Formation Of Surface Oxide Layer Research Articles

Surface Reconstruction on Metal Nitride during Photo-oxidation.

The efficient conversion and storage of solar energy for chemical fuel production presents a challenge in sustainable energy technologies. Metal nitrides (MNs) possess unique structures that make them multi-functional catalysts for water splitting. However, the thermodynamic instability of MNs often results in the formation of surface oxide layers and ambiguous reaction mechanisms. Herein, we present on the photo-induced reconstruction of a Mo-rich@Co-rich bi-layer on ternary cobalt-molybdenum nitride (Co3 Mo3 N) surfaces, resulting in improved effectiveness for solar water splitting. During a photo-oxidation process, the uniform initial surface oxide layer is reconstructed into an amorphous Co-rich oxide surface layer and a subsurface Mo-N layer. The Co-rich outer layer provides active sites for photocatalytic oxygen evolution reaction (POER), while the Mo-rich sublayer promotes charge transfer and enhances the oxidation resistance of Co3 Mo3 N. Additionally, the surface reconstruction yields a shortened Co-Mo bond length, weakening the adsorption of hydrogen and resulting in improved performance for both photocatalytic hydrogen evolution reaction (PHER) and POER. This work provides insight into the surface structure-to-activity relationships of MNs in solar energy conversion, and is expected to have significant implications for the design of metal nitride-based catalysts in sustainable energy technologies.

Passivation bypassed Zero valent Iron based heterostructures for effective removal of persistent antibiotics by solar driven photo-Fenton process

The natural passivation phenomena in nano zerovalent iron (nZVI) through formation of surface oxide layer is a bottleneck for exploring the complete potential of the material. This problem is addressed by the in-situ synthesis of nZVI/Cu bimetallic heterostructure and exploring its improved potential towards solar-driven photodegradation of persistent antibiotics and their mixture in contaminated water. Effect of passivation bypassing by Cu0 on the stability of nZVI as a photo-Fenton candidate is scrutinized through aging of the materials and their effect on degradation performance. Post-aging studies reveal a higher performance of nZVI/Cu as compared to bare nZVI. A systematic comparative study of photo-Fenton activity for nZVI and nZVI/Cu is estimated through degradation of toxic organic pollutants such as Tetracycline (TET) and Ciprofloxacin (CIP) and also a mixture of both antibiotics using High performance Liquid Chromatography (HPLC). The degradation efficiencies achieved with nZVI/Cu for TET and CIP were 99.99% and 98.59% with rate constant of 0.0631 min−1 and 0.032 min−1 respectively. Furthermore, nZVI/Cu showed a removal efficiency of 99.34% and 99.9% for TET and CIP in antibiotic mixture. Hydroxyl radicals are found to be the prominent active species from radical trapping experiments. The degradation pathway is traced with LC-MS/MS analysis to identify the intermediates and byproducts formed during degradation process. The reusability of nZVI/Cu catalyst is evaluated with five successive cycles of CIP and TET degradation. The inherent magnetic property of nZVI/Cu sample is employed for the effortless retrieval of catalyst. Finally, a plausible explanation for the photo-Fenton degradation mechanism of nZVI/Cu for the antibiotics is put forward.

Effect of structural disorder on the oxidation of Zr-based amorphous alloys: A focused review

Structural disordering leads to outstanding mechanical properties of amorphous alloys. Many properties are strongly related to formation of surface oxide layers. An in-depth understanding of effect of structural disordering of amorphous alloys on oxide evolution is essential for applications. This paper presents a review of recent comparative oxidation studies of Zr-based amorphous and single-phase crystalline counterpart alloys. Three representative model alloy systems are selected: Zr–Cu and Zr–Al representing alloys with components of different and similar oxygen affinities, respectively, and Zr–Cu–Al representing multi-component alloys. In combination with molecular dynamic simulations, the fundamental role of the structural disordering of parent alloys in the oxidation processes is revealed. The oxidation of Zr–Cu–Al–Ni amorphous alloys at the supercooled liquid temperature region is further reviewed to clarify the relationship between oxidation and substrate crystallization.

