Dynamic processes in a silicate liquid from above melting to below the glass transition

Author(s)

Ferreira Nascimento, Marcio Luis ^[1] ; Fokin, Vladimir Mihailovich ^[2] ; Zanotto, Edgar Dutra ^[3] ; Abyzov, Alexander S. ^[4]

Affiliation

^[1] Univ Fed Bahia, Inst Humanities Arts & Sci, BR-40170115 Salvador, BA - Brazil

^[2] Vavilov State Opt Inst, St Petersburg 193171 - Russia

^[3] Univ Fed Sao Carlos, Vitreous Mat Lab, Dept Mat Engn, BR-13595905 Sao Carlos, SP - Brazil

^[4] Ctr Nat Sci, Kharkov Inst Phys & Technol, UA-61108 Kharkov - Ukraine

• Document type: Journal article
• Source: Journal of Chemical Physics; v. 135, n. 19 NOV 21 2011.
• Web of Science Citations: 40

Abstract

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 (Li(2)O center dot 2SiO(2)) over a wide temperature range, from above T(m) to 0.98T(g) where T(g) approximate to 727 K is the calorimetric glass transition temperature and T(m) = 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, D(eff)(U), and completed the analyses by looking at the ionic diffusion coefficients of Li(+1), O(2-), and Si(4+) estimated from experiments and molecular dynamic simulations. These values were then employed to estimate the effective volume diffusion coefficients, D(eff)(V), resulting from their combination within a hypothetical Li(2)Si(2)O(5) ``molecule{''}. The similarity of the temperature dependencies of 1/eta, where eta. is shear viscosity, and D(eff)(V) corroborates the validity of the Stokes-Einstein/Eyring equation (SEE) at high temperatures around T(m). Using the equality of D(eff)(V) and D(eff)(eta), we estimated the jump distance lambda similar to 2.70 angstrom from the SEE equation and showed that the values of D(eff)(U) have the same temperature dependence but exceed D(eff)(eta) by about eightfold. The difference between D(eff)(eta) and D(eff)(U) 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 eta(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(d)(U) approximate to 1.1 -1.2T(g), when D(eff)(eta) begins decrease with decreasing temperature faster than D(eff)(U). A similar decoupling occurs between D(eff)(eta) and D(eff)(tau) (estimated from nucleation time-lags) but at a lower temperature T(d)(tau) approximate to T(g). For T > T(g) the values of D(eff)(tau) exceed D(eff)(eta) only by twofold. The different behaviors of D(eff)(tau)(T) and D(eff)(U)(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. The origin of decoupling is assigned to spatially dynamic heterogeneity in glass-forming melts. (C) 2011 American Institute of Physics. {[}doi:10.1063/1.3656696] (AU)

• FAPESP's process: 07/08179-9 - Kinetic processes in glasses and glass ceramics
• Grantee: Edgar Dutra Zanotto
• Support Opportunities: Research Projects - Thematic Grants