Annihilation of E′ center defects in silica glass by hydrogen treatment

It was found, using electron paramagnetic resonance (EPR), that the signal of E′ centers in silica glass totally disappeared following a 1 h heat-treatment at 1000 °C under hydrogen atmosphere. However, by subsequent heat-treatment at the same temperature under a dry nitrogen atmosphere, some of the E′ centers re-appeared.     

27Al NMR structural study on aluminum coordination state in bismuth doped silica glass

The aluminum coordination state in bismuth doped silica glass, which has new broad infrared emission at 1.3 μm regions, was investigated by using ²⁷Al NMR, and it is demonstrated that 6-fold coordinated aluminum ions with corundum structure are dominant in bismuth doped silica glass until Bi₂O₃ concentrations of 1.0 mol% with Al₂O₃. The aluminum ion efficiently affects the creation of a Bi luminescent center at an intensity of Bi₂O₃ (1.0 mol%)–Al₂O₃ (2.3 mol%)–SiO₂ (96.7 mol%); the sample is three orders of magnitude larger than the Bi₂O₃ (1.0 mol%)–SiO₂ (99.0 mol%) sample. Aluminum ions with corundum structure in silica glass have a very important role for the configuration of peculiar Bi luminescent centers.     

High resolution O MAS and triple-quantum MAS NMR studies of gallosilicate glasses

Gallosilicate (or ‘galliosilicate’) glasses have been widely studied as analogs of aluminosilicates, and the variations with composition in properties are known to be similar in both systems. We have applied ¹⁷O MAS and triple-quantum MAS (3QMAS) NMR spectroscopy to investigate the oxygen local environments in Na-, Li-, Ca- and Y-gallosilicate glasses. Signals due to several different oxygen species can be resolved and their concentrations quantified and NMR parameters determined. The NMR spectra generally resemble those of aluminosilicate glasses, indicating that the current model of gallium ions occupying the same types of sites as aluminum ions is a good first approximation. Quadrupolar coupling constants for the various oxygen sites tend to be larger than those for aluminosilicates, however, and vary less among different types of sites. Broader ¹⁷O spectra at low to medium external magnetic fields result. In detail, several types of differences between the observed oxygen species populations for gallosilicate and aluminosilicate are consistent with the larger radius of Ga³⁺ in comparison to Al³⁺, and possibly with a somewhat greater tendency for the former to form groups with oxygen coordination numbers greater than four. These include more Si-Ga disorder than Si-Al disorder in corresponding sodium gallo- vs. aluminosilicates, and more ‘non-stoichiometric’ non-bridging oxygen in a calcium gallosilicate than in a calcium aluminosilicate glass.     

Reduction of copper in soda-lime-silicate glass by hydrogen

The reduction of copper in soda-lime-silicate glasses by hydrogen was measured as a function of time and copper content within the temperature range 440 to 650°C. Copper is reduced to the metallic state in a surface layer whose thickness increases with time and temperature and decreases with copper content. A well developed periodical precipitation of copper particles is observed in the reduced layer. The reduction is accompanied by an alteration of the glass composition in which there is an increase of the copper content and a reduction of the calcium content. The growth kinetic of the reduced layer, followed by optical spectroscopy and measurement of the layer thickness, is analyzed with regard to the tarnishing model.     

Vacancy-induced densification of silica glass

We investigate vacancy-induced densification of silica glass using molecular dynamics simulations. Equilibration of defective glasses initially with various concentrations of vacancies yields glasses denser than the intact glass. The structural and vibrational properties of the densified glasses are characterized. Densification is related to structural changes induced by atomic rearrangement near vacancies, and increases with the concentration of vacancies. Vacancies may cluster and form voids, and the maximum densification for void-structured glasses occurs at a critical radius of about 0.44 nm. The glasses densified by vacancies and by simulated UV-laser irradiation display nearly identical structural and vibrational properties. These results appear to support the Douillard–Duraud point defect model as a common mechanism for radiation densification of silica glass.     

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.     

A MAS NMR structural study of cadmium phosphate glasses

Cadmium phosphate glasses, of general formula xCdO(1−x)P₂O₅ (0.25?x?0.6), have been prepared by melting in alumina crucibles, with resulting dissolution of up to 6.4 mol% Al₂O₃. The local structure in these glasses has been studied using ³¹P, ²⁷Al and ¹¹³Cd nuclear magnetic resonance. The distribution of [PO₄]Qⁿ species as a function of composition has been shown to follow the simple binary model. The rate of change of the chemical shift of the ³¹P species in the Q² environment depends on the bond order, which in turn reflects the extent of double bonding between phosphorus and oxygen.     

