Resumo:
We collect and critically analyze extensive literature data, including our own, on three important kinetic processes—viscous flow, crystal nucleation, and growth—in lithium disilicate (Li2O· 2SiO2) over a wide temperature range, from above Tm to 0.98Tg where Tg ≈ 727 K is the calorimetric glass
transition temperature and Tm =1307 K, which is the melting point.We found that crystal growthmediated by screw dislocations is the most likely growth mechanism in this system. We then calculated the diffusion coefficients controlling crystal growth, DU eff , and completed the analyses by looking at the ionic diffusion coefficients of Li+1, O2−, and Si4+ estimated from experiments and molecular dynamic simulations. These values were then employed to estimate the effective volume diffusion coefficients, DV eff , resulting from their combination within a hypothetical Li2Si2O5 “molecule”. The
similarity of the temperature dependencies of 1/η, where η is shear viscosity, and DV
eff corroborates the validity of the Stokes-Einstein/Eyring equation (SEE) at high temperatures around Tm. Using the
equality of DV eff and Dη eff , we estimated the jump distance λ ∼ 2.70 Å from the SEE equation and showed that the values of DU
eff have the same temperature dependence but exceed Dη eff by about eightfold. The difference between Dη eff and DU
eff indicates that the former determines the process of mass transport in the bulk whereas the latter relates to the mobility of the structural units on the crystal/ liquid interface.We then employed the values of η(T) reduced by eightfold to calculate the growth
rates U(T). The resultant U(T) curve is consistent with experimental data until the temperature decreases to a decoupling temperature T U d ≈ 1.1 − 1.2Tg, when Dη
eff begins decrease with decreasing temperature faster than DU eff . A similar decoupling occurs between Dη eff and Dτ
eff (estimated from nucleation time-lags) but at a lower temperatureT τ d ≈ Tg. For T > Tg the values of Dτ eff exceed Dη eff
only by twofold. The different behaviors of Dτ eff (T ) and DU eff (T ) are likely caused by differences in the mechanisms of critical nuclei formation. Therefore, we have shown that at low undercoolings, viscosity data can be employed for quantitative analyses of crystal growth rates, but in the deeply supercooled liquid state, mass transport for crystal nucleation and growth are not controlled by viscosity.
