Thin Surface Oxide Layer Research Articles

Nano-scale corrosion mechanism of T91 steel in static lead-bismuth eutectic: A combined APT, EBSD, and STEM investigation

T91 steel is a candidate material for structural components in lead-bismuth-eutectic (LBE) cooled systems, for example fast reactors and solar power plants [1]. However, the corrosion mechanisms of T91 in LBE remain poorly understood. In this study, we have analysed the static corrosion of T91 in liquid LBE using a range of characterisation techniques at increasingly smaller scales. A unique pattern of liquid metal intrusion was observed that does not appear to correlate with the grain boundary network. Upon closer inspection, electron backscatter diffraction (EBSD) reveals a change in the morphology of grains at the LBE-exposed surface, suggesting a local phase transition. Energy dispersive X-ray (EDX) maps show that Cr is depleted in the T91 material near the LBE interface. Furthermore, we observed the dissolution of all Cr-enriched precipitates in this region. Although the corrosion is conducted in an oxygen deficient environment, both scanning transmission electron microscopy (STEM) and atom probe tomography (APT) reveal a thin surface oxide layer (presumably wüstite) at the LBE-steel interface. Using electron energy loss spectroscopy (EELS) in the STEM, as well as APT, the atomic scale elemental redistribution and 3D morphology of the corrosion interface is investigated. By combining results from these different techniques, several types of oxide phases and structures can be identified. Based on this detailed nano-scale information, we propose potential mechanisms of T91 corrosion in LBE.

Effect of oxygen plasma treatment on the performance of recessed AlGaN/GaN Schottky barrier diodes

The treatment effect of the oxygen plasma on the performance of recessed AlGaN/GaN Schottky barrier diodes has been investigated. After the oxygen plasma treatment, the turn-on voltage and reverse leakage current are slightly changed, while the current collapse could be effectively mitigated. The X-ray photoelectron spectroscopy results suggest that a thin surface oxide layer is formed by the oxygen plasma treatment, which is responsible for the reduced current collapse. In addition, the device treated by oxygen plasma has a relatively more inhomogeneous barrier height.

Failure modes and bonding strength of ultrasonically-soldered glass joints

While useful for optics and solar energy applications, solder-glass joints are difficult to fabricate due to poor wettability and adhesion, but reliable joints can be achieved with the use of ultrasonic-assisted soldering (UAS). In this study, an automated UAS system was used to investigate the effects of processing parameters on the solder joint quality so that the technology can be more suitably scaled for future widespread use in industry. Through mechanical shear strength testing of soldered joints and microstructure failure surface analysis with XRD, a more comprehensive understanding of the relationships between processing parameters and joint quality was developed. Two novel failure surface types with distinct failure behavior were observed which could be predicted based on soldering tip speeds. The differences between the two failure surfaces appear to be due to the growth of a thin surface oxide layer on the solder lines from prolonged sonication, as determined from EDS and XRD microstructure analysis. These observations and analysis highlight a complex relationship that determines the quality of the solder-glass joint and suggest further research is necessary to refine the UAS process specifically for glass substrates and their varied applications in industry to prevent premature failure.

Tailoring the activation behaviour and oxide resistant properties of TiFe alloys by doping with Mn

Ti–Fe alloy is the most investigated material for H2 storage, however, the poor activation kinetics and surface oxide formation limits its practical application. Herein, Mn substituted Ti–Fe alloys are investigated for hydrogen storage application and the effect of air exposure on their performance is evaluated. The alloys were synthesized using arc melting method and characterized for structure, composition and morphology analysis. The XRD analysis confirmed the partial substitution of Fe by Mn in the TiFe1-xMnx alloys. The activation kinetics of the alloys are improved by Mn substitution, and the rate of reaction increased with Mn concentration. The desorption PCIs showed a distinct but dual plateau for the low Mn content and the slope of plateau increased with Mn content. The surface oxide layer formation upon air exposure was analysed by XPS technique. The combined XRD and XPS results illustrated a thin surface oxide layer formation. It was also observed that Mn acts as a sacrificial element to prevent the bulk oxidation of alloys. The overall study depicts synergetic effect of Mn addition on hydrogen absorption kinetics of TiFe1-xMnx alloys.

