Determination of thermodynamic and microscopic origins of the Soret effect in sodium silicate melts: Prediction of sign change of the Soret coefficient

Abstract

The Soret effect in silicate melts has attracted attention in earth and material sciences, particularly in glass science and engineering, because a compositional change caused by the Soret effect modifies the material properties of silicate melts. We investigated the Soret effect in an Na2O–SiO2 system, which is the most common representative of silicate melts. Our theoretical approach based on the modified Kempers model and non-equilibrium molecular dynamics simulation was validated for 30Na2O–70SiO2(mol. %). The sign and order of the absolute values of the calculated Soret coefficients were consistent with the experimental values. The positive Soret coefficient of SiO2 in the SiO2-poor composition range was accurately predicted. Previous experimental studies have focused on SiO2-rich compositions, and only the negative sign, indicating SiO2 migration to the hot side, has been observed. In the SiO2-poor composition range, the Q0 structure was dominant and had four Si–O–Na bonds around an SiO4 unit. The Si–O–Na bond had high enthalpic stability and contributed to the large negative enthalpy of SiO2 mixing. According to our model, components with a large negative partial molar enthalpy of mixing will concentrate in the cold region. The microscopic and thermodynamic origins of the sign change in the Soret effect were determined.

Competition Effects during Femtosecond Laser Induced Element Redistribution in Ba- and La-Migration Based Laser Written Waveguides

“…Similarly, to the mixed modifier effect that accounts for non-linear property changes in the presence of different size cations, it is proposed that here applies what could be called a "mixed valence effect" and that would be worth validating in other glass composition systems. Whatsoever the origin of the migration competition effects is, they could potentially be used to inhibit thermo-migration effects in fs-laser writing applications where the preservation of the composition of the glass can be needed as a requirement [13,20,[22][23][24][25].…”

“…At the end of the day, these parameters condition the local chemical potential of the different glass constituents [12]. Indeed, the prediction of the sign and magnitude of the Soret coefficient of a given component in a multicomponent mixture is still an open problem that requires to consider microscopic parameters and numerical modeling [13]. Still, FLIER can certainly be controlled in practice [14,15] and exploited for producing high-performance photonic devices, as indicated above.…”

“…However, not all of the glass constituents necessarily move upon laser excitation. This depends on the material composition [13], the excitation conditions and type of ions involved [20], the shape of the focal volume and the correlated thermal gradients involved [8,21].…”