An X-ray photoelectron spectroscopy study of the chemical changes in oxide and hydroxide surfaces induced by Ar+ ion bombardment

In the course of X-ray photoemission studies on the oxidation of metals and alloys and on bulk oxides, we found that in addition to physical sputtering Ar⁺ ion bombardment can in many cases reduce an oxide to a mixture of the original oxide, lower oxides and metal. The effect is even more pronounced in systems containing hydroxyl groups which are readily destroyed by the ion beam. Specific examples for oxidized cobalt, nickel and iron surfaces and their bulk oxides and hydroxides are given. The relative reduction rates of Co^II and Fe^III in CoFe₂O₄ are also examined. From these observations, it is clear that any depth compositional profiling using ion sputtering in conjunction with Auger or X-ray photoelectron spectroscopy should be treated with extreme caution. The mechanism for the chemical changes induced by ion bombardment is briefly discussed.     

Methodology for analysing the degradation of Mg-PSZ

For expanded applications and ease of manufacture, joining ceramics to other ceramic and metal components is a subject of intense interest, especially for heat engine applications. Magnesia partially stabilized zirconia (Mg-PSZ) is one possible material for various desired applications, due to its toughness and thermal and mechanical shock resistance. During processing of the join and during the lifetime of the ceramic component, thermal and chemical potential gradients are expected to cause complex reactions at the zirconia-metal interface. Particularly important reactions are the oxidation of the metal-joining agents and their diffusion/migration into the ceramic. Because of the small spatial scales of both the complex reactions and the interface, identifying mechanisms of degradation due to particular metals or metal oxides would be difficult. This research focuses on a methodology to identify whether the reaction of the metal oxides with Mg-PSZ would cause degradation. The methodology for investigating these reactions of Mg-PSZ to oxidized metals was developed by adapting a conventional metallurgical technique known as the temperature-time-transformation diagram. The metals selected for investigation were copper, tin and zinc (typical brazing agents), titanium and aluminium (reactive metals), and cobalt and nickel (super alloys and typical interlayer metals). To model the reaction at the interface layer, oxides of the metals were mixed with Mg-PSZ powder and its effect on precipitation analysed. All metal oxides accelerated the precipitation rate of the tetragonal phase, thereby shifting the nose of the temperature-time-transformation diagram to shorter times as compared to undoped Mg-PSZ. Additionally, these oxides enhanced growth of the monoclinic phase with increasing time at temperature.     

Silazane derived ceramics and related materials

This review highlights the synthesis, processing and properties of non-oxide silicon-based ceramic materials derived from silazanes and polysilazanes. A comprehensive summary of the preparation of precursor compounds containing Si–N–Si units, including commercially available materials, is followed by the discussion of various processing techniques. The fabrication of dense bulk ceramics in the Si/E/C/N systems is reported which involves cross-linking of the polymeric ceramic precursor followed by a polymer-to-ceramic transformation step. The cross-linked precursor can be milled, compacted and pyrolysed to form dense, additive-free, amorphous silicon carbonitride monoliths or polycrystalline composites which withstand oxidation in air at 1600°C. Furthermore, an overview is given on the fabrication of silazane derived powders and coatings involving chemical vapour deposition (CVD) methods utilising volatile precursors. Fibre spinning and fibre properties, as well as other processing techniques like infiltration of preforms, the preparation of porous ceramics and joining are briefly discussed. A state of the art of the mechanical properties of polymer derived amorphous Si/C/N and Si/B/C/N ceramics with respect to hardness as well as high-temperature creep and oxidation resistance is summarised. Finally, some important aspects of industrial applications will be considered. The review is in part based on our own work related to the polysilazane derived ceramics, but will also cover a comprehensive state of the art including the published literature in this field.     

Ceramic-metal reaction welding

In bonding ceramic oxides to metals, two reaction mechanisms have been utilized. These are (1) a surface or microscopic reaction, between close-packed ceramic oxides, quartz, and glasses and the noble metals (e.g. Pt, Pd, Au, Ag), maintaining a sharp discontinuity at the interface, (2) a bulk or macroscopic reaction between the same oxides and other transition metals (e.g. Fe, Co, Ni) producing a diffusional type interface. Both have common reaction conditions and occur: (a) below the melting point of the lowest melting component, (b) in any atmosphere compatible with all components at the operating temperature, (c) under little or no pressure, and (d) without deformation.The progress of several Type 1 reactions has been followed by direct observation at elevated temperatures and high resolution in a modified transmission electron microscope. It has thereby been established that this reaction proceeds with the formation of an intermediate phase, liquid-like at temperatures below the melting point of any of the components.Bonds have also been examined using electron scanning microscopy, electron probe microanalysis, and optical microscopy. Vacuum tight bonds can be produced and shear tests indicate that strength is generally limited by the strength of oxide components.     

Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: a review

Catalytic wet air oxidation (CWAO) is one of the most economical and environmental-friendly advanced oxidation process. It makes a promising technology for the treatment of refractory organic pollutants in industrial wastewaters. Various heterogeneous catalysts including noble metals and metal oxides have been extensively studied to enhance the efficiency of CWAO. The present review is concerned about the literatures published in this regard. Phenolics, carboxylic acids, and nitrogen-containing compounds were taken as model pollutants in most cases, and noble metals such as Ru, Rh, Pd, Ir, and Pt as well as oxides of Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Ce were applied as heterogeneous catalysts. Reports on their characterization and catalytic performances for the CWAO of aqueous pollutants are reviewed. Discussions are also made on the reaction mechanisms and kinetics proposed for heterogeneous CWAO and also on the typical catalyst deactivations in heterogeneous CWAO, i.e. carbonaceous deposits and metal leaching.     

Electrophoretic deposition of alumina,yttria, yttrium aluminium garnet and lutetium aluminium garnet

The electrophoretic deposition (EPD) method has been adapted for the deposition of ceramic green bodies from aqueous nanodispersions of alumina, yttria, yttrium aluminium garnet and lutetium aluminium garnet. These materials have been selected, since they are promising candidates for optically transparent ceramics. Films as well as cylindrical bodies have been successfully prepared by application of pulsed direct current (pDC) EPD. To guarantee constant deposition yield during pDC, a variant with variable pulse widths and pulse heights has been developed. The obtained green bodies were studied by surface area analysis, scanning electron microscopy, optical transmittance measurements, determination of pycnometric density and sintering behaviour. The effect of colloid-chemical dispersion properties on green and sintered ceramics is discussed as well. The green ceramics received are nanoporous and dense, providing excellent properties for further processing under mild conditions to optical materials. For comparison, EPD-formed green bodies were either processed directly to ceramic bodies or after additional compacting by hot-pressing in a piston-cylinder apparatus.     

New high-temperature materials and methods of investigating their thermophysical properties at temperatures up to 3500° C

Summary To summarize, we may conclude that the various new high-temperature materials such as refractory metals and their alloys, metal borides, carbides, nitrides, and silicides, oxides, refractory coatings, and heat shields, can be studied by the methods of high-temperature thermophysics.     

The dependence of catalytic activity of cobalt and copper oxides in the soot oxidation reaction on the method of adding the catalysts

The paper addresses the kinetics of air oxidation of soot in the presence of copper and cobalt oxides which were added in the form of metal acetates or nitrates. The activity of oxides is assessed by the catalytic constants for only the catalytic reaction picked out from the overall process. The oxides derived from nitrates are more active, and the activity ratio between the copper and cobalt oxides depends on the precursor from which they were produced.     

Preparation of Al_3Sc Intermetallic Compound by FFC Method

The FFC Cambridge process is a direct electrodeoxidation process used to reduce metal oxides for metals or alloys in molten salts.Al-Sc compound oxides are used as a precursor which formed upon blending and sintering Al2O3,Sc2O3 and Al powders and are successfully reduced by using the FFC Cambridge process at 973 K with a constant cell voltage of-3.2 V.This method is applied to the preparation of fine Al3Sc particles, which can give another new view for aluminum industry.     

Impregnated carbon as a susceptor material for low loss oxides in dielectric heating

Dielectric heating of materials may have distinct advantages over conventional heating in view of the fact that it is a bulk heating technique, and does not rely on conduction or convection, the driving force of which are temperature gradients. Dielectric heating is currently predominantly practised in food and ceramic processing. Research is conducted also in the areas of chemistry and in solid catalysed reactions. Base materials used in these applications, like food containers, catalysts and green ceramic products, may not demonstrate sufficiently high dielectric loss factors to allow dielectric heating to be feasible. In such cases microwave susceptor materials may be added. A whole variety of such susceptor materials are known, such as zirconia, silicon carbide and carbon. In this work, the impregnation with carbon was investigated as a way to enhance the dielectric loss factor for low loss porous oxidic materials. To this end a variety of porous oxides was impregnated with poly furfuryl alcohol which was subsequently carbonised. Impregnation conditions, dielectric heating behaviour, calculations on dielectric properties and oxidation resistance will be reported.     