Cover Picture: Stabilization of Undercooled Metals via Passivating Oxide Layers (Angew. Chem. Int. Ed. 11/2021)

Cool frustration. Engineering composition across a thin surface oxide layer of a metal particle, and stabilizing it with appropriate ligands, leads to a sharp surface stress that frustrates solidification. Chemical potential gradients from compositional variation and ligand inductive effects all contribute to stabilization of a metastable state. An undercooled liquid metal particle is formed, highlighting a new paradigm in engineering energy landscape of a material using thin surface layer, as described by Martin Thuo et al. in their Research Article on page 5928.

Diffusionless FCC to BCC phase transformation in CoCrCuFeNi MPEA thin films

The aim of the present work is growing CoCrCuFeNi HEA thin films of 40–50 nm thickness and annealing them in-situ in the electron microscope for a better understanding and controlling the early stages of phase and compositional instability of as deposited films at temperatures up to 550 °C.Carrying out in-situ electron microscopic experiments of isochronal annealing at temperatures of 450 °C and 550 °C we could follow the structural changes in thin (40–50 nm thick) CoCrCuFeNi MPEA films. We observed that at temperatures 450 and 550 °C the first change in the structure is the martensitic transformation of the FCC to BCC phase. We have shown by EDS mapping of the transformed area that no compositional changes occur during this process and, specific for martensitic transformation orientation, relationships between the two phases are present. The changes occur after about 20 min annealing time at these temperatures.Longer anneals at the same temperatures lead to separation of components. Above 40 min annealing time at 450 °C voids appear in the film, Cr will be partly bonded to oxygen (impurity in the vacuum system) in the form of a thin surface oxide layer. The other components tend to form binary like compositions of Cu-Ni and Fe–Co pairs. We also estimated the diffusion coefficient at 450 °C in this alloy to be around ~6 × 10−19 m2/s.

Evolution of surface chemistry during sintering of water‐atomized iron and low‐alloyed steel powder

Water‐atomized iron and steel powder is commonly used as the base material for powder metallurgy (PM) of ferrous components. The powder surface chemistry is characterized by a thin surface oxide layer and more thermodynamically stable oxide particulates whose extent, distribution, and composition change during the sintering cycle due to a complex set of oxidation–reduction reactions. In this study, the surface chemistry of iron and steel powder was investigated by combined surface and thermal analysis. The progressive reduction of oxides was studied using model sintering cycles in hydrogen atmospheres in a thermogravimetric (TG) setup, with experiments ended at intermediate steps (500–1300°C) of the heating stage. The surface chemistry of the samples was then investigated by means of X‐ray photoelectron spectroscopy (XPS) to reveal changes that occurred during heating. The results show that reduction of the surface oxide layer occurs at relatively lower temperature for the steel powder, attributed to an influence of chromium, which is supported by a strong increase in Cr content immediately after oxide layer reduction. The reduction of the stable oxide particulates was shifted to higher temperatures, reflecting their higher thermodynamic stability. A complementary vacuum annealing treatment at 800°C was performed in a furnace directly connected to the XPS instrument allowing for sample transfer in vacuum. The results showed that Fe oxides were completely reduced, with segregation and growth of Cr and Mn oxides on the particle surfaces. This underlines the sequential reduction of oxides during sintering that reflects the thermodynamic stability and availability of oxide‐forming elements.

Effects of metal dusting relevant exposures of alloy 601 surfaces on carbon formation and oxide development

Ni and Fe are excellent catalysts for carbon formation, and industrial alloys are therefore susceptible to metal dusting corrosion; a costly issue in e.g. synthesis gas manufacture. The objective of this work is to better understand the initial reaction phenomena leading to metal dusting, and thereby minimize the corrosion through optimum alloy selection and pretreatment. Pre-oxidized alloy 601 samples were subjected to carburizing gaseous environments at 750 °C, and carbon formation and surface oxide layer development were investigated by SEM, optical microscopy, AES and Raman spectroscopy. Thin (S)TEM/EDS cross-section lamellae were prepared by Focussed Ion Beam milling.Beyond the initial incubation period, less carbon is formed under 10% CO/Ar than under synthesis gas with finite low carbon activity. Cr2O3 evolves as a thin surface oxide layer with only CO reacting and more ordered carbon develops with increasing exposure time. In contrast, oxidation yields (Ni, Fe, Cr)3O4 spinel formation while the materializing carbon remains its disorder during prolonged exposure to synthesis gas. The metal dusting corrosion rate is hence lowered due to Cr2O3 stabilization, while the spinel represents an unstable redox state that continuously yields new carbon. A fine-grained alloy surface structure is also found beneficial to the Cr2O3 formation.

