Pressure effects on aluminium oxidation kinetics using X-ray photoelectron spectroscopy and parallel factor analysis

The effects of water vapour pressure on oxidation kinetics of aluminium have been studied using X-ray photoelectron spectroscopy (XPS) and three-way parallel factor analysis (PARAFAC). While the first technique is a powerful experimental tool for surface oxidation studies, the PARAFAC technique is a sophisticated analytical tool for analysing XPS data. The XPS Al(2p) and O(1s) core level have been used to follow the oxide film growth on clean surfaces at room temperature as a function of oxidation time (ranging from 1 to 60 min) and pressure of water vapour (ranging from 2.0×10⁻⁶ to 6.5×10⁻⁴ Pa). The growth of thin oxide films on aluminium surfaces has been found to follow the Cabrera–Mott inverse logarithmic law in all pressure ranges studied. The pressure effects have shown that the defect formation reaction at the oxide film/gas interface is the rate determining process in the aluminium oxidation. The pressure dependence of oxidation kinetics can be explained on the basis of metal vacancies in the defect structure of thin aluminium oxide films.     

Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation

The paper reports on a systematic investigation into the effects of process parameters on the growth kinetics and associated changes in the structure, phase composition and mechanical properties of surface layers formed on Ti–6Al–4V alloy by plasma electrolytic oxidation (PEO) treatment in 0.05–0.2 mol l⁻¹ solutions of sodium aluminate. Methods of gravimetric, SEM and XRD analysis, as well as microhardness and scratch testing, are employed to investigate mass transfer and phase-structure transformations in the surface layer. The probable mechanisms of layer formation are discussed, which comprise electrochemical oxidation of the Ti-electrode by OH⁻ anions, complimented by chemical precipitation of Al(OH)₃ and plasma-induced transformations in the surface discharges. Running with a total yield efficiency of 20–30%, these processes lead to the formation of predominantly the Al₂TiO₅ phase with heterogeneous precipitation of Al₂TiO₅·TiO₂ and 3Al₂TiO₅·Al₂O₃ eutectics. Al- and Ti-enriched constituents of this structure show hardnesses of 1050–1480 and 300–845 H_K, 0.02, respectively. The layer growth rate increases with increasing electrolyte concentration, providing a maximum thickness of over 60 μm and a surface roughness (R_a) of 3–4 μm. Increasing the electrolyte pH from 12.0 to 12.8 results in smoothing and thickening of the surface layer but a lower sample weight gain, associated with an enhancement of the Ti electro-oxidation process. Morphological changes during PEO formation of the surface layer include gradual transformation of the original fine grained but porous structure into a dense, fused morphology which is adversely affected by discharge-induced thermal stresses, causing a degradation of the layer adhesion strength.     

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminium ignition.     

Catalytic soot oxidation in microscale experiments: Simulation of interactions between co-deposited graphitic nanoparticle agglomerates and platinum nanoparticles

Continuously regenerating catalytic soot traps are under development to reduce particulate emissions from diesel exhaust. A good understanding of the processes that take place during soot oxidation is needed to optimize diesel soot trap performance. To gain insight into these processes from the perspective of nanoparticle technology, the effects of catalyst particle size and the interparticle distance between soot and catalyst particles were measured. A model catalyst was prepared by depositing Pt nanoparticles on a SiO/SiO₂-coated transmission electron microscope (TEM) grid. A soot surrogate composed of graphitic nanoparticle agglomerates generated by laser ablation was deposited on the same surface. This system simulates, morphologically, catalytic soot traps used in practice. The reaction was carried out in a tubular flow reactor in which the gas phase simulated diesel exhaust gas, composed of a mixture of 10% O₂ and 1000 ppm NO with the remainder N₂. The progress of the carbon nanoparticle oxidation was monitored off-line by analysis of electron microscopy images of the agglomerates before and after reaction. This experimental method permitted the correlation of reaction rate with particle sizes and separation distances as well as catalyst surface area in the direct environs of the soot particles. The experimental results revealed no effect of Pt catalyst particle size in the range 7–31 nm on the rate of reaction. Also observed were a decrease in the rate of reaction with increasing distance between carbon agglomerates and catalyst particles and a linear dependence of the reaction rate on the fractional catalyst surface area coverage.     