Comparative assessment of the characteristics and Cr(VI) removal activity of the bimetallic Fe/Cu nanoparticles pre- and post-coated with carboxymethyl cellulose

High redox activity and superior magnetic property of nano-scale zero valent iron (nZVI) leads to the formation of surface oxide layers and large agglomerates, limiting its application in remediation activities. To inhibit surface oxidation and to overcome the aggregation challenge, copper (Cu) doped bimetallic iron/copper (Fe/Cu) nanoparticles with/without polymeric stabilization (carboxymethyl cellulose (CMC)) were chemically synthesized and compared with bare iron (Fe) nanoparticles for hexavalent chromium (Cr(VI)) removal. CMC stabilization of Fe/Cu nanoparticles was carried out in both post and pre-grafting pathways to evaluate the role of stabilization method on nanoparticle characteristics and reactivity. Detailed material characterization and CMC bonding mechanisms onto nanoparticle surface were explored by performing TEM, FTIR, XRD, XPS, and TGA analysis. TEM results showed spherical morphology of nanoparticles with better stability and smaller particle size for CMCpre-Fe/Cu (∼25 nm). Batch experiment results exhibited lower Cr(VI) removal efficiency for stabilized nanoparticles in contrast with bare nanoparticles (CMCpost-Fe/Cu (20.68%) < CMCpre-Fe/Cu (41.79%) < Fe/Cu (58.19%) < Fe (62.09%)) – attributable to the electrostatic repulsion between stabilized nanoparticles and Cr(VI) species. Contrary to other materials, despite the low Cr(VI) removal efficiency of stabilized nanoparticles, CMCpre-Fe/Cu showed a faster initial adsorption rate (22.23 mg·g−1.min−1). Acidic pH and particle dispersion were found to impact the removal process positively. XPS analysis of the materials before and after the reaction described the removal mechanism as a synergistic effect of adsorption, reduction, and precipitation. In conclusion, Cu doping can boost the removal process by facilitating reductive pathways, and polymer coating may result in reduced Cr(VI) removal from wastewater due to surface charge properties.

On a possibility of replacing grey cast iron for manufacturing cast elements of electrolyzer gas collecting bell

The main process leading to the destruction of the cast elements of gas-collection bell of electrolyzer, made of grey cast iron, is the oxidation of iron by oxygen, SO2 gas and sulfur vapors to form magnetite, hematite and pyrrhotin. The simultaneous formation of iron oxides and sulfides does not prevent further corrosion, since scale is formed with a loose structure that does not have protective properties. Reducing the length of the interfacial boundaries inside the material of the cast enables to reduce the rate of corrosion destruction, which can be achieved by modifying the cast iron to change the shape of graphite inclusions, i.e. obtaining high-strength cast iron with a spherical shape of graphite inclusions. However, the obtaining spherical graphite in cast iron using magnesium modification does not exclude the access of aggressive gases to the surface of the products and the possibility of their diffusion along the grain boundaries. It was shown that alloying can be an alternative, which leads not only to the exclusion of lamellar secretions of graphite in the structure of cast iron, but also to the formation of surface oxide layers based on the alloying element preventing the corrosion. Alloying with chromium gives cast iron high abrasive resistance due to the presence of a carbide component in the structure, as well as corrosion resistance due to the alloying of the metal base, heat resistance due to increasing the electrochemical potential of the metal base and creating a strong neutral oxide film on the surface of the castings, heat resistance, etc. An experimental comparative analysis of the corrosion resistance of cast iron used for manufacturing of gas collecting bell of electrolyzers showed that chromic cast iron ЧХ3 has a higher corrosion resistance than high-strength cast iron with spherical graphite ВЧ50 and much higher than grey cast iron with lamellar graphite. However, chromic cast iron ЧХ3 has low casting properties, is very sensitive to the cooling rate and has a large heterogeneity in structure, which makes it difficult to use it for the manufacture of gas collecting bell of electrolyzers.

