Letter to the editor

Critical cooling rates in alkali silicate systems

The composition dependence of glass formation is examined in a variety of silicate systems that include alkali and alkaline earth alumino-, titano-, ferro- and ferrisilicates. Empirically, there is a clear correlation between wide extent of glass formation, possible crystallization from the melt of numerous compounds, and moderate liquidus temperatures. Vitrification with usual cooling rates is in contrast impossible when binary and ternary compounds are scarce and liquidus temperatures are high. These correlations imply that vitrification is favored by moderately negative enthalpies of mixing in the melt but made difficult by high configurational heat capacities. The close connection between glass formation and viscosity is reviewed in the light of these melt properties. That bulk viscosity is in general not directly relevant to the kinetics of crystal nucleation in particular indicates that vitrification theories cannot be considered as by-products of crystallization theories.

Glass-forming ability (GFA) is the easiness to vitrify a liquid on cooling, while glass stability (GS) is the glass resistance against devitrification on heating; but it is questionable if there is any direct relationship between these two parameters. Therefore, to test this possibility, we assess and compare GFA and several GS parameters through quantitative criteria. GFA and GS were calculated for six stoichiometric glass forming oxides that only present surface (heterogeneous) crystallization in laboratory time scales: GeO₂, Na₂O · 2SiO₂, PbO · SiO₂, CaO · Al₂O₃ · 2SiO₂, CaO · MgO · 2SiO₂ and 2MgO · 2Al₂O₃ · 5SiO₂; plus Li₂O · 2SiO₂ and Li₂O · 2B₂O₃ that, in addition to surface nucleation, also present homogeneous (internal) crystallization. We gauge GFA by the critical cooling rate, q_cr, which was calculated from an estimated number of heterogeneous nucleation sites per unit surface, N_s, and from experimental crystal growth rates, u(T), assuming a detectable surface crystallized fraction X_c = 0.001. We define GS parameters by fourteen different combinations of the following characteristic differential thermal analysis (DTA) or differential scanning calorimetry (DSC) temperatures: the glass transition temperature (T_g), the onset crystallization temperature on heating $(T_x^{\mathrm{h}})$, the peak crystallization temperature on heating $(T_{\mathrm{c}}^{\mathrm{h}})$, and the melting point (T_m). To obtain the experimental GS parameters for each glass we carried out DSC runs using coarse and fine powders, and completed the necessary data with literature values for T_m. The results for fine and coarse particles were quite similar. Most of the GS parameters that consist of three characteristic DSC temperatures show excellent correlation with GFA, however, rather poor correlations were observed for parameters that use only two characteristic temperatures. We thus demonstrated that certain, but not all GS parameters can be used to infer GFA.

A new differential thermal analysis (DTA) experimental method has been developed to determine the critical cooling rate for glass formation, R_c. The method, which is found especially suitable for melts that, upon cooling, have a small heat of crystallization or a very slow crystallization rate, has been verified using a 38Na₂O–62SiO₂ (mol%) melt with a known R_c (∼19 °C/min), then used to determine R_c for two complex lithium silicate glass forming melts. The new method is rapid, easy to conduct and yields values for R_c that are in excellent agreement with the R_c values measured by standard DTA techniques.

Experimental nucleation and growth rates for some glasses that nucleate in the bulk were used to calculate critical cooling rates for glass formation (R_c) by the TTT method. Resulting critical cooling rates were consistent with laboratory melting and quenching and also with experimental data of critical cooling rates for lithium disilicate glass. Strong correlation between the Hrubÿ parameter of glass stability (K_gl) and the glass-forming ability, evaluated by R_c, was found. Therefore, K_gl can be used to estimate the glass-forming ability for glasses that exhibit volume nucleation.

This article presents a model of nucleation and growth in three dimensions which can be applied to the investigation of glass systems. The occurrence of an amorphous network as a solution of statistical evolution equations based on the agglomeration of typical SRO (short-range order) structures, which exist in the glass-forming liquid, is analyzed in terms of trajectories in an appropriate phase space. An application to binary glasses (1 − x)XY − xM₂Y in the extreme high alkali limit (0.5 < x < 0.66) is performed and the predictions discussed (existence of the crystalline M₂XY₃ and M₄XY₄ compounds, of the glassy phase M₆X₂Y₇ and the concentration dependence of the SRO structures).

Glass-forming regions in the Li₂ONa₂OmSiO₂ (m = 2 or 1.5) and Li₂ONa₂OK₂OmSiO₂ (m = 2 or 1.5) systems were determined with various cooling rates. Viscosity and liquidus temperature were measured. Eutectics were observed in these systems. The critical cooling rate shows a minimum and the liquidus viscosity shows a maximum. It was found that Li₂ONa₂OmSiO₂ showed more extensive glass-forming ability than Li₂OmSiO₂, Na₂OmSiO₂ or K₂OmSiO₂. Li₂ONa₂OK₂OmSiO₂ shows an even larger glass-forming ability than Li₂ONa₂OmSiO₂. The significant role of mixing entropy or liquidus viscosity in the glass-forming kinetics of mixed alkali silicate systems is recognized.