G. Sh. Boltachev

Nucleation in superheated liquid argon–krypton solutions

We report nucleation-rate measurements in metastable liquid argon–krypton solutions at pressures of 1.0 and 1.6 MPa over a wide temperature and concentration range. These measurements were performed with the use of a superheated liquid lifetime measurement method. The experimental results are compared with the homogeneous nucleation theory data both using a macroscopic (capillary) approach and taking into account the dependence of critical bubble surface tension on interface curvature. The size effect in nucleation is considered in the framework of the Van-der-Waals, Cahn–Hilliard method. The experimental data indicate that the homogeneous nucleation theory quantitatively describes the kinetics of a first order phase transition in binary solutions of simple liquids if the size effect is taken into account and nucleation rates are J≳106 m−3 sec−1. At J≲106 m−3 sec−1 there is initiated nucleation. A diffusion spinodal of a solution is approximated. The attainable superheating temperature data are presented.

Extended version of the van der Waals capillarity theory

An extended version of the van der Waals capillarity theory describing the liquid-vapor interface in the temperature range from the triple to the critical point is suggested. A model functional of thermodynamic potential for a two-phase Lennard-Jones system taking into account the effect of the highest degree terms of gradient expansion has been constructed. The identity of the thermodynamic and the mechanical definition of Tolmans length has been proved in the framework of the adopted form of functional. The properties of nuclei of the liquid and the vapor phase are described. The paper determines: the work of formation of a nucleus, density profiles, size dependences of the surface tension, and the parameter delta in the Gibbs-Tolman-Koenig-Buff equation.

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.

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.

Analytical model of a corona discharge from a conical electrode under saturation

Exact partial solutions are found for the electric field distribution in the outer region of a stationary unipolar corona discharge from an ideal conical needle in the space-charge-limited current mode with allowance for the electric field dependence of the ion mobility. It is assumed that only the very tip of the cone is responsible for the discharge, i.e., that the ionization zone is a point. The solutions are obtained by joining the spherically symmetric potential distribution in the drift space and the self-similar potential distribution in the space-charge-free region. Such solutions are outside the framework of the conventional Deutsch approximation, according to which the space charge insignificantly influences the shape of equipotential surfaces and electric lines of force. The dependence is derived of the corona discharge saturation current on the apex angle of the conical electrode and applied potential difference. A simple analytical model is suggested that describes drift in the point-plane electrode geometry under saturation as a superposition of two exact solutions for the field potential. In terms of this model, the angular distribution of the current density over the massive plane electrode is derived, which agrees well with Warburg’s empirical law.

Effect of long-range interactions on the surface tension

The effect of long-range interactions on the surface tension at a liquid-gas interface was considered. An analytical expression for the correction to the surface tension for the cutoff of the particle interaction potential at the distance rc was derived based on a step density profile. For the Lennard-Jones fluid, this correction was calculated numerically from the results of computer simulations of the density profiles. It was established that, in the vicinity of the triple point, the correction is as great as ∼6% at the potential cutoff radius rc=6.78 molecular diameters, a quantity insensitive to the form of the density profile in the interfacial layer.

Model of corona discharge from a thin point in a space-charge-limited current regime

Exact self-similar solutions for the problem of electric field and charge density distribution in the external region of a unipolar corona discharge from a thin point emitter have been found. The obtained solutions are valid outside the framework of the traditional Deutsch approximation based on the small influence of a volume charge on the shape of equipotential surfaces. The value of a saturated discharge current is estimated assuming that the corona discharge is generated by a certain part of the point.

Compaction and elastic unloading of nanopowders under the granular dynamic method

The method of granular dynamics is used to study the quasistatic uniaxial compaction of nanopowders with a particle size from 10 nm to several hundred nanometers. The interaction of individual particles includes Hertz elastic forces, Cattaneo–Mindlin friction forces, and van der Waals dispersion forces of attraction. The influence of the model cell size on simulation results is analyzed. The curves of uniaxial compression and elastic unloading in the “axial pressure–density” coordinates are plotted. The generalization of the traditional Hertz law in the range of relatively high strains is discussed.