Role of partial molar enthalpy of oxides on Soret effect in high-temperature CaO-SiO2 melts

Ultra-high temperature Soret effect in a silicate melt: SiO2 migration to cold side

The Soret effect, temperature gradient driven diffusion, in silicate melts has been investigated intensively in the earth sciences from the 1980s. The SiO2 component is generally concentrated in the hotter region of silicate melts under a temperature gradient. Here, we report that at ultra-high temperatures above ∼3000 K, SiO2 becomes concentrated in the colder region of the silicate melts under a temperature gradient. The interior of an aluminosilicate glass [63.3SiO2-16.3Al2O3-20.4CaO (mol. %)] was irradiated with a 250 kHz femtosecond laser pulse for local heating. SiO2 migrated to the colder region during irradiation with an 800 pulse (3.2 ms irradiation). The temperature analysis indicated that migration to the colder region occurred above 3060 K. In the non-equilibrium molecular dynamics (NEMD) simulation, SiO2 migrated to the colder region under a temperature gradient, which had an average temperature of 4000 K; this result supports the experimental result. On the other hand, SiO2 exhibited a tendency to migrate to the hotter region at 2400 K in both the NEMD and experimental study. The molar volume calculated by molecular dynamics simulation without a temperature gradient indicates two bends at 1650 and 3250 K under 500 MPa. Therefore, the discontinuous (first order) transition with coexistence of two phases of different composition could be related to the migration of SiO2 to colder region. However, the detailed mechanism has not been elucidated.

Composition-dependent sign inversion of the Soret coefficient of SiO2 in binary borosilicate melts

Using a laser-induced local-heating experiment combined with temperature analysis, we observed the composition-dependent sign inversion of the Soret coefficient of SiO2 in binary silicate melts, which was successfully explained by a modified Kempers model used for describing the Soret effect in oxide melts. In particular, the diffusion of SiO2 to the cold side under a temperature gradient, which is an anomaly in silicate melts, was observed in the SiO2-poor compositions. The theoretical model indicates that the thermodynamic mixing properties of oxides, partial molar enthalpy of mixing, and partial molar volume are the dominant factors for determining the migration direction of the SiO2 component under a temperature gradient.

Molecular Dynamics Simulation of the Soret Effect on Two Binary Liquid Solutions with Equimolar n-Alkane Mixtures

Molecular dynamics is employed to simulate the Soret effect on two binary liquid solutions with equimolar mixtures: normal pentane (n-pentane, nC-5) and normal heptane (n-heptane, nC-7) molecules plus normal decane (n-decane, nC-10) and normal pentane molecules. Moreover, two coarse-grained force field (the CG-FF) potentials, which may depict inter-/intramolecular interactions fairly well among n-alkane molecules, are developed to fulfill such investigations. In addition, thermal diffusion for the mass fraction of each of these n-alkane molecules is simulated under an effect of a weak thermal gradient (temperature difference) exerting on solution systems from their hot to cold boundary sides. Finally, quantities of the Soret coefficient (SC) for two binary solutions are calculated by means of the developed CG-FF potentials, so as to improve the calculation rationality. As a result, first, it is found that molecules with light molar masses will migrate toward the hot boundary side, while those with heavy molar masses will migrate toward the cold boundary one ; second, the SC quantities indicate that they match relevant experimental determinations fairly well, i.e., trends of these SC quantities show inverse proportionality to the thermal gradient on the systems.

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

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.

Revisiting ultrafast laser inscribed waveguide formation in commercial alkali-free borosilicate glasses

Alkali-free borosilicate glasses are one of the most used dielectric platforms for ultrafast laser inscribed integrated photonics. Femtosecond laser written waveguides in commercial Corning Eagle 2000, Corning Eagle XG and Schott AF32 glasses were analyzed. They were studied in depth to disclose the dynamics of waveguide formation. We believe that the findings presented in this paper will help bridge one of the major and important gaps in understanding the ultrafast light-matter interaction with alkali-free boroaluminosilicate glass. It was found that the waveguides are formed mainly due to structural and elemental reorganization upon laser inscription. Aluminum along with alkaline earth metals were found to be responsible for the densification and silicon being the exchanging element to form a rarefied zone. Strong affinity towards alkaline earth elements to form the densified zone for waveguides written with high feed rate (>200 mm/min) were identified and explained. Finally we propose a plausible solution to form positive refractive index change waveguides in different glasses based on current and previous reports.