Edgar Dutra Zanotto

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.

Experimental tests of the classical nucleation theory for glasses

Abstract New data for crystal nucleation rates and viscosities were obtained for Li2O·2SiO2 and BaO·2SiO2 glasses. Special efforts were made to minimise impurities in the glasses. Good general agreement with previously published nucleation results was found. The new and previous results were used to test the applicability of classical nucleation theory. For both lithium and barium disilicates the temperature dependence of nucleation rates was satisfactorily described by theory, ln (Iη/T) vs. 1/ΔG2T plots yielding straight lines over a wide range of temperatures (ΔG is the bulk free energy difference per mole between crystal and liquid at temperature T). However, for lithium disilicate, where experimental ΔG data were available, the experimentally determined preexponential factor A in the nucleation equation was much higher than the theoretical value, in agreement with previous studies. This was the case even allowing for reasonable variations in ΔG. A similar result was obtained for barium disilicate using calculated ΔG values. Possible reasons for the discrepancy in A between theory and experiment are discussed including transient nucleation, experimental errors in the nucleation rates and heterogeneous nucleation. None of these effects can account for the observed discrepancy. Other possible explanations are considered.

Crystallization mechanism and properties of a blast furnace slag glass

The complex crystallization process of a Brazilian blast-furnace slag glass was investigated using differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy, transmission electron microscopy (TEM), selected area diffraction (SAD), energy dispersive spectroscopy (EDS) and micro-Raman spectroscopy. Three crystalline phases (merwinite, melilite and larnite) were identified after heat treatment between Tg (742°C) and the DSC crystallization peak (T=1000°C). Merwinite was identified as a metastable phase. A small amount (0.004 wt%) of metallic platinum was found in the glass composition. Particles of Pt3Fe, detected by EDS and SAD–TEM, were the starting points of crystallization acting, therefore, as heterogeneous nucleating sites. Only melilite and larnite precipitated in a glass sample heat-treated at 1000°C for 1 h. The flexural strength of this crystallized sample was less than that of the glass, probably due the allotropic phase transformation of larnite.

ISOTHERMAL AND ADIABATIC NUCLEATION IN GLASS

A summary of both classical (isothermal) and adiabatic theories of nucleation is presented. Using experimentally determined parameters, such as viscosity, specific heat, glass transition temperature (Tg), melting temperature (Tf) and heat of fusion, the temperatures of maximum crystal nucleation rates, Tmax, were calculated for several glass forming systems and compared with experimental data. It is shown that both theories give a good estimate for Tmax. For systems which do not show volume (homogeneous) nucleation, Tmax is lower than Tg. For systems which show volume nucleation, Tmax >Tg. It is concluded that both theories can be used to predict the occurrence of internal crystal nucleation in glasses and that, in general, a high value of Tg (Tg/Tf > 0.58) indicates the absence of internal nucleation.

29Si MAS–NMR studies of Qn structural units in metasilicate glasses and their nucleating ability

The purpose of this work is to verify the possible existence of a relationship between the similarity of the local structure of the network-forming cation Si4+ (Qn units and chemical shifts) in glasses and isochemical crystals and the nucleating ability of these glasses. Four metasilicate glasses with widely different volume nucleation rates: Na2Ca2Si3O9 and Na4CaSi3O9 (very large), CaSiO3 (intermediate) and CaMgSi2O6 (undetectably small) were chosen. We present magic angle spinning nuclear magnetic resonance spectroscopy (MAS–NMR) data for Na2Ca2Si3O9 and Na4CaSi3O9 glasses and for their respective isochemical crystalline phases for the first time. Additionally, we repeat NMR measurements of glasses and crystals previously studied by other authors (CaSiO3 and CaMgSi2O6) to test the consistency of our experimental techniques and method of analysis. Different central chemical shifts of Q2 resonances in parent glasses and their isochemical crystals were measured, indicating structural differences. The relative amount of Qn groups in each glass was obtained from the deconvolution of the 29Si MAS–NMR spectra. The shape of the Qn distribution for each system was considered as a measure of the similarity of the connectivities of SiO4 tetrahedra in each glass with respect to its isochemical crystal (which has only Q2 groups). A correlation was found between the shape of the Qn distribution and the nucleation tendency of these glasses, indicating that similarities between the tetrahedra connectivities in glass and isochemical crystal has a role in determining the internal nucleation tendency of the metasilicate glasses studied.

