AFM investigations of self-assembled DOPA–melanin nano-aggregates

The formation of longitudinal, fibril-like structures upon drop drying of the synthetic DOPA–melanin colloidal suspension was investigated. Synthetic DOPA–melanin comprises mainly molecular species derived from the coupling of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid monomers that aggregate forming granules. AFM images have shown that for different substrates (mica, glass, gold), melanin granules have a tendency to form aggregates on nanometer-size scale, which in turn self-assemble into fibril-like structures. The deposits are about 250 nm in width and 50 nm in height. The formation of observed deposits has been explained assuming Van der Waals interactions between stacked planar aromatic structures present in melanin protomolecules. These results can support the recent stacked oligomer model for eumelanin protomolecules and provide experimental evidence for formation of organic self-assembled nano-aggregates of melanin polymer.     

Incorporation of Copper in SiBEA Zeolite as Isolated Lattice Mononuclear Cu(II) Species and its Role in Selective Catalytic Reduction of NO by Ethanol

The Cu _x SiBEA zeolites prepared by a two-step postsynthesis method are active in selective catalytic reduction of NO by ethanol and their catalytic activity is related to the presence of mononuclear Cu(II) species. Zeolite with isolated lattice mononuclear Cu(II) species is active in this reaction with selectivity toward dinitrogen close to 70–90%.     

Is work function a surface or a bulk property?

We discuss whether the electronic work function (WF) can be considered as a surface property of a given conductor or a bulk property as we have demonstrated in the past that WF is a fundamental surface property which can be derived solely from bulk properties like the Wigner-Seitz radius and Fermi energy. The term “bulk” applies already to a very thin layer at the surface, where several monolayers (ML) is usually a sufficient thickness to go beyond the quantum size effects and into the bulk properties regime. In the case of large-size atoms like K, Ba, Ca, Th, etc. even 1 ML shows WF of the bulk material. At the same time, WF is an environment-sensitive property subject to large variations due to the sorption-desorption phenomena in the vacuum atmosphere. Our considerations are supported by a number of experimental results.     

A model for interphase precipitation in V-microalloyed structural steels

A model for interphase precipitation, with a predictive capacity, is presented. This article deals with its application to V-microalloyed steels. The model rests on an analysis of the growth of the V-depleted zone ahead of a sheet of V(C,N) particles and the simultaneous advance of the γ/α interface in which it was nucleated. It is shown that volume diffusion of V cannot explain the observed intersheet spacings and that a faster diffusion process is required. It is postulated that the γ/α boundary will bow out some time after a sheet of V(C,N) particles has formed in it. Part of the V in the γ will then be fed to V(C,N) particles in the sheet by boundary diffusion as the γ transforms to α. The V content at the front will, thus, be lower than the initial content in the austenite. However, the reduction will be less the further the interface has moved away from the sheet of V(C,N) particles. At a sufficient distance, the V content is again high enough to allow new V(C,N) particles to nucleate, and a new sheet of particles will form. Between the two sheets, there will be a ledge (or superledge) that will advance along the first sheet. The height of the ledge will, thus, be determined by the distance in which V(C,N) particles can again be nucleated. The model exhibits reasonably good agreement with observed values of intersheet spacing, with its temperature dependence and transition from interphase to general precipitation, and with its dependence on C, V, and N content. It also provides physically sound explanations of these dependencies.     

Regioselectivity in acid- or base-catalysed acetalation of sucrose: selection of [OH-2, OH-3] or [OH-4, OH-6] diols

Regioselective monoacetalation of sucrose can be achieved without prior protection. Under acidic catalysis, and notably with the use of lanthanide-exchanged cation resins as heterogeneous catalysts, sucrose monoacetals of α,β-unsaturated carbonyl compounds involving OH-4 and OH-6 are obtained. On the other hand, the reaction of unprotected sucrose with a α-chloromethyl ketone in the presence of base provided a β-hydroxymethyl 5-membered ring acetal involving OH-2 and OH-3 as the major product, illustrating the pre-eminent reactivity of OH-2 in sucrose and its consequences on the product distribution for reactions under kinetic control. This revised version was published online in June 2006 with corrections to the Cover Date.     

The microstructure and plasticity of Ni-AI-Fe alloys

Although intermetallic compounds form a very important group of materials, their properties have been insufficiently investigated yet. The nonstoichiometric intermetallic NiA1 compound is mentioned among useful constructive materials for high temperature applications. Many attempts to increase its poor ductility were undertaken by micro- and macroalloying, most promising results were reported for the Fe addition. The present investigation was carried out to determine the influence of the Fe and small titanium and boron additions on the phase composition, microstructure and mechanical chracteristic, particularly with respect to the feasibility to high temperature deformation on Ni-AI-Fe-Ti-B alloys. The Ni-A1-Fe-Ti-B alloys, containing 35.8 at.% A1 and 3,6-8,6-17.6 at.% Fe were prepared from high purity components and Al master alloy containing Ti2B particles, by melting in a Balzers furnace and casting. After the homogenization in the β-phase temperature range and long-time annealing at 1023 K, high temperature uniaxial compression test were performed. The sequence of structural changes has been correlated with mechanical characteristics of high temperature deformation process, determined in uniaxial compression tests. Two ranges of work hardening were identified on the stress-strain curves. It has been found that the first range of the deformation corresponds to intergranular slip system operating within individual grains, while the second one is connected with transgranular slip.     

Maximal loads acting on legs of powered roof support unit in longwalls with bumping hazards

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Variational setting for reversible and irreversible fluids with heat flow

This paper derives and discusses variational formulations for heat flows subject to physical constraints that involve the (generally) non-conserved balance of internal energy and the entropy representation kinetics in the form of the Cattaneo equation of heat. Another approach is also outlined which uses the (generally) non-conserved balance of the entropy and the energy-representation counterpart of the Cattaneo equation called Kaliski’s equation. Results of nonequilibrium statistical mechanics (Grad’s theory) lead to nonequilibrium corrections to entropy and energy of the fluid in terms of the nonequilibrium density distribution function, f. These results also yield coefficients of the wave model of heat such as: relaxation time, propagation speed and thermal inertia. With these data a quadratic Lagrangian and a variational principle of Hamilton’s type follows for a fluid with heat flux in the field representation of fluid motion. For an irreversible heat transfer we show that despite of generally non-canonical form of the matter tensor the coefficients in source terms of the variational conservation laws can be suitably adjusted, so that physical (source-less and canonical) conservation laws are obtained for the energy and momentum. We discuss canonical and generalized conservation laws and show the satisfaction of the second law under the constraint of canonical conservation laws.