Surface nature of nanoparticle zinc-titanium oxide aerogel catalysts

Nanoparticle zinc-titanium oxide materials were prepared by the aerogel approach. Their structure, surface state and reactivity were investigated. Zinc titanate powders formed at higher zinc loadings possessed a higher surface area and smaller particle size. X-ray photoelectron spectroscopy (XPS) revealed a stronger electronic interaction between Zn and Ti atoms in the mixed oxide structure and showed the formation of oxygen vacancy due to zinc doping into titania or zinc titanate matrices. The 8-45 nm aerogel particles were evaluated as catalysts for methanol oxidation in an ambient flow reactor. Carbon dioxide was favorably produced on the oxides with anion defects. Titanium based oxides exhibited a high selectivity to dimethyl ether, so that a strong Lewis acidic character suggested for the catalysts was associated primarily with the Ti⁴⁺ center. Both methanol conversion and dimethyl ether formation rates increased with increasing the zinc content added to the oxide support. Results demonstrate that cubic zinc titanate phases produce new Lewis acid sites having also a higher reactivity and that the nature of the catalytic surface transforms from Lewis acidic to basic characters due to the presence of reactive oxygen vacancies.     

Oxidation study of silicon nanoparticle thin films on HOPG

Thin films of silicon nanoparticles (diameter 5-10 nm) were deposited on highly oriented pyrolytic graphite (HOPG) by low-pressure DC magnetron sputtering. The effect of different room-temperature oxidation techniques was investigated using XPS sputter-depth profiling. Both oxygen treatment during deposition (using an argon-oxygen mixture in the sputter gas) as well as post-deposition oxidation techniques (exposure to oxygen plasma beam, ambient air conditions) were studied. In all cases oxidation was found to involve the whole film down to the film/substrate interface, indicating a network of open pores. Depending on the type of oxidation between 15 and 25 at% of oxygen, mostly associated with low oxidation states of silicon, were detected in the interior of the film and attributed to oxidized surfaces of the individual silicon nanoparticles. The highest oxygen concentrations were found at the very film surface, reaching levels of 25-30% for films exposed to air or prepared by reactive magnetron sputtering. For the oxygen plasma-treated films even oxygen surface concentrations around 45% and fully oxidized silicon (i.e., SiO₂) were achieved. At the Si/HOPG interface formation of silicon carbide was observed due to intermixing induced by Ar-ion beam used for sputter-depth profiling.     

Forced bursting and transition mechanism in CO oxidation with three time scales

The mathematical model of CO oxidation with three time scales on platinum group metals is investigated, in which order gaps between the time scales related to external perturbation and the rates associated with different chemical reaction steps exist. Forced bursters, such as point-point type forced bursting and point-cycle type forced bursting, are presented. The bifurcation mechanism of forced bursting is novel, and the phenomenon where two different kinds of spiking states coexist in point-cycle type forced bursting has not been reported in previous work. A double-parameter bifurcation set of the fast subsystem is explored to reveal the transition mechanisms of different forced bursters with parameter variation.     

