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High-temperature oxidation of steels can be relatively fast when exposed to air. Consequently, elucidating the effect of different parameters on the oxidation mechanism and kinetics is challenging. In this study, short-time oxidation was investigated to determine the oxidation mechanism, the affecting parameters, and the linear-to-parabolic growth transition of different Fe–Mn alloys in various oxygen partial pressures (10–30 kPa) and gas flow rates (26.6 and 53.3 sccm) in a temperature range of 950–1150 °C. Oxidation kinetics was investigated using a thermogravimetric analyzer (TGA) under controlled atmosphere. Linear oxide growth was observed within the first 20 minutes of oxidation. The linear rate constant was significantly increased by increasing the oxygen partial pressure or the flow rate of the oxidizing gas. The morphology of the oxide layer was determined by scanning electron microscopy (SEM). The crystal structure of the oxides formed was followed by in-situ X-ray diffraction (XRD), confirming that the growing layer consists of wustite mainly, which upon slow cooling to room temperature, transformed into magnetite. Energy-dispersive X-ray spectroscopy (EDS) showed that the atomic ratio of Fe+Mn to O was ~ 1.03:1 in the oxide scale, corresponding to Fe(Mn)O formation. Based on the characterization and a model for linear growth kinetics, it is concluded that the oxidation rate is controlled by the diffusion of oxidizing molecules through the gas layer to the sample's surface. The findings led to a better understanding of initial oxidation behavior and provided a pathway for improved insight into the high-temperature oxidation behavior for more complex alloys. [ABSTRACT FROM AUTHOR]

A method is presented to determine the photoelectron surface core-level shift (SCLS) of 3d transition metals using x-ray photoelectron spectroscopy. The experimental difficulties arising from the relatively large broadening of photoemission lines in the 3d transition series can be overcome by the analysis of the angular dependence of photoemission spectra. The proposed method has been demonstrated using well-defined single-crystal surfaces of copper. The observed values of the SCLS for copper are compared with those predicted by both ab initio calculations and a macroscopic atom model. The experimental determination of SCLSs opens alternative routes for collecting thermochemical data for surfaces/interfaces. [ABSTRACT FROM AUTHOR]

The growth kinetics and mechanisms of thin aluminum-oxide films formed by the dry, thermal oxidation of a bare Al(431) substrate at a partial oxygen pressure of 1.33 × 10[sup -4] Pa in the temperature range of 373-773 K were studied using x-ray photoelectron spectroscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (T ≤ 573 K), an amorphous oxide film develops that attains a limiting (uniform) thickness. At high temperatures (T>573 K), growth is not impeded at a limiting thickness. Kinetic analysis established the occurrences of two different oxide-film growth regimes: an initial regime of very fast oxide-film growth and a second, much slower oxidation stage that is observed only at T > 573 K. These results could be discussed in terms of electric-field controlled, interstitial, outward transport of Al cations through a close packing of O anions in the amorphous films, and inward diffusion of O along grain boundaries in the crystalline films, respectively. For the electric-field controlled Al cation motion, a value of 2.6 eV was determined for the rate-limiting energy barrier, which is located at the metal/oxide interface. This corresponds with a Mott potential of -1.6 V. [ABSTRACT FROM AUTHOR]

Focuses on a study with which analyzed the annealing-induced variations in compositions, microstructure, resistivity and internal stresses of amorphous molybdenum/silicon. Specimen preparation; Composition and architecture of the molybdenum silicate layers; Crystalline phase formation.

The high chromium nickel base alloy 602 CA is used as construction material for high temperature components in aggressive environments such as gas burners or heat exchangers. The high temperature oxidation behaviour of this alloy is characterised by the formation of an external Cr2O3 scale and internal precipitation of alumina. The main goal of the present investigation was to elucidate the effect of aluminum on the subscale chromium depletion phenomena. The chromium concentration profiles were found to be flat and sometimes even increasing towards the oxide-alloy interface. The experimental observations were interpreted in terms of alloy thermodynamics, i.e. effect of oxidation induced compositional changes on chemical potentials of the alloy constituents. The concentration profiles in alloy 602 CA observed during high temperature exposure were modelled using the CALPHAD approach for thermodynamic and kinetic calculations. Thermodynamic calculations revealed that the Al depletion in the alloy subsurface due to internal alumina precipitation leads to enhanced Cr diffusion towards the alloy surface. [ABSTRACT FROM AUTHOR]

