R. Pascova

Surface-induced devitrification of glasses : the influence of elastic strains

The influence of elastic strains on devitrification processes in glasses is investigated. It is known that in the vicinity and below the vitrification temperature, Tg, thermodynamic inhibition of crystallization has to be taken into account in addition to kinetic inhibition of crystallization connected with the extremely low values of the mobility (large values of the viscosity). This thermodynamic inhibition results from the evolution of elastic fields due to a difference in the specific volumes of the vitreous and crystalline states of the substance. The inhibiting term is of the same order of magnitude as the thermodynamic driving force of crystallization (and may even exceed it). It is considerably diminished for devitrification at the surface of a glass. In this way, elastic strains can be considered as the origin both for the absence of bulk devitrification and for preferential surface crystallization of glasses in systems with different specific volumes of the vitreous and crystalline states.

The kinetics of surface induced sinter crystallization and the formation of glass-ceramic materials

A thorough analysis is given of a process which is of great importance for the formation of many present day glass ceramic materials: sinter-crystallization. In the first part of the paper the problems determining surface induced nucleation of glasses are analyzed, emphasis being given to the influence of elastic strains and surface contamination by active substrates. The second stage of the analysis is centred on the dependence of crystal growth and overall crystallization kinetics on the mean size of an ensemble of sintering glass grains. Here a formalism is derived, connecting overall crystallization with the mean size of the crystallizing system of glass particles. In the third part the interdependence sintering – crystallization is investigated. Several cases of this interrelation are analyzed in details for different mechanisms of growth of nuclei, athermally formed on the grain surface.

Surface energy and structure effects on surface crystallization

Abstract The shape of a critical cluster formed at a planar surface of a solid was determined taking into account both interfacial contributions and elastic terms in the expression for the free enthalpy of cluster formation. The shape of critical clusters may change continuously from a sphere in the ambient solid tangent to the interface via a hemisphere to a circular nearly planar layer depending on the values of the specific surface energies (cluster-air, ambient phase-air and ambient phase-cluster phase). The work of formation of critical clusters formed at or near planar interfaces is significantly reduced compared with cluster formation in the bulk of the matrix. This reduction is even more significant for solids with a rough surface structure. As an application of the outlined theoretical analysis, the preferential surface crystallization of glasses, often observed experimentally, can be explained. For isoconcentration crystallization of glass-forming melts, nucleation catalysis on surfaces is caused mainly by the relative decrease of the elastic strains for crystallization at the surface compared with the bulk both for planar and rough surfaces.

Evolution of cluster size distribution in nucleation and growth processes

Abstract Based on classical nucleation theory, a Fokker-Planck-type partial differential equation for the time evolution of the cluster size distribution is formulated. The numerical solution of this equation leads to a general scenario of first order phase transitions in systems with molecule conservation proceeding via nucleation and growth. For the initial stage of nearly constant supersaturation, the proportionality of the mean cluster radius, 〈R〉, to the square root of time, t, is verified. The following transition to the stage of Ostwald ripening may proceed either by a continuous decrease of the growth rate or via a transient stage of 〈R〉-growth considerable slower than t 1 3 . The explanation of such different possibilities is given detail. The analysis shows that, for growth processes proceeding via an attachment of single monomers and non-overlapping diffusion zones of the clusters, only one population of clusters with an unimodal size distribution should evolve. The small-angle-X-ray scattering (SAXS) experiment data from the present study show, however, the existence of two cluster populations with significantly different mean radii. Additional physical assumptions are deduced from which an explanation of a bimodal size distribution is proposed. To support this general scenario, a series of experimental results from electron microscopic, SAXS and other measurements are given for the case of silver halide precipitation from glass-forming sodium borate melts. They show the time evolution of characteristic kinetic quantities such as relative supersaturation, mean cluster size and total cluster number.

Kinetics of phase segregation in elastic and viscoelastic media

Abstract A general theory of cluster growth and Ostwald ripening under the influence of elastic strain evolving as the result of cluster formation and growth is developed. This general theory can be applied to the description of cluster growth independent of the particular model applied for the calculation of the strain fields. Three models for the description of the elastic strain are discussed in detail. It is shown that the elastic response of the matrix may lead to a total inhibition of a cluster growth and Ostwald ripening, also in the absence of direct elastic interactions between the clusters both in elastic and in viscoelastic media if the mobility of the matrix particles (for viscoelastic media) is small compared with the mobility of the segregating component or equal to zero (elastic bodies). The developed theory is applied to an interpretation of experimental results on segregation processes in glass-forming melts, showing a quite different asymptotic behavior in the stage of Ostwald ripening as compared with the Lifshitz-Slyozov power laws. The agreement between the theory developed here and the experimental results is found to be good.

