Metastable equilibrium density of hydroxyl-free synthetic vitreous silica

It is shown that the volume behaviour of hydroxyl-free vitreous silica (type IV silica glass) is similar to that of the so-called natural vitreous silica (fused quartz, type I and II silica glasses) with low hydroxyl content. Thus, in view of that, a distinction may be made between two groups, types I, II and IV on the one hand and type III, the hydroxyl-rich silica glasses, on the other hand, although a distinction may also be made between the group of “natural” (type I and II) and “synthetic” silica glasses (type III and IV) in view of production method and starting material (quartz or SiCl₄). It is further shown that the type III silica glasses give an example for a change from the anomalous volume behaviour of “pure” silica glasses (thermal expansion, high-temperature minimum of volume) to the common behaviour of silicate glasses.     

Effect of heat treatment on the low-temperature thermal conductivities of vitreous silica and neutron-irradiated vitreous silica

Heat treating neutron-irradiated vitreous silica erases the effects of irradiation on the thermal conductivity at temperatures below 1 K. This indicates that the density of low-energy, localized excitations in vitreous silica is restored to its pre-irradiation value.     

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.     

The optical constants of quartz,vitreous silica and neutron-irradiated vitreous silica (I)

The infrared reflectivity of three forms of silica, α-quartz, vitreous silica and neutron-irradiated vitreous silica (≈ 2.7 × 10²⁰ neutrons cm^?2) has beenmeasured from 400–2000 cm^?1. These data have been analysed by a Kramers-Kronig transform to give the real and imaginary parts of the complex dielectric constant. Considerable care has been taken to identify and minimize errors arising in the measurements of the reflectivity spectra and in the subsequent analysis. Data are presented for the optical constants, oscillator frequencies, band strengths and halfwidths of each band. The spectra for vitreous silica and neutron-irradiated silica show two regions of absorption which are not present in the crystalline form — a strong band is observed near 950 cm^?1 and a broad band from 600–800 cm^?1. A difference spectrum obtained by subtracting the spectrum of the imaginary part of the dielectric contrast for vitreous silica from the corresponding data for neutron-irradiated silica reveals more detailed structure in the form of a weak but sharp band at 620 cm^?1. Interpretation of these results is contained in a companion paper.     

The interstitial structure of vitreous silica

A careful analysis of gas solubility data provides a unique tool for characterizing glass network geometry. The density of interstitial (solubility) sites in vitreous silica and its crystalline analog, high cristobalite, was reviewed. Close inspection of high cristobalite indicates that there are eight solubility sites per unit cell giving a bulk density of 2.34 × 10²² sites/cm³. A statistical thermodynamic analysis of experimental solubility data for helium and neon in high cristobalite correlates with this calculated value. Correcting this value for the slightly lower bulk density of vitreous silica gives an estimated density of solubility sites in vitreous silica of 2.22 × 10²² sites/cm³. Literature data for solubility site density (obtained from the same statistical thermodynamic analysis) depend strongly on the dissolved gas species. This implies a statistical distribution of effective site diameters. The probability distribution function is adequately represented as log-normal.     

Raman spectroscopic study of hydrogen-containing vitreous silica

It is shown by Raman spectroscopy that high-temperature hydrogen-impregnated vitreous silica contains, in addition to physically dissolved hydrogen, ≡SiOH and ≡SiH groups. A Raman-scattering peak with a wave-number shift of 3685 cm^?1 is assigned to the OH stretching vibration, a peak with a wave-number shift of 2254 cm^?1 to a SiH stretching vibration and a peak with a wave-number shift of 4135 cm^?1 to the presence of physically dissolved hydrogen. When hydrogen was replaced by deuterium, appropriate shifts of the peaks mentioned were found.     

Thermal properties of vitreous silica with He impurities

The specific heat and thermal conductivity of vitreous silica (Suprasil W) doped with He at 420°C and 200 bar has been measured at low temperatures (0.5 K to 8 K). While with respect to an undoped sample the specific heat shows an excess contribution which varies as T^1.7, no effect of doping on the thermal conductivity is observed. This shows that the coupling to phonons of the excitations introduced by He doping is weak compared to that of intrinsic low energy excitations common to glasses. The excess specific heat can be attributed to excitations of He atoms in cavities.     

Microwave assisted diffusion of Au-nanoclusters in vitreous silica

Gold nanocluster composite glasses were prepared via the evaporation of a thin gold layer, followed by microwave-assisted diffusion. Increased diffusion using a microwave (MW) oven as compared to the traditional diffusion in a muffle furnace is observed and measured. The influence of diffusion temperature and time on diffusion depth and surface plasmon resonance (SPR) absorption bands of the so-obtained gold nanocluster have been studied. The diffusion parameters have been shown to influence the size of the metal nanoclusters and hence, the position of the SPR absorption bands.     