Oxidation of chromia forming molybdenum-tungsten based alloys

The oxidation behaviour of tungsten and molybdenum based, chromia-forming alloys prepared by powder sintering activated with group VIII metals has been investigated. The influences of the alloy composition, nature of the sintering agent and oxidation temperature have been studied. A good oxidation resistance is observed with palladium as sintering agent. This metal is rejected at the grain boundaries and allows a fast diffusion of chromium to the metal-oxide interface. Contrary to palladium, nickel leads to a catastrophic oxidation of the sample. The formation of a two-phase interface enriched in nickel leads to a non-protective oxide layer constituted with Cr₂O₃ and NiWO₄.Catastrophic oxidation is observed when the refractory metals are oxidised into volatile oxides, i.e. in the case of the alloys with a high molybdenum content. Contrary to molybdenum, a high tungsten level leads to high oxidation resistance, even at temperature as high as 1300°C. In this latter case, alloys are two-phase: this result has led to the investigation of the ternary section of the Cr-Mo-W system at 1300°C.     

Metal Immiscibility Route to Synthesis of Ultrathin Carbides,Borides, and Nitrides

Ultrathin ceramic coatings are of high interest as protective coatings from aviation to biomedical applications. Here, a generic approach of making scalable ultrathin transition metal‐carbide/boride/nitride using immiscibility of two metals is demonstrated. Ultrathin tantalum carbide, nitride, and boride are grown using chemical vapor deposition by heating a tantalum‐copper bilayer with corresponding precursor (C₂H₂, B powder, and NH₃). The ultrathin crystals are found on the copper surface (opposite of the metal–metal junction). A detailed microscopy analysis followed by density functional theory based calculation demonstrates the migration mechanism, where Ta atoms prefer to stay in clusters in the Cu matrix. These ultrathin materials have good interface attachment with Cu, improving the scratch resistance and oxidation resistance of Cu. This metal–metal immiscibility system can be extended to other metals to synthesize metal carbide, boride, and nitride coatings.     

Perovskite-type catalytic materials for environmental applications

Abstract

Perovskites are mixed-metal oxides that are attracting much scientific and application interest owing to their low price, adaptability, and thermal stability, which often depend on bulk and surface characteristics. These materials have been extensively explored for their catalytic, electrical, magnetic, and optical properties. They are promising candidates for the photocatalytic splitting of water and have also been extensively studied for environmental catalysis applications. Oxygen and cation non-stoichiometry can be tailored in a large number of perovskite compositions to achieve the desired catalytic activity, including multifunctional catalytic properties. Despite the extensive uses, the commercial success for this class of perovskite-based catalytic materials has not been achieved for vehicle exhaust emission control or for many other environmental applications. With recent advances in synthesis techniques, including the preparation of supported perovskites, and increasing understanding of promoted substitute perovskite-type materials, there is a growing interest in applied studies of perovskite-type catalytic materials. We have studied a number of perovskites based on Co, Mn, Ru, and Fe and their substituted compositions for their catalytic activity in terms of diesel soot oxidation, three-way catalysis, N₂O decomposition, low-temperature CO oxidation, oxidation of volatile organic compounds, etc. The enhanced catalytic activity of these materials is attributed mainly to their altered redox properties, the promotional effect of co-ions, and the increased exposure of catalytically active transition metals in certain preparations. The recent lowering of sulfur content in fuel and concerns over the cost and availability of precious metals are responsible for renewed interest in perovskite-type catalysts for environmental applications.     

Perovskite-type catalytic materials for environmental applications

Perovskites are mixed-metal oxides that are attracting much scientific and application interest owing to their low price, adaptability, and thermal stability, which often depend on bulk and surface characteristics. These materials have been extensively explored for their catalytic, electrical, magnetic, and optical properties. They are promising candidates for the photocatalytic splitting of water and have also been extensively studied for environmental catalysis applications. Oxygen and cation non-stoichiometry can be tailored in a large number of perovskite compositions to achieve the desired catalytic activity, including multifunctional catalytic properties. Despite the extensive uses, the commercial success for this class of perovskite-based catalytic materials has not been achieved for vehicle exhaust emission control or for many other environmental applications. With recent advances in synthesis techniques, including the preparation of supported perovskites, and increasing understanding of promoted substitute perovskite-type materials, there is a growing interest in applied studies of perovskite-type catalytic materials. We have studied a number of perovskites based on Co, Mn, Ru, and Fe and their substituted compositions for their catalytic activity in terms of diesel soot oxidation, three-way catalysis, N₂O decomposition, low-temperature CO oxidation, oxidation of volatile organic compounds, etc. The enhanced catalytic activity of these materials is attributed mainly to their altered redox properties, the promotional effect of co-ions, and the increased exposure of catalytically active transition metals in certain preparations. The recent lowering of sulfur content in fuel and concerns over the cost and availability of precious metals are responsible for renewed interest in perovskite-type catalysts for environmental applications.     

