N. B. Volkov

Simulation of generation of ultradisperse particles upon irradiation of metals by a high-power electron beam

A model of formation of ultradisperse particles in the plasma torch emerging during evaporation of a metal target by a high-power electron beam is described. A model of heterogeneous media is proposed for describing the plasma torch dynamics taking into account heat conduction, heat transfer and friction between components, relaxation of components to equilibrium, condensation, and evaporation and coagulation of drops as a result of their collisions. Numerical simulation of the generation of ultradisperse particles in the plasma torch formed during irradiation of a metal target by a powerful electron beam is performed. The size distribution of ultradisperse particles is obtained for various regimes of irradiation and cooling.

Size effect in nanopowder compaction

The process of nanopowder compaction has been studied by the method of granular dynamics. The interaction of particles involves the elastic Hertz forces, tangential friction forces, and dispersive attraction forces. The curves of uniaxial compaction on the “axial pressure-compact density” plane are analyzed. Calculations have been performed for powders with particle sizes from 10 to 100 nm. It is shown that the allowance for dispersive attraction forces ensures adequate description of the size effect in nanopowder compaction.

On the Mechanism of Cratering on Solid Surfaces Exposed to an Intense Charged Particle Beam

A physical mechanism of cratering on the surface of solid targets exposed to intense charged particle fluxes is suggested. According to this mechanism, the craters form due to surface gravitational waves and Richtmyer-Meshkov instability of the plasma torch free surface [1–4]. The crater sizes and shapes predicted theoretically agree well with experimental observations. It is shown that the stresses arising in the target are the highest under the crater, which explains the localization of structure modification observed in the experiments.

Simulation of nanopowder compaction in terms of granular dynamics

The uniaxial compaction of nanopowders is simulated using the granular dynamics in the 2D geometry. The initial arrangement of particles is represented by (i) a layer of particles executing Brownian motion (isotropic structures) and (ii) particles falling in the gravity field (anisotropic structures). The influence of size effects and the size of a model cell on the properties of the structures are studied. The compaction of the model cell is simulated with regard to Hertz elastic forces between particles, Cattaneo-Mindlin-Deresiewicz shear friction forces, and van der Waals-Hamaker dispersion forces of attraction. Computation is performed for monodisperse powders with particle sizes ranging from 10 to 400 nm and for “cohesionless” powder, in which attractive forces are absent. It is shown that taking into account dispersion forces makes it possible to simulate the size effect in the nanopowder compaction: the compressibility of the nanopowder drops as the particles get finer. The mean coordination number and the axial and lateral pressures in the powder systems are found, and the effect of the density and isotropy of the initial structure on the compressibility is analyzed. The applicability of well-known Rumpf’s formula for the size effect is discussed.

Simulation of radial pulsed magnetic compaction of a granulated medium in a quasi-static approximation

Compression adiabats for alumina-based nanopowders are obtained experimentally, various conditions of pulsed magnetic cylindrically symmetric radial compaction of the nanopowders are tested, and the density distribution in the compacted powders are measured. Using the compression adiabats obtained, quasi-static compaction of a granulated (porous) medium, which is considered to be compact, is simulated. The conditions of uniform and equilibrium compaction on a rigid rod are analyzed. The voidage distribution, stress tensor, and amount of accumulated deformation are calculated. The features of nanopowder compaction, specifically, the presence (absence) of voidage nonuniform radial distribution, are explained.

The peculiarities of uniaxial quasistatic compaction of oxide nanopowders

The processes of the compaction of alumina-based nanopowders are studied experimentally and theoretically. The effect that size has on the compaction process (the density decrease with a decrease in particle size) is investigated. The processes investigated are modeled in terms of the granular-dynamics method. It is shown that accounting for the disperse attraction forces between the powder particles makes it possible to achieve a qualitative agreement with the experimental data. In terms of the theoretical model, the process of multiloading the powder body up to the necessary level by axial pressure is analyzed. It was found that, in the latter case, it becomes possible to substantially increase the final density of the compact.

Mechanisms of metallic nanoparticle generation during an electric explosion of conductors

The mechanisms of conductor fracture and the generation of metallic nanoparticles during an electric explosion are discussed. The fracture of polycrystalline conductors with a crystallite size smaller than 100 nm during rapid introduction of energy is shown to occur due to its localization at grain boundaries. The dynamics of the explosion products (droplets, vapor) flying into a buffer gas is numerically simulated in terms of the mechanics of heterogeneous media with allowance for condensation and evaporation. The calculated size distributions of particles agree with the experimental distributions both qualitatively and quantitatively.

The formation and structure of current cells in a vacuum arc cathode spot

A physical model describing the structurization and localization of electric current in the surface layer of a cathode in the initial stage of a vacuum arc discharge is proposed. According to this model, each current cell on the cathode surface represents a dislocation cell containing a ring electron vortex with the axis perpendicular to the electrode surface. It is shown that localization of the electric current at the vortex center leads to the formation of a spatial structure of hot spots acting as precursors of the explosive emission centers, the dynamics of which determines operation of the vacuum discharge.

The nonlinear dynamics of the interface between media possessing different densities and symmetries

A time-saving method is proposed for the study of a nonlinear vortexless stage of a Rayleigh-Taylor (RT) instability development at the free surface of a heavy liquid or a Richtmyer-Meshkov (RM) instability at the interface between continuous media possessing different densities and symmetries. The efficacy of the proposed method is demonstrated by the results of a special numerical experiment. It is shown that instability development at the solid-plasma interface gives rise to stresses in the solid.

Thermocapillary convection in a target irradiated by an intense charged particle beam

A mathematical model is proposed for the heat-and-mass transfer in a target irradiated by an intense charged particle beam. It includes mechanics of continua equations and a kinetic equation for fast particles that are closed by a wide-range equation of state. A method for solving the model equations, which is based on the division of motion into vortex and potential flows, is proposed, and a numerical experiment is performed. Thermocapillary convection is shown to be the main mechanism of liquid-phase mixing in the target. Convective mixing is found to be effected when the pulse duration is much shorter than the characteristic thermal diffusivity time. Thermocapillary convection is shown to provide mixing on scales of 1–20 μm depending on the irradiation conditions.