Size Effects in the Surface Properties of Electroplated Alloys between Iron Group Metals and Tungsten

Factors leading to size-dependent surface properties of electroplated coatings obtained by induced codeposition of iron group metals and tungsten are investigated. The size effect in microhardness described in earlier studies and the size effect in the corrosion resistance revealed in this work are shown to stem from a common origin, which is the formation of surface oxides. Removing surface oxides by abrasive processing leads to a higher corrosion rate and cancels the size effect in microhardness. Factors contributing to the formation of surface oxide layers during induced codeposition of considered alloys are studied.

Formation mechanism of surface oxide layer of grain-oriented silicon steel

The surface oxide layer of grain-oriented electrical steels was investigated by scanning electron microscopy. The formation mechanism and the influence on the glass film of the surface oxide layer were analyzed by the calculation of thermodynamics and kinetics. The surface oxide layer with 2.3 μm in thickness is mainly composed of SiO2, a small amount of FeO and Fe2SiO4. During the formation of surface oxide layer, the restriction factor was the diffusion of O in the oxide layer. At the initial stage of the decarburization annealing, FeO would be formed on the surface layer. SiO2 and silicate particles rapidly nucleated, grew and formed a granular oxide layer in the subsurface. As the oxidation layer thickens, the nucleation of new particles decreases, and the growth of oxide particles would be dominant. A lamellar oxide layer was formed between the surface oxide layer and the steel matrix, and eventually formed a typical three-layer structure. During the high temperature annealing, MgO mainly reacted with SiO2 and Fe2SiO4 in the surface oxide layer to form Mg2SiO4 and Fe2SiO4 would respond first, thus forming the glass film with average thickness of 4.87 μm.

Crystal-plane-dependent redox reaction on Cu surfaces

The dynamic redox process of surface oxide layers on metal surfaces is of great significance for understanding the active phase in catalytic reactions. We studied the formation of surface oxide layers on Cu(111) and Cu(110) in O-2, as well as the subsequent reduction by CO using in situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). By monitoring and comparing the oxidation process of Cu(111) and Cu(110) surfaces, we found a crystal-plane-dependent reaction mechanism, which also applies to the reduction of surface oxide layers on Cu surfaces. We found XPS Cu spectra cannot be used to identify the various surface oxide layer on Cu surfaces, suggesting their presence in catalytic reactions might have been overlooked. The combination of STM and XPS studies are thus advantageous in identifying surface oxide structures and pinpointing the active phases in the redox process, which paves the way for engineering the catalyst and reaction environment for optimized catalytic performances.

Improving Ni/GaN Schottky diode performance through interfacial passivation layer formed via ultraviolet/ozone treatment

Electrical passivation has a significant effect on metal-semiconductor (MS) device operations including performance and reliability. In this study, the improvement in performance of Ni/GaN Schottky diodes (SDs) through an ultraviolet/ozone (UV/O3) interface treatment is investigated and the mechanism of carrier conduction at the MS junction interfaces is analyzed. The formation of surface oxide layer at the MS interface through the UV/O3 treatment is confirmed by the measurements using X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy. The atomic intensity and surface energy increased and surface roughness improved through the implementation of oxide layer. Electrical measurements reveal reduced leakage and improved breakdown voltage and are used to determine the Schottky barrier height and Richardson constant of the Ni/GaN MS SDs. The enhancement in the entire performance of the MS SDs is attributed to the passivation of defect centers at the dislocation-related pits through the formation of oxide layer with the UV/O3 treatment, which thereby improves the carrier transfer properties of Ni/GaN SDs.

Improvement of Fuel Cell Durability Performance by Avoiding High Voltage

In order to increase the durability of fuel cell stack, various operating technologies such as voltage cut-off and lean air operation was verified. With an application of upper voltage cut-off, cell performance loss was effectively diminished, which suggests that a formation of surface oxide layer on Pt was suppressed resulting in less irreversible catalyst deterioration such as Pt dissolution. In addition, it was found that voltage sweeping to lower reductive potential with a lean air supply was beneficial in preventing a power loss. With a simulated vehicle driving mode, it was confirmed that performance durability was 3 times improved with an application of both voltage cut-off and lean air supply which might be ascribed to the maintaining the electrochemical acive area of Pt catalyst. The suggested operating conditions based on Pt degradation mechanism will contribute the durability enhancement of fuel cell stack for FCEVs.

