Abstract
Understanding the conditions that favour crystallisation and vitrification has been a longstanding scientific endeavour. Here we demonstrate that the extremely high glass-forming ability of unseeded supercooled Na2O·Al2O3·6SiO2 (Albite) and B2O3—known for decades as “crystallisation anomaly”—is caused by insufficient crystal nucleation. The predicted temperatures of the maximum homogeneous nucleation rates are located well below their glass transition temperatures (Tg), in a region of very high viscosity, which leads to extremely long nucleation time-lags and low nucleation rates. This behaviour is due to the remarkably small supercoolings where the glass transition occurs for these liquids, which correspond to a very small driving force for crystallisation at and above the Tg, where crystallisation is normally observed. This meagre nucleation ability is caused by the significant difference in the structures of the supercooled liquids and their isochemical crystals. These findings elucidate the cause behind the crystallisation anomaly, and could be used for the design of other oxide glasses that are extremely stable against crystallisation.
Highlights
Where α is a non-dimensional constant and NA is Avogadro’s number. From both theoretical arguments and the fitting of the Classical Nucleation Theory equation to experimental crystal nucleation rates in oxide glass-formers that show measurable homogeneous nucleation, α is expected to be in the range of 0.4–0.620
The values of equilibrium viscosity, given by Eq (5) were used even below the Tg of each glass because the structural relaxation times, τrel, are significantly shorter than the crystal nucleation time-lags, which is similar to the average time of formation of the first critical nucleus, .the temperature where τrel =is the so-called supercooled liquid metastability limit (SCLML) or kinetic spinodal, TKS48–50
Our analysis clearly demonstrates that the lack of detectable crystallisation (“crystallisation anomaly”) in both supercooled liquids is meagre homogeneous crystal nucleation, solving an enigma that persisted for 80 decades since the pioneering work of Kracek
Summary
Reluctant crystallisation could be due to either exceptionally low steady-state crystal nucleation rates, very long induction periods for nucleation, and/or extremely low crystal growth rates. We will discuss the governing equations for these three kinetic properties, and at their main controlling parameters, that can be calculated or measured, which are: the crystallisation driving force, the nucleus-melt interfacial energy, and the effective diffusion coefficient. We will start with a brief definition of these parameters, before discussing the crystal nucleation and crystal growth models. The driving force for crystallisation is defined as Δμ≡−ΔG =Gl −Gc and is positive for temperatures below the melting point[14,15]. The absolute value of Δμincreases with decreasing temperature from the melting point (that is, increasing undercooling). Equation (1) provides a way to calculate Δμfor a closed system under isobaric condition[14,15]
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