MCrAlY (M = Ni, Co) coatings are commonly used to protect the underlying superalloy component in industrial gas turbines and aeroengines from oxidation attack. They also function as bond coats for thermal barrier coatings (TBC). The chemical life time of the coated component is mainly governed by the depletion of the β-phase in the bond coat as a result of simultaneously occurring surface oxidation and interdiffusion between the coating and the substrate. A new computational approach to model the microstructural evolution in MCrAlY (M = Ni, Co) coatings on Ni base superalloys was undertaken in the present study. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and wavelength dispersive X-ray spectroscopy (WDX) was employed to characterise MCrAlY coated superalloy samples after exposure at 1000 and 1100°C. Phases were identified by electron backscatter diffraction (EBSD) and correlated with SEM/EDX/WDX analyses. Modelling of the microstructural evolution was carried out considering simultaneously occurring surface oxidation and interdiffusion processes. A flux based calculation of the concentration profiles and stable phases was performed, taking into account diffusion of all elements in the γ, γ' and β-NiAl phases. Good agreement was found between the observed and computed phase distributions after specific time intervals. The computed widths of the β-depletion zones showed satisfactory agreement with the measurements. Additionally, the model was able to predict the formation of a TCP-phase and its penetration into the substrate with increasing exposure time in one of the investigated coating systems. [ABSTRACT FROM AUTHOR]

Since the oxidation reactions in the process of steel production occur in harsh conditions (i.e., high temperatures and gas atmospheres), it is practically impossible to observe in situ the compositional changes in the steel and the formed oxide scale. Hence, a coupled thermodynamic-kinetic numerical model is developed that predicts the formation of oxide phases and the composition profile of the steel alloy's constituents in a short time due to external oxidation. The model is applied to high-temperature oxidation of Fe–Mn alloys under different conditions. Oxidizing experiments executed with a thermogravimetric analyzer (TGA) on Fe–Mn alloys with different Mn contents (below 10 wt %) are used to determine kinetic parameters that serve as an input for the model. The mass gain data as a function of time show both linear and parabolic regimes. The results of the numerical simulations are presented. The effect of different parameters, such as temperature, Mn content of the alloy, oxygen partial pressure, and oxidizing gas flow rate on the alloy composition and oxide phases formed, is determined. It is shown that increasing the temperature and decreasing the oxygen partial pressure both lead to a thicker depleted area. [ABSTRACT FROM AUTHOR]

We study the stability in time of the current-voltage characteristics of organic thin-film devices on glass substrates. We find for poly(3-hexylthiophene) and for pentacene that the resistance of the devices gradually changes under the application of an electrical bias depending on the sodium content of the glass substrates used in the experiment. For devices on a very common type of glass (with a Na[sub 2]O content of about 6%) and on sodalime glass ( 14% Na[sub 2]O) substrates, the prolonged application of a voltage bias results in a substantial decrease (up to two orders of magnitude) of the bulk and contact resistances, whereas for sodium-free glass substrates the gradual changes in current-voltage characteristics are much smaller. A systematic study of the electrical behavior complemented by chemical analysis shows that the instabilities observed are due to Na[sup +] ions diffusing from the substrate into the organic film, and moving inside the organic material as a result of the applied electric field. Our results show in detail how ion motion in organic materials results in substantial hysteresis and device instabilities. [ABSTRACT FROM AUTHOR]

MXenes are a new family of 2D carbides or nitrides that have attracted attention due to their layered structure, tunable surface groups and high electrical conductivity. Here, we report for the first time that the Ti2CTx MXene catalyses the selective oxidative dehydrogenation of n‐butane to butenes and 1,3‐butadiene. This catalyst showed higher intrinsic activity compared to a commercial TiC and TiO2 samples in terms of C4 olefin formation rate. We propose that the stabilisation of structural vacancies and the change in composition (from a carbide to a mixed phase oxide) in the MXene causes its higher catalytic activity. These vacancies can lead to a higher concentration of unpaired electrons in the MXene‐derived material, enhancing its nucleophilic properties and favouring the production of olefins. [ABSTRACT FROM AUTHOR]

A method is presented that allows determination of the work of adhesion between oxides and metals using a macroscopic atom model. The method allows complex interfaces to be modeled easily and the influence of additives and impurities to be assessed. The model is used to study the work of adhesion between α-Al2O3 and β-NiAl. This interface is of importance for the performance of thermal barrier coatings as applied to jet turbines. The model shows that the work of adhesion is not significantly altered by so-called reactive element additions. Reactive elements are known to improve the durability at the alloy/oxide interface and include elements such as Zr, Y, and Hf added in concentrations of less than 1 at.%. A significant weakening of the interface is predicted when impurities such as sulfur and carbon are present. The model also predicts a large interaction enthalpy between the reactive elements and impurities. It is proposed that the primary effect of reactive element additions is impurity scavenging. The impurities are fixed in the bulk of the alloy by the reactive elements and cannot diffuse to the oxide/metal interface to weaken it. [ABSTRACT FROM AUTHOR]

