Stability of bars made of linear elastic materials with voids

Summary Dynamic stability of an elastic bar with voids is considered. Using the Lyapunov approach some new sufficient stability conditions are obtained and explicit expressions for the critical load are derived.     

A Constitutive Model in Finite Viscoelasticity of Particle-reinforced Rubbers

Two series of tensile relaxation tests are performed on natural rubber filled with high abrasion furnace black. To fit observations, constitutive equations are derived for the nonlinear viscoelastic behavior of a particle-reinforced elastomer. A filled rubber is modeled as a composite medium, where inclusions with low concentrations of junctions are randomly distributed in the host matrix. The inclusions are treated as equivalent networks of macromolecules, where strands can separate from temporary junctions as they are thermally agitated. The bulk medium is thought of as a permanent network of chains. Unlike conventional concepts of transient networks, the concentration of strands in inclusions is assumed to be affected by mechanical factors: under active loading, inter-chain interactions weaken and some strands that were prevented from detachment from their junctions in a stress-free compound become free to separate from the junctions in a deformed medium. Unloading strengthens interactions between macromolecules, which results in an increase in the number of permanent strands. By using the laws of thermodynamics, stress–strain relations for a particle-reinforced rubber are developed. Adjustable parameters in the constitutive equations are found by fitting the experimental data. It is demonstrated that mechanical pre-loading and annealing of specimens at an elevated temperature noticeably affect concentrations of inclusions with various activation energies for rearrangement of strands.     

Effect of bismuth nanolayers on the oriented growth of fullerene C<Subscript>60</Subscript> on an amorphous substrate

The effect of a bismuth sublayer with an effective thickness of 0.5 to 4 nm on the structure of C₆₀ fullerene films grown on amorphous substrates (silicon covered with a natural oxide layer; glass) using the quasi-closed-volume method is studied. An x-ray diffraction study of fullerene films showed that the intensity ratio between the (220) and (111) peaks depends nonmonotonically on the sublayer thickness. In the bismuth sublayer thickness range 0.5–2.0 nm, fullerene films are found to exhibit a growth texture with the 〈110〉 axis; the average crystallite size was ～20 µm. The quality of the texture can be improved by varying the fullerene growth temperature.     

Photoluminescence of Ge(Si)/Si(0 0 1) self-assembled islands in the near infra-red wavelength range

The dependence of photoluminescence spectra of structures with GeSi/Si(0 0 1) self-assembled nanoislands on growth temperature has been investigated. It was shown that the redshift of the island-related photoluminescence peak with a decrease of the growth temperature is associated with suppression of Si diffusion in the islands and an increase of Ge content in them. For the first time a photoluminescence signal from SiGe islands was observed at energies much lower than the Ge band gap. The energy position of the island-related photoluminescence peak is well described by the model of optical transition, which is indirect in real space. The photoluminescence signal at 1.55 μm from GeSi/Si(0 0 1) self-assembled islands was obtained up to room temperature.     

Oxidation of Zinc Sulfide in the System HNO3-Fe(NO3)3-H2O

The behavior of zinc sulfide in the system HNO₃-Fe(NO₃)₃-H₂O at 25, 55, and 80°C was studied using the simplex-lattice design method.     

X-ray diffraction studies of the state of components in Pt?Ce/Al2O3 catalysts

X-ray diffraction studies of supported Ce/Al₂O₃ and Pt–Ce/Al₂O₃ catalysts have shown that besides a bulk phase CeO₂ a product of the interaction with the support cerium aluminate having a perovskite structure, is usually formed. Platinum in unreduced modified samples forms an oxide and is statistically distributed in the perovskite phase of cerium aluminate. After reduction, only half of the metal is in the metallic state with a particle size of 10 Å.

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, CeO₂, - . . 10 Å.

     

Comparative study of the activity of Al?Pd and Al?Pt catalysts in deep oxidation of hydrocarbons

Atomic catalytic activity of palladium has been revealed to be higher than that of platinum in deep oxidation of methane and lower in the case of butane. According to the experimental data and the quantum-chemical calculation, the observed effects are ascribed to the different bond strength of the surface oxygen due to different reaction conditions.

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, _{Pd Pt}, . _{Pd Pt} - , .

     

Texture–Strength Properties of the Alumina–Montmorillonite Composite

The texture characteristics and strength properties of the molded alumina–montmorillonite composite are studied. The mixture is obtained by mixing suspensions of activated clay and pseudoboehmite followed by drying (at 293 and 393 K) and calcination (at 873 K). It is a promising support for cracking and hydrotreating catalysts. The changes in the specific surface area; the volumes of micro-, meso-, macropore, and medium-size pores; and the strength of samples are studied by varying the montmorillonite concentration in the composite. The addition of 20–35 wt % clay largely results in a sharp in reduction of the macropore region and, correspondingly, provides the alumina oxide average strength of the extrudates of 10–12 MN/m², which is sufficient for industrial supports. This excludes the stage of standard acidic peptization from the technological scheme of alumina production. The complex study of the porous structure of the composite by adsorption and mercury porosimetry combined with elements of percolation theory makes it possible to predict the texture–strength properties of derivative materials.     

Properties of MnSb/GaAs heterostructures

MnSb layers are deposited on GaAs(100) via the laser ablation of Mn and Sb targets. The magnetic field dependences of the transverse Kerr effect display magnetic anisotropy and magnetization jumps. Measurements of the Hall effect reveal a hysteresis loop at temperatures of up to 300 K.