Inhomogeneous Oxide Research Articles

Color of Copper/Copper Oxide.

Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidation front in single-crystal Cu thin film is demonstrated to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain-boundary-free and atomically flat films prepared by atomic-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full-color red/green/blue indices is realized by precise control of the oxide-layer thickness; the samples cover ≈50.4% of the standard red/green/blue color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from the oxide "pigment" (complex dielectric functions of Cu2 O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. Furthermore, laser-oxide lithography is demonstrated with micrometer-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.

A Novel Mechanism of the Formation of White and Brown Etching Layer in Bainitic Rail Steel

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle White and Brown Etching Layer (WEL and BEL) on the rail raceway during service. The bainitic rail steels with excellent strength-toughness balance are promising candidates for the next generation of rail steels. It is generally stated that the hard and brittle WEL/BEL is difficult to generate in the bainitic rails because of low carbon content. However, we find a distinct multilayer WEL/BEL composite structure on the worn surface/subsurface of the bainitic rail after in-field testing. The multi-scale characterizations from the microscopic to the atomic scale reveal that the WEL and BEL exhibit different microstructural, chemical, and mechanical features. The oxide particles preferentially form in the BEL rather than the WEL. The local oxidation generates the Mn, Si, and Cr depletion but C enrichment in the BEL matrix. Consequently, the BEL exhibits hard but brittle features due to the formation of brittle twinning martensite, cementite, and oxide particles. In contrast, the WEL is relatively soft and ductile. Under plastic deformation and temperature rise upon wheel-rail contact, the heterogeneous microstructure and chemical micro-segregation in parent material lead to the inhomogeneous microstructural refinement, oxidation, and localized embrittlement, which is responsible for the formation of multilayer WEL/BEL structure in bainitic rail steels.

Evolutions of morphology and electronic properties of few-layered MoS2 exposed to UVO

Ultraviolet-Ozone (UVO) treatment has potentially promoted the long-term stability and the high performance of two-dimensional-material-based devices. However, the detailed evolutions of materials upon UVO treatment are less reported and unclear. Herein, we have systematically investigated the evolutions of morphology and electronic properties of the UVO treated few-layered MoS2. A weak p-doping effect on a 1-hour-UVO-exposed bulk MoS2 is revealed by photoelectron spectroscopy, being attributed to the charge transfer from MoS2 to the newly-formed MoOx. Optical microscopy, atomic force microscopy, Raman and PL mapping measurements reveal an inhomogeneous change of MoS2 morphology after UVO treatment. The oxidation is initiated at intrinsic defect sites and dislocations, and then expands in-plane from dislocations to both sides to form one dimensional standing-wave-like features. The dangling-bond-free surface regions remain smooth, being attributed to the higher energetic barrier for O2 dissociation and chemisorption. A model is proposed to explain the mechanism of inhomogeneous oxidation. The rather different oxidation behavior of MoS2 compared with MoSe2 and MoTe2 can be attributed to the difference in the in-plane chemical bonds. Our findings remind that the device design with UVO oxidation need to take the microstructure changes after treatment into account instead of only taking notice of the doping effect for performance improvement.

Inhibitive effect of sodium molybdate on corrosion behaviour of AA6061 aluminium alloy in simulated concrete pore solutions

Corrosion inhibition of MoO42− for AA6061 aluminium alloy in simulated concrete pore solutions was investigated. Electrochemical methods were adopted to assess corrosion behaviour of AA6061 and surface characterizations were employed to obtain composition and morphology of oxide layers. Results show that low concentration (6 mM) of MoO42− cannot inhibit corrosion, while MoO42− of 10–30 mM effectively inhibits corrosion, which obeys Langmuir adsorption isotherm. MoO42− acts as an anodic inhibitor. 10 mM MoO42− exhibits an optimum corrosion inhibition efficiency. A uniform oxide layer consisting of Al-Ca-Mo(V)-O(H) complexes forms in moderate concentration of MoO42− (10 mM). Inhomogeneous oxide layer consisting of Al-Mg-Si-O(H) and Al-Ca-Mo(VI)-O(H) complexes forms in high concentration of MoO42− (30 mM).

