Molecular dynamics simulation of high-pressure densification of fluorozirconate glass

Molecular dynamics simulations have been performed for a fluorozirconate glass and its model crystals in order to investigate the mechanism of the peculiar high-pressure densification phenomenon of the glass. All the polymorphs and the pressure-induced phase-transition of the model crystals were satisfactorily reproduced. The changes in the density, the F⁻ coordination number and the connectivity of ZrF_n polyhedra during the compression-decompression process are investigated under the glassy state. The density increases under high pressure, accompanying the increases in the coordination number and the connectivity. The effect of annealing treatment near T_g is also investigated. The effect was significant around 20 GPa, around which a maximum was found in the treatment-pressure dependence of density after decompression, only for the samples with the annealing treatment before decompression.     

Local microstructure of silica glass

We present a microstructure study of different amorphous states of SiO₂, which are constructed by compress–decompress procedure. The local microstructure is analyzed through the coordination number, bond-angle statistic and the characteristic of void. The simulation reveals that the number of SiO₄, SiO₅ and SiO₆ units varies in different silica states, but their topology is identical. The densest model (highest density) is obtained upon compress pressure around 20 GPa. The change in void space for different silica states is also calculated and discussed here.     

Structural relaxation in sputter-deposited silica glass

We investigated structural relaxation below the glass transition temperature in sputter-deposited silica glass. Structural relaxation was obtained from annealing behavior of the IR reflection structural band position. Results were compared with that of bulk silica glass. Results showed the following. (1) The structural relaxation time is 10⁶ times shorter than that of bulk silica glass. (2) The activation energy is close to that of bulk silica glass. (3) Once the structural relaxation reaches a steady state, the structure of silica glass film resembles that of bulk silica glass.     

Thermodynamics of fluorine incorporation into silica glass

Fluorine incorporation from the gaseous phase into silica glass was investigated both during the deposition of synthetic silica layers and during the sintering of pre-deposited porous silica layers, using different fluorine-containing compounds. The fluorine content dependence on the gas concentration and the influence of fluorine on the deposition efficiency of silica were determined. The results could be interpreted and quantitatively described by a simplified model of the equilibrium chemistry, valid for different fluorine sources.     

Tunable visible photoluminescence of powdered silica glass

Intense photoluminescence in the visible region was observed at room temperature in standard soda-lime-silica glass powder, mechanically milled in a high-energy attrition mill. The emission band maximum shows an interesting dependence on the exciting wavelength, suggesting the possibility to tune the PL emission. These findings indicate that the photoluminescence may be directly related to unsatisfied chemical bonds correlated with the high surface area. The Raman scattering and ultraviolet–visible optical reflectance measurements corroborate this assertion. Transmission electron microscopy measurements indicate that samples milled more than 10 h present the formation of nanocrystallites with about 10–20 nm.     

Effects of permanent densification on the vibrational density of states of vitreous silica

We present a detailed Raman scattering study of the boson peak evolution in vitreous silica as a function of density, from the normal one up to ~ 22% of densification. We show that the distribution of low frequency modes in the boson peak range does not change as a function of density, at least until the densification process starts to modify the glassy network structure from a fourfold coordinated structure into a sixfold coordinated one.     

New description of structural disorder in silica glass

Structural changes in cristobalite and silica glass and melt are investigated at high temperatures by molecular dynamics simulations. The calculated interaction energies are analyzed employing a new method called ‘atomistic energy distribution analysis’. Each pair-interaction energy is divided into equal halves which are allocated to each atom. Therefore, each atom has the summed-up value (‘atomistic potential energy’) of half the pair-interaction energies. First, an analysis of atomistic energy distribution functions show a correspondence with structural changes such as α–β phase transition, cristobalite melting and glass transition of SiO₂, in harmony with the results obtained in our preceding study. Moreover, this study demonstrates the different roles of oxygen and silicon in terms of structural changes. Finally, newly defined order parameters offer the possibility of distinguishing structures resulting from different thermal histories through introduction of higher moments.     

