Search

To elucidate the atomistic origin of volume relaxation in soda-lime silicate glass annealed below the glass transition temperature (Tg), the experimental and calculated Raman spectra were compared. By decomposing the calculated Raman spectra into specific groups of atoms, the Raman peaks at 800, 950, 1050, 1100, and 1150 cm−1 were attributed to oxygen and silicon in Si–O–Si, non-bridging oxygen in the Q2 unit, bridging oxygen in low-angle Si–O–Si, non-bridging oxygen in the Q4 unit, and bridging oxygen in high-angle Si–O–Si, respectively. Based on these attributions, we found that by decreasing the fictive temperature by annealing below Tg − 70 K, a homogenization reaction Q2 + Q4 → 2Q3 and an increase in average Si–O–Si angle occurred simultaneously. By molecular dynamics simulation, we clarified how the experimentally demonstrated increase in average Si–O–Si angle contributes to volume shrinkage; increasing Si–O–Si angles can expand the space inside the rings, and Na can be inserted into the ring center. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We demonstrate that the modified Kempers model, a recently developed theoretical model for the Soret effect in oxide melts, is applicable for predicting the composition dependence of the Soret coefficient in three binary molecular liquids with negative enthalpies of mixing. We compared the theoretical and experimental values for water/ethanol, water/methanol, water/ethylene glycol, water/acetone, and benzene/n-heptane mixtures. In water/ethanol, water/methanol, and water/ethylene glycol, which have negative enthalpies of mixing across the entire mole fraction range, the modified Kempers model successfully predicts the sign change of the Soret coefficient with high accuracy, whereas, in water/acetone and benzene/n-heptane, which have composition ranges with positive enthalpies of mixing, it cannot predict the sign change of the Soret coefficient. These results suggest that the model is applicable in composition ranges with negative enthalpies of mixing and provides a framework for predicting and understanding the Soret effect from the equilibrium thermodynamic properties of mixing, such as the partial molar volume, partial molar enthalpy of mixing, and chemical potential. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Microscopic dynamical features in the relaxation of glass structures are one of the most important unsolved problems in condensed matter physics. Although the structural relaxation processes in the vicinity of glass transition temperature are phenomenologically expressed by the Kohlrausch–Williams–Watts function and the relaxation time can be successfully interpreted by Adam–Gibbs theory and/or Narayanaswamy's model, the atomic rearrangement, which is the origin of the volume change, and its driving force have not been elucidated. Using the microsecond time-scale molecular dynamics simulations, this study provides insights to quantitatively determine the origin of the thermal shrinkage below Tg in a soda-lime silicate glass. We found that during annealing below Tg, Na ions penetrate into the six-membered silicate rings, which remedies the acute O–O–O angles of the energetically unstable rings. The ring structure change makes the space to possess the cation inside the rings, but the ring volume is eventually reduced, which results in thermal shrinkage of the soda-lime silica glass. In conclusion, the dynamical structural relaxation due to the cation displacement evokes the overall volume relaxation at low temperature in the glassy material. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Room-temperature magnetization of ZnFe2O4 nanoparticles (NPs) dispersing in silica was successfully enhanced by a femtosecond laser irradiation. This drastic increase in magnetization can be explained in terms of the metastable phase that exhibits ferrimagnetic high magnetization. The silica composite material was prepared by heat treatment of xerogels made by a sol–gel reaction of TEOS containing zinc nitrate and iron nitrate. When normal spinel ZnFe2O4 NPs precipitated inside silica are irradiated with a femtosecond laser, a random cation distribution in spinel structure is instantly formed due to a local temperature rise, and it would be frozen as a metastable phase, resulting in an enhancement of magnetization at room temperature. When the irradiated sample was reheated at 800 °C, a decrease in saturation magnetization was observed by approximately 40%. This is ascribed to the cation rearrangement from the random spinel to the stable normal spinel structure. The size of the change region was estimated from the saturation magnetization, assuming that the cation distribution of ZnFe2O4 NPs in the focal centered sphere was almost completely randomized. The calculated size was in close agreement with the laser spot size, suggesting that the induced magnetization increase is due to the formation of a metastable phase of ZnFe2O4 in the local region. The femtosecond laser process using a silica/ZnFe2O4 nanocomposite material enables three-dimensional patterning of magnetic materials. In the near future, it can be expected to be applied as waveguide-type magneto-optical devices and microphotonics based on magnetic materials. [ABSTRACT FROM AUTHOR]