The corrosion behaviour of Cr(III) based conversion coatings on electrogalvanised steel was studied using polarisation and electrochemical impedance spectroscopy measurements in 0.01M NaCl solution (pH 6). The Cr(III) based conversion treatments were carried out in two different commercial trivalent chromium baths (pH 1.8–2.0). With an immersion time of 1 min, the thickness of the first type of Cr(III) based coating (blue) was about 94 nm and the thickness of the second type of coating (iridescent yellow) was about 250 nm; these thicknesses were determined by spectroscopic ellipsometry. As a comparison, a Cr(VI) based coating on electrogalvanised steel was also studied. The morphologies and compositions of the coatings were investigated by means of scanning electron microscopy, Fourier transform infrared spectroscopy and Auger electron spectroscopy. The Cr(III) coatings contained mainly a mixture of zinc oxides and chromium oxides/ hydroxides, and no Cr(VI) was detected in these coatings. The polarisation measurements in NaCl solution showed a retarded cathodic reaction for both the Cr(III) cmd the Cr(VI) based coatings. The corrosion resistance of the thicker Cr(III) coating was greater than that of the thinner Cr(III) coating. However, in the cases studied, the Cr(III) coatings stiff did not inhibit the corrosion of the electrogalvanised steel as effectively as did the Cr(VI) based coatings. The reason may be that the Cr(III) coatings do not possess the ‘self-healing’ properties that the Cr(VI) based coatings do. [ABSTRACT FROM AUTHOR]

The precipitation of supersaturated solutes at free surfaces in ternary Fe–3Au–4W and binary Fe–3Au and Fe–4W alloys (composition in weight percentage) for different ageing times was investigated at a temperature of 700 °C. The time evolution of the surface precipitation is compared among the three alloys to investigate the interplay between the Au and W solutes in the ternary system. The Au-rich grain-interior surface precipitates show a similar size and kinetics in the Fe–Au–W and Fe–Au alloys, while the W-rich grain-interior surface precipitates show a smaller size and a higher number density in the Fe–Au–W alloy compared to the Fe–W alloy. The kinetics of the precipitation on the external free surface for the ternary Fe–Au–W alloy is compared to the previously studied precipitation on the internal surfaces of the grain-boundary cavities during creep loading of the same alloy. [ABSTRACT FROM AUTHOR]

Growth stress in tungsten carbide-diamond-like carbon coatings, sputter deposited in a reactive argon/acetylene plasma, has been studied as a function of the acetylene partial pressure. Stress and microstructure have been investigated by wafer curvature and transmission electron microscopy (TEM) whereas composition and energy distribution functions of positive ions were obtained by electron probe microanalyzer, elastic recoil detection analysis, and mass-energy analyzer (MEA). It has been observed that the compressive stress decreases with increasing acetylene partial pressure, showing an abrupt change from -5.0 to -1.6 GPa at an acetylene partial pressure of 0.012 Pa. TEM micrographs show that by increasing the acetylene partial pressure in the plasma from 0 to 0.012 Pa, the microstructure of the coating changes from polycrystalline to amorphous. MEA results show that the most probable energy of positive ions bombarding the substrate during deposition in pure argon and argon/acetylene atmosphere is the same. Based on the results, it is concluded that the huge variation in the compressive stress at low acetylene partial pressures is due to a change in the microstructure of the coating from polycrystalline to amorphous and not to the energy of positive ions bombarding the film. [ABSTRACT FROM AUTHOR]

Focuses on a study which grew amorphous silicon layers by krypton plasma sputter deposition. Experimental setup and procedure; Model for ion assisted deposition; Background on Mössbauer spectroscopy.

The effect of particle size on the oxidation kinetics of TiC powders is studied. Different sizes of TiC powder ranging from nanometre to submillimetre sizes are investigated. The samples are heated at different heating rates from room temperature up to 1200 °C in dry synthetic air. The Kissinger method for analysis of non-isothermal oxidation is used to estimate the activation energy for oxidation of the powders and to identify the active temperature window for efficient self-healing. The master curve plotting method is used to identify the model which best describes the oxidation of TiC powders, and the Senum and Yang method is used to approximate the value for the Arrhenius constant. The oxidation of TiC proceeds via the formation of oxycarbides, anatase and then finally the most stable form: rutile. The activation energy is found to be a strong function of the particle size for particle sizes between 50 nm and 11 µm and becomes constant at larger particle sizes. The data demonstrate how the minimal healing temperature for oxide ceramics containing TiC as healing particles can be tailored between 400 and 1000 °C by selecting the right average TiC particle size. [ABSTRACT FROM AUTHOR]

Carbon-coated monolithic catalysts were prepared by dipcoating a cordierite substrate in a Furan resin, after which the ruthenium was incorporated by impregnation with a [RuCl5]2- complex. Immobilization of the precursor proceeded particularly via weak physisorption. Since the carbon-coated monolithic supports have a broad pore-size distribution, redistribution of the ruthenium precursor occurred upon drying due to capillary forces, resulting in low dispersion. A significant amount of ruthenium is present on the surface of the carbon inclusions located deep in the cordierite walls, making the diffusion path of the reactants to the active ruthenium sites considerably larger than the average carbon-coating thickness. For successful application of the carbon-coated monolithic catalysts in gas–liquid reactions, the deposition of carbon in the walls of the monolith should be prevented and maldistribution of ruthenium upon drying should be solved. [ABSTRACT FROM AUTHOR]