Alexander S. Abyzov

Nucleation versus spinodal decomposition in confined binary solutions.

Basic features of spinodal decomposition, on one side, and nucleation, on the other side, and the transition between both mechanisms are analyzed within the framework of a generalized thermodynamic cluster model based on the generalized Gibbs approach. Hereby the clusters, representing the density or composition variations in the system, may change with time both in size and in their intensive state parameters (density and composition, for example). In the first part of the analysis, we consider phase separation processes in dependence on the initial state of the system for the case when changes of the state parameters of the ambient system due to the evolution of the clusters can be neglected as this is the case for cluster formation in an infinite system. As a next step, the effect of changes of the state parameters on cluster evolution is analyzed. Such depletion effects are of importance both for the analysis of phase formation in confined systems and for the understanding of the evolution of ensembles of clusters in large (in the limit infinite) systems. The results of the thermodynamic analysis are employed in both cases to exhibit the effect of thermodynamic constraints on the dynamics of phase separation processes.

Size and rate dependence of crystal nucleation in single tin drops by fast scanning calorimetry

The experimentally accessible degree of undercooling of single micron-sized liquid pure tin drops has been studied via differential fast scanning calorimetry. The cooling rates employed ranged from 100 to 14,000 K/s. The diameter of the investigated tin drops varied in the range from 7 to 40 μm. The influence of the drop shape on the solidification process could be eliminated due to the nearly spherical shape of the single drop upon heating and cooling and the resultant geometric stability. As a result it became possible to study the effect of both drop size and cooling rate in rapid solidification experimentally. A theoretical description of the experimental results is given by assuming the existence of two different heterogeneous nucleation mechanisms leading to crystal nucleation of the single tin drop. In agreement with the experiment these mechanisms yield a shelf-like dependence of crystal nucleation on undercooling. A dependence of crystal nucleation on the size of the tin drop was observed and is discussed in terms of the mentioned theoretical model, which can possibly also describe the nucleation for other related rapid solidification processes.

Influence of detector surface processing on detector performance

Abstract Characteristics of gamma ray semiconductor detectors essentially depend on properties of crystal surface. The status of a lateral surface influences surface leakage current of the detector, and the status of a surface, on which the contacts are made, influences properties of contacts and, thus, a volume leakage current and the highest possible bias voltage. In this connection several ways of processing the lateral surface of CdZnTe and CdTe crystals grown by a high-pressure Bridgman method were investigated: chemical etching, ion cleaning, passivation. Influence of a preliminary processing of a crystal surface on the properties of ohmic contacts is investigated. An analysis of electrophysical properties of crystals subjected to surface processing is carried out.

Thermodynamic analysis of nucleation in confined space: Generalized Gibbs approach

A general thermodynamic analysis of nucleation-growth processes in confined space in initially metastable states of the ambient phase is performed based on the generalized Gibbs approach to the description of heterogeneous systems. In particular, it is shown analytically how the parameters of critical clusters and clusters in stable equilibrium with the ambient phase depend on the volume of the system for initially fixed intensive state parameters of the ambient phase. Qualitatively, the results are shown to be similar independent on the boundary conditions employed. It is demonstrated further that the behavior of systems in confined space is directly related to the kinetics of phase transformation processes in spatially extended systems, when ensembles of clusters are formed. The results of the thermodynamic analysis of cluster formation and growth in a confined space are employed then, in particular, to the derivation of kinetic equations for the description of the process of coarsening or Ostwald ripening. In the analysis of both the nucleation in confined space and the description of Ostwald ripening, no specific assumptions concerning the equations of state of the system under consideration and the number of components both in the ambient and newly evolving phases are made. Consequently, the results are of very general nature and hold always as far as the necessary condition for the possibility of a phase transformation is fulfilled.

Crystallization of glass-forming liquids: Specific surface energy

A generalization of the Stefan-Skapski-Turnbull relation for the melt-crystal specific interfacial energy is developed in terms of the generalized Gibbs approach extending its standard formulation to thermodynamic non-equilibrium states. With respect to crystal nucleation, this relation is required in order to determine the parameters of the critical crystal clusters being a prerequisite for the computation of the work of critical cluster formation. As one of its consequences, a relation for the dependence of the specific surface energy of critical clusters on temperature and pressure is derived applicable for small and moderate deviations from liquid-crystal macroscopic equilibrium states. Employing the Stefan-Skapski-Turnbull relation, general expressions for the size and the work of formation of critical crystal clusters are formulated. The resulting expressions are much more complex as compared to the respective relations obtained via the classical Gibbs theory. Latter relations are retained as limiti...