Characterization of surface layers formed on DU10Mo ingots after processing steps and high humidity exposure

The design of monolithic UMo fuel elements fabricated from low-enriched uranium for use in high-power research reactors requires bonding of the fuel foil to either Al cladding or a Zr barrier layer. Processing of the UMo ingot to final foil form has the potential to generate surface layers on the foil that differ from the bulk, metallic UMo. The interfacial properties between the UMo and Zr or Al cladding layers will then be determined by these surface layers. We use x-ray photoelectron spectroscopy, cross-sectional scanning electron microscopy, and atomic force microscopy to characterize the composition, oxidation state, and morphology of the surface layers that form after hot rolling and cold rolling depleted U–10 wt% Mo alloy (DU10Mo). A thick uranium nitride layer is observed after hot rolling, although its origin is likely from a previous processing step. The efficacy of acid etching in HNO3 is compared to that of electropolishing in H2SO4 to remove surface nitride and oxide layers, and both methods are found to be similarly effective. Both laboratory (low humidity) air exposure and longer rinse times in water are shown to promote the formation of surface oxide layers. Exposure of both acid-etched and electropolished DU10Mo foils to humid air (97% relative humidity) for six weeks results in formation of a thick oxide layer due to corrosion. The oxide layer on the acid-etched foil is thicker and more highly oxidized than the oxide layer that forms on the electropolished foil, and these differences in oxidation behavior are attributed to higher surface roughness on the acid-etched foil. In general, Mo is found to play a role as a sacrificial element, typically exhibiting a larger ratio of Mo6+/Mo4+ than U6+/U4+. This is unexpected, given the greater thermodynamic driving force to form U oxides than Mo oxides.

Enhancement of Rate Capabilities for All-Solid-State Batteries Using Surface Oxidized Sulfide Solid Electrolyte

Recently, the demand for rechargeable batteries with higher capacity has been increasing. Bulk-type all-solid-state batteries (ASSBs) are expected as candidates for one of the next generation batteries. Among solid electrolytes used for ASSBs, sulfide solid electrolyte (SSE) have attracted much attention because of its high ionic conductivity and high deformability [1]. One of the main drawbacks of ASSBs using SSE is high interfacial resistance between cathode material and SSE, which restricts high rate performance [2]. We previously demonstrated that formation of surface oxide layer on sulfide solid electrolyte (O-SSE) reduced the interfacial resistance and improved the ASSBs performance due to improvement of the oxidation stability of the SSE [3]. For further enhancement of rate capability, we herewith better controlled the formation of surface oxide layer to reduce the interfacial resistance while suppressing the bulk conductivity reduction. It is known that in the system Li2S-P2S5, the composition of Li2S and P2S5 affects not only ionic conductivity but also the interfacial resistance [4]. So, first, we optimized Li2S-P2S5 ratio of O-SSE. SSE with three different compositions, x Li2S – (100 – x) P2S5 (x = 70, 75, 80), were synthesized, and then the surface oxide layer was formed by annealing in O2 atmosphere (to be referred as O-7030, O-7525, O-8020, respectively). Among batteries using these electrolytes in cathode layer, the battery using O-7525 showed the lowest interfacial resistance with LiNiCoAlO2 cathode. In the next step, the new synthesis process of O-SSE was developed to maximize exchange yield of the near-surface sulfur with oxygen and to minimize the thickness of oxide layer so that the abrupt oxide-sulfide junction was to be formed. In the previously reported process, SSE powder was sealed in a vessel in an Ar-filled glove box and O2 gas was introduced into the vessel during annealing. It took several hours to react the entire powder with O2 gas, and after annealing, ionic conductivity decreased due presumably to oxygen diffusion deep into SSE particles during long annealing time. For improving reactivity, the vessel was sealed under vacuum before O2 gas introduction, so that O2 gas uniformly spread into the powder. Thanks to this new process, significantly shorter anneal time, only within 10 min, was needed to have the same level of total oxygen content per SSE particle. This prevented the oxygen atoms from diffusing into inside the SSE particles and resulted in more abrupt sulfide core / oxide shell structure with thin oxide layer, and the ionic conductivity of O-SSE improved from 3.7×10-5 S cm-1 to 1.8×10-4 S cm-1. These composition optimizations and the new synthesis process of O-SSE improved the battery discharge capacity from 40.7 mAh g-1 to 86.3 mAh g-1 at a high current density of 2.0 mA cm-2. This improvement can be attributed to the simultaneous improvement of both the interface stability against cathode materials and the ionic conductivity of O-SSE. In conclusion, we demonstrated that the composition and synthesis process optimization of O-SSE improved the ASSBs performance, especially discharge characteristic at a high current density. [1] A. Hayashi et al, Solid State Ionics, 175 (2004) 683. [2] N. Ohta et al, Electrochem. Commun., 9 (2007) 1486. [3] I. Sasaki et al, 233rd ECS Meeting (2018). [4] T. Ohtomo et al, J. Power Sources, 233 (2013) 231.