Epitaxial growth of rock salt MgZrN2 semiconductors on MgO and GaN

Ternary nitride compound semiconductors have attracted recent attention as electronic materials since their properties can be tuned by cation stoichiometry and ordering. A recently discovered example is MgZrN2, a ternary analog to the rock salt semiconductor ScN. MgZrN2 has a larger bandgap and stronger dielectric response than the binary compound. Polycrystalline thin films of MgZrN2 have been studied, but demonstration of high-quality growth is still required to establish its suitability for technological applications. Here, we report on epitaxial growth of MgZrN2 thin films on (100) and (111) MgO substrates and (001) GaN templates. The MgZrN2 composition is confirmed by Rutherford backscattering spectrometry, showing no oxygen in the film except for a thin surface oxide layer. Epitaxial growth results in MgZrN2 with x-ray diffraction rocking curves with a full-width at half-maximum in the range of 0.3–3.0°, depending on the substrate. Transmission electron microscopy analysis of the MgZrN2 film grown on a (111) MgO substrate confirms epitaxial growth and shows a sharp film/substrate interface. In-plane temperature-dependent Hall effect measurements show that the material is an n-type semiconductor with a relatively high concentration (n300K ≈ 1019–1020 cm−3) of thermally activated electrons. Room-temperature transport measurements show a conductivity of 25 S cm−1 and a Seebeck coefficient of −80 μV K−1. Overall, these results provide an important step toward integration of rock salt MgZrN2 with other technological nitrides for device applications.

Lignocellulose-stabilized iron-palladium nanomagnetic biocomposites

In this study we report the synthesis and characterization of superparamagnetic Fe-Pd (1–75 at.% Fe) nanomagnets grown within a lignocellulose fiber matrix for future advanced material applications. The small (~5 nm diameter) Fe-Pd nanoparticles are crystalline with spherical shape and evenly dispersed throughout the lignocellulose support while preserving the structure of the biopolymer matrix. Material characterization of the as-formed biocomposites confirms primarily zero-valent Pd-Fe alloy with a thin surface oxide layer. Normal superparamagnetic behavior consistent with extremely small nanoparticle size is measured, including increasing measured blocking temperature with increasing Fe concentration. Finally, important mass loading effects of metallic nanoparticles within the lignocellulose matrix are measured and discussed.

Electrocatalytic and Optoelectronic Characteristics of Exfoliated Two-Dimensional Titanium Nitride Ti4N3Tx mxene

A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic and electrical properties of the 2D Ti4N3TxMXene (Tx= basal plane surface terminating groups) and show this material exhibits both metallic and semiconducting behavior. We provide complete structural characterization of exfoliated Ti4N3TxMXene and assign Tx= O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti4N3TxMXene forms a hybrid with a thin surface oxide layer resulting in visible light absorption at energies greater than ~2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ~2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti4N3Txshows an overpotential of ~300 mV at –10 mA cm–2and a Tafel slope of ~190 mV/decade. Finally, we observe a clear semiconductor-to-metal transition at ~90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti4N3TxMXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.

