevity of the electrorefiner vessel and electrode assemblies. Haynes C276, Inconel 600, AISI 316L stainless steel, and 42CrMo low-alloy steel were exposed to a LiCl-KCl melt at 500 °C for 500 h in an argon atmosphere. All alloys suffered from dissolution attacks with the presence of oxide islands or a porous oxide layer on the surface. AISI 316L, T91 steel, and tungsten specimens were submitted to corrosion tests in liquid cadmium at 500 °C for 120 h. The corrosion of AISI 316L and T91 stainless steel was predominated by chemical oxidation, with the additional occurrence of severe Ni dealloying and Cd penetration on AISI 316L. Destabilization of the Cr oxide layer by cadmium was discovered, resulting in the formation of CdCrO4. Tungsten only suffered from a dissolution attack at a rate of 0.50 mm/a.

Enhanced corrosion inhibition of copper in acidic environment by cathodic control of interface formation with 2-mercaptobenzothiazole

Electrochemistry along with advanced surface analyzes (X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry) were implemented to investigate copper corrosion inhibition by 2-mercaptobenzothiazole (2-MBT) in hydrochloric aqueous solution and the effects of electrochemical control on the formation of the inhibiting interface. It is shown that reducing the native surface oxide by cathodic reduction prior to exposure to the inhibitor drastically decreases the rate of anodic dissolution. Additionally, increasing the exposure time of the metal surface to the inhibitor results in even better protection against dissolution. 2-MBT adsorbs as multilayers and bonds to both metallic copper and residual copper oxide islands via its sulphur atoms. Partial dissociation of the molecule, releasing the exocyclic sulphur, which then bonds to metallic Cu, is observed. Further dissociation of the molecule, releasing the endocyclic sulphur, is induced by anodic polarization, forming defects in the organic layer and thus compromising the barrier properties of the organic layer.

Nested Metal Catalysts: Metal Atoms and Clusters Stabilized by Confinement with Accessibility on Supports.

Supported catalysts that are important in technology prominently include atomically dispersed metals and metal clusters. When the metals are noble, they are typically unstable-susceptible to sintering-especially under reducing conditions. Embedding the metals in supports such as organic polymers, metal oxides, and zeolites confers stability on the metals but at the cost of catalytic activity associated with the lack of accessibility of metal bonding sites to reactants. An approach to stabilizing noble metal catalysts while maintaining their accessibility involves anchoring them in molecular-scale nests that are in or on supports. The nests include zeolite pore mouths, zeolite surface cups (half-cages), raft-like islands of oxophilic metals bonded to metal oxide supports, clusters of non-noble metals (e.g., hosting noble metals as single-atom alloys), and nanoscale metal oxide islands that selectively bond to the catalytic metals, isolating them from the support. These examples illustrate a trend toward precision in the synthesis of solid catalysts, and the latter two classes of nested catalysts offer realistic prospects for economical large-scale application.

Extending powder lifetime in additive manufacturing: Chemical etching of stainless steel spatter

• Chemical etching can eliminate most oxides from 316 L spatter. • A reduction in oxide coverage from 7–0.7% was possible. • Removal is thought to occur via initial dissolution of oxide-surrounding metal. • Etched spatter powder performed better in LPBF. • Reduced slag layer, and better wetting was seen for LPBF etched spatter. In laser powder bed fusion, unconsolidated metal powder on the build plate tends to comprise a proportion of oxidised powder after repeated use. This is generally caused by the generation of oxidised spatter particles during processing which can contaminate the powder bed, and be incorporated into future builds, ultimately undermining part integrity. Oxidised metal powder often results in porosity, poor layer-layer bonding and detrimental oxides in the printed part. This work uses a new chemical etching approach to remove oxides from the surface of oxidised stainless steel spatter powder. It was shown that a ten-fold reduction in oxide area coverage on spatter powder was possible through submersion in a solution of heated Ralph's etchant for one hour. Oxide removal is thought to occur mostly via dissolution of the metal surrounding and underneath oxide islands on the powder, allowing more aggressive oxide removal. LPBF processing was performed using spatter, etched and virgin powder sieved to an identical powder size range. Etched spatter showed a reduced oxide slag layer on track surfaces compared to spatter. In addition, incorporation of powder into tracks appeared improved after chemical etching of the powder. This work demonstrates that chemical etching has the potential to be used to increase the re-usability and lifetime of spatter or heavily used powder from widely used and corrosion resistant stainless steel powder.

