Influence of dispersion aluminum powders on the regularities of their interaction with nitrogen

Regularities of the interaction of widely dispersed industrial powders and electroexplosive aluminum nanopowder with nitrogen under atmospheric pressure in a linearly rising temperature and isothermal conditions have been investigated. The effect of the powder’s dispersion and composition, and the structure of the oxide shell and metal core of the aluminum particles on the temperature range of reaction and its kinetic parameters is shown. The relationship between the process of dehydration of hydroxides that are part of the superficial oxide shell of Al particles and the initial temperature of interaction between the powders and nitrogen was investigated. It is shown that the staged nature of weight gain for powders in a nitrogen atmosphere that is observed under conditions of linear heating results from a change in the macrokinetic regime, due to the effect of the Al nitride layer.     

Ellipsometric and IR spectroscopic study of the surface interaction of binary Al-REM alloys with water

The effect of Al-REM binary alloys’ composition on the kinetics of their surface oxidation by water is studied by means of ellipsometry and IR spectroscopy. Intermetallides with ～21 at % rare-earth metal in aluminum are oxidized in distilled water at 100°C, along with alloys of eutectic composition (Al-～2 at % REM) in solid and disperse state. It is found that the kinetic dependences of the thickness of films formed on pollycrystalline alloy samples and the IR transmission spectra of dispersed samples show that increasing the quantity of dopant REM in aluminum lowers the reactive capability of aluminum under certain conditions. It is found that the experimental ellipsometric results are described by a two-layer model consisting of a substrate, an inner oxide layer and an outer layer of hydroxide.     

Preparation and TG—DTA analysis of manganese silicon nitride

Manganese silicon nitride was prepared quantitatively as a precipitated phase by treating a Mn; Si-alloy (Mn: 1.84 w/o, Si: 1.12 w/o) in a mixture of 2% NH₃ and H₂ at 700°C. Nitriding was carried out in situ in a thermobalance and the nitrogen uptake was recorded as a function of time. The nitride phase was isolated and investigated by means of the combined TG-DTG-DTA technique both in an atmosphere of nitrogen at 25–1600°C and in a mixture of Ar+O₂ (p_O2 = 0.20 atm) at 25–1000°C. In the nitrogen atmosphere MnSiN₂ appears to be stable up to 1000°C. Oxidising the nitride in the Ar/O₂ mixture caused three distinct exothermic processes to occur at characteristic temperatures. The final oxidation products as identified by diffractometry and IR-spectroscopy are manganese oxide silicate (braunite) and silicon dioxide.     

Study of Aluminum–Oxygen Binary Systems: Determination of Partial Interatomic Distances

This paper reports on an Auger-electron study of the oxidation of the perfect surface of polycrystalline Al in residual gas atmosphere at a partial pressure of oxygen of 10 ^-10 torr. Under supervacuum conditions, layer-by-layer oxidation of the aluminum surface takes place along with stabilization of the chemical composition of the oxide film. Oxygen can penetrate into the oxidation region not only by deposition from the residual atmosphere but also by other mechanisms. After the chemical composition of the oxide film was stabilized, its local atomic structure was studied by Al and O K-edge extended electron energy loss fine structure (EELFS) spectroscopy. The estimated partial interatomic distances for Al–Al, Al–O, O–Al, and O–O atomic pairs permitted us to conclude that an oxide-like phase is formed in the subsurface layers of aluminum oxidized under supervacuum conditions and to construct a model of the local atomic structure of this oxide-like phase.     

Calorimetric study of the oxidation of Al–Mg alloys for the prediction of healing of the double oxide film defect

The oxidation of Al alloys containing 0.3–4.5 wt% Mg in an atmosphere with a very low oxygen partial pressure (<0.5 ppm, to depict the atmosphere within a double oxide film defect) was studied using differential scanning calorimetry and scanning electron microscopy. The results showed that a newly formed Al₂O₃ layer held in an Al–Mg melt first transformed to MgAl₂O₄ spinel and then to MgO. This mechanism was the same for all the Al alloys containing 0.3–4.5 wt% Mg, but the kinetics of the transformations were different and depended on the Mg content of the melt. The results also suggest that the two layers of a double oxide film defect that is held in an Al melt containing 0.3–4.5 wt% Mg can heal (i.e. bond to each other) if held in the liquid metal for a long enough period of time.     