Arc evaporated TiN films with reduced macroparticle contamination

Abstract Through effective magnetic arc steering and optimization of the modes of reactive deposition and ion assistance, TiN films with reduced droplet contamination were produced. Arc steering was accomplished by two small-sized rotating electromagnets with low power consumption ( The maximum size and the number density of the droplets greater than 1 μm on the surface of the deposited TiN films did not exceed 4 μm and 4000 mm −2 , respectively. These characteristics make the TiN coatings obtained in the present contribution superior to TiN films produced by arc deposition methods currently used for industrial applications. Moreover, the TiN films investigated are characterized with dense structure, smooth surface, good adhesion, high microhardness and corrosion resistance. These TiN films could find successful application as wear- and corrosion-resistant coatings on cutting and forming tools.

Thermodynamics and diffusion of iron in homogeneous and phase separated sodium borosilicate melts

Abstract Voltammetric measurements were carried out at temperatures in the range of 600 to 1200°C in both homogeneous and phase separated glass melts of different compositions in the Na 2 OB 2 O 3 SiO 2 system doped with 0.5 mol% Fe 2 O 3 . At high temperatures, the current-potential curves were fairly similar to those recorded, e.g., in soda-lime-silica glasses. At temperatures below the immiscibility temperature T c , peak splitting in the curves occurs, the two different states can be attributed to iron in the silica-rich and in the borate-rich phase. Peak splitting, however, also occurs up to temperatures about 250°C higher than T c . This behaviour is interpreted by a layer adjacent to the electrode formed by a boron-rich phase.

A model investigation of the process of phase formation in photochromic glasses

The problems of phase formation and Ostwald ripening in materials behaving as viscoelastic media are considered. The current theories of phase formation and crystal growth have been developed for two limiting cases only: for Newtonian viscous fluids and for Hooke’s elastic solids. In solving the problems of phase formation and growth in real systems combining viscous and elastic properties a formal approach is used based on the relationships of phenomenological rheology and the models of viscoelastic media. The growth equations of the newly formed phase in viscoelastic systems are derived from the respective dependences in the case of a Newtonian fluid and the elastic reactions of the matrix are accounted for by introducing appropriate correction functions. The nature of these functions is determined assuming an analogy in the growth equations and the rate of deformation in viscoelastic media. The relationships thus obtained describe the kinetics of AgCl separation in photochromic glasses investigated in a previous contribution. The application of the results of the theory thus developed to other cases of phase segregation in viscoelastic systems especially in glass forming melts is discussed.

Growth kinetics of silver chloride precipitation in a Na2O-B2O3 glass investigated by means of small-angle X-ray scattering and transmission electron microscopy

The kinetics of silver chloride cluster formation in a sodium borate glass heat treated isothermally at different temperatures above the glass transition temperature has been studied with small-angle X-ray scattering (SAXS). It is established that the size distributions of particle number and particle volume have bimodal shapes, i.e. the system of AgCl droplets consists of two populations with significantly different mean radii. The bimodality of the size distribution functions is confirmed by results of transmission electron microscopy (TEM) even if different techniques of sample preparation are used. The resolution limits of the TEM techniques applied amount to 2 nm and are comparable with the smallest particle diameters detectable by the SAXS method. The evolution of the size distributions is discussed in the framework of the theories of nucleation, growth and Ostwald ripening.

Catalyzed nucleation under microgravity conditions:: results with a model glass

Abstract In the present paper results are reported on the influence of microgravity on the heterogeneous crystallization in a glass-forming melt (Grahams glass (NaPO 3 ) x ) in which nucleation is induced by Pd microcrystals. In its crystallization behavior the system resembles a model of the catalyzed nucleation used in glass-ceramics synthesis. Moreover, heterogeneous nucleation guarantees maximal reproducibility in experimentation and conditions, at which the absence of convection plays a decisive role in crystallization. The microgravity experimentation was performed on the MIR Cosmic Station and simultaneously repeated at the MUSC Laboratory in Cologne in an arrangement guaranteeing a DTA-like comparison between cosmic and terrestrial crystallization kinetics. Both series of samples crystallized were additionally examined at terrestrial conditions using different techniques. The thermodynamic, rheologic and crystallization characteristics of the NaPO 3 glass are known from previous terrestrial experiments. From the devitrification kinetics of NaPO 3 glassy samples in heating run experiments it is found that microgravity considerably enhances the induced nucleation of the glass-forming melt in the vicinity of glass transition temperature T g . This effect is determined mainly by an increased impingement rate z of the ambient-phase building units caused by an enhanced rise of the sample temperature. At microgravity conditions convection and heat transfer are reduced and this causes a steep temperature rise, viscosity decrease and thus, increased z values. In crystallization experiments at low undercoolings, where nucleation barrier and not attachment rate z is decisive, no difference is observed between terrestrial and microgravity samples.