Directional growth of quasi-2D Cu2O monocrystals on rGO membranes in aqueous environments.

The preparation technology of unconventional low-dimensional Cu2O monocrystals, which exhibit specific crystal planes and present significantly unique interfacial and physicochemical properties, is attracting increasing attention and interest. Herein, by integrating a high-temperature oxidation process under vacuum and a pure-water incubation process under ambient conditions, we propose the self-assembled growth and synthesis of quasi-two-dimensional Cu2O monocrystals on reduced graphene oxide (rGO) membranes. The prepared Cu2O crystals have a single (110) crystal plane, regular rectangular morphology, and potentially well conductivity. Experimental and theoretical results suggest that this assembly is attributed to the pre-nucleation clusters aggregation and directional attachment of Cu and O on the rGO membranes in aqueous environment and cation-π interactions between the (110) crystal plane of Cu2O and rGO surface. Our findings offer a potential avenue for the discovery and design of advanced low-dimensional single-crystal materials with specific interfacial properties in a pure aqueous environment.

Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)

Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)

Highly stretchable island‐structure metal oxide thin‐film transistor arrays using acrylic adhesive for deformable display applications

AbstractStretchable electronics are a promising technology for introducing new styles of display applications and for realizing displays that can be adapted to various free shapes, such as domes and spheres, to suit various viewing styles. To fabricate a stretchable display, it is necessary to form thin‐film transistors (TFTs) and light‐emitting devices on stretchable substrates. We demonstrate high‐performance stretchable metal oxide TFT islands on polyimide film using an acrylic adhesive structure. We developed mass transfer technology for metal oxide island TFTs on a polyimide film to be transferred on a stretchable substrate in a batch and a device structure with a passivation layer that stabilizes device characteristics when the substrate is stretched. The TFT showed stable switching characteristics with a mobility of 30 cm2/Vs without any fluctuation, even when the substrate was stretched from 0% to 50% and released to 0%.

Strain-modulated initial oxidation of Al(1-x)Nix alloy surface

Despite of extensive studies about surface oxidation of metals and alloys, the impact of external loadings on nanoscale initial oxidation remains relatively unclear. Here, we demonstrate that surface prestrains and dynamic loadings can effectively modulate the initial oxidation of Al(1-x)Nix alloys. We find that prestrains can not only suppress or promote outward and inward ionic diffusions, but change the oxygen Ehrlich-Schwöbel (ES) barrier across oxide islands. The former affects the oxidation kinetics while the latter leads to a oxide growth mode transtion, which comprehensively determines the evolution of oxide nanostructure. In addition, we propose a theoretical chemo-mechanical coupling model to predict the internal stress evolution and the stress relaxation in Al0.7Ni0.3 oxidation. Finally, the study of Al and Al0.7Ni0.3 oxidations under dynamic strains shows that alloying nickel in aluminum effectively enhances the oxidation resistance under cyclic loadings by intensifying the chemo-mechanical coupling effect. Findings in this work may shed the light on the modulation of surface oxidation through applying mechanical loadings, and is helpful to the materials design and protection.

Oxidation of a Platinum–Tin Alloy Surface during Catalytic CO Oxidation

We have investigated the surface composition of a well-ordered Pt3Sn(111) surface during CO oxidation with ambient pressure X-ray photoemission spectroscopy. Oxidation of tin in the surface coincides with the onset of catalytic conversion, and we observe significant differences in the oxidation state and morphology of the oxide formed depending on the gas composition, with an oxygen-rich mixture leading to formation of 2D wetting layers and a CO-rich mixture leading to formation of 3D oxide islands. Spontaneous oscillations in conversion are observed at 300 °C in the oxygen-rich gas mixture and attributed to the combined effects of site blocking by tin oxides and by CO. The results highlight the importance of gas–surface interactions in determining the nature of oxides formed and thus the type and number of interfacial sites under reaction conditions.

