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.     

Hydrogen diffusion in fused silica at high temperatures

Hydrogen diffusion in a silica glass at high temperature (750–1500 °C) was investigated. The starting material was a silica glass that was melted in hydrogen atmosphere and contained a high concentration of hydroxyl and hydride. After heat treatment at various temperatures in air, the effective hydrogen diffusion coefficients were evaluated from the removal rates of hydroxyl and hydride. The obtained diffusion coefficients exhibited an unusual temperature dependence, with the lower diffusion coefficient being observed at higher temperature in the temperature range of 1000–1500 °C. This was attributed to the higher reactivity of hydrogen with the glass network at higher temperatures, effectively slowing down the hydrogen mobility. Below 1000 °C, the effective hydrogen diffusion coefficient exhibited a normal temperature dependence. This reversal of the temperature dependence of the effective hydrogen diffusion coefficient appears to be related to the reversal of the temperature dependence of the hydroxyl (or hydride) solubility in silica glass.     

Optical attenuation in titania-silica glasses

The bleaching of bulk titania-silica glasses in the annealing temperature range is investigated. The sample-size kinetics can be predicted from the hydrogen diffusion coefficient. Hydrogen gas is evolved from both titania-silica and silica glass but in larger amount from the former. In the bleaching process, Ti³⁺ ions disappear from the titania-silica glass. These results are consistent with the coloration caused by Ti³⁺ which is oxidized by OH- in the glass to produce gaseous hydrogen.     

Sodium tracer diffusion in glasses of the type (CaO·Al2O3)x(2 SiO2)1−x

Tracer diffusion coefficients of the radioactive isotope Na-22 were measured in glasses of the type (CaO·Al₂O₃)_x(2 SiO₂)_1−x to study the diffusion of sodium as a function of glass composition, x, temperature and initial water content. The diffusion of Na-22 in glasses diffusion-annealed in dry air can always be well described by a single tracer diffusion coefficient, but sometimes not in samples annealed in common air. It was found that the sodium tracer diffusion coefficient decreases by about six orders of magnitude when the glass composition x changes from 0 to 0.75 at 800 °C. The temperature dependence of the diffusion of sodium seems to decrease as the silica content increases. Variations of the initial water content in some of the glasses investigated did not very significantly influence the rate of the tracer diffusion of sodium.     

Size effect on surface structural relaxation kinetics of silica glass sample

It is known that surface structural relaxation takes place more rapidly than bulk structural relaxation, especially in the presence of water vapor. The surface structural relaxation kinetics of the silica glass fiber and plates was compared at 950 °C and the surface structural relaxation kinetics of silica glass fiber was found to be faster than that of the silica glass plate, even though the composition and initial fictive temperatures of the samples were the same. The observed difference of the surface structural relaxation kinetics between silica glass fiber and silica glass plate can be accounted for using a diffusion equation with time-dependent surface concentration. The analysis indicates that there is a general size effect on the surface structural relaxation kinetics, with smaller sized samples exhibiting faster relaxation kinetics.     

Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5 μm grown on silicon can be taken before and after etching a thin surface layer of 20–30 nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2 μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.     

Source of optical loss in tellurite glass fibers

Non-silica-based optical fibers generally suffer from high optical loss and low strength, whereas they have other optical properties superior to that of silica for certain device applications. Tellurite glasses offer one of the best compromises among optical, mechanical, and processing properties. The goal of this study was to achieve low loss tellurite fibers for active and non-linear applications. In this paper, single mode tellurite fibers doped with KNbO₃ were made with losses varying from 1.3 to 6 dB/m. The sources of loss were striation, dust particles, and bubbles. The decrease of striation was observed by employing lower pouring temperature, and the increase of fiber strength was achieved by hydrochloric acid etching of preform.     

Photoluminescence in as-drawn and irradiated silica optical fibers: an assessment of the role of non-bridging oxygen defect centers

The spectral and temporal characteristics of the photoluminescence in as-drawn and irradiated silica and doped silica fiber-optic waveguides have been investigated. The extended pathlength available with a fiber-optic geometry has offered the opportunity to make high sensitivity emission measurements on high silica glasses under both steady state and pulsed laser excitation. The analyses of the fiber data coupled with emission studies on selectivity doped bulk glasses suggest that the dominant emission band centered near 650 nm is intrinsic to defects in the SiO network, specifically, dangling non-bridging oxygens ions which can be generated by irradiation, fiber drawing or by the introduction of network modifying ions such as alkali.     