Improvement of Electrochemical Performance for All-Solid-State Batteries By Surface Oxidation of Sulfide Solid Electrolyte

Recently, more and more plug-in hybrid and electric vehicles have been put to practical use and the demand for rechargeable batteries with higher capacity has been increasing. Bulk-type all-solid-state batteries (ASSBs) are expected as candidates for next generation batteries, because their volumetric energy density may be improved by a bi-polar stacked structure. Among solid electrolytes used for ASSBs, sulfide solid electrolyte (SSE) have attracted much attention because of its high ionic conductivity and high deformability [1]. One of the main drawbacks of ASSBs using SSE is high interfacial resistance between cathode material and SSE. Calculations of the stability window of SSE have revealed that SSE is electrochemically oxidized and decomposed above 2.5 V vs. Li/Li+ and the interfacial resistance is mainly originated from the decomposition layer including an elemental sulfur [2]. These results suggest that suppression of electrochemical decomposition of SSE will reduce the interfacial resistance and, as a result, improve the electrochemical performance of ASSBs. In order to suppress the electrochemical decomposition of SSE, a method introducing oxide such as Li2O into SSE has been proposed [3,4]. Although introducing oxide improves the electrochemical stability of SSE, the ionic conductivity decreases because lithium ions are more deeply bound to oxide ions whose electronegativity are higher than sulfide ions. So, it has been difficult to realize the high electrochemical stability and high ionic conductivity at the same time. In this study, we focused on the synthesis and electrochemical evaluation of surface oxidized sulfide solid electrolyte (O-SSE). O-SSE has the SSE structure with high ionic conductivity inside the particle and has the oxide layer with high electrochemical stability on its surface, which cathode material directly contacts with. We synthesized O-SSE by annealing SSE (80Li2S-20P2S5) in O2 atmosphere. For comparison, we synthesized Li2O-SSE (56Li2S-20Li2O-24P2S5) by conventional mechano-chemical process. In order to verify the oxygen distribution within the particles, Li2O-SSE and O-SSE were analyzed by XPS depth profiling with C60 cluster ion sputtering. P2p peak of Li2O-SSE showed ca. 132.1 eV, which matched P-S bond, regardless of sputtering time. On the other hand, as sputtering time proceeded, P2p peak of O-SSE shifted from ca. 133.3eV, which matched P-O bond, to ca. 132.1 eV. It was confirmed that the surface oxide layer including P-O bond was formed on the surface of O-SSE. The ionic conductivity of Li2O-SSE and O-SSE showed 1.1×10-4 Scm-1 and 2.0×10-4 Scm-1, respectively. O-SSE showed higher ionic conductivity than Li2O-SSE. Discharge capacities of ASSBs, using LiNiCoAlO2 (NCA) as a cathode material and Li2O-SSE or O-SSE as a solid electrolyte, were 122.8 mAhg-1 and 158.5 mAhg-1, respectively. Discharge capacity of ASSBs improved significantly by using O-SSE as a solid electrolyte. Therefore, we demonstrated that formation of surface oxide layer on SSE improved the ASSBs performance by improvement of the oxidation stability without reducing bulk ionic conductivity. [1] A. Hayashi et al, Solid State Ionics, 175 (2004) 683 [2] G. Ceder et al, Chem. Mater., 28 (2016) 266 [3] J. E. Trevey et al, Solid State Ionics, 214 (2012) 25 [4] T. Ohtomo et al, J. Solid State Electrochem., 17 (2013) 2551