XPS study of a laser-nitrided iron surface using a focused pulsed Nd:YAG laser under various conditions

Laser nitridation of a pure iron (Fe) surface was conducted using a focused pulsed Nd:YAG laser under a nitrogen atmosphere, and the effects of nitrogen gas pressure, laser power, and repetition number of laser shots on the surface characteristics were analyzed using XPS. The laser-irradiated surface consisted of the topmost surface layer of Fe oxynitride (FeO_xN_y) and the underlayer beneath, which mainly comprised Fe nitride (Fe₄N). The topmost surface layer is a post-formed layer due to the oxidation of the nitride layer. The thickness of the underlayer corresponding to the original nitride layer drastically increased under nitrogen gas at atmospheric pressure. Increasing the repetition number of laser shots enhanced layer thickness up to 5 shots, after which no change was observed. Moreover, the layer thickness increased monotonically with increasing laser power. Nitridation through pulsed laser irradiation was likely predominated by the melting and resolidification of a specific surface area, as well as the convection of nitrogen therein. Thickness variation under various conditions can be explained appropriately using this assumption.     

Oxygen scavenging enhances exothermic behavior of aluminum-fueled energetic composites

The theoretical heat of combustion for aluminum (Al) and copper oxide (CuO) under ideal conditions is roughly 4,100 kJ kg^?1, but in practice only a fraction of this energy is available for application. Approaching the theoretical value requires innovative strategies for exploiting reaction kinetics. One strategy is to force Al to react CuO as opposed to oxygen from the environment and this approach was shown here to improve the measured heat of combustion significantly. This was achieved through tailoring the mixture using an additive with oxygen affinity prior to Al ignition. The additive is hafnium hydride (HfH₂) and this study examines reaction kinetics for HfH₂ 0–60 mass% concentration range using simultaneous thermal-gravimetric analysis. Results reveal that 0–10 mass% HfH₂ additive enhanced the heat of combustion of Al+CuO by 12 %. The observed enhancement is attributed to HfH₂ creating oxidation competitions that directly influence the combustion performance of Al+CuO. The transition of this equilibrium-based observation to a non-equilibrium fast heating setup is also investigated using plate deformation tests. Exploding bridge wire heads initiated by a capacitive discharge unit are used to dent plates and the deformation energies are estimated to observe the effects of varying composites in a non-equilibrium setting. The presence of HfH₂ showed improved deformation energies for Al+CuO in the 10 % mass HfH₂ range. This work introduces a novel method of using a catalyst in thermite composites to enhance combustive behaviors through oxygen scavenging. Using the same concept as getter materials, the atmosphere is scavenged of free oxygen allowing for the fuel to react exclusively with the solid oxidizer.     

Formation and dielectric properties of anodic oxide films on Zr–Al alloys

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO₂ with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO₂ phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.     

Morphological development and oxidation mechanisms of aluminum nitride whiskers

Hexagonal aluminum nitride (AlN) whiskers have been synthesized at 1873 K under a flowing nitrogen atmosphere. The synthesized whiskers are long straight filaments with diameters between 1 and 5 μm and length in the cm range. In order to investigate its “oxidation resistance”, a series of experiments have been performed. The oxidation behavior was quite different in the experimental temperature range assigned, which can be attributed to the kinetic factor and the morphological development during oxidation process. It was chemical controlled at lower temperature while both chemical reaction and diffusion controlled at medium temperature. Further accelerating of temperature to 1473 K, AlN whiskers was peeled into smaller parts, which increased the oxidation rate and hence showed powder-like oxidation behavior. Our new kinetic theory has been applied to study the oxidation behavior of AlN whiskers. The comparison of the experimental data with the theoretical ones validates the applicability of the new model.     