Influence of different concentration of hydrogen peroxide on the corrosion behavior of Ti-6Al-4V alloy immersed in physiological solution

The materials used as implants are expected to be highly non toxic and should not cause any inflammatory or allergic reactions in the human body. The success of the biomaterials is mainly dependent on the reaction of the human body to the implant. Titanium and some of its alloys are used as biomaterials for dental and orthopedic applications. The most common grades used are commercially pure titanium and the Ti-6Al-4V alloy. Frequent utilization of titanium and titanium alloys as biomaterials due to their superior biocompatibility, and high corrosion resistance as to form a thin surface oxide layer, and good mechanical properties, as a certain elastic modulus and low density that make that these metals present a mechanical behavior close to those of bones. Strong and totally biocompatible, titanium is one of the few materials that naturally match the requirements for implantation in the human body. Hydrogen peroxide appears in the human body when an inflammation occurs. This research work aims to assess the influence of different concentrations of hydrogen peroxide on the corrosion behavior of Ti-6Al-4V alloy submerged in Hank's physiological solution. Electrochemical methods as Open Circuit Potential and Electrochemical Impedance Spectroscopy were applied for corrosion investigations. The results provide evidence that Ti-6Al-4V corrosion resistance is affected by inflammatory conditions presence in Hank solution.

Electrocatalytic and Optoelectronic Characteristics of the Two-Dimensional Titanium Nitride Ti4N3Tx MXene.

A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic, and electrical properties of the 2D Ti4N3Tx MXene (Tx = basal plane surface terminating groups) and show that this material exhbits both metallic and semiconducting behaviors. We provide complete structural characterization of exfoliated Ti4N3Tx MXene and assign Tx = O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti4N3Tx MXene forms a hybrid with a thin surface oxide layer resulting in visible light absorbtion at energies greater than ∼2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ∼2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti4N3Tx shows an overpotential of ∼300 mV at -10 mA cm-2 and a Tafel slope of ∼190 mV dec-1. Finally, we observesemiconductingbehavior at temperatures below∼90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti4N3Tx MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.

Oxidation and corrosion behavior of commercial 5 wt% Al-Zn coated steel during austenitization heat treatment

A commercial 5 wt% Al-Zn coated steel was employed to detail the microstructural evolution, oxidation, and the related corrosion behavior changes during the austenitization heat treatment. During the early stages of austenitization, Fe2Al5Znx phase forms between the coating and the steel substrate. With continued austenitization, the inner part of the coating transforms to the Γ-Fe3Zn10 phase and the outer part of the coating to the FeAl phase, accompanying with the formation of a thin surface oxide layer mainly composed of Al2O3 with minor ZnO. Subsequently, the coating transforms to the Γ-Fe3Zn10 phase and α-Fe(Zn), and finally to the single α-Fe(Zn) phase. The Al in the coating is distributed mainly on the surface of the alloy layer and suppresses the oxidation of the alloy layer. The austenitization treatment enriches the coating with Fe, which, in turn, ennobles the corrosion potential and reduces the corrosion current density of the coating. Conversely, the potential difference between the coating and the steel substrate is reduced after austenitization.

Mobilization of tungsten dust by electric forces and its bearing on tritiated particles in the ITER tokamak

Experimental investigations on the electric field strength required to overcome the adhesion forces of micron size tungsten metallic dust as well as silver and aluminum oxide in powdery form deposited on a conductive surface are presented. The results reveal a strong influence of particulate surface properties on the detachment threshold, especially for tungsten particles which exhibit a thin surface oxide layer that screens their electrical conductivity. Such tungsten particles makes possible that electrostatic self-charging occurs when tritiated because of their dielectric properties even if there are deposited on a conductive grounded surface.

Application of chemical mechanical polishing process on titanium based implants

Modification of the implantable biomaterial surfaces is known to improve the biocompatibility of metallic implants. Particularly, treatments such as etching, sand-blasting or laser treatment are commonly studied to understand the impact of nano/micro roughness on cell attachment. Although, the currently utilized surface modification techniques are known to improve the amount of cell attachment, it is critical to control the level of attachment due to the fact that promotion of bioactivity is needed for prosthetic implants while the cardiac valves, which are also made of titanium, need demotion of cells attachment to be able to function. In this study, a new alternative is proposed to treat the implantable titanium surfaces by chemical mechanical polishing (CMP) technique. It is demonstrated that the application of CMP on the titanium surface helps in modifying the surface roughness of the implant in a controlled manner (inducing nano-scale smoothness or controlled nano/micro roughness). Simultaneously, it is observed that the application of CMP limits the bacteria growth by forming a protective thin surface oxide layer on titanium implants. It is further shown that there is an optimal level of surface roughness where the cell attachment reaches a maximum and the level of roughness is controllable through CMP.

