Surface composition of the steel powders pre-alloyed with manganese

The determination of the surface oxide layer composition is vital to facilitate the adjustment of the sintering conditions for sufficient removal of the surface oxides for providing strong metal bonding between the metal particles during sintering. To systematically investigate the composition, morphology and thickness of the surface oxide the influence of manganese content from 0.3 to 1.8 wt.% on the surface products composition in the case of water atomized steel powder was evaluated. Analysis of the powder surfaces by X-ray photoelectron spectroscopy and high-resolution scanning electron microscopy in combination with X-ray microanalysis showed that powder particles in all cases are covered by heterogeneous oxide layer, composed of particulate features of thermodynamically stable oxides (Cr-Mn-Si) and homogeneous iron surface oxide layer in between. For increasing alloying content the fraction of stable oxide cations in the surface layer increases linearly, whereas the thickness of the iron oxide layer decreases. Moreover, from the investigation of the sintering and degassing behavior by thermal analysis coupled with mass-spectrometry (TG/DTA + MS), three different stages of carbothermal reduction process were observed and their correlation with surface oxides composition was established during sintering in argon.     

A model for the initiation of reaction sites during the uranium–hydrogen reaction assuming enhanced hydrogen transport through thin areas of surface oxide

A model for the initiation of hydride sites on uranium metal is described for conditions of constant hydrogen pressure. The model considers variations in hydrogen permeation through the surface oxide film due to intrinsic variations in the oxide thickness. It is proposed that thin areas of surface oxide favour enhanced hydrogen permeation through the oxide and lead to the more rapid initiation of hydride sites. The time and spatial dependence of the hydrogen concentration field in the metal underlying thin areas of oxide is calculated in terms of the local oxide film thickness, the hydrogen diffusion coefficients in the oxide and metal and the hydrogen concentration in the oxide at the gas–oxide interface. The time to precipitate hydride at any location is calculated by assuming that precipitation occurs once the hydrogen concentration in the metal attains the terminal solubility limit of the metal at the prevalent temperature. The model is compatible with the reported temperature and pressure dependence of the hydride induction time. The model can also explain observations such as precipitation of hydride at or beneath the oxide–metal interface and the arrested growth of hydride sites. Finally, an expression is derived for the number of hydride sites initiated on an entire sample surface in any given time by assuming a Gaussian oxide film thickness distribution over the entire sample surface.     

Supercritical isopropanol as a reducing agent for inorganic oxides

Preparative methods for the reduction of simple and complex metal oxides by supercritical isopropanol (SCI) were developed. Procedures for effective work with SCI under usual laboratory conditions were suggested. Optimum reaction conditions (temperature and pressure) and reagent ratios for reactions between SCI and metal oxides were found. Disperse oxides coated by fine-dispersity metals that could be used as catalysts were prepared. Simple methods for obtaining metal nanoparticles by the reduction in situ of metal oxide nanoparticles stabilized in polyethylene and synthetic silica (opal) matrices with SCI were developed.     

Features of film growth during plasma anodizing of Al 2024/SiC metal matrix composite

Plasma anodizing is a novel promising process to fabricate corrosion-resistant protective films on metal matrix composites. The corrosion-resistant films were prepared by plasma anodizing on SiC reinforced aluminum matrix composite. The morphology and microstructure of films were analyzed by scanning electron microscopy. Specifically, the morphology of residual SiC reinforcement particles in the film was observed. It is found that the most SiC reinforcement particles have been molten to become silicon oxide, but a few tiny SiC particles still remain in the film close to the composite/film interface. This interface is irregular due to the hindering effect of SiC particles on the film growth. Morphology and distribution of residual SiC particles in film provide direct evidence to identify the local melt occurs in the interior of plasma anodizing film even near the composite/film interface. A model of film growth by plasma anodizing on metal matrix composites was proposed.     

