Characterization of various aluminium oxide layers by means of spectroscopic ellipsometry

This paper reports the characterization of both barrier type and porous type anodic oxide films on aluminium by means of spectroscopic ellipsometry (SE). In order to show the capabilities of the technique for quantitative determination of the layer characteristics, results based on ellipsometric data are correlated with complementary information from the analytical techniques transmission electron microscopy (TEM) and Auger electron spectroscopy (AES). It is concluded that ellipsometry yields an accurate characterization for the thicknesses and the interfacial properties of both the barrier layer and the porous layer. The porosity of the porous layer, determined with SE, is found to be in good agreement with the results obtained from TEM.     

Evolution of microcrystalline growth pattern by ultraviolet spectroscopic ellipsometry on Si:H films prepared by Hot-Wire CVD

Ultra-violet spectroscopic ellipsometry has been applied successfully to determine the evolution of μc-Si:H film structure, including incubation layer, bulk layer and the growth zone and surface over-layer, their individual thickness and composition. In view of the availability of a significantly high atomic H density in H₂-diluted SiH₄ ensemble in Hot-Wire CVD, microcrystallization seems to be achieved easily at a low substrate temperature. Atomic H induced etching at the growing network has been identified as instrumental in controlling the microcrystallization in Si:H. Sharp elimination of the incubation layer as well as surface roughness and significant improvement in the overall crystallinity was obtained on increase in H₂ dilution to SiH₄. A H₂ dilution limited sharp transition from an amorphous dominated (a+μc)-Si mixed phase to a virtually amorphous free μc-Si phase has been identified. However, enhanced atomic H reactivity at the growth zone beyond the attainment of the amorphous free bulk and the surface layer induces porosity in the network and a gradual deviation from crystallinity.     

The aza-Wittig reaction: an efficient tool for the construction of carbon-nitrogen double bonds

Recent advances in the aza-Wittig reaction of phosphazene derivatives with several carbonyl compounds are reviewed. Phosphazenes afford inter- and intramolecular aza-Wittig reactions with different compounds such as aldehydes, ketones, esters, thioesters, amides, anhydrides and sulfimides. One of the most important applications of this reaction is the synthesis of a wide range of acyclic and heterocyclic compounds, ranging from simple monocyclic compounds to complex polycyclic and macrocyclic systems.     

The formation of glassy alloys by irradiation with intense proton beams

Using a 50 ns pulse of an intense proton beam 1.5 J cm^–2 of energy was deposited in a 1 m thick surface layer of glass forming alloys. In Fe₈₀B₂₀, the formation of a glassy surface layer of 1.9 m thickness was observed by x-ray diffraction. Etching experiments performed with alloys containing phosphorus yielded similar results. Applying a mask technique amorphous and crystalline zones were structured with a resolution of better than 2 m.On leave from Central Electronics Engineering Research Institute, Pilani, Rajasthan, India     

Atomistic simulation on the site preference and mechanical properties of Th3Co4+xAl12−x and U3Co4+xAl12−x

An atomistic study is presented on the phase stability, site preference and lattice constants of the actinide intermetallic compounds Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x. Calculations are based on a series of interatomic pair potentials related to the actinides and transition metals, which are obtained by a strict lattice inversion method. The lattice constants of Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x are calculated for different values of x. The site preference of Co atoms at Al sites is also evaluated and the order is given as 6h, 4f, 2b and 12k for Th₃Co_4+xAl_12−x, and 6h, 4f, 12k and 2b for U₃Co_4+xAl_12−x. In addition, some simple mechanical properties such as the elastic constants and bulk modulus are investigated for the actinide compounds with complex structures.     

Corrosion of stainless steels in chloride solution: An XPS investigation of passive films

Five commercial steels ranging from the martensitic stainless steel containing 12% chromium to the superferrite containing 29% chromium, 4% molybdenum, and 2% nickel have been studied by XPS. In addition, a pure iron-chromium alloy containing 7% chromium has been investigated. Armco iron and pure chromium (99.99%) were included as references. The formation of the passive films (or corrosion) occurred in deoxygenated 0.1 M NaCl solution (pH=5.6), from which the samples were transferred directly to the XPS chamber under controlled atmosphere (Ar). Concentration profiles (at.-%) of the alloy constituents in their oxidized and metallic states have been determined separately from the measured XPS depth profiles. Forc= 12% chromium the passive films have the following structure: there is a depletion of Cr in the inner region, followed by an enrichment (concentration maximum) in the central region of the films. The height of this maximum increases, and its position shifts towards the surface with increasing chromium content in the alloy. The outermost monolayers are rich in water and hydroxyl groups. Various significant properties of the films change drastically at the critical chromium concentration of about 12%. This behaviour is rather independent of the other components (Mo, Ni, Cu) present in the alloys and is discussed in terms of a phase transition in the films which is controlled by the chromium concentration.     

Acylpyrazolone ligands: Synthesis, structures, metal coordination chemistry and applications

This review summarizes the literature on 4-acyl-5-pyrazolone ligands, their synthesis, characterization and coordination chemistry toward main group, transition, lanthanide and actinide metals and relevant applications of their metal complexes.     

Nucleophilic substitution at phosphorus: stereochemistry and mechanisms

This review is devoted to the stereochemistry of nucleophilic substitution reactions at phosphorus. The study of the reactions of phosphoryl group transfer is important for biological and molecular chemistry. The stereochemistry and mechanisms of S_N1(P) monomolecular and S_N2(P) bimolecular nucleophilic substitution reactions of organophosphorus compounds are discussed. It has been shown that hydrolysis of many natural phosphates proceeds according to the monomolecular S_N1(P) mechanism via the formation of metaphosphate intermediate (PO₃^?). S_N2(P) nucleophilic substitution at chiral trivalent or pentavalent phosphorus compounds proceeds via the formation of penta-coordinated transition state or pentacoordinate intermediate.     

A model of binary assemblies of spheres

We propose a theoretical model of random binary assemblies of spheres at any packing fraction. We use the notion of geometrical neighborhood between grains that is defined through two generalizations of the Vorono? tessellation: the radical (or Laguerre) tessellation and the navigation map. The model is tested on different numerical packings. We find a weak local segregation for high packing fraction. We also find that the higher the size ratio of the particles, the more important the segregation. Received 19 February 2001 and Received in final form 27 June 2001     

Icosahedral and dense random cluster packing in metallic glass structures

It has recently been shown that metallic glass structures can be idealized as inter-penetrating solute-centered atomic clusters that are packed with essentially periodic symmetry. The present work applies the same methodology to explore whether experimental observations can be matched by inter-connected solute-centered clusters that are organized in space via dense random cluster packing, Bergman icosahedral cluster packing or Mackay icosahedral cluster packing. Idealized partial pair distribution functions are developed where the symmetry of the solute positions in the structure is derived from the cluster-packing symmetry and the solute concentration, which establishes occupation of inter-cluster sites, especially β structural sites enclosed by an octahedron of solute-centered clusters. While each of the three models matches major features of the measured solute-solute partial pair distribution functions, the arrangement of clusters with Mackay icosahedral ordering provides the best fit. However, this model is not able to match an essential feature in solute-lean glasses and does not provide the same overall agreement as does periodic cluster packing for solute-rich glasses. Strong similarities between the structure factors in the Mackay icosahedral and periodic cluster-packing models, along with expected deviations from the idealized solute positions studied here, are likely to hinder an unambiguous distinction between these two models.