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

TL;DR

This study reveals that at ultra-high temperatures above 3000 K, SiO2 in silicate melts migrates towards colder regions under a temperature gradient, contrary to behavior at lower temperatures, supported by experiments and molecular dynamics simulations.

Contribution

It demonstrates a temperature-dependent reversal in SiO2 migration direction in silicate melts at ultra-high temperatures, supported by experimental laser heating and NEMD simulations.

Findings

SiO2 migrates to colder regions above 3060 K

Migration direction reverses around 2400-3000 K

A second-order phase transition occurs near 2000-3400 K

Abstract

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 approximately 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 around 2400 K in both the NEMD and experimental study. The second-order like phase transition was observed at ~2000-3400 K when calculated using MD without a temperature gradient. Therefore, the second-order phase transition could be related to the migration of SiO2 to colder region. However, the detailed mechanism has not been elucidated.