Nucleation and Post-Nucleation Growth in Diffusion-Controlled and Hydrodynamic Theory of Solidification

Two-step nucleation and subsequent growth processes were investigated in the framework of the single mode phase-field crystal model combined with diffusive dynamics (corresponding to colloid suspensions) and hydrodynamical density relaxation (simple liquids). It is found that independently of dynamics, nucleation starts with the formation of solid precursor clusters that consist of domains with noncrystalline ordering (ringlike projections are seen from certain angles), and regions that have amorphous structure. Using the average bond order parameter q¯6, we distinguished amorphous, medium range crystallike order (MRCO), and crystalline local orders. We show that crystallization to the stable body-centered cubic phase is preceded by the formation of a mixture of amorphous and MRCO structures. We have determined the time dependence of the phase composition of the forming solid state. We also investigated the time/size dependence of the growth rate for solidification. The bond order analysis indicates similar structural transitions during solidification in the case of diffusive and hydrodynamic density relaxation.

Формирование кремниевых нанокластеров при диспропорционировании моноокиси кремния

In this work, the processes of disproportionation of solid-phase silicon monoxide, accompanied by the formation of nanocrystalline silicon precipitates in the medium of amorphous SiOx suboxide (initial composition SiO0.9), have been studied. Based on the data of X-ray diffraction analysis and transmission electron microscopy, the dynamics of changes in the amount, concentration and size of phase precipitates of silicon with an increase in the temperature of isochronous annealing from 800 °C to 1200 °C is traced. It was found that with a monotonic increase in the total mass of the precipitated silicon, the number of its crystallization centers per unit volume nonmonotonically depends on temperature. The activation energy of diffusion of silicon atoms in the SiOx matrix was determined to be Ea1= 1.64 eV, and the activation energy of their transfer from the formed precipitates to the growth medium of SiOx was Ea2 = 2.38 eV. Anisotropic deformation of silicon crystallites precipitated during the disproportionation of SiO has been revealed for the first time. This phenomenon is associated with the difference in the specific volumes of the separated phases and the anisotropy of the growth rate of silicon precipitates formed in a solid amorphous medium.

О возможном механизме внешнего инфразвукового механического стимулирования процесса формирования нанокристаллов в аморфной металлической пленке

A kinetic model, a physical reason and a condition for stimulated by external infrasonic mechanical vibrations of the formation of nanocrystals in an amorphous metal film. The nanostructural elements of the amorphous medium are responsible for these processes: locally ordered nanoclusters and nanoregions containing free volume, which contain two-level systems. When glass is deformed, two-level systems are excited, due to which they make a significant contribution to inelastic deformation, structural relaxation, formation of nanoclusters and nanocrystals. The physical mechanism of nanocrystallization of metallic glass during mechanical exposure includes, in addition to the mechanism of local thermal fluctuations, also athermal mechanism of quantum tunneling of atoms or atomic groups stimulated by inelastic deformation.

Recognition of the Crystal Structure Clusters in Fast-Cooled Amorphous Medium

A series of computer experiments using the molecular dynamics method, imitating the processes of the argon particles system cooling at different cooling rates, was carried out. As a result of modeling, solid phases of varying degrees of order were obtained, a comparative analysis of structures was performed using the calculation of the radial distribution functions of particles and using the author’s algorithm for recognizing clusters of a crystal structure.

Enabling random lasing in an ultrabroad spectral range with robust platforms based on amorphous media

An amorphous medium is proposed to serve as the base of alternative strongly scattering centers for random lasing.