Vladimir M. Fokin

Surface crystallization of silicate glasses : nucleation sites and kinetics

In this paper we review some pertinent research aimed at understanding surface nucleation from both qualitative and quantitative points of view. The majority of quantitative studies discuss the crystal nucleation kinetics of soda-lime-silica glasses and alkali-free silicate (cordierite, anorthite and diopside) glasses. We emphasize the kinetics of surface nucleation and consider the effects of surface quality, tips, cracks and scratches, foreign particles and surrounding atmosphere on crystallization. Related nucleation mechanisms are discussed.

Recent studies of internal and surface nucleation in silicate glasses

This article reviews recent findings on internal and surface nucleation in silicate glasses. The internal homogeneous nucleation rates sharply decrease and the induction times increase with the Tg/TL ratio (Tg is the glass–transition temperature and TL is the liquidus temperature). Only systems that have Tg/TL < 0.58 display measurable internal nucleation rates on a laboratory time–scale. Numerous tests of the classical nucleation theory have demonstrated that the theory fails to describe nucleation rates in glasses quantitatively. Possible explanations for this failure are tested and discussed. Surface nucleation depends strongly on the surface quality, e.g. tips, cracks and scratches, elastic stresses, foreign particles and surrounding atmosphere. The mechanisms of surface nucleation are still not fully understood, but some of the key factors are now known and the surface–nucleation density can thus be controlled for the development of sintered glasses or glass ceramics.

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 and volume nucleation and growth in TiO2-cordierite glasses

Abstract Systematic measurements and analyses of surface nucleation and growth of μ-cordierite on diamond-polished surfaces and on fractured surfaces of cordierite glasses containing 0.3, 1.2, 3.4, 6.2 and 8.1 wt% TiO 2 were carried out. All the glasses exhibit surface nucleation, however, the glass having 8.1 wt% TiO 2 also exhibits volume nucleation. The maximum surface nucleation rate is located at a temperature considerably higher than the maximum volume nucleation rate. Both surface and volume crystallization occur by heterogeneous nucleation. The average nucleus/substrate wetting angles, θ , were estimated from surface and volume nucleation measurements. The widely different values of θ indicate that the active catalyzing sites are different for surface and volume crystallization. It is likely that Al 2 TiO 5 crystals induce volume crystallization of μ-cordierite, while surface crystallization is affected by TiO 2 , defects and relicts of polishing power. Nucleation on fractured surfaces was induced mainly by solid particles, most likely by broken glass particles. The crystal growth velocity of μ-cordierite crystals is well fitted by the 2D-surface nucleation growth model.

Method to estimate crystal/liquid surface energy by dissolution of subcritical nuclei

Abstract Within the framework of the classical nucleation theory (CNT) the nucleus–liquid surface energy ( σ ) is one of the most important parameters to predict or analyze nucleation kinetics in undercooled liquids. However, the few methods proposed to determine σ are based on the direct use of nucleation rate data and, thus, do not allow an independent test of the theory. In the present paper, this parameter is estimated without direct use of nucleation rate data. Instead, we provide further experimental evidence for the dissolution of sub-critical nuclei at several development temperatures and use this effect to estimate the nucleus–liquid surface energy. The results of the present analysis demonstrate that, in agreement with a new approach for the description of nucleation processes, not only the surface energy but also the driving force for nucleation have quite different values from those of the evolving macrophases. The possible implications of this result for the understanding of crystal nucleation in liquids are discussed.

The effect of pre-existing crystals on the crystallization kinetics of a soda–lime-silica glass. The courtyard phenomenon

New data are presented on bulk crystallization in a glass of composition close to the stoichiometric phase Na2OAE2CaOAE3SiO2. The eAects of pre-existing crystals on the nucleation and growth behavior of secondary crystals are described for the first time. Primary crystals, grown at high temperatures, dramatically hinder the formation of new crystals at lower temperatures, in their vicinity (the courtyard eAect). The crystal growth velocity decreases with increasing crystal size. Chemical analyses by energy-dispersive X-ray spectroscopy demonstrate that the above-mentioned eAects result mainly from compositional changes of the glass matrix, in the neighborhood of the primary crystals. ” 1999 Elsevier Science B.V. All rights reserved.

Diffusion coefficients for crystal nucleation and growth in deeply undercooled glass-forming liquids

We calculate, employing the classical theory of nucleation and growth, the effective diffusion coefficients controlling crystal nucleation of nanosize clusters and the subsequent growth of micron-size crystals at very deep undercoolings, below and above Tg, using experimental nucleation and growth data obtained for stoichiometric Li2O.2SiO2 and Na2O.2CaO.3SiO2 glasses. The results show significant differences in the magnitude and temperature dependence of these kinetic coefficients. We explain this difference showing that the composition and/or structure of the nucleating critical clusters deviate from those of the stable crystalline phase. These results for diffusion coefficients corroborate our previous conclusion for the same glasses, based on different experiments, and support the view that, even for the so-called case of stoichiometric (polymorphic) crystallization, the nucleating phase may have a different composition and/or structure as compared to the parent glass and the evolving macroscopic crystalline phase. This finding gives a key to explain the discrepancies between calculated (by classical nucleation theory) and experimentally observed nucleation rates in these systems, in particular, and in deeply undercooled glass-forming liquids, in general.

Comments on DTA/DSC Methods for Estimation of Crystal Nucleation Rates in Glass-Forming Melts

Detailed information about crystallization kinetics is important for glass-ceramic (GC) production, which, in most cases, is based on controlled internal crystallization. In this context, kinetic parameters such as crystal nucleation rate and time-lag (or induction period) for nucleation are of great interest because they can be used to define the crystal number density, N (nuclei/m3), which in turn limits the maximal average size, \( {{\bar{R}}_{{{ \max }}}} \), of the crystals in the resulting microstructure. Both quantities determine, to a great extent, the properties and applications of GCs. The traditional method to estimate the number density of nucleated crystals (supercritical nuclei) consists of the development of these nuclei at a relatively high temperature (higher than the previous nucleation temperature) up to a detectable size by optical or electron microscopy (Fokin VM, Zanotto ED, Yuritsyn NS, Schmelzer JWP. J Non-Cryst Solids 352:2681, 2006). This method, developed by Gustav Tammann (Tammann’s method) more than 100 years ago to measure crystal nucleation rate in organic liquids (Tammann G. Z Phys Chem 25:441, 1898), was successfully applied to inorganic glasses for the first time by Ito et al. (Ito M, Sakaino T, Moriya T. Bull Tokyo Inst Technol 88:127, 1968) and Filipovich and Kalinina (Filipovich VN, Kalinina AM. Izv Akad Nauk USSR. Neorgan Mater 4:1532 (in Russian), 1968). It provides an estimation of the number of supercritical nuclei needed for the determination of the steady-state nucleation rate and the time-lag for nucleation. However, Tammann’s method is laborious because it includes image analysis of crystallized samples. The foregoing method is valid for the cases of stoichiometric (when crystal and glass have the same composition) and nonstoichiometric crystallization.