Abstract
The present study explores the temperature and time dependence of heterogeneous crystal nucleation in a lithium disilicate glass using the stochastic approach. In particular a single lithium disilicate sample was repeatedly (284 runs) undercooled to 1173 K in a PtRh-crucible and the crystallization onset time during an isothermal hold was detected in each run. The statistical distribution of the times elapsed before crystallization is described by a first order reaction with a heterogeneous crystal nucleation rate of (9.19 ± 0.04) × 10-4 s-1 while individually shaped crystallization exotherms of each run were recorded which indicate growth of a single or only few crystals during crystallization of the entire volume. The data were used together with results of previous stochastic experiments and those of double-stage heat treatments to calculate the crystallization time of a fraction of 10-4 percent for all temperatures between glass transition and melting. The derived TTT diagram shows a double nose of crystallization in the volume at large undercoolings (0.53–0.61 Tm) and crystallization at the surface at small undercoolings (0.62–0.92 Tm) initiated by homogeneous and heterogeneous crystal nucleation, respectively. The critical cooling rate at the heterogeneous nose is approximately 73 K s-1.
Highlights
In the last 40 years, crystal nucleation in silicate glasses has been addressed by counting crystal number densities in the volume of glass specimens subjected to double-stage heat treatments (Fokin et al, 2006)
In the light of the above, this study aims at establishing nucleation kinetics by its time average of repeated single crystal nucleation events rather than using the ensemble average of crystals of double-stage heat treatments
Our results show that it is applicable to high viscous silicate melts and it results in heterogeneous (HET) crystal nucleation rates from which TTT representations and critical cooling rates can be derived
Summary
In the last 40 years, crystal nucleation in silicate glasses has been addressed by counting crystal number densities in the volume of glass specimens subjected to double-stage heat treatments (Fokin et al, 2006). The techniques bears the advantage of direct determination of the crystal nucleation rate from calculating the first time derivative of the number density curve but has the drawback that it is limited to a narrow temperature range near the glass transformation temperature (Tg ≤ T ≤ 1.2 Tg) where driving forces are high and crystal nucleation is occurring frequently in the volume. Another drawback is the dissolution of nuclei that are critical at the first stage but subcritical at the second stage. Birth- (from crystal size distribution) (Müller et al, 2000) and impingementtimes (Krüger and Deubener, 2015a) of crystals at the surface have been analyzed to deduce
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