Mechanical\u2013chemical coupling phase-field modeling for inhomogeneous oxidation of zirconium induced by stress\u2013oxidation interaction

A phase-field model is proposed to study the inhomogeneous growth of zirconia induced by the stress–oxidation interaction, which captures the complex interplay among diffusion, oxidation kinetics, interfacial morphology evolution, and stress variation in an oxidation process. Through this numerical model, many experimentally observed but insufficiently understood phenomena can be well explained. Specifically, the numerical simulations reveal quantitatively the causes of interface roughening or smoothening during the inward oxide growth, the roughness-dependent oxide growth rate, and the nucleation sites of premature cracking. These numerical findings can be used as the theoretical references for the improving the durability of oxide scale and prolonging the service life of zirconium-based alloy cladding used in the nuclear power plant.

Effects of ethanol and 2, 5-dimethylfuran addition on the morphology and nanostructure evolution of soot in gasoline primary reference fuel-air coflow diffusion flames

The sediment morphology and oxidation activity of soot at different heights above the burner are studied for three fuels. The three fuels are n-heptane/iso-octane mixtures (0% 2, 5-dimethylfuran and 0% ethanol, DE0), n-heptane/isooctane mixture with 15% ethanol (E15) and n-heptane/iso-octane mixture with 15% 2, 5-dimethylfuran (DMF15). Oxygenated fuel increases flame height and decreases flame brightness. DMF15 has the highest flame and has a brighter waist edge. From the field emission scanning electron microscopy (FESEM) images, ethanol and 2–5, dimethylfuran (DMF) make the deposits particle size of soot smaller and uniform. The soot deposits at the height of the flame became more orderly, E15 soot with the smallest fibrous deposits and pore structure. “Donut” and “Steamed bun” structures are found in soot sediments of DE0 and DMF15, which had not been previously reported. The results of thermogravimetric analysis (TGA) show that the oxidation activity of soot in different flame positions is very different. Generally, the activation energy of soot increases with the increase of sampling position, but the activation energy of DMF15 soot decreases first and then increases. Finally, the soot activation energy of E15 and DMF15 is much less than that of DE0, and E15 has the highest oxidation activity. It is inferred that the structure of the special soot deposits is caused by the inhomogeneous oxidation activity on the surface of the deposits, therefore, a free radical erosion sediments model is proposed.

Amorphous nonstoichiometric oxides with tunable room-temperature ferromagnetism and electrical transport

Material functionalities strongly depend on the stoichiometry, crystal structure, and homogeneity. Here we demonstrate an approach of amorphous nonstoichiometric inhomogeneous oxides to realize tunable ferromagnetism and electrical transport at room temperature. In order to verify the origin of the ferromagnetism, we employed a series of structural, chemical, and electronic state characterizations. Combined with electron microscopy and transport measurements, synchrotron-based grazing incident wide angle X-ray scattering, soft X-ray absorption and circular dichroism clearly reveal that the room-temperature ferromagnetism originates from the In0.23Co0.77O1−v amorphous phase with a large tunable range of oxygen vacancies. The room-temperature ferromagnetism is tunable from a high saturation magnetization of 500 emu cm−3 to below 25 emu cm−3, with the evolving electrical resistivity from 5 × 103 μΩ cm to above 2.5 × 105 μΩ cm. Inhomogeneous nano-crystallization emerges with decreasing oxygen vacancies, driving the system towards non-ferromagnetism and insulating regime. Our work unfolds the novel functionalities of amorphous nonstoichiometric inhomogeneous oxides, which opens up new opportunities for developing spintronic materials with superior magnetic and transport properties.