Surface crystallization kinetics in soda-lime-silica glasses

Abstract A review of previous research on surface nucleation in glasses demonstrates that these are mostly qualitative and that strong discrepancies exist regarding the nucleation mechanism. In this article, the surface crystallization kinetics of several glasses — a Na20.3Ca0.6SiO2 (devitrite), a non-stoichiometric devitrite, and two commercial soda-lime-silica (a float and a microscope slide) glasses — were determined in a wide range of temperatures and time. An analysis of the average number of crystals per crystallization arises from a fixed number of special sites, Ns. The number of crystals nucleated strongly depends on the unit area, Ns, crystal growth rates and viscosity data indicates that the surface nucleation rates are very high and that surface condition (e.g., fire polished versus mechanically polished or as-received; clean versus dirty), on the chemical composition of the parent glass, and also on the nature of the crystallizing phase. However, Ns does not depend on time or temperature. The experimental evidence indicates that the surfaces ‘per se’ do not alter the thermodynamic barrier for nucleation (the interfacial energy or the chemical potential). The enhanced nucleation rates at the external surfaces are the result of the catalytic effect of some (unknown) solid impurity particles and faster surface diffusion rates.

Glass sintering with concurrent crystallization

Abstract We critically review and discuss the main glass-sintering models: Frenkel, Mackenzie–Shuttleworth, Scherer and the recently developed Clusters model, and focus on the problem of sintering with concurrent crystallization. The Clusters model is tested under various practical conditions. Isothermal tests are carried out on a widely polydispersed alumino-borosilicate (ABS) glass having jagged particles, which is stable against devitrification, and on a soda–lime–silica (SLS) glass with a narrow spherical particle distribution, which crystallizes easily. The algorithm for non-isothermal processes is also tested with two distinct systems: the same ABS glass and a narrow-sized cordierite glass, which is devitrification-prone. In addition to physical parameters such as viscosity, surface tension, particle-size distribution, crystal growth rate and number of nucleation sites, microscopic-particle-packing data are introduced into the model and it is demonstrated that the evolution of both density and pore size distribution can be reasonably predicted. All the results are discussed taking into account the assumptions made in the derivations and other complicating factors, such as irregular particle shape, compositional shifts due to crystallization, temperature gradients and degassing during sintering. Finally, we discuss the physical and processing parameters that determine whether sintering will be favorable over crystallization. We demonstrate that the Clusters model and related algorithm provide a powerful simulation tool to design the isothermal or non-isothermal densification of devitrifying or stable glass compacts with any particle-size distribution, thus minimizing the number of time-consuming laboratory experiments.

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.

Model for sintering polydispersed glass particles

We propose a model to describe the sintering kinetics of polydispersed glass particles, having no adjustable parameter. The model is based on three sintering stages: a pure ‘Frenkel’ (F) first step, a mixed ‘Frenkel/Mackenzie‐ Shuttleworth’ stage, and a third, pure ‘Mackenzie‐Shuttleworth’ (MS) step. The model considers sample shrinkage as the sum of the partial shrinkage of several clusters, each consisting of equally sized particles and each showing independent F or MS behavior. The overall set of clusters mimics the specimen’s real particle size distribution. We then introduce the concept of neck forming ability ‐ nr, which allows the formation of necks among particles of diAerent sizes, relaxing the clustering condition. Using experimental physical parameters: particle size distribution, viscosity, surface energy, and the theoretical nr, the model describes well the sintering kinetics of an alumino-borosilicate glass powder having polydispersed, irregular shaped particles in a variety of temperatures. The sintering kinetics of the real powder is slower, but not far from the calculated kinetics of a monodispersed distribution containing only particles of average size. Thus the model provides a tool for estimating the sintering kinetics of real glass powders, for any size distribution and temperature, thus minimizing the number of laboratory experiments. ” 2001 Elsevier Science B.V. All rights reserved.