Fictive temperature dependence of subcritical crack growth rate of normal glass and anomalous glass

Subcritical crack growth rates of soda–lime–silicate glass, which is a typical normal glass, and silica glass, which is a typical anomalous glass, with different fictive temperatures were measured by the double-cleavage-drilled-compression (DCDC) fracture mechanics technique under both dry and humid atmospheres in order to clarify the effect of the fictive temperature on mechanical strength and fatigue. In the humid atmosphere, the soda–lime–silicate glass with a higher fictive temperature showed a slower crack growth rate than the same glass with a lower fictive temperature while the silica glass with a higher fictive temperature showed a faster crack growth rate than the silica glass with a lower fictive temperature. These results imply that normal glass with a higher fictive temperature is expected to show a higher mechanical strength compared with the same glass with a lower fictive temperature and anomalous glass with a higher fictive temperature is expected to show a lower mechanical strength than the same glass with a lower fictive temperature when tested in ambient air if the flaw size is the same. In the dry atmosphere, the fictive temperature effects on the crack growth rate in both glasses were small and within the experimental error.     

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.     

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.     

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.     

Structural relaxation and optical properties in transparent nanocrystalline β-quartz glass-ceramic

We studied structural relaxation in a nanocrystalline β-quartz solid solution (s.s.) glass-ceramic through density measurement, and the effect of structural relaxation on its optical properties. The density of the glass-ceramic changed as structural relaxation proceeded in the glass phase. It was found that the glass-ceramic whose glass phase has a high fictive temperature shows a high optical transmittance because of less optical scattering. We also demonstrated that the refractive index difference between the crystal and glass phases can be calculated from the scattering coefficients of the specimens with different fictive temperatures. The calculated refractive index of the crystalline phase was in fair agreement with the indices of a high-quartz crystal.     

Fictive temperature measurement of alumino-silicate glasses using IR spectroscopy

A simple IR reflection method was used in a previous study to determine the fictive temperature of silica glasses and a soda-lime glass. The IR method is based upon the fact that the silica structural band of a glass takes a unique wavenumber at a particular fictive temperature. When this method was applied to an alumino-silicate glass in this study, however, the IR reflection peak wavenumber of the glass surface was found to be strongly affected by the reaction of the glass with water vapor in the atmosphere. Still, it was possible to measure the fictive temperature of the alumino-silicate glass using IR spectroscopy by taking an IR reflection of the bulk sample after eliminating the surface layer affected by the reaction with water vapor. The IR peak wavenumber of the silica structural band decreased with increasing fictive temperature for the alumino-silicate glass, similar to silica glass.     

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.     

A glass with high crack initiation load: Role of fictive temperature-independent mechanical properties

The crack initiation load of a series of calcium aluminosilicate glasses and selected commercial glasses were evaluated using Vickers indentation. The results showed that a calcium aluminosilicate glass containing 80 mol% SiO₂, 10 mol% Al₂O₃ and 10 mol% CaO exhibited a high crack initiation load comparable to that of the less-brittle glass (LB glass) developed by Asahi Glass Co., Ltd. It has previously been determined that glasses experience a fictive temperature increase by indentation. The indented region of a glass, therefore, acquires, in general, different mechanical properties, such as hardness and elastic moduli, from the original, unindented glass. The extent of these mechanical property changes depends upon the glass composition and a certain glass composition with fictive temperature-independent mechanical properties can have the deformed region with matching mechanical properties to those of the undeformed region of the glass. It was found that the calcium aluminosilicate glass having no fictive temperature dependence on elastic moduli gave the highest crack initiation load. However, this composition did not coincide with fictive temperature-independence of hardness or density.     

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.     

Deceleration of surface structural relaxation in chlorine-containing silica glass due to chlorine volatilization

Both surface and bulk fictive temperatures of chlorine-containing silica glass were measured using the IR method, after thermal, mechanical and chemical treatments. A metastable equilibrium state at 1200 °C was first established for the glass by heat-treatment and a uniform fictive temperature was observed except for the sample surface created by polishing after the heat-treatment. The densified layer of the polished surface