Электродинамическая модель редокс-сорбции кислорода металлсодержащими нанокомпозитами

Предложена электродинамическая модель редокс-сорбции кислорода из воды на зернистых слоях металл-ионообменных нанокомпозитов. Особенностью является одновременное описание процесса восстановления кислорода по электрохимическому и химическому маршрутам. В связи с неоднородным окислением наночастиц металла кислородом проведен учет меняющегося по высоте и во времени омического сопротивления зернистого слоя нанокомпозита. Проверена адекватность модели на примере медьсодержащего нанокомпозита.

Method of Spectral Ellipsometric Evaluation of the Phase Composition of Multilayer Films and Metal-Oxide Structures in the Process of their Growth

We develop and test a spectral ellipsometric method of nondestructive testing of the phase compositions of multilayer films and metal-oxide structures in the course of their growth on the surfaces of metals and alloys. The method is based on the preliminary thermodynamic and spectral analyses of the phase compositions of inhomogeneous surface oxides. The permanent monitoring of the spectra of ellipsometric parameters makes it possible to trace the distribution of phase composition of nanosized metal-oxide structures over the volume of inhomogeneous surface oxide (film) in the process of its growth. The proposed spectral-ellipsometric method can be useful for the nondestructive testing and, especially, for the in-situ automated investigations with subsequent classification of the structures, compositions, and types of multilayer metal-oxide structures in the course of their growth.

Effect of inhomogeneous oxidation on the mechanical degradation of anode supported solid oxide fuel cell

A typical failure mode of the anode-supported solid oxide fuel cell (SOFC) is the cracking of electrolyte due to the excessive tensile stress caused by the expansion of anode during re-oxidation. This paper builds a three-dimensional model based on the finite element method (FEM) to investigate the effect of inhomogeneous oxidation on mechanical degradation of SOFC. The stress distributions and critical oxidation strains with considering inhomogeneous oxidation are compared to those of supposed homogeneous oxidation. The results indicate that the gradient of oxidation strain induces a large stress gradient and bending moment in anode, which increase the stresses and curvature of the cell, leading to the deterioration of SOFC degradation. The critical oxidation strains are increased in the SOFC with considering the inhomogeneous oxidation except for the anode. By calculating the stress under homogeneous re-oxidation, the resistance to failure is underestimated for cathode, electrolyte and glass-ceramic (GC), but the resistance to failure of the anode is overestimated. The effects of creep and thickness of the graded-oxidized zone on the redox stability are also investigated. This study helps to understand the effect of inhomogeneous oxidation and enhance the performance and lifetime of SOFC.

ELECTRODYNAMIC MODEL OF OXYGEN REDOX SORPTION BY METAL-CONTAINING NANOCOMPOSITES

An electrodynamic model of the redox sorption of oxygen from water on metal-ion-exchange nanocomposites’ (NCs) granular layers is proposed. It features a simultaneous description of the oxygen reduction process along the electrochemical and chemical routes. Due to the inhomogeneous oxidation of metal nanoparticles by oxygen, the granular layer of the NC ohmic resistance, which varies in height and time, is taken into account. The adequacy of the model was tested on a copper-containing NC.

Low Density Multi-principal Element Alloy from Al-Mg-Ca-Si-B System

The scientific paper presents a numerical modeling of the chemical composition for the optimization of the multicomponent light alloys in the Al-Mg-Ca-Si-B system. The effects of the proportion of each chemical element on the main characteristics of the alloy based on the mixture rule and the correlation between the melting temperature and the modulus of elasticity were analyzed numerically. The model results has reveals that even other factors must be taken into account, i.e. the mechanical characteristics which varied significantly with changing of chemical compositions. A compromise was set, by slightly increasing the density to acquire better mechanical characteristics. The selected chemical composition was then used to obtain the new low density alloy. In current research stage we conclude that the as cast alloy comprises an inhomogeneous solid solution and complex oxides. Further studies are ongoing on the experimental alloy in various states (homogenization annealed and processed by plastic deformation).