Catalytic Activity of Metal Ions in Redox Processes in Polymer–Salt Systems during Synthesis of Mixed Oxides

Data are presented on the catalytic activity of metal ions in the oxidative destruction of poly(vinyl alcohol) in polymer–salt (nitrate) systems. The results indicate that the catalytic activity is due to the coexistence of oxidized and reduced forms of metals and that the oxidation potential for the conversion from one oxidation state to another must be high enough for catalysis. To assess catalytic activity, one can also compare the Gibbs energies of redox reactions involving oxides. Metal ions exhibit catalytic activity in solutions (of iron nitrate) and films (of molybdenum, tungsten, and silver salts) and also during the pyrolysis of polymer–salt systems in the synthesis of mixed oxides or in the presence of silver nitrate.     

The Auger analysis of contaminants that influence the thermocompression bonding of gold

Thin layers of metallic impurities and hydrocarbon residues on the surface of gold-plated headers and lead frames were found to degrade the mechanical strength of thermocompression bonds used for attaching wire leads. Any contamination of the lead wire or of the gold on the header or lead frame can prevent intimate gold-to-gold contact and result in unreliable bonding. These impurities can codeposit from solution during plating and can then segregate to the surface during thermal processing or can diffuse through the gold from the back side. At the surface, the metals oxidize and act as barriers to contact between the gold wire and the plated metal surface.Such impurities are often present in submonolayer quantities which preclude detection by most analytical techniques other than Auger and X-ray photoelectron spectroscopy (XPS). Both these analytical techniques can be used to study the surface composition to a depth of three or four monolayers with a bulk sensitivity of about 0.1 at.%. Surface concentrations of Fe, Ni, Cu, Ag, Si and other metals were observed in the 1–5 at.% range. This is attributed to the fact that, for example, 100 ppm Ni in the bulk of an Au film 5000 Å thick represents 10 at.% if it diffuses to the surface. Recent results indicate that the composition of the plating bath as determined by emission spectroscopy can be sorrelated with the surface composition and subsequently the thermocompression bondability. This information can be used to establish the maximum tolerable impurity levels in the Au plating bath.     

Relationship between activation energies for the thermal oxidation of metals and the semiconductivity of the oxides

The activation energies for the thermal oxidation of metals appear to be inversely related to the band gaps, heats of formation per equivalent and the electronic conductivity of the oxides formed. An attempt is made to develop the theoretical significance of these correlations in the context of a general discussion of some fundamental aspects of the thermal and the anodic oxidation of metals.     

Oxide layers on III–V compound semiconductors

Results of an investigation of the thermal and anodic oxidation of the III–V compound semiconductors are presented. First the growth models for the thermal oxidation of metals and alloys are reviewed. From these, a model for the growth of thermal oxides on the III–V compounds is proposed. It is shown that the diffusion rates of the elements, the mutual solubilities of the mixed oxides and the thermodynamic stabilities of the oxides are important factors in the oxide growth. Auger compositional profiles of the thermal oxides show that some oxides partition into two layers while others are nearly uniform. The insulating properties of these oxides are shown to be related to the chemical composition. The Auger profiles of the anodic oxides are contrasted with those of the thermal oxides.     

Sintering mechanism for high-density NZP ceramics

We have studied the shrinkage kinetics and mechanism of NaZr₂(PO₄)₃ containing inorganic additives. The sintering of NaZr₂(PO₄)₃ containing ZnO microadditives is shown to follow a liquid-phase mechanism. Effective sintering aids include oxides of metals in the oxidation states ²⁺ and ³⁺ that are capable of reacting with sodium zirconium phosphate to form solid solutions. Ceramics having a relative density of 96–99% and isostructural with NaZr₂(PO₄)₃ can be prepared by adding 0.75–2.0 wt % ZnO as a sintering aid capable of influencing the structure of grain boundaries in ceramic materials, pressing green bodies at 200–300 MPa, and sintering them at 1000–1100°C for 7–15 h.     

Protective coatings for carbon fibers

Protective refractory nanocoatings on carbon bundles and tapes have been investigated. Processes have been developed for growing thin layers of refractory oxides—alumina, zirconia, and silica—on continuous carbon fibers and tapes using sol-gel processing. ZrC/ZrO₂ bilayer coatings have been produced by chemical vapor transport. The surface morphology, phase composition, and elemental composition of the coatings have been studied by x-ray diffraction, high-resolution scanning electron microscopy, and qualitative energy-dispersive x-ray microanalysis. The results demonstrate that the refractory oxide coatings are homogeneous along the length and perimeter of individual fibers and adhere well to the fiber surface, with no peeling. Their thickness is within 200–300 nm. The effect of the nature of the coating on the oxidation resistance of carbon materials is analyzed.