Characterization of transition metal nitride formation in rapid thermal processing (RTP)

The potential of RTP for the preparation of transition metal nitrides by reaction of metal thin films in molecular nitrogen was investigated. The films and the nitridation process were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive x-ray analysis (EDX) in a scanning electron microscope (SEM) and secondary neutral mass spectrometry (SNMS). The chemical states of vanadium at the utmost surface, detected by XPS, are related to V₂O₅ before RTP and to vanadium nitride, oxide and oxynitride after RTP. The deposition of a 3 nm Si top layer prevents V from oxidation and its selective removal before RTP enhances the proportion of nitride determined by XPS after RTP. From comparative experiments in a conventional tube furnace the advantages of RTP became obvious. With short process times of the RTP technique the integral amount of residual oxygen is kept low and oxide formation is largely avoided. The nitrogen content and the different polycrystalline phases formed by varying process time and temperature provide information about reactivity and the nitridation process. The nitrogen to vanadium ratio was determined by EDX and SNMS, revealing that the N content reaches saturation after only 5 seconds at 1100?°C.     

Cathodic electrochemiluminescence at double barrier Al/Al2O3/Al/Al2O3 tunnel emission electrodes

Double insulating barrier tunnel emission electrodes were fabricated by adding a new pure aluminum layer upon oxidized aluminum electrodes by vacuum evaporation and thermally oxidizing the new aluminum layer in air at room temperature. Resulting Al/Al₂O₃/Al/Al₂O₃ electrodes allow the use of various aluminum alloys in the electrode body necessary for hardness or shaping ability of the electrode while obtaining the luminescence properties of pure aluminum oxide. During electrical excitation of luminescent labels by cathodic hot electron injection into aqueous electrolyte solution, the background noise is mainly based on high-field-induced solid-state electroluminescence and F-center luminescence of the outer aluminum oxide film. The more defect states and/or impurity centers the outer oxide film contains, the higher is the background emission intensity. The present electrode fabrication method provides a considerable improvement in signal-to-noise ratio for time-resolved electrochemiluminescence (TR-ECL) measurements when the original native oxide film of the electrode body contains luminescence centers displaying long-lived luminescence. The excellent performance of the present electrodes is demonstrated by extremely low-level detection of Tb(III) chelates, luminol, Pt(II) coproporphyrin and Tb(III) labels in an immunometric immunoassay by time-resolved electrochemiluminescence.     

Nitridation of niobium oxide films by rapid thermal processing

The possibility of forming niobium oxynitride through the nitridation of niobium oxide films in molecular nitrogen by rapid thermal processing (RTP) was investigated. Niobium films 200 and 500 nm thick were deposited via sputtering onto Si(100) wafers covered with a thermally grown SiO₂ layer 100 nm thick. These as-deposited films exhibited distinct texture effects. They were processed in two steps using an RTP system. The as-deposited niobium films were first oxidized under an oxygen atmosphere at 450 °C for various periods of time and subsequently nitridated under a nitrogen atmosphere at temperatures ranging from 600 to 1000 °C for 1 min. Investigations of the oxidized films showed that samples where the start of niobium pentoxide formation was detected at the surface and the film bulk still consisted of a substoichiometric NbO_x phase exhibited distinctly lower surface roughness and microcrack densities than samples where complete oxidation of the film to Nb₂O₅ had occurred. The niobium oxide phases formed at the Nb/substrate interface also showed distinct texture. Zones of niobium oxide phases like NbO and NbO₂, which did not exist in the initial oxidized films, were formed during the nitridation. This is attributed to a “snow-plough effect” produced by the diffusion of nitrogen into the film, which pushes the oxygen deeper into the film bulk. These oxide phases, in particular the NbO₂ zone, act as barriers to the in-diffusion of nitrogen and also inhibit the outdiffusion of oxygen from the SiO₂ substrate layer. Nitridation of the partially oxidized niobium films in molecular nitrogen leads to the formation of various niobium oxide and nitride phases, but no indication of niobium oxynitride formation was found. Figure Schematic representation of the phase distribution in 200 nm Nb film on SiO₂/Si substrate after two steps annealing using an RTP system. The plot below represents the SIMS depth profiles of the nitridated sample with the phase assignment     