Doped Transition Metal Nitride Supercapacitors (DTMSCs): Mechanisms of Charge Storage

Charge storage behavior in nitride nanomaterials depends on a number of particle properties and also on the nature of the electrode (1-9). Vanadium nitride is a promising supercapacitor material on account of pseudocapacitance arising from the surface oxide/oxynitride phase interactions. We have previously reported excellent charge storage behavior of nanoparticulate vanadium nitride on account of the surface reactions occurring on the surface of the oxide exo-shell. Vanadium oxide is a superior pseudocapacitor material but suffers from poor electronic conductivity(10). Forming a thin surface oxide layer on nanoparticle nitrides yields the benefits of the charge storage along with charge storage occurring in the nitride core itself. However, there is evidence showing that the surface oxide suffers from instability in highly alkaline solutions on account of side-reactions. In addition, the core nitride itself becomes a poor conductor when prepared as fine nanoparticles (<10 nm)(3). In this study, we tailor nitride nanoparticles to engineer architectures similar to that shown in Figure 1by the use of suitable dopants. By using wet chemical procedures, we generate a number of doped nitrides with doped surface oxide structure. We use ab-initio first principle studies to guide our dopant selection with improved electronic conductivity of surface oxide/core nitride and stability of surface oxide being parameters driving our dopant selection. We demonstrate in this study doped VN with superior electronic conductivity, high capacity and long cycle life. In-depth surface characterization using XPS and electrochemical impedance spectroscopy (EIS) to probe change in charge-storage mechanisms are conducted herein to obtain fundamental understanding into the nature of the various doped nitride materials. Change in nanoparticle morphology and surface composition are also examined and related to charge storage behavior. Results of these studies will be presented and discussed.

Oxidation of Water Atomized Metal Powders

This study focuses on the oxidation of water atomized metal powders. Pilot plant experiments were performed using liquid iron alloyed with manganese and carbon. The powder particle shape and the oxides were determined using optical and scanning electron microscopy with energy dispersive X‐ray microanalysis. The oxygen in the atomized powders was mainly present as thin surface oxide layers, which were determined to increase from 10 to 40–60 nm, at increased particle sizes from 10 to 750 μm. In addition, manganese oxides were observed to be unevenly distributed at the surface of several particles for iron powders alloyed with 0.3 mass% Mn. Experimental data indicated that between 10 and 20% of the manganese was present as oxides in the powders. However, equilibrium calculations predicted a strong driving force for oxidation of manganese. More specifically, it was estimated that only 4% of the initial manganese content remained in the final atomized powders.

Electrical contact resistance of a thin oxide layer with a low mechanical load

The electrical contact resistance of a vertical binary contact between stainless steel balls with a low mechanical load was investigated. Using a statistical approach, we measured the voltage at which the dielectric breakdown occurs within a thin surface oxide layer and the distribution of the contact resistance. Electrical load-bearing conduction through a thin insulating layer was found to occur through two possible sequential processes. In both cases, once a conduction path is formed, the melting of bridges as in conventional contact theory is involved. This suggests that conduction through an oxide layer with a low mechanical load depends mainly on breakdown-induced bridges. Furthermore, the distribution of such path’s resistance shows the log-normal distribution with a long tail toward high resistance.

Gold/Wüstite Core–shell Nanoparticles: Suppression of Iron Oxidation through the Electron‐Transfer Phenomenon

Plasmonic Au and magnetic Fe are coupled into uniform Au@Fe core-shell nanoparticles (NPs) to confirm that electron transfer occurred from the Au core to the Fe shell. Au NPs synthesized in aqueous medium are used as seeds and coated with an Fe shell. The resulting Au@Fe NPs are characterized by using various analytical techniques. X-ray photoelectron spectroscopy and superconducting quantum interference device measurements reveal that the Fe shell of the Au@Fe NPs mainly consists of paramagnetic Wüstite with a thin surface oxide layer consisting of maghemite or magnetite. Electron transfer from the Au core to the Fe shell effectively suppresses iron oxidation from Fe(2+) to Fe(3+) near the interface between the Au and the Fe. The charge-transfer-induced electronic modification technique enables us to control the degree of iron oxidation and the resulting magnetic properties.