Interdiffusion of hydrogen and alkali ions in glass surfaces

The Boksay-Doremus theory on the interdiffusion of hydrogen and alkali ions in glasses was extended by taking into account the association of hydrogen ions and non-bridging oxygen ions. In the derivation of the interdiffusion coefficient the bulk composition of the glass is taken into account. Theoretical concentration profiles were compared with experimental data, reported in literature, of alkali-alkaline earth-silicate glasses and of a sodium-aluminium-silicate glass. An essential feature of this extended B-D theory is, that the relative order of chemical durability of alkaline earth containing glasses is found by this theory without assuming excessive changes in individual diffusion coefficients.     

Light scattering in a number of optical grade glasses

Light scattering measurements have been made on a number of commercially available optical grade glasses. By measuring the spectrum as well as intensity of the scattered light it was found possible both to obtain the total scattered intensity and to identify the origin of the scattering. The optical attenuation coefficient due to scattering was evaluated. It was found that in all glasses studied the total scattered intensity exceeds that in fused silica by at least a factor of two.     

Interfacial reactions between tellurite melts and silica during the production of microstructured optical devices

Interfacial reactions between silica glass and tellurite melts were studied under confined conditions in the temperature regime of 400-700 °C, applying two different sampling techniques: isothermal heat-treatment of a several micrometer thick tellurite film, confined in a silica/tellurite/silica sandwich, and capillary filling of tellurite melts into silica microcapillaries. The sandwich technique provides detailed ex situ insights on the interface chemistry, microstructure and diffusion after given treatment times and temperatures. Data on dynamic viscosity, surface tension, wetting behaviour and eventual scaling effects was obtained from the capillary filling technique. For temperatures > 500 °C, silica is completely wet by the considered tellurite melts. At T > 600 °C and for a treatment time of 20 min or longer, cationic diffusion of Na⁺ and Te⁴⁺ into the silica substrate occurs to a depth of several micrometers. At the same time, the tellurite melt attacks the silica surface, leading to the formation of a stationary silica-tellurite reaction layer and silica dissolution. Dissolved silica was observed to re-precipitate from the tellurite melt by liquid-liquid phase separation. In the early reaction stages, as a result of alkali diffusion into the silica substrate, β-quartz crystallizes at the interface (what can be avoided by using alkali-free filling glasses). Obtained data set the boundary conditions for the generation of tellurite-silica all-solid fiber waveguides by melt infiltration of silica photonic crystal fibers or microcapillaries.     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

Remark on the optical gap in ZnO-Bi2O3-TeO2 glasses

Four ZnO-Bi₂O₃-TeO₂ glasses were prepared from high purity (4N5) oxides. From measurements of the optical transmission on very thin bulk samples the optical gap was determined at around 3.55 eV for the glasses studied. The temperature dependence of the optical gap was also determined from the room temperature close up to 500 K. Preliminary Raman scattering measurements indicate that with a decrease in TeO₂ content, TeO₄ trigonal bipyramid transformation proceeds into TeO₃ trigonal pyramids.     

Influence of morphology on in vitro compatibility of bioactive glasses

In vitro bioactivity and biocompatibility of fiber and bulk bioactive glasses were studied. After synthesis with tetraethoxysilane, individual bulk and fiber samples were soaked in a simulated body fluid solution and removed at intervals of 5 and 30 days, respectively. Fourier transform infrared spectroscopic analysis showed hydroxyapatite characteristic P–O vibrational bands for simulated body fluid soaked fibers and bulk samples. Osteoblast viability and alkaline phosphatase production were evaluated by cell incubation in the presence of samples and compared to controls. Fiber glass-incubated osteoblasts showed lower viability but the same alkaline phosphatase production when compared to bulk sample and controls.     

Probing the erbium ion distribution in silica optical fibers with fluorescence based measurements

Accurate determination of the rare earth dopant distribution in optical fibers enhances our understanding of the fiber manufacture process and enables further improvement in the design of fiber based products such as optical fiber lasers and amplifiers. Here a simple theoretical model consisting of an ensemble of rate equation systems, characteristic of the most likely electronic transitions that take place in the vicinity of erbium (Er³⁺) doped silica glasses, is developed and solved. Through this theoretical study it is established that information about the relative Er³⁺ ion distribution in fibers can be inferred by simply monitoring the backscattered fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. Following these theoretical studies a fluorescence intensity confocal optical microscopy (FICOM) scheme was employed to investigate the Er³⁺ ion distribution profiles in a range of silica optical fibers. The validity of the proposed theoretical model was confirmed through a comparison of the Er³⁺ ion distribution profiles acquired using the FICOM technique and those obtained from the application of a powerful analytical ion probe.     