Surface analysis of nitride layers formed on Fe‐based alloys through plasma nitride process

Nitriding phenomena that occur on the surfaces of pure Fe and Fe? Cr alloy (16 wt% Cr) samples were investigated. An Ar + N₂ mixture‐gas glow‐discharge plasma was used so that surface nitriding could occur on a clean surface etched by Ar⁺ ion sputtering. In addition, the metal substrates were kept at a low temperature to suppress the diffusion of nitrogen. These plasma‐nitriding conditions enabled us to characterize the surface reaction between nitrogen radicals and the metal substrates. The emission characteristics of the band heads of the nitrogen molecule ion (N₂⁺) and nitrogen molecule from the glow‐discharge plasma suggest that the active nitrogen molecule is probably the major nitriding reactant. AES and angle‐resolved XPS were used to characterize the thickness of the nitride layer and the concentration of elements and chemical species in the nitride layer. The thickness of the nitride layer did not depend on the metal substrate type. An oxide layer with a thickness of a few nanometers was formed on the top of the nitride layer during the nitriding process. The oxide layer consisted of several species of N_x‐Fe_y‐O, NO⁺, and NO₂^?. In the Fe? Cr alloy sample, these oxide species could be reduced because chromium is preferentially nitrided. Copyright © 2009 John Wiley & Sons, Ltd.     

Adsorption of nitrogen on aluminum, Al2O3, and AlN powders at 78 K

The adsorption of nitrogen on aluminum powders of ASD-4 and UDA grades, aluminum oxide (α-Al₂O₃), and aluminum nitride is studied at 78 K in the adsorbate relative pressure range (P _a /P ₀) of 0 to 1. It is shown that the nature of the chemical bonds and the structure and state of the adsorbent determine the share of the adsorption isotherms and their attribution to a particular type of isotherms. With an increase in the fraction of metal bonds in an adsorbent, the isotherms become more convex, indicating enhancement of the effect of lateral interactions. The specific surfaces of samples are calculated.     

Electrodeposition of aluminum–hafnium alloy from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride molten salt

The electrochemistry of Hf(IV) and the electrodeposition of Al–Hf alloys were examined in the Lewis acidic 66.7–33.3 mol% aluminum chloride-1-ethyl-3-methylimidazolium chloride molten salt containing HfCl₄. When cyclic staircase voltammetry was carried out at a platinum disk electrode in this melt, the deposition and stripping waves for Al shifted to negative and positive potentials, respectively, suggesting that aluminum stripping is more difficult due to the formation of Al–Hf alloys. Al–Hf alloy electrodeposits containing ~13 at.% Hf were obtained on Cu rotating wire and cylinder electrodes. The Hf content in the Al–Hf alloy deposits depended on the HfCl₄ concentration in the melt, the electrodeposition temperature, and the applied current density. The deposits were composed of dense crystals and were completely chloride-free. The chloride-induced pitting corrosion potential of the resulting Al–Hf alloys was approximately +0.30 V against pure aluminum when the Hf content was above 10 at.%.     

Composition and Structure of Anodic Oxide Films on Titanium–Aluminum Alloys by Fast Electron Reflection Diffraction, Rutherford Backscattering, and Secondary Neutral Particle Mass Spectrometry

Anodic oxide films on some Ti–Al alloys are studied using fast-electron reflection diffraction, Rutherford backscattering, and secondary neutral-particle mass spectrometry. The films are amorphous, with a small amount of crystalline phases, and comprise a mixture of TiO₂ and Al₂O₃. The Ti/Al ratio in an anodic film corresponds to that in the alloy matrix. Constants of anodic oxidation of the alloys are determined.     