Micro-photoluminescence study on defects induced by ion microbeam in silica glass

We have evaluated the irradiation effects by ion microbeam on silica glass for various ion species by means of a micro-photoluminescence technique. Defect generation and refractive index change were observed for silica at the area of 10 μm × 50 μm scanned by ion microbeam of H⁺, He⁺, N⁴⁺, C⁴⁺, O⁴⁺, and Si⁵⁺ with energy from 1.7 to 18 MeV. The μ-PL spectroscopy measurements were performed along the side surface perpendicular to the microbeam irradiated surface. Based on the comparison with a result of SRIM (stopping and range of ions in matter) simulation, the defect generation mechanism was discussed in terms of the energy deposition processes due to electronic and nuclear stopping powers. We conclude that the electronic stopping power is responsible for the defect generation at the track of ions. The effect of the nuclear stopping power is also not negligibly small at the end of range.     

Refractive index and density changes in silica glass by halogen doping

The refractive index and density of halogen-doped silica glasses were measured as a function of fictive temperature, and the relationship between the refractive index and density was discussed on the basis of the Lorentz–Lorenz formula. The effect of halogen doping on the change in the refractive index was divided into two parts. The refractive index change was found to be strongly affected by structural rearrangement resulting from the termination of Si–O–Si bondings by halogens, as well as by the mean polarizability change caused by replacing oxygen with halogen.     

Preparation of silver nanoparticles in soda-lime silica glass by an electrochemical procedure

An electrochemical technique for producing silver particles of nanometric sizes in soda-lime silica glass is presented. The procedure is based on the use of silver iodide as solid electrolyte membrane.     

A large-size silica glass produced by a new sol-gel process

A large-size silica glass was fabricated with a new sol-gel process involving the following procedures: (a) hydrolysis of Si(OC₂H₅)₄ with HCl, (b) addition of silica powders and their high dispersion, (c) adjustment of the pH value by adding ammonia solution, (d) gellation, (e) drying to dry-gel, and (f) sintering to silica glass. With this process a dry-gel plate as large as 520 x 360 mm² was obtained, which was sintered to a 420 x 290 mm² sized silica glass. The effect of pH on the gelation time of the sol, and the correlation between the weight percentage of silica powder and porosity of dry-gel were examined. In addition, fiber preforms were produced by this sol-gel process.     

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.     

IR investigation of density changes of silica glass and soda-lime silicate glass caused by microhardness indentation

Density changes of silica glass and soda-lime silicate glass caused by ball indentation and Vickers indentation were investigated. The IR reflection peak shift of the silica structural band was monitored to determine the extent of the fictive temperature change and the corresponding density change. Under the central portion of the ball indentation, the density of silica glass increased while a change in the soda-lime silicate glass structure was not clear. On the other hand, in the vicinity of the Vickers indentation, the opposite trend was observed. Namely, soda-lime silicate glass exhibited the structural change corresponding to the density decrease, while the structural change of the silica glass was uncertain. The initial density of the silica glass influenced the change of density under ball indentation in such a way that the initial density difference of the glass samples was reduced.     

On the glass transition in vitreous silica by differential thermal analysis measurements

Scanning calorimetry measurements of the glass transition in vitreous SiO₂ (about 120 wt ppm. OH groups) are reported. Data were obtained upon heating after controlled cooling through the glass transition, and after annealing at temperatures between 990 and 1292 K. The onset of the glass transition is at 1247 K, and the supercooled liquid state is reached at 1475 K. The step in the specific heat is (2.9 ± 0.7) J mol⁻¹ K⁻¹. This value, lower than the results of drop calorimetry experiments, agrees with the calculated value from viscosity data. The glass transition is nearly twice as wide as expected from the temperature dependence of the viscosity. Annealing reduces the enthalpy of glasses as usually expected, and the corresponding entropy decrease is in agreement with results for other network glasses. In vitreous silica, depending on the annealing temperature, both exothermic and endothermic processes take place. Based on Davis’ and Tomozawa’s results, endothermic processes upon annealing are attributed to the diffusion of the OH groups.     

Simulation of surface structural relaxation kinetics in silica glass accelerated by water vapor

It is known that surface structural relaxation of silica glass takes place more rapidly than bulk structural relaxation, especially in the presence of water vapor. The effect of water vapor pressure, heat-treatment temperature and initial fictive temperature on the surface structural relaxation kinetics in silica glasses was investigated by measuring the change of the surface fictive temperature determined from the IR reflection peak shift of silica structural bands. The superimposed component of bulk structural relaxation was subtracted from the measured surface structural relaxation data to isolate the true surface structural relaxation kinetics. The obtained surface structural relaxation data as a function of fictive temperature, heating temperature and water vapor pressure were simulated with a model based on the diffusion equation with time-dependent surface concentration. The simulation model was used to predict the surface structural relaxation kinetics of the optical fiber having a high fictive temperature of ～ 1650 °C at 950 °C under 355 torr of water vapor, and it was confirmed that the present model can simulate surface structural relaxation of the fiber reasonably well.     

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.     

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.