Catalyst design using an inverse strategy: From mechanistic studies on inverted model catalysts to applications of oxide-coated metal nanoparticles

Metal-oxide interfaces are of great importance in catalytic applications since each material can provide a distinct functionality that is necessary for efficient catalysis in complex reaction pathways. Moreover, the synergy between two materials can yield properties that exceed the superposition of single sites. While interfaces between metals and metal oxides can play a key role in the reactivity of traditional supported catalysts, significant attention has recently been focused on using “inverted” oxide/metal catalysts to prepare catalytic interfaces with unique properties. In the inverted systems, metal surfaces or nanoparticles are covered by oxide layers ranging from submonolayer patches to continuous films with thickness at the nanometer scale. Inverse catalysts provide an alternative approach for catalyst design that emphasizes control over interfacial sites, including inverted model catalysts that provide an important tool for elucidation of mechanisms of interfacial catalytic reactions and oxide-coated metal nanoparticles that can yield improved stability, activity and selectivity for practical catalysts.This review begins by providing a summary of recent progress in the use of inverted model catalysts in surface science studies, where oxides are usually deposited onto the surface of metal single crystals under ultra-high vacuum conditions. Surface-level studies of inverse systems have yielded key insights into interfacial catalysis and facilitated active site identification for important reactions such as CO oxidation, the water-gas shift reaction, and CO₂ reduction using well-defined model systems, informing strategies for designing improved technical catalysts. We then expand the scope of inverted catalysts, using the “inverse” strategy for preparation of higher-surface area practical catalysts, chiefly through the deposition of metal oxide films or particles onto metal nanoparticles. The synthesis techniques include encapsulation of metal nanoparticles within porous oxide shells to generate core-shell type catalysts using wet chemical techniques, the application of oxide overcoat layers through atomic layer deposition or similar techniques, and spontaneous formation of metal oxide coatings from more conventional catalyst geometries under reaction or pretreatment conditions. Oxide-coated metal nanoparticles have been applied for improvement of catalyst stability, control over transport or binding to active sites, direct modification of the active site structure, and formation of bifunctional sites. Following a survey of recent studies in each of these areas, future directions of inverted catalytic systems are discussed.     

Flame synthesis of 1-D complex metal oxide nanomaterials

One dimensional (1-D) complex metal oxide nanomaterials, such as ternary oxides, doped oxides, and hierarchical structures containing several oxides, not only benefit from large aspect ratios, but also offer exciting opportunities to design materials with desired properties by tuning their chemical compositions and tailoring their sizes and morphologies at the nanometer scale. Flame synthesis is an attractive method to grow 1-D complex metal oxide nanostructures because of its high temperature, scalability, low-cost and rapid growth rate. Here, we present three new combined flame synthesis methods: (1) simultaneous vapor–vapor growth, (2) simultaneous solid–vapor growth, and (3) sequential solid–vapor growth, to grow 1-D complex metal oxide nanostructures with well-defined compositions and morphologies. These three methods combine the previously reported flame vapor deposition and solid diffusion growth methods that were separately used to grow 1-D simple binary metal oxide nanostructures, and significantly advance the capabilities of existing flame synthesis methods for the growth of 1-D nanomaterials. The first method, simultaneous vapor–vapor growth, combines the flame vapor deposition growth of two different metal oxides by oxidizing and evaporating two different metal sources. With this we have successfully grown W-doped MoO₃ nanoplates and nanoflowers. In the second method, simultaneous solid–vapor growth, one precursor is again provided by oxidizing and evaporating metal oxide from a metal, while the other precursor diffuses out from a different growth substrate. With this we have successfully grown ternary Cu₃Mo₂O₉ nanowires. The third method, sequential solid–vapor growth, essentially uses the 1-D nanostructures firstly grown by solid diffusion as the substrates for subsequent flame vapor deposition. With this we have successfully grown hierarchical CuO/MoO₃ core/shell nanowires and MoO₃-branched CuO nanowires. We believe that these three new combined flame synthesis methods will provide a general platform for the synthesis of 1-D complex metal oxide nanostructures with tailored properties.     

Comparison of the electrochemical behaviour of SnO2 and PbO2 negative electrodes for lithium ion batteries

In this paper we report our structural and electrochemical investigations of tin dioxide and lead dioxide electrodes in order to highlight the difference observed between them. The electrochemical reactions of these two oxides are known: the reduction of the metal oxide and the reversible formation/decomposition of the lithium-metal alloys. The reversible capacity of these systems is based on the alloy formation. The first reaction is supposedly irreversible (formation of Li₂O), but the X-ray diffraction analysis and especially¹¹⁹Sn Mössbauer spectrometry show a possible re-oxidation of the metal particles in the case of tin dioxide electrodes. However, this reaction is not fully reversible and occurs at a high potential vs. Li. For lead dioxide electrodes, the re-oxidation of the metal particles seems more difficult in spite of the similar structure of both oxides.     