On the theoretical description of nucleation in confined space

In a recent paper, Kozisek et al. [J. Chem. Phys. 134, 094508 (2011)] have demonstrated for four different cases of phase formation that the work of formation of critical clusters required to form in the system in some given time a first experimentally measurable cluster of the new phase depends in a logarithmic way on the volume of the system. This result was obtained based on the numerical solution of the kinetic equations describing nucleation and growth processes and the obtained in this way steady-state cluster size distributions. Here a straightforward alternative analytical interpretation of this result is proposed by computing directly the mean expectation times of formation of supercritical clusters. It is proven strictly that this result is generally independent of the kind of nucleation (homogeneous or heterogeneous) or specific realization (condensation, cavitation, crystallization, segregation, etc.) considered. It is shown that such behavior is simply a consequence of the linear dependence o...

Rapid solidification behavior of nano-sized Sn droplets embedded in the Al matrix by nanocalorimetry

Al-10Sn (wt.%) melt-spun ribbons with nano-sized Sn droplets (20–400 nm in diameter) embedded in the Al matrix and bulk Sn distributed at Al grain boundaries were prepared. Differential fast scanning calorimetry (DFSC) based on nanocalorimetry and thin film technique was successfully applied to investigate the rapid solidification behavior of the embedded nano-sized Sn droplets at cooling rates ranging from 103 to 104 K s−1. Two broad exothermic peaks were observed in the DFSC curves. They were ascribed to the solidification of nano-sized Sn droplets with various catalytic activity factors f(θ). The cooling rate dependence of undercooling of nano-sized Sn droplets has been studied experimentally. The two series of undercooling which correspond to the two exothermic peaks increase slightly with the increases of cooling rate. Furthermore, a theoretical description of the experimental DFSC curves based on classical heterogeneous nucleation theory is developed. It is performed advancing a previously developed approach by assuming a smooth dependence of the droplet mass fraction on contact angle, m(θ), with a double Gaussian distribution during the nucleation process. This modified theoretical model is believed to be relevant also for other related rapid solidification processes.

Beating Homogeneous Nucleation and Tuning Atomic Ordering in Glass-Forming Metals by Nanocalorimetry

In this paper, the amorphous Ce68Al10Cu20Co2 (atom %) alloy was in situ prepared by nanocalorimetry. The high cooling and heating rates accessible with this technique facilitate the suppression of crystallization on cooling and the identification of homogeneous nucleation. Different from the generally accepted notion that metallic glasses form just by avoiding crystallization, the role of nucleation and growth in the crystallization behavior of amorphous alloys is specified, allowing an access to the ideal metallic glass free of nuclei. Local atomic configurations are fundamentally significant to unravel the glass forming ability (GFA) and phase transitions in metallic glasses. For this reason, isothermal annealing near Tg from 0.001 s to 25,000 s following quenching becomes the strategy to tune local atomic configurations and facilitate an amorphous alloy, a mixed glassy-nanocrystalline state, and a crystalline sample successively. On the basis of the evolution of crystallization enthalpy and overall latent heat on reheating, we quantify the underlying mechanism for the isothermal nucleation and crystallization of amorphous alloys. With Johnson-Mehl-Avrami method, it is demonstrated that the coexistence of homogeneous and heterogeneous nucleation contributes to the isothermal crystallization of glass. Heterogeneous rather than homogeneous nucleation dominates the isothermal crystallization of the undercooled liquid. For the mixed glassy-nanocrystalline structure, an extraordinary kinetic stability of the residual glass is validated, which is ascribed to the denser packed interface between amorphous phase and ordered nanocrystals. Tailoring the amorphous structure by nanocalorimetry permits new insights into unraveling GFA and the mechanism that correlates local atomic configurations and phase transitions in metallic glasses.

Heterogeneous nucleation on rough surfaces: Generalized Gibbs’ approach

Heterogeneous nucleation (condensation) of liquid droplets from vapor (gas) on a defective solid surface is considered. The vapor is described by the van der Waals equation of state. The dependence of nucleating droplet parameters on droplet size is accounted for within the generalized Gibbs approach. As a surface defect, a conic void is taken. This choice allows us to simplify the analysis and at the same time to follow the main aspects of the influence of the surface roughness on the nucleation process. Similar to condensation on ideal planar surfaces, the contact angle and catalytic factor for heterogeneous nucleation on a rough surface depend on the degree of vapor overcooling. In the case of droplet formation on a hydrophilic surface of a conic void, the nucleation rate considerably increases in comparison with the condensation on a planar interface. In fact, the presence of a defect on the hydrophilic surface leads to a considerable shift of the spinodal towards lower supersaturation in comparison with heterogeneous nucleation on a planar interface. With the decrease in the void cone angle, the heterogeneous spinodal approaches the binodal, and the region of metastability is diminished at the expense of the instability region.

The cahn—hilliard equation with “frozen-in” fluctuations of mobility

Abstract Early time kinetics of a system with conserved order parameter quenched into an unstable two-phase region of the phase diagram is considered. We study the process of phase separation under the influence of random “frozen-in” inhomogencities of mobility. The combined effect of this “frozen-in” randomness and persistent thermal fluctuations on the structure factor is discussed. The maximum instability of the Cahn—Hilliard type shifts to larger wavelengths, which may be interpreted as “coarsening” even for small times.