Effects of various aluminum surface treatments on the basalt fiber metal laminates interlaminar adhesion

Fiber metal laminates (FMLs) are a new class of advanced materials consisting of metal and fiber reinforced polymer layers. Surface treatment of the metal layers can have significant effects on the interlaminar properties and thus the overall properties of FMLs. The aim of this study was to apply four different surface treatment methods on the aluminum layers and investigate their effects on the flexural behavior of a FML made up of Al2024, basalt fibers and epoxy Epon 828. To study the surface morphologies and fracture surface of samples, scanning electron microscopy (SEM), optical microscopy (OM) and profilometry were utilized. Results of the four methods applied, showed that the mechanical abrasion and alkaline etching treatments were not found suitable for enhancing the flexural properties due to weak interfacial adhesive bonding. OM images revealed that the adhesive failure was the failure type of mechanical abrasion and alkaline treated samples. On the other hand, Forest Products Laboratory etching (FPL-etching) and anodizing treatments caused significant improvements in the flexural properties. The improvements are attributed to the formation of surface oxide layers containing microscale pits. The microscale pits provided appropriate sites for the penetration of the polymer resin, which caused strong interfacial bonding by mechanical interlocking mechanism. Moreover, SEM and OM images indicated that the main failure type of FPL-etching and anodized samples was the cohesive failure.

Structure and microwave properties of Fe75Si25 alloys produced by high-energy wet ball milling in organic media

Fe-Si alloys are known for excellent soft magnetic properties and are widely used as fillers of composite for microwave applications. A change in the size and the morphology of the powder particles has a significant influence on the microwave properties of composites filled with such powders. In this paper, Fe75Si25 powders were produced by wet ball milling. A solution of stearic acid in petroleum ether (a surfactant solution) and acetone were used as milling media. The influence of wet ball milling conditions on the morphology and the structure of the produced powders was studied by scanning electron microscopy, X-ray diffraction, Mossbauer spectroscopy and X-ray photoelectron spectroscopy. Wet ball milling of the alloy in an acetone leads to the formation of the particles of greater thickness and smaller size in comparison with the particles produced by wet ball milling in a surfactant solution. In both cases after milling the alloy is disordered. During the milling process Si concentration in the alloy decreases due to the formation of surface oxide layers enriched with silicon. The frequency dependence of microwave permittivity and permeability of composites filled with produced powders was studied. The depolarization factor, percolation threshold, and intrinsic permeability of produced powders particles were determined. The frequency dependence of intrinsic permeability of the particles was shown to depend strongly on the milling conditions. A change in the milling conditions makes it possible to produce Fe75Si25 powders with the required microwave properties.

Graphite–Aluminium Composite Anode for Li-Ion Battery by High Energy Ball Milling

Graphite–30% aluminium composite powder is prepared by high energy ball milling for different time intervals to improve the capacity over the conventionally used natural graphite powder as anode in Li-ion battery. Optimum milling time of 10 h results in higher capacity than either lower or higher milling duration. Initial increase in capacity with milling time is attributed to the reduction in crystallite size and alloying of Al and C. The formation of surface oxide layer on Al particles reduces the conductivity and capacity of the powder milled beyond 10 h. Formation of cracks during charge–discharge cycling of the anode causes fading in the capacity.

