Investigation of Metallic and Ceramic Materials by Small-Angle Neutron Scattering

Small-angle neutron scattering measurements on a double-crystal spectrometer with perfect monochromator and analyzer crystals were used to follow microstructural changes in the aluminum alloy VD-17, refractory alloy ZhS-6, and dispersion-hardened zirconia-based ceramics with yttria additions. The measurements were carried out by the Warren method, involving determination of the small-angle neutron attenuation coefficient. The experimentally determined total macroscopic small-angle scattering cross section is correlated with different parameters characterizing external influences on the material: the number of loading cycles, heat-treatment temperature, and fracture stress. The potential of the proposed approach for optimizing processing technologies is discussed.     

Surface Activation of Diamond Particles in the Process of Chemical Coppering

The effect of the composition of sensitizing–activating solutions on the activation efficiency and the chemical coppering of diamond powders is studied. The results demonstrate that stabilizing additives improve the stability of the solutions, influence the average coppering rate, and play a key role in determining the morphology and quality of the resultant coatings. The most effective stabilizing additive is synthamid-10, which ensures the deposition of high-quality copper coatings.     

On energy arguments applied to the hydrodynamic impact force

The hydrodynamic impact problem is investigated within the framework of potential-flow theory. The vertical load acting on the rigid body is derived based on either momentum or energy conservation, and using the concept of added mass together with a homogeneous Dirichlet condition for the potential on the free surface as usually done to model an impact problem. It is demonstrated that the use of this simplified dynamic free-surface condition, instead of the fully nonlinear one, has a direct influence on the computation of the loads. In particular, the equivalence of momentum and energy analysis is in general not recovered. The situation is then highlighted by performing an asymptotic analysis of the two-dimensional blunt-body asymmetric impact problem. The asymptotic solution is given explicitly and validated through comparisons with experimental results. The energy distribution is then studied. It is shown that the contradiction between momentum and energy analysis can be removed, provided that the flux of energy through the jets is taken into account in the energy balance. If the simplified free-surface condition is indeed valid in the far-field, nonlinear terms must be retained near the body, in the spray-root domains. To leading order, the energy distribution during the gravity-free inertia stage does not depend on the blunt-body shape. The general analysis based on momentum or energy conservation suggests that this result also applies for arbitrary body shape as soon as a homogeneous Dirichlet condition can be applied as a dynamical free-surface boundary condition. In this case, and for a constant vertical impact velocity, half the work performed by the body would seem to be transferred to the fluid as kinetic energy within the spray.     

Modelling of Diffuse Interfaces with Temperature Gradients

The work is devoted to capillary phenomena in miscible liquids under the assumption that they have a constant and the same density. The model consists of the heat equation, diffusion equation, and the Navier-Stokes equations with the Korteweg stress. We study several configurations corresponding to the microgravity experiments planned for the International Space Station. The basic conclusion of the numerical simulations is that transient capillary phenomena in miscible liquids exist and can produce convective flows sufficiently strong to be observed experimentally. In particular, there exists a miscible analogue to the Marangoni convection where the temperature gradient is applied along the transition zone between two fluids. Convection also appears if, instead of the temperature gradient, the case where the width of the transition zone varies in space is considered. Finally, similar to the immiscible case, miscible drops move in a temperature gradient.     

Corner flow in free liquid films

A lubrication-flow model for a free film in a corner is presented. The model, written in the hyperbolic coordinate system ξ = x ² −y ², η = 2xy, applies to films that are thin in the η-direction. The lubrication approximation yields two coupled evolution equations for the film thickness and the velocity field which, to lowest order, describes plug flow in the hyperbolic coordinates. A free film in a corner evolving under surface tension and gravity is investigated. The rate of thinning of a free film is compared to that of a film evolving over a solid substrate. Viscous shear and normal stresses are both captured in the model and are computed for the entire flow domain. It is shown that normal stress dominates over shear stress in the far field, while shear stress dominates close to the corner. This revised version was published online in October 2005 with corrected page numbers.     

Ugtracold Alkali Atoms in Optical Lattices: A New Type of Quantum Crystals?

Similarities between alkali gases in optical lattices with non-integer occupation of the lattice sites and quantum crystals are explored. The analogy with the vacancy liquid provides an agternative explanation to the Mott transition for the recent experiment on the phase transition in the lattice. The vacancy liquid can undergo BEC with T _c within experimental reach. Direct and vacancy-assisted mechanisms of the band motion for hyperfine impurities are discussed. The presence of vacancies can resugt in the spatial decomposition of the system into pure hyperfine components. Below BEC for the vacancies, the impurity component resembles ³He in ³He-Hell mixtures.     

The reaction of ceria coatings on mica with H2S: An in-situ X-ray diffraction study

Thin layers of ceria were deposited on the surface of mica platelets in solution. The reaction of such particles with hydrogen sulfide yields a red colored special effect pigment. The ceria layer reacts with H₂S to produce a variety of sulfide and oxysulfide phases. The reaction path discovered in situ by time and temperature resolved X-ray diffraction is CeO₂→CeS₂→C-Ce₂S₃→Ce₁₀S₁₄O. The reaction itself is extremely variable depending on gas flow, heating rates and decomposition atmospheres. Effects on the thin film are recorded by scanning electron microscopy (SEM) and revealed a destruction of the layer once red Ce₁₀S₁₄O was formed. The product layer then reveals the typical nonwetting behaviour of a liquid on a surface.     

On the role of high-energy electrons in the formation of a plasma-beam discharge structure in a diode with a slit cathode

The spatial structure of visible emission from plasma of a transverse nanosecond pulsed electric discharge in a gas-filled diode with a hollow (slit) cathode has been experimentally studied. A relation is established between the regime of electron energy relaxation and the plasma-beam discharge structure formation. High values of the electron emission coefficient are observed in the experiment, which cannot be explained using the notion of electron emission from a cathode bombarded by fast heavy particles.     

High-strain-rate superplasticity of nanocrystalline aluminum alloy 1570

The structure and mechanical properties of nanocrystalline aluminum alloy 1570 obtained by means of severe plastic deformation have been studied. Being tested in a temperature range from 300 to 400°C, the alloy exhibits high-strain-rate superplasticity. At 400°C, the superplasticity is manifested in a very broad range of strain rates, extending from 5 × 10^?3 to 1 s^?1.     

Studying chaotic microwave oscillations in a TWO with depressed collector

Broadband chaotic oscillations were monitored in a depressed potential collector of a traveling-wave oscillator (TWO). This signal appears due to the oscillations of a virtual cathode, which is formed in the spent electron beam under the action of retarding fields existing in the multistage depressed collector.