Automated Laser Ablation of Inhomogeneous Metal Oxide Films to Manufacture Uniform Surface Temperature Profile Electrical Heating Elements

This paper presents automated laser ablation strategies to improve the temperature distribution across the surface of inhomogeneous Ni-Fe-Cr-NiO electrical heating elements during joule heating. A number of iterative closed-loop laser control algorithms have been developed and analyzed in order to assess their impact on the efficacy of the heating element, in terms of homogeneous temperature control, and on the implications for automated fabrication of inhomogeneous metal oxide films. Analysis shows that the use of the leading method, i.e., use of a temperature-dependent variable-power approach with memory of previous processes, showed a 68% reduction in the standard deviation of the temperature distribution of the heating element and a greater uniformity of temperature profile as compared to existing manual methods of processing.

A model for oxidation-induced stress analysis of Ni-based anode-supported planar solid oxide fuel cell

A critical hurdle in realizing commercially viable anode-supported solid oxide fuel cell (SOFC) is the re-oxidation of cermet anode during the cell abnormal operation. In this paper, the analysis of energy dispersive X-ray spectroscopy on a partially oxidized half-cell demonstrates a particular inhomogeneous oxidation mechanism that a portion of anode near the air/anode interface is oxidized with a graded NiO content, while the remaining region hold a reduced status. Based on this observation, an analytical oxidation-induced stress model is developed to provide in-depth information about the mechanical behavior of the half-cell suffered from various oxidation. The dependences of the mechanical performance of half-cell on the NiO distribution and thickness of oxidation-graded zone are revealed. The results show that an increase in oxidation-graded zone elevates the stress level of electrolyte, but decreases the curvature under the same global degree of oxidation (DoO). In addition, the influence of thickness of oxidation-graded zone on the electrolyte failure probability is also investigated. According to these results, we conclude that the thickness of oxidation-graded zone should be as thin as possible in order to delay the electrolyte cracking.

Comparative study on oxidation behavior of Ti2AlN coatings in air and pure steam

The oxidation behavior of the Ti2AlN coatings is comparatively investigated at 750 °C in air and in pure steam conditions, respectively. The linear kinetics composed of two stages is observed for each case, suggesting a chemical reaction determining process, and the second stage behaves a slightly higher oxidation rate. In air, the oxidation behavior exhibits an inhomogeneous oxidation due to the presence of macro-particles and impurity phases in the coating, while a transition from inhomogeneous oxidation to homogeneous oxidation occurs in pure steam. Compared to that in air, the oxidation process in pure steam is accelerated, leading to higher mass gains. XRD and EDX analyses indicate that Al easily diffuses to the surface and thus forms α-Al2O3 layer under the two atmospheres, which result from the high Al activity within MAX phase. Additionally, the scaling mechanism of Ti2AlN coatings is discussed in terms of the microstructure evolution.

Minimizing Trap Charge Density towards an Ideal Diode in Graphene-Silicon Schottky Solar Cell.

Photovoltaic device performance of graphene/n-Si Schottky diodes is largely affected by inhomogeneous oxide formation at the interface that suppresses the tunneling current of injected and photoexcited charges. The accumulated trap charges at low current induce charge recombination at the interface and degrade the ideality factor of the diode and the fill factor (FF) of the solar cell. This consequently gives rise to a nonlinear current-voltage ( I- V) feature in solar cells, commonly known as an S-shaped kink, which can be engineered by optimizing the interface barrier thickness or by increasing the carrier mobility. Here, we present chemical and electrochemical doping methods to increase the conductivity of graphene that transforms nonlinear kink photodiodes with a low FF and solar cell efficiency towards trap-free linear photovoltaic I- V. Space-charge-limited-current manifested Ohmic I- V diode behavior with enhanced conductance in graphene by injecting homogeneous ionic liquid; confirming the significant reduction of trap charge density. This was further congruent with the disappearance of the nonlinear kink in photodiodes with a high FF and nearly ideal diodes. The solar cell efficiency obtained with our strategy is around 13.6% and suggests possibilities to reach the theoretical limit of 19% by tailoring parameters such as conductance of graphene, carrier density of Si, and oxidation of the interfaces.

