I. Gutzow

Activity of substrates in the catalyzed nucleation of glass-forming melts. II. Experimental evidence

Abstract Methods for determining the nucleating activity of substrates are considered. Experimental data on the activity of nucleating catalysts in various organic and inorganic glass-forming melts are summarized and discussed in terms of the theoretical approach developed in Part I of the present investigation. The nucleation of the same melt induced by different substrates is analysed. In this respect, the data for sodium metaphosphate, lithium metaphosphate, poly(ethylene terephthalate), poly(vinyl chloride), sodium tetraborate nucleated with noble metal cores, for poly(decamethylene terephthalate), lithium disilicate, poly(ethylene) and water initiated by oxide and halide catalysts give a direct proof for the validity of the concepts, developed in Part I.

Activity of substrates in the catalyzed nucleation of glass-forming melts. I. Theory

Abstract The nucleating activity of different insoluble additives in the heterogeneous nucleation of organic and inorganic glass-forming melts is treated from a generalized thermodynamic standpoint. The activity of a nucleating catalyst is defined through the adhesion energy and calculated from the forces of cohesion in the substrate and in the overgrowing crystal. An estimate is also made of the relative contribution of structure mismatch to the bonding energy by evaluating the total energy of misfit dislocations at the deposit/substrate interface. This approach extends and specifies the limits of application of existing concepts of crystallographic matching and mismatching. By this generalized method, there is a possibility of predicting the nucleating activity of various crystallization cores in the induced crystallization of glass-forming melts by using data on the heat of sublimation (or the temperature of melting) of the respective nucleation catalyst or the value of its thermal expansion coefficient.

Nucleation and crystallization in glass-forming melts: Olds problems and new questions

Abstract A review is given of basic problems and results in the field of nucleation and crystallization in glass-forming melts. The temperature dependence of the driving force for crystallization and of the free energy of the crystal/melt interface, as well as the connection between the melt viscosity and entropy are considered in some detail. The influence of various factors on the rates of nucleation, crystal growth and overall crystallization is discussed and existing evidence for nonstationary nucleation is surveyed. A generalized kinetic criterion for vitrification is formulated accounting for the effect of nucleation nonstationarity. Finally, an approximate expression is obtained for the minimum cooling rate for glass formation.

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.

Glass formation and crystallization

Abstract A survey of the different approaches to the determination of the glass-forming ability of substances is given. A generalized criterion for glass formation, based on the kinetics of the overall crystallization process, is used to calculate the minimum cooling rate for glass formation. This criterion accounts for non-steady-state effects in nucleation and for the presence of active substances and surfactants in the melt. The classical structural criteria for glass formation are derived as limiting cases of the generalized kinetic treatment. The glass-forming ability of a number of substances (metals, halides and liquified noble gases) is discussed.

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.

Structure, thermodynamic properties and cooling rate of glasses

Abstract The concepts of the thermodynamics of irreversible processes are used in order to derive expressions describing the dependence of the structure and thermodynamic properties of glasses on cooling rate. The temperature dependence of the thermodynamic potential of a frozen-in system is calculated and estimates are made for the deviation from equilibrium in vitrified melts. The theoretical results thus obtained are compared with experimental evidence on the thermodynamic properties of typical glass-forming substances and for vitreous metallic alloys. In this way basic kinetic and thermodynamic properties of the vitrification process are determined.

Crystallization of glassforming melts under hydrostatic pressure and shear stress: Part I Crystallization catalysis under hydrostatic pressure: possibilities and limitations

This is the first part of a thorough study of the kinetics of melt crystallization under applied static pressure, P, and under shear stress. The thermodynamic and kinetic consequences of increased external pressure on nucleation rate, non-steady-state time lag, rate of crystal growth and overall crystallization kinetics in undercooled melts are analysed. Two types of undercooled liquids (with either positive or negative volume dilatation upon crystallization) are considered. Particular attention is given to the effect of pressure on the specific interface energy, σ, at the crystal/melt phase boundary. Using an appropriate thermodynamic model it is shown that for one-component systems, (∂σ/∂p)<0 is to be expected as a rule. Thus an additional decrease of the thermodynamic barrier of nucleation in pressurized melts is to be expected. However, it is also shown that the increase of melt viscosity with pressure in most cases reduces the effect of this decrease. Thus increased pressure has a limited effect as a nucleation catalyst. The possibilities in this respect are analysed and conditions under which static pressure may lead to enhanced crystallization are outlined.