The anodic oxidation of gold + palladium alloys

The anodic oxidation of Au+Pd alloys has been studied in solutions of 1 M H₂SO₄ and 0.1 M K₂SO₄ by voltammetric methods. A linear relationship between oxide reduction maximum and bulk alloy composition, often used to determine the surface composition of homogeneous alloys, could be shown to hold only for alloys up to 60 at% gold. At higher gold content the Au oxide peak must be additionally evaluated. With continuous cycling in acid solution the anodic dissolution of Pd, especially from gold-rich places, leads to a rather heterogeneous surface. The O--chemisorption is not governed by a transfer mechanism from Pd to Au surface atoms. The alloys are able to absorb the oxygen species generated in the positive potential region; however, this ability decreases with increase of the gold content.     

Use of rare earth metal salt solutions for corrosion protection of aluminum alloys and mild steel

The rare earth metal salt (REMS) compounds which are non-toxic and inexpensive have been used successfully for the corrosion protection of commercial aluminum alloys as well as of mild steel. The resistance to localized corrosion of aluminum alloys such as Al 2024, Al 6061 and Al 7075 has been greatly improved by immersion in hot cerium salt solutions. Cerium oxides/hydroxides were formed at sites where intermetallic compounds containing copper were located and eliminated the formation of local cathodes. REMS solutions were also used for sealing of anodized layer on Al alloys replacing toxic chromates. In addition to increased corrosion resistance excellent paint adhesion was observed. Cerium nitrate and yttrium sulfate solutions produced the most satisfactory results. Cerium salts were also applied successfully as inhibitors and as pretreatment for the corrosion protection of carbon steel in hot NH₃/water solutions used in absorption heat pumps. Factorial design experiments have been used to determine the optimum concentrations of CeCl₃ and H₂O₂ as well as the treatment time used in the cerating process for mild steel. The corrosion protection provided by the cerated layer was further improved by cathodic polarization in cerium chloride. A dual corrosion protection strategy employing cerating and addition of a REMS to the working solution is expected to provide long-lasting corrosion protection of mild steel. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth. This article was submitted by the author in English.     

Oxidation Behaviour of A Boron Carbide Based Material in Dry and Wet Oxygen

The oxidation behaviour of a B₄C based material was investigated in a dry atmosphere O₂(20 vol.%)-CO₂(5 vol.%)-He and also in the presence of moisture H₂O (2.3 vol%) as boron oxide is very sensitive to water vapour. The mass changes of samples consisting of a chemical vapour deposit of B₄C on silicon nitride substrates were continuously monitored in the range 500–1000°C during isothermal experiments of 20 h. The stability of boron oxide formed by oxidation of B₄C was also studied in dry and wet atmospheres to explain the kinetic curves. In both atmospheres, oxidation is diffusion controlled at 700 and 800°C and enhanced by water vapour. At 900°C and higher temperatures, boron oxide volatilisation and consumption by reaction with water vapour modifies the properties of the oxide film and the material is no more protected. At 600°C, B₄C oxidation is weak but the process remains diffusion controlled in dry conditions as boron oxide volatilisation is negligible. However, in the presence of water vapour, B₂O₃ consumption rate is significant and mass losses corresponding to this consumption and to the combustion of the excess carbon are observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fe2O3/Al2O3催化氧化苯偶姻制备苯偶酰

考察了几种常用载体负载金属氧化物催化分子氧氧化苯偶姻制备苯偶酰的性能 ，发现氧化铁、三氧化二铝催化活性较高，稳定性较好。以吡啶为溶剂，用483K下 活化的含铁14．8％的氧化铁／三氧化二铝作催化剂，当其用量为苯偶姻用量20％ （质量分数），253K下反应1h,苯偶酰平均产率98．1％。用IR，MS，和^1H NMR光 谱对其结构进行了表征。