Influence of thermal treatments on the low temperature specific heat of Suprasil-W vitreous silica

We report specific heat results between 0.04 and 4 K in Suprasil-W vitreous silica submitted to thermal treatments at 1100 and 1300°C, on samples of different sizes. In the very low temperature range, i.e. below 0.5 K, the specific heat mainly due to the two-level systems is only slightly sensitive to the annealing. On the contrary, the large anomaly localized at 1–2 K is very sensitive to the conditions of the thermal treatments and to the shape of samples. Comparison is made with a similar anomaly induced in crystalline quartz by fast neutron irradiation at moderate doses.     

Raman spectroscopic investigations of Mn2+ doping effects on the densification of acid-catalyzed silica xerogels

Undoped and Mn²⁺-doped silica xerogels were prepared from hydrolysis and condensation of tetramethyl orthosilicate (TMOS). The xerogels were characterised by density measurements and fluorescence and Raman spectroscopies. Raman measurements over the range 4–1200 cm⁻¹ showed that the number of three- and four-membered rings in the xerogel network depends on the thermal treatment and on the concentration of Mn²⁺ ions. Indeed, both structures are found to be more numerous in the gel network of the doped samples than in the undoped one, showing that doping with Mn²⁺ hampers the destruction of three- and four-membered rings. In the low-wave number region (4–100 cm⁻¹), doping with manganese ions was found to affect the position of the boson peak. The boson peak profiles were used to deduce that the sizes of the cohesive domains in the gel-derived silica network are much larger for doped samples (11 nm for 500 ppm) than for undoped ones (2.1 nm).     

Molecular dynamics study of UV-laser-induced densification of fused silica. II. Effects of laser pulse duration,pressure, and temperature,and comparison with pressure-induced densification

Numerical simulations on UV-laser-induced densification of fused silica have been performed using classical molecular dynamics. The effect of laser irradiation is modeled by the energy transfer from the absorbed laser photons to the bonded silicon and oxygen atoms, i.e., the thermal effects of laser-irradiation on silica glass. Results from simulations at various laser pulse duration, pressure, and temperature conditions show that longer laser pulse duration, higher pressure, and higher temperature cause larger densification. We have also compared the microstructural and elastic properties of fused silica densified by UV-laser and hydrostatic pressure, respectively. Similar changes are observed in both cases; several notable differences are noticed, too, and include Si–O bond length change, number of over-coordinated atoms, and ring distributions.     

Study of fast-neutron irradiated vitreous silica by positron annihilation methods

The nature of the glass-like structure as obtained by fast neutron irradiation of vitreous silica has been investigated by positron annihilation lifetime and Doppler broadening experiments. Comparing the annihilation parameters obtained in the unirradiated sample to those obtained in irradiated ones we observed that in the irradiated SiO₂ glasses the probability of positronium formation is reduced and the orthopositronium lifetime is increased. The decrease of the positronium formation probability is mainly attributed to the reduction of the amount of free-volume in compacted domains. The orthopositronium lifetime increase is explained by large free spaces generated along trajectories of fast knocked-on particles.     

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.     

Low-temperature specific heat of vitreous silica containing hydrogen or helium solutes

The specific heat of vitreous silica has been measured in the temperature range 0.1–0.6 K to determine if solute gases might contribute to the low-temperature anomaly which is common to all amorphous materials. The specific heat was found to be independent of hydrogen or helium impurities intentionally introduced into the samples.     

Anomalous heat capacity of vitreous silica: Effect of interstitial 4He

⁴He (gas or liquid) sorbs into SiO₂ glass at liquid helium temperatures. The term in the heat capacity of vitreous silica which is linear in temperature is increased by the presence of interstitial ⁴He. (The linear term is believed to be generally characteristic of glasses). A number of experiments on SiO₂ glass are suggested using H₂, ³He, and ⁴He as probes of mass 2, 3, and 4 to explore the mechanism which is responsible for the anomalous heat capacity.     

Fluorine doped vitreous silica analysis of fiber optic preforms by vibrationnal spectroscopy

Raman spectra of fluorine doped silica samples have been investigated. A model for dopant incorporation into the silica network is suggested, involving the formation of a SiO₃F tetrahedron. The relationship between refractive index and intensity of the γ (SiF) vibration band is presented.     

Indentation crack initiation behavior of vitreous silica glasses containing different hydroxyl concentration

The indentation crack initiation behavior of eight vitreous silica specimens containing bulk OH concentrations ranging from 0.2 to 754 wt ppm was investigated. A recording microindentation instrument equipped with optical observation and acoustic emission detection was used to study, in situ, the cracking behavior from indentation with a Vickers diamond. No significant differences in the threshold loads for various types of cracking behavior of the specimens were found. In addition, the polishing medium was found to have little influence on the cracking behavior. The lengths of median-radial cracks around indentations varied little between specimens. The Vickers hardness of the specimens measured at 0.98 N ranged from 6.6 ± 0.3 GPa to 7.5 ± 0.7 GPa, and no trend with the OH concentration was apparent. In addition, the Vickers hardness of the specimens measured while under a 9.81 N maximum load (LVH_max), showed little variation, and no apparent trend with the OH concentration.