Effects of duty ratio at low frequency on growth mechanism of micro-plasma oxidation ceramic coatings on Ti alloy

The aim of this work is to study the effects of duty ratio on the growth mechanism of the ceramic coatings on Ti-6Al-4V alloy prepared by pulsed single-polar MPO at 50 Hz in NaAlO₂ solution. The phase composition of the coatings was studied by X-ray diffraction, and the morphology and the element distribution in the coating were examined through scanning electron microscopy and energy dispersive spectroscopy. The thickness of the coatings was measured by eddy current coating thickness gauge. The corrosion resistance of the coated samples was examined by linear sweep voltammetry technique in 3.5% NaCl solution. The changes of the duty ratio (D) of the anode process led to the changes of the mode of the spark discharge during the pulsed single-polar MPO process, which further influenced the structure and the morphology of the ceramic coatings. The coatings prepared at D = 10% were composed of a large amount of Al₂TiO₅ and a little γ-Al₂O₃ while the coatings prepared at D = 45% were mainly composed of α-Al₂O₃ and γ-Al₂O₃. The coating thickness and the roughness were both increased with the increasing D due to the formation of Al₂O₃. The formation of Al₂TiO₅ resulted from the spark discharge due to the breakdown of the oxide film, while the formation of Al₂O₃ resulted from the spark discharge due to the breakdown of the vapor envelope. The ceramic coatings improved the corrosion resistance of Ti-6Al-4V alloy. And the surface morphology and the coating thickness determined the corrosion resistance of the coated samples prepared at D = 45% was better than that of the coated samples prepared at D = 10%.     

Initial oxidation of duplex stainless steel

Three different techniques were used to produce thin oxide layers on polished and sputter-cleaned duplex stainless-steel samples. These samples were exposed to 10⁻⁵ mbar of pure oxygen inside the vacuum chamber, exposed to ambient conditions for 24 h, and plasma oxidized. The oxide layers thus produced were analysed using XPS depth profiling in order to determine the oxide layers’ compositions with depth. We found that all the techniques produce oxide layers with different traces of metallic components and with the maximum concentration of chromium oxide shifted towards the oxide-layer-bulk-metal interface. A common characteristic of all the oxide layers investigated is a double-oxide stratification, with regions closer to the surface exhibiting higher concentrations of iron oxide and those more in-depth exhibiting higher concentrations of chromium oxide. A simple non-destructive Thickogram procedure was used to corroborate the thickness estimates for the thinnest oxide layers.     

Sensitivity of styrene oxidation reaction to the catalyst structure of silver nanoparticles

This study shows how different morphologies of silver nanoparticles affect the selective oxidation of styrene in the gas phase using oxygen as oxidant. Silver nanoparticles (nanowires and nanopolyhedra), prepared using the polyol process, were supported on α-Al₂O₃. For comparison, a conventional catalyst obtained by wet impregnation was also prepared. Phenylacetaldehyde (Phe) and styrene oxide (SO) were the main products for nanoparticles catalysts. The promotion effect on the catalytic activity of potassium and cesium on the silver nanowires catalysts was also studied. At 573 K, the styrene conversion and selectivity to styrene oxide with the silver nanowires catalyst were 57.6 and 42.5%, respectively. Silver nanopolyhedra catalyst showed 57.5% conversion and 30.8% selectivity to styrene oxide. The promotion by cesium played an important role in improving the epoxidation of styrene. The samples were structurally characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) were applied to characterize the oxygen species detected (O_β, O_γ) on the silver surface.     

A study on the law of oxidation rate in GaAs-based VCSELs

In order to accurately control the oxidation aperture in high power vertical cavity surface emitting lasers (VCSELs), the selective oxidation process is studied with experiments. Selective oxidation experiments are carried out upon the simulate wafer of VCSELs at different temperature. Oxidation products are examined at different oxidation depths of oxidation layer and each component content is analyzed. The results of the experiments are analyzed with the kinetics of thermal diffusion. The analyzed results show that the concentration of oxidant is e exponentially declined with increasing depth of oxidation in high-power VCSELs. The oxidation depth stability and precision can be improved by lowering the oxidation temperature and prolonging the oxidation time.     