Current Collapse Reduction in AlGaN/GaN HEMTs by High-Pressure Water Vapor Annealing

We have demonstrated for the first time a remarkable reduction of current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) by high-pressure water vapor annealing (HPWVA). The device subjected to HPWVA exhibited considerably low dynamic ON-resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> ), suggesting highly improved performance of these devices. Analyses of the results on normalized dynamic R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> experiments have shown the elimination of deeper traps by HPWVA, leading to the substantially reduced current collapse. X-ray photoelectron spectroscopy (XPS) studies revealed a significant increase in the oxygen core-level O 1s peak. Moreover, angle-resolved XPS suggested the formation of surface oxide layer. These results indicate that the effective reduction of current collapse in the HPWVA-processed samples is likely due to the incorporation of active oxygen species generated by the HPWV into the AlGaN surface. These oxygen atoms eventually fill up near-surface nitrogen vacancies and promote the formation of Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> native oxide and possibly Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O suboxide, which is known to be an excellent III-V surface passivant. HPWVA is a relatively simple, low-damage, and low-temperature process, and hence, it is found to be a highly feasible and promising alternative for realizing AlGaN/GaN HEMTs with improved performance.

Effect of oxide layers and near surface transformations on friction and wear during tool steel and boron steel interaction at high temperatures

Recent years have seen a continuously growing interest in high temperature tribological research. A significant part of this is driven by the need for improved understanding and knowledge pertaining to friction and wear and their control in the context of hot forming of high strength steels. Friction and wear characteristics of a sliding system are highly dependent on the properties of the two interacting surfaces. At high temperatures, the surface and material properties become extremely important since these systems often operate under unlubricated conditions. High temperature tribological processes are highly complex as these involve changes in mechanical properties due to microstructural changes; thermal softening; surface chemical and morphological changes due to oxidation and diffusion; deterioration of the surface and bulk material as a result of adhesive/abrasive wear and thermal fatigue. Many of these changes occur on the surfaces and/or in the near surface region. The formation of surface oxide layers and near surface layers with a highly refined microstructure (nano-structured) has been reported to have a significant influence on the tribological behaviour. An improved understanding of these effects is a prerequisite in an attempt towards controlling friction and wear at high temperatures. The main aim of this work is to investigate the formation of oxide layers and near surface transformed layers during tool steel and boron steel interaction at elevated temperatures and their relation to the friction and wear response. The results from sliding wear tests showed that under favourable conditions of temperature and load, a reduction of wear by three orders of magnitude and reduced friction by 50% was obtained. This was attributed to the formation of a composite layer structure involving a refined workhardened layer and a protective oxide layer on top. In the case of three body abrasive wear of boron steel, a reduction in wear rate when temperature increased (100–200°C) has also been found. This reduction in three-body wear is due to the formation of a workhardened layer with a mechanically mixed layer of wear debris and fragmented silica particles on top. At higher temperatures (>500°C), the softer matrix due to recrystallisation and phase transformations was unable to maintain a lower wear rate despite the presence of embedded fragmented silica particles.

Amphiphilic coatings on cobalt boride nanocatalysts for stability in hydrogen generation applications

