Jürn W. P. Schmelzer

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.

Homogeneous nucleation versus glass transition temperature of silicate glasses

Abstract This paper provides experimental and theoretical evidence for a correlation between the maximum internal nucleation rate, Imax=I(Tmax) [where Tmax is the temperature of maximum nucleation rate] and the reduced glass transition temperature, Tgr, for 51 glass-forming liquids. In addition, it demonstrates an analogous correlation between Tmax, the time-lag of nucleation at Tmax and the reduced glass transition temperature. An explanation is given for these remarkable trends.

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.

Thermodynamics of Heterogeneous Systems

As it was already mentioned in chapter 1 the following premises of thermodynamic nature are underlying classical nucleation theory, a number of its modifications and generalizations; first, the existence of a critical cluster size rc; second, the possibility to express the work of formation of such critical clusters by

Nucleation versus spinodal decomposition in confined binary solutions.

W = \frac{1}{3}\sigma {A_c}\quad {A_c} = 4\;\pi \;r_c^2

Evolution of cluster size distribution in nucleation and growth processes

(2.1) where б is the surface tension for a planar interface between the both phases; third, the possibility to describe the bulk contributions to the thermodynamic properties of the cluster by the macroscopic characteristics of the corresponding phases; fourth, that the probability P of formation of the critical clusters is proportional to

Nucleation and growth of AgCl clusters in a sodium borate glass: Numerical analysis and SAXS results

P \sim \exp ( - \frac{W}{{kT}})