The influence of treatment duration on multi-walled carbon nanotubes functionalized by H2SO4/HNO3 oxidation

Variation in the nature of multi-walled carbon nanotubes (MWCNTs) subjected to different degrees of oxidation was investigated. The microstructure was determined by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) methods, and the surface chemistry was evaluated in terms of the functional groups determined by X-ray photoelectron spectroscopy (XPS) and thermal analysis-mass spectroscopy (TA-MS). In addition, TGA was used to indicate the thermal stability of the nanotubes. Results demonstrate that the graphitic structure of nanotubes oxidized with a mild mixture of H₂SO₄/HNO₃ was preserved. Decrease in the degree of crystallinity started with widening of the C(0 0 2) XRD diffraction peak, followed by this peak shifting towards lower angles. The oxygen content increased with increasing treatment time. A defect peak incorporated in deconvolution of XPS C1s spectra was helpful for detecting the generation of defect sites. The predominant surface functionalities of the nanotubes have been changed from basic to acidic groups after treatment for one day. The samples oxidized for two days had the most abundant surface -COOH and the highest oxidation resistance. The oxidation mechanism of MWCNTs in mild H₂SO₄/HNO₃ mixture was proposed, which was a successive and iterative process, including the initial attack on active sites, and next the hexagon electrophilic attack generating new defects and introducing more oxygen, and then the tubes becoming thinner and shorter.     

Fabricating tunable nanoparticle density gradients with the contact printing based approach

Recently, the generation of spatial gradient nanoparticle assemblies has attracted much attention. Such assemblies can be intriguing templates for building novel molecular architectures, and be employed as a combinatorial tool for quick determination of interaction selectivity for nanoparticles. In this communication, we report on convenient contact printing based techniques for generating lateral gradients containing nanoparticles with tunable geometry, scale and steepness. In the first method, octadecyltrichlorosilane (OTS) gradient surfaces were generated via the contact printing approach, and then spaces un-occupied by OTS molecules were back-filled with an amine-terminated silane, which allows the grafting of nanoparticles that were surface functionalized with carboxylic acid. By varying the size and geometry of the stamp, different geometrical gradients were generated. In addition, by changing the stamping procedures, either well-defined stepwise gradients or continuous gradients can be achieved. Furthermore, the contact printing based technique can be utilized in combination with diffusion of the aminosilane molecules to directly create its gradient, and consequently a nanoparticle density gradient.     

Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

MnO₂-based catalysts have attracted great attention in the field of elemental mercury (Hg⁰) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg⁰ oxidation by oxygen species (gas-phase O₂, chemisorbed oxygen, and lattice oxygen) on MnO₂ surface. The results indicate that Hg⁰ and HgO are chemically adsorbed on MnO₂ surface with the adsorption energies of ?69.50 and ?226.48?kJ/mol, respectively. The adsorption of O₂ on MnO₂ surface belongs to chemisorption. O₂ can decompose on MnO₂ surface with an energy barrier of 97.46?kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O₂ and dissociative adsorbed O₂ are more favorable for Hg⁰ catalytic oxidation than lattice oxygen, and perpendicular adsorbed O₂ is the most active oxygen for Hg⁰ oxidation. The reaction pathway of Hg⁰ oxidation by perpendicular adsorbed O₂ includes three reaction steps: Hg⁰?→?Hg(ads)?→?HgO(ads)?→?HgO. The third step (HgO(ads)?→?HgO) is endothermic by 168.17?kJ/mol with an energy barrier of 179.48?kJ/mol, and it is the rate-limiting step of the whole Hg⁰ oxidation reaction.     

In situ oxidation study of MgO(1 0 0) supported Pd nanoparticles

We have investigated the oxidation behavior of Pd nanoparticles grown epitaxially on MgO(1 0 0) single crystal substrates. We find that the interaction of oxygen with octahedral Pd nanoparticles at 500 K can be subdivided in three stages: above 10⁻⁶ mbar O₂ pressure, the particles start to flatten; above 10⁻³ mbar, the particles begin to shrink laterally and to be less truncated at the corners. The formation of epitaxial bulk PdO sets in at oxygen pressures above 0.1 mbar, which is accompanied by a continuous shrinkage of the Pd particles. Our results point to a novel nanoparticle oxidation mechanism: the Pd particles act as dissociation centers for O₂ and serve at the same time as source for Pd atoms resulting in epitaxial PdO growth on MgO(1 0 0).     