Stress influence on high temperature oxide scale growth: Modeling and investigation on a thermal barrier coating system

In thermal barrier coating (TBC) systems, an oxide layer develops at high temperature below the ceramic coating, leading in the long term to the mechanical failure of the structure upon cooling. The mechanism of stress-affected oxidation likely to induce the growth of a non-uniform oxide scale detrimental to the TBC lifetime was investigated. A continuum thermodynamics formulation is derived accounting for the influence of the stress and strain situation at the sharp metal/oxide phase boundary on the local oxidation kinetics. It specially includes the contributions of the large volumetric strain and the mass consumption associated with metal oxidation. A continuum mechanics/mass diffusion framework is used along with the developed formulation for the interface evolution to study the growth of an oxide layer coupled with local stress development. The implementation of the model has required the development of a specific simulation tool, based on a finite element method completed with an external routine for the phase boundary propagation. Results on an electron-beam physical vapor deposited (EB-PVD) TBC case are presented. The processes resulting in a non-uniform oxide scale growth are analyzed and the main influences are discussed.     

Mechanisms of oxidation-reduction processes on metal and oxide surfaces under ion bombardment

Surface oxidation occurs if metals are bombarded with low-energy (1–5 keV) ions of a chemically active gas (oxygen) in vacuum. It is ascertained that ion bombardment leads to the generation of lower, intermediate, and higher oxides. The composition and thickness of an oxidized layer depend on the metal reactivity and the dose and energy of oxygen ions. The mechanism underlying the ion-beam oxidation of metal surfaces is proposed. Surface reduction is observed if higher oxides are bombarded with low-energy (1–5 keV) ions of inert gases (argon and helium) in vacuum. It is revealed that ion bombardment not only generates intermediate and higher oxides but sometimes gives rise to surface metallization. The composition and thickness of the reduced layer are determined by the oxide type, the kind of inert gases, and the dose and energy of bombardment. The mechanism describing the ion-beam reduction of higher metal oxide surfaces is proposed.     

Side reaction in the hydrogen and carbothermal reductions of BaO and BaCO3: The role of an infinitesimal amount of water

Alkaline earth (AE) metals are irreplaceable ingredients in the synthesis of AE metal-based antiperovskite oxides, and it can be achieved by a chemical reduction of a stable AE metal compound. In this study, hydrogen and carbothermal reduction of BaO and BaCO₃ were attempted, and we here report an undesirable side reaction creating barium hydroxide (Ba(OH)₂) as the product of the reaction with the small amount of water in ultra-high purity inert gas used in the reduction processes. Such side reaction pathways and products are hardly identifiable in a high-temperature reaction; yet, systematic investigations on phase evolutions using X-ray diffraction, IR spectroscopy, and thermogravimetric analysis enabled the detection of Ba(OH)₂·xH₂O. Unintentional creation of alkaline earth metal oxides in intermediate and subsequent hydration even under a negligible amount of H₂O may lead to an unexpected loss of alkaline earth metal element and, consequently, its deficiency in a desired final product.     

Cadmium sulphide clusters grown on zeolite and MCM-41-type matrices

Cadmium sulphide clusters were grown on mordenite and Al-MCM41 matrices by using hydrothermal synthesis. X-ray fluorescence and diffraction, Raman spectroscopy, N₂-adsorption isotherms, UV-visible and photoluminescence spectroscopies were used for sample characterisation. The results confirm the growth of CdS clusters mainly on the mordenite surface, whereas they could grow inside the mesoporous MCM-41 matrix. The final products showed approximately the same band-gap value (2.35 eV) as that calculated for bulk CdS synthesised in this work for comparison. Refractive indexes and extinction and absorption coefficients of the samples were evaluated by using a computational program. The mean values of those magnitudes and their connections with the structure, morphology and composition of the CdS-doped matrices were analysed. The relation between optical non-linearities and the growth of other species, like SiO_x, during the synthesis processes is also discussed. PACS 81.05.Dz; 81.40.Tv; 82.75.Vx     

On the use of scanning tunneling microscopy to investigate surface structure and interface formation in transition metal oxides: SrTiO3 and TiO2

Since transition metal oxides are wide bandgap, low conductivity materials compared to conventional semiconductors, surface analysis by scanning tunneling microscopy (STM) is expected to be problematic. This paper considers the factors that affect atomic scale imaging of transition metal oxides and demonstrates how STM can be exploited to examine the geometric and electronic structures of SrTiO₃ and TiO₂ surfaces, their variations with thermochemical history, and the mechanisms of metal/oxide interface formation. The development of periodic atomic scale surface structure with variations in surface compositions are documented for both oxides. Further, the interactions of these surfaces with metal are examined by characterizing the morphologies that develop upon deposition of Cu on SrTiO₃ and Al on TiO₂.     