While the idea of using hydrogen as a fuel has been in circulation for hundreds of years, only in recent decades have viable hydrogen generation and storage mechanism become widely appealing for a future hydrogen economy. In addition, the field of energy research continues to make great strides in the utilization of hydrogen fuel for stationary and mobile power systems. Transport of hydrogen gas itself is inherently bulky and with regards to safety concerns may be preferable in a less flammable formulation, such as stored within an advanced carbon material or more commonly as a chemical hydride. More important than ever is the exploration of an effective catalytic system for on-demand hydrogen fuel generation from such chemical hydride precursors for effective use in applications including proton exchange membrane fuel cells. Among the chemical hydrides, sodium borohydride (NaBH4) has been praised for its hydrogen storage density, stability in air and in basic solutions, and the recyclability of its sideproduct sodium metaborate (NaBO2). In prior works, cobalt has proven to be an effective and low-cost catalyst for the hydrolysis of stable NaBH4 solutions into H2 gas and NaBO2 as an alternative to high priced noble metal based catalysts. However, like many metals, cobalt is highly susceptible to surface oxide layer formation, which when in the form of a nanomaterial catalyst means a dramatic change in catalytic properties. Therefore, a system which can eliminate or slow down the passivation of cobalt to its catalytically inactive form will offer significant benefit for hydrogen generation systems. Strategies that have been applied to improve the stability of Co nanoparticles have implemented protective coatings of aluminum oxide, silica, and gold among others; however, these methods may not offer an effective catalytic surface for the hydrolytic generation of hydrogen from sodium borohydride and moreover the processing is inherently expensive. Therefore, we present in this work our analysis of a technique for generating an inexpensive Co-based catalyst for hydrolysis of sodium borohydride which makes use of stabilizing amphiphilic molecules to hinder formation of a surface oxide layer. Numerous systems have been conceived for the on-demand generation of hydrogen, taking on various forms including cobalt and nickel foam catalysts and cobalt alloy nanoparticle catalysts. Cobalt nanoclusters are often supported on poly(vinyl pyrrolidone) by reductive processes and have yielded catalytic scaffold with high hydrogen generation rates. Other recent works have shown Co2B nanoparticle catalysts to be competitive with noble metals and have revealed that particle size and dispersion characteristics could yield high catalytic rates owing to their increased accessibility to active sites. In order to continue to improve the stability and activity of such nanoscale catalysts with respect to oxidative passivation, we have explored the use of amphiphiilic components (particularly polydiacetylene and polysorbate 20) in order to protect the surface of cobalt nanoparticles. By utilizing the inherent magnetic properties of the colloidal cobalt for separation and collection of cobalt, this approach may also offer controlled magnetic removal of catalyst for on-demand catalytic generation of hydrogen gas from stable sodium borohydride precursor solution. In detail, we explore the use of amphiphilic polysorbate or amphiphilic polydiacetylene for coating of the cobalt particles in order to hinder their transformation to the non-magnetic, poorly catalytic oxidized state (Figure 1). The formation of cobalt boride (Co2B) nanoparticles was carried out by a routine technique involving the reduction of a cobalt salt solution by dropwise addition to a solution of sodium borohydride. Similar metal boride nanoparticles are commonly prepared in such a technique owing to the simple synthesis and low cost. Related reports confirm that shortly after cobalt nanoparticle particle synthesis, there is a gradual, complete loss in magnetization due to the progressing oxidation process. In this work, we proceed to collect the Co2B nanoparticles after synthesis by magnet and subsequently expose the nanoparticles to amphiphilic polysorbate 20 and 10,12-pentacosadiynoic acid (PCDA), referred to in its polymerized form as polydiacetylene (PDA). The role of the polysorbate 20 and liposomal PDA was assessed with respect to their effect on the hydrogen generation rate of the nanoparticle catalysts. In addition, we identified interesting chemisorptions effects upon mixing of Co2B nanoparticles with PDA at particular pH conditions, which was explored by spectrophotometer and surface charge measurements. In the DOI 10.1007/s13233-015-3048-7

Development of an aluminide coating layer on Alloy 617 by Al sputtering and inter-diffusion heat treatments

To improve the oxidation resistance in high temperature environments, an aluminide coating layer was developed on the surface of Alloy 617 by Al sputtering, aluminizing and inter-diffusion heat treatment (IDHT). After the IDHT at 1000°C and 1150°C, β-NiAl and γ′-Ni3Al layers were formed on Alloy 617, respectively. Also, an inter-diffusion zone (IDZ) was formed between the aluminide layer and Alloy 617 substrate, where Cr-rich M23C6 carbide and Cr and Mo-rich σ phase were extensively precipitated. The formation of IDZ and precipitates were explained with the low solubility of alloying elements in aluminide phases and diffusion of Ni. Isothermal oxidation tests in air at 900°C showed transient oxidation kinetics for the aluminized+IDHTed Alloy 617, resulting in much lower oxidation rate than the as-received Alloy 617. A thin Al-rich oxide layer was formed on top of aluminide coating layer and re-precipitation of σ phase and M23C6 carbide was observed below the IDZ. However, despite the formation of surface oxide layer and re-precipitation below the IDZ, the overall size and composition of aluminide layer were not significantly altered during the oxidation tests for Alloy 617 with outer γ′-Ni3Al layer.