Double layer oxidation resistant coating for carbon fiber reinforced silicon carbide matrix composites

Double layer coatings, with celsian-Y₂SiO₅ as inner layer and Y₂Si₂O₇ as outer layer, were prepared by microwave sintering on the surface of carbon fiber reinforced silicon carbide matrix composite. Both celsian, Y₂SiO₅ and Y₂Si₂O₇ were synthesized by in situ method using BAS glass, Y₂O₃ and SiO₂ as staring materials. The sintering temperature was 1500 °C, and little damage was induced to the composite. The composition and micrograph of the fired coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation and thermal shock resistance of samples with doubled-layered coating were characterized at 1400 °C in air. After 150 min oxidation and thermal cycling between 1400 °C and room temperature for 15 times, the weight loss of double layer-coated sample was 1.22% and there were no cracks in the coating.     

Supersonic laser spray of aluminium alloy on a ceramic substrate

Applying a ceramic coating onto a metallic substrate to improve its wear resistance or corrosion resistance has attracted the interest of many researchers during decades. However, only few works explore the possibility to apply a metallic layer onto a ceramic material. This work presents a novel technique to coat ceramic materials with metals: the supersonic laser spraying.In this technique a laser beam is focused on the surface of the precursor metal in such a way that the metal is transformed to the liquid state in the beam-metal interaction zone. A supersonic jet expels the molten material and propels it to the surface of the ceramic substrate.In this study, we present the preliminary results obtained using the supersonic laser spray to coat a commercial cordierite ceramic plate with an Al-Cu alloy using a 3.5 kW CO₂ laser and a supersonic jet of Argon.Coatings were characterized by scanning electron microscopy (SEM) and interferometric profilometry.     

Infrared induced visible emission from porous silicon: the mechanism of anodic oxidation

The visible luminescence caused by anodic oxidation of p-type porous silicon has been studied. It is shown that similar luminescence can be observed in n-type material by illumination with near-infrared light. Addition of a suitable reducing agent to the electrolyte solution can both suppress the oxidation of the porous layer and quench its luminescence. These results confirm a previously suggested mechanism, in which the capture of a valence band hole in a surface bond of the porous semiconductor gives rise to a surface state intermediate capable of thermally injecting an electron into the conduction band.     

Femtosecond laser micro-structuring of aluminium under helium

The interaction of 180 fs, 775 nm laser pulses with aluminium under a flowing stream of helium at ambient pressure have been used to study the material re-deposition, ablation rate and residual surface roughness. Threshold fluence F_th0.4 J cm⁻² and the volume ablation rate was measured to be 30<V<450 μm³ per pulse in the fluence range 1.4<F<21 J cm⁻². The presence of helium avoids gas breakdown above the substrate and leads to improved surface micro-structure by minimising surface oxidation and debris re-deposition. At 1 kHz rep. rate, with fluence F>7 J cm⁻² and >85 W cm⁻² average power density, residual thermal effects result in melt and debris formation producing poor surface micro-structure. On the contrary, surface micro-machining at low fluence F1.4 J cm⁻² with low power density, 3 W cm⁻² produces much superior surface micro-structuring with minimum melt and measured surface roughness R_a1.1±0.1 μm at a depth D50 μm. By varying the combination of fluence/scan speed during ultra-fast ablation of aluminium at 1 kHz rep. rate, results suggest that maintaining average scanned power density to <5 W cm⁻² combined with single pulse fluence <4 J cm⁻² produces near optimum micro-structuring. The debris under these conditions contains pure aluminium nanoparticles carried with the helium stream.