Inverse oxide/metal catalysts: A versatile approach for activity tests and mechanistic studies

There is a general desire to improve the configuration of industrial catalysts to take advantage of the intrinsic properties of metal oxides. In recent years, a series of studies has been published examining the growth of oxide nanoparticles on metal substrates. These studies have revealed structures for the supported oxide which are different from those found in bulk phases. In addition, the oxide ? metal interactions can alter the electronic states of the oxide producing new chemical properties. On an inverse oxide/metal catalyst, the reactants can interact with defect sites of the oxide nanoparticles, metal sites, and the metal–oxide interface. In these systems, one can couple the special reactivity of the oxide nanoparticles to the reactivity of the metal to obtain high catalytic activity. Furthermore, an oxide/metal system is also an attractive model for fundamental studies. It can be used to investigate the role of the oxide in a catalytic process, and how the stability of different reaction intermediates depends on the nature of the oxide.     

Specific features of the formation of vanadium oxide micro- and nanocrystals during gas-phase synthesis

A modified method of gas-phase synthesis of vanadium oxide micro- and nanocrystals was developed. The morphology of the crystals obtained and its dependence on the synthesis conditions are analyzed. The method is shown to permit the growth of rod-shaped crystals rectangular in cross section, whiskers, crystals with a rectangular cavity, and also lamellar crystals and planar ordered textures consisting of oriented rodshaped crystals. The microcrystals synthesized were used to study the mutual reversible transformation of various vanadium oxides using oxidation and reduction reactions.     

From ceramic–matrix nanocomposites to the synthesis of carbon nanotubes

The selective reduction in H₂ of oxide solid solutions produces nanocomposite powders in which transition metal nanoparticles are dispersed inside and on the surface of the oxide matrix grains. When using a H₂/CH₄ reducing atmosphere, the metal nanoparticles that form on the surface of the oxide grains act as catalysts for the CH₄ decomposition and, because of their small size at high temperatures (>800^○C), favor the in-situ nucleation and growth of single-wall and thin multiwall carbon nanotubes. This article reviews our results on the synthesis and characterization of M-MgAl₂O₄ (M=Fe, Fe/Co, Fe/Ni) nanocomposite powders, without and with carbon nanotubes, emphasizing the information that can be derived from Mössbauer spectroscopy as a complement to other characterization techniques.     

溶胶凝胶自燃烧法合成金属与合金材料研究进展

本文综述了溶胶凝胶自燃烧法制备金属与合金材料的研究进展, 详细介绍了该方法的实验原理和技术路线, 通过实例介绍了该方法在制备金属和合金材料中的具体应用. 通过这一系列的工作介绍, 我们证实可以把传统的溶胶凝胶法制备氧化物材料的技术拓展到金属与合金材料的制备, 希望能够对材料研究的实验工作有所帮助.     

An aqueous solution approach to advanced metal oxide arrays on substrates

Materials chemistry has emerged as one of the most consistent fabrication tools for the rational delivery of high purity functional nanomaterials, engineered from molecular to microscopic scale at low cost and large scale. An overview of the major achievements and latest advances of a recently developed growth concept and low temperature aqueous synthesis method, for the fabrication of purpose-built large bandgap metal oxide semiconductor materials and oriented nano-arrays is presented. Important insight of direct relevance for semiconductor technology, optoelectronics, photovoltaics and photocatalysis for solar hydrogen generation, are revealed by in-depth investigations of the electronic structure of metal oxide nanostructures with new morphology and architecture, carried out at synchrotron radiation facilities. PACS 73.22.-f; 73.61.Ga; 73.61.Le; 73.63.Bd     

Influence of surface orientation and defects on early-stage oxidation and ultrathin oxide growth on pure copper

We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300?K and 600?K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600?K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surface defects. (110) shows modest oxidation at 300?K but the highest oxidation is observed at 600?K. By surface disorder and reconstruction, the oxide layer is produced continuously. The (111) surface is sensitive to ambient temperature and surface defects, showing that surface reconstruction is a key element for further oxidation. The charge distribution of oxidized Cu atoms indicates multiple groups of stoichiometric oxides, while the fraction of CuO-like characteristics increases significantly on the (110) and (111) orientations at higher temperature (600?K). The energetics and mechanisms of oxidation on Cu metal substrates at the nanoscale are discussed in detail, and comparisons with available experimental and other theoretical studies are presented wherever possible.