Crack-free high load Vickers indentation of silica glass

The surface of pure silica glass was prepared to be resistant to cracking from application of Vickers indentation loads of up to one kilogram force (1 kgf). Sample preparation, testing environment, and water content of the glass sample were found to be critical in producing crack-free indentations. Sample preparation consisted of a careful polishing and etching to remove the densified surface layer. Indentation was performed under a nitrogen environment which was also necessary to produce crack-free specimens. Water introduced into the silica glass sample mechanically as well as water introduced into the glass during melting or heat-treatment, were found to be detrimental to crack resistance.     

The study of removing hydroxyl from silica glass

The removal of hydroxyl from silica glass produced by melting quartz powder under an atmosphere containing hydrogen was investigated. After heat-treatment at the temperature range (700–1200 °C) in nitrogen atmosphere, the effective hydrogen diffusion coefficients were evaluated based on the law of nonsteady-state diffusion. The activation energy obtained is 254 kJ mol⁻¹ for the dehydroxylation process in the heat-treatment temperature range of 700–900 °C, and a different activation energy calculated is 32 kJ mol⁻¹ in the temperature range of 900–1200 °C. The activation energies for the dehydroxylation process at the temperature (700–900 °C) and the higher temperature (900–1200 °C) correspond to the binding energy of SiO–H bond and the activation energy for the diffusion of hydrogen in silica glass respectively, which indicate there is a change of mechanism for dehydroxylation with heat-treatment temperature.     

Rayleigh scattering in silica glass with heat treatment

We investigated Rayleigh scattering in silica glass with heat treatment under various conditions, including quenching and slow cooling, and its relation to fictive temperature. The Rayleigh scattering intensity varied according to the conditions of the heat treatment. The scattering intensity in slowly cooled samples is less than that in quenched samples after heating at the same temperature. We evaluated fictive temperatures based on measurements of infrared absorption and Raman scattering. The Rayleigh scattering intensity was related to the fictive temperature regardless of the heat treatment conditions, and a linear relation between them was obtained. In addition, we suggest that the decrease in scattering intensity in slowly cooled samples results from structural relaxation due to viscous flow during the cooling process.     

Multilayer colloid formation in soda lime silica glass

Silver and copper colloids are formed on the tin-rich surface of float glass at elevated temperatures via reaction with the tin present from the float process. Optical absorption measurements of samples initially stained with copper and then subsequently stained with silver indicate that the second treatment with silver eliminates the optical absorption due to the copper colloids formed during the first treatment. If silver is applied before copper, both silver and copper colloid absorption bands increase in intensity for initial heat treatments at lower temperatures and shorter times, but no copper colloids are formed if the initial heat treatment produces a very large amount of silver colloids. It is proposed that copper colloids formed during the initial heat treatment serve as nuclei for the subsequent growth of a layer of silver, eventually covering the nuclei of copper with a layer of silver which is thick enough to mask the optical absorption of the original colloids. The reverse does not occur if silver colloids are formed before copper colloids.     

Water diffusion into a silica glass optical fiber

The diffusion coefficient and solubility of water in silica glass optical fiber cladding were measured in the temperature range of 600-800 °C and were compared with the corresponding values of bulk silica glasses. It was found that the diffusion coefficient was slightly lower and the solubility was appreciably higher in optical fiber, especially at low temperatures, compared with those in bulk silica glasses. The observed trend was consistent with the expected effect of fictive temperature.     

Doubly positively charged oxygen vacancies in silica glass

The geometric and electronic structures of doubly positively charged oxygen vacancies in a-SiO₂ are calculated. We have found four types of relaxed configurations in the amorphous matrix, corresponding to puckered and unpuckerd configurations of Si atoms of the vacancy. The predicted optical absorption strongly depends on the atomic configuration of particular center and transition energies are distributed in the range from 4.5 to 6.5 eV.     

Irradiation effects on the absorption edge of silica glass

Vacuum ultraviolet absorption experiments were carried out on a variety of specimens of amorphous silica β-irradiated at different doses from ∼10³ to 5 × 10⁶ kGy. Changes in the width of the absorption (Urbach) edge were investigated. These changes strongly depend on the kind of silica considered: in particular the Urbach energy of silica of industrial manufacture increases in the irradiated samples, whereas in sol–gel silica it is poorly influenced by the irradiation. The fictive temperature of the different materials before and after irradiation was also monitored. The changes of the Urbach energy and of the fictive temperature are tentatively discussed considering the disorder degree induced by irradiation.