Adequacy test of the fictive temperatures of silica glasses determined by IR spectroscopy

Adequacy of the fictive temperature determined by the IR spectroscopy method was examined for various silica glasses by measuring the extent of the memory effect. The IR method of fictive temperature measurement of silica glasses relies upon the assumption that a silica glass has a silica structural band with a unique wavenumber when it has a particular fictive temperature. Silica glasses with various impurities or added components such as Cl, F, and OH were given a cross-over heat-treatment and the extent of the resulting deviation of the wavenumber during the second stage heat-treatment at a constant temperature corresponding to the apparent fictive temperature of the sample was determined. Silica glasses containing high concentrations of water or fluorine exhibited distinct memory effects while glasses with low water concentration or Cl did not. These results seem to indicate that the main source of the memory effect in silica glasses is the composition fluctuation rather than density fluctuation. Thus, the IR method of determining the fictive temperature is adequate for high purity silica glasses even when the glass samples have unknown thermal history.     

Water diffusion in silica film during silicon wet oxidation

Water diffusion in silica glass is explained in terms of the migration of molecular water and its reaction with the silica network to form immobile hydroxyl water. It is assumed that the reaction is fast at high temperatures, and a local reaction equilibrium is maintained during water diffusion. While this mechanism appears to explain water diffusion in silica glasses determined from hydroxyl profiles reasonably well, we found that water diffusion-promoted phenomena, such as the oxidation of silicon, require a small quantity of molecular water diffusing into silica glasses ahead of the hydroxyl diffusion front, unrestricted by the local equilibrium. This was demonstrated by studying oxidation kinetics of silicon with a thick (∼15 μm) oxide layer and measuring the delay time before the steady-state oxidation.     

Chemical annealing of oxygen hole centers in bulk glasses

Chemical annealing of the oxygen hole centers in bulk glasses by using hydrogen (or deuterium) has been extensively reported in the literature. Hydrogen chemically anneals these defect centers by reacting with them to form hydroxyl species. We have here presented a reaction-diffusion model to predict the chemical annealing rates of these defect centers in bulk glasses. The model considers diffusion of hydrogen (or its isotopes, e.g., deuterium) through the glass and its simultaneous reaction with the oxygen hole defect centers, resulting in the formation of the hydroxyl species in the glass. The predictions from the model are in excellent agreement with the experimental data presented in [J. Appl. Phys. 60 (12) (1986) 4325]. The model also establishes the precise connection between the dependence of effective hydrogen diffusivity on its intrinsic diffusivity, temperature, hydrogen and defect concentration. We also discuss methods for understanding chemical annealing behavior at high temperatures, where hydrogen not only reacts with the oxygen hole centers, but also with the silica matrix. For the case where hydrogen interaction with the silica matrix is the dominant reaction, a reaction-diffusion model is presented and the equilibrium constant (between 500 and 900 °C) for the reaction is estimated by comparing the predictions from the model with the hydroxyl profiles reported in [Appl. Phys. Lett. 47 (3) (1985) 328; J. Appl. Phys. 61 (12) (1987) 5447].     

Surface stress relaxation and resulting residual stress in glass fibers: A new mechanical strengthening mechanism of glasses

Glass surfaces exhibit faster relaxation than the bulk at a constant temperature in the presence of either a liquid water or water vapor atmosphere. Permanent bending of glass fibers at temperatures lower than their glass transition temperatures in an atmosphere containing water vapor or in liquid water was attributed to and analyzed in terms of surface stress relaxation and the resulting surface residual stress. It was found that the fiber bending kinetics are controlled by water diffusion into the glass surface and that glasses with a faster transition crack velocity from region I (linear relation between logarithm of crack velocity and stress intensity factor) to the fatigue limit exhibited a faster rate of surface stress relaxation. It was suggested that this surface stress relaxation and resulting residual stress is the cause of various strengthening phenomena of glasses such as static fatigue limit and coaxing.     

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.