Influence of thin surface oxide films on hydrogen isotope release from ion-irradiated tungsten

We studied the influence of thin, electro-chemically grown tungsten (W) surface oxide films on hydrogen isotope release from W. As deuterium (D) reservoir underneath the oxide, we used a defect-rich, ion-irradiated W layer that was filled with D prior to oxidation. Several oxide films with thicknesses between 5 and 100 nm were studied and compared with tungsten with a natural oxide film. The release of D through the oxide film was analyzed with thermal desorption spectroscopy (TDS). The depth-resolved concentration profiles of D in the sample were measured with nuclear reaction analysis at all experimental steps. Changes of the morphology of the oxide film due to the release of D were investigated with scanning electron microscopy (SEM).In TDS studies, we found that the thin oxide films significantly influence the release behavior of D from W. The first D release peak (at 560 K) is shifted towards higher temperature (or later times) with increasing oxide thickness. This indicates that the oxide film acts as both a D reservoir and a transport barrier that delays D release at temperatures above 475 K. At this temperature, D also starts to interact chemically with the oxide film and is released not only as HD or D2 but also in the form of heavy water (HDO and D2O). Above 700 K, D is released only in form of heavy water as long as enough oxide is available. Accordingly, SEM images after TDS show a strong modification of the oxide film. For film thicknesses of 5–10 nm, all oxide is removed from the surface and smooth metallic W remains. For 15 nm, the surface is still partially covered by oxide islands with several micrometer of metallic W between them. From the fact that D is still only released as heavy water at high temperatures, we conclude that the mobility of D atoms at the surface is very high. Even D atoms that surface far from an oxide island apparently travel along the surface to form an O-D group at the W oxide before they recombine with another D atom to form water.Our results indicate that the oxide film becomes relevant for the D release during TDS if the ratio of O atoms on the surface to D atoms in the sample is larger than 5–10 %. Consequently, even the natural oxide film (1–2 nm) that forms on W upon contact with air may significantly influence the D release spectra from TDS for experiments with low D retention.

An investigation on the oxidation behavior of spatters generated during the laser powder bed fusion of 316L stainless steel

The characterization of the oxide islands on the surface of the spatter contributes to a fundamental understanding of the formation and oxidation of spatter in laser powder bed fusion (LPBF). In the present study, the surface and cross-section characteristics of spatters with oxide islands are investigated using high resolution scanning electron microscopy combined with energy dispersive spectroscopy, scanning transmission electron microscopy and selected area electron diffraction. The oxide islands with different configurations on the spatter surface are divided into three main categories: hemispherical, disc-like and film-like. Moreover, the primary component of oxide islands is an amorphous SiO2, which may also contain MnO, MnSiO3, and Mn2SiO4. Furthermore, a transition layer (enriched in Si, Mn, and O) is found at the oxide islands || spatter substrate interface, and the width of which is 4 μm and 20 nm on the spatter surface and within the spatter cross-section, respectively. Besides, the composition and sources of the circular oxide nano-inclusions (50-600 nm) internal to the spatters are also discussed in detail. Eventually, the driving forces affecting the formation of oxide islands on the surface of spatters are summarized, and a schematic diagram of the formation procedure is developed.

Influence of Al content on the oxidation behavior of CrAl coating on Zry-4 alloys in 1200 °C steam

The oxidation behavior of CrAl coatings with three representative compositions deposited on Zircaloy-4 in 1200 °C steam environment has been studied. The Cr0.90Al0.10 coating shows bettter performance than the other CrAl coatings due to the protectiveness of both the outer Cr2O3 oxide layer and the internal Al2O3 oxide layer. The Cr0.95Al0.05 coating shows partial-protective performance, exhibiting a continuous Cr2O3 oxide layer and Al2O3 oxide islands. However, the oxidation resistance of the Cr0.68Al0.32 coating is poor, exhibiting a discontinuous Al2O3 layer on the surface and cracks in the coating. The effect of Al concentration on the oxidation resistance of CrAl coatings is discussed in detail.