The Influence of Thermal Oxidation Parameters on Structural, Friction, and Wear Characteristics of Oxide Layers Produced on the Surface of Ti–6Al–7Nb Alloy

The effects of thermal oxidation of a biomedical titanium alloy (Ti–6Al–7Nb) on its morphology, structure, mechanical properties, and sliding friction and wear against alumina were investigated. It was found that at 600 °C, the surface of the alloy was characterized with a thin inhomogeneous oxide scale. Increasing the temperature of oxidation to 700 °C and 800 °C allowed obtaining homogeneous layers, which fully covered the examined surfaces. By contrast, the oxide scale obtained at 800 °C was composed of big oxide particles with a developed surface. Thermal oxidation process allows a fourfold increase in the hardness of Ti–6Al–7Nb alloy. It was shown that the oxide scale on the examined alloy efficiently enhances its resistance to sliding wear against alumina.

Cap-layer-dependent oxidation of ultrathin cobalt films and its effect on the magnetic contrast in scanning electron microscopy with polarization analysis

We investigate the influence of a non-magnetic cap layer on the magnetic contrast of a Co film by scanning electron microscope with polarization analysis (SEMPA). Ex-situ sputter-deposited [Ir\Co\Pt]2 samples are terminated with wedge-shaped Pt layers with thicknesses ranging from 0 to 2 nm. The magnetic contrast almost vanishes at low capping, as well as towards the maximum Pt thickness. The highest contrast is found around 1 nm Pt cap layer, where about 25% of the contrast of a clean uncapped Co film is obtained. This contrast is sufficient to record high-quality SEMPA images. Energy-dispersive X-ray spectroscopy (EDX) reveals complete protection against oxidation from 1.1 nm onwards. Analysis of the contrast reduction towards higher coverages yields a spin-decay length of 0.7 nm in Pt for electrons with energies of roughly 6–10 eV above Fermi level. Quantitative EDX analysis indicates the formation of cobalt(II)hydroxide Co(OH)2 at a lower Pt coverage. The oxidation causes a roughening of the surface (4.5 Å RMS), while the surface of the closed Pt film is smooth (1.2 Å RMS), as proven by atomic force microscopy. Island growth of the Pt film on Co is observed, yielding a laterally inhomogeneous oxidation of uncovered patches up to an effective Pt coverage of about 0.9 nm.

The compatibility between thermocouple wires and sheath alloy for small diameter mineral-insulated metal-sheathed (MIMS) thermocouples in long-term aging

In this paper, the influence of long-term aging on the electromotive force (EMF) drift of small diameter mineral-insulated metal-sheathed thermocouples has been investigated on the two typical commercial thermocouple alloys for the nuclear reactor application. The results show that the stability of the de2-316L thermocouples is superior to that of de1 thermocouples. The drift is attributed to inhomogeneous oxidation, especially the KN thermoelement. The alloying elements diffuse towards the surface of thermocouple wire causing inhomogeneous composition even in the un-oxidized area. However, a premature failure is shown in the de2-Inconel thermocouples. This is because of the precipitation of Cr and Fe elements, which reduces the concentration of Cr and Fe in the Ni matrix. Fe generates a larger distortion of the crystal lattice and increases the instability of the solid solution. The deep understanding of the compatibility between the thermocouple alloys and sheath alloys is of crucial important in terms of the manufactures of thermocouple sensors and the safety operation systems in the nuclear reactor.

Inhomogeneous metal oxide films with a fractal structure as an object for observation of quantum size effects

The energy spectrum of charge carriers in inhomogeneous metal oxide films is studied. The principle of specifying the potential on the boundary of nanomaterial/substrate, nanomaterial/atmosphere is demonstrated using the theory of fractals. The energy of the dimension quantization levels in the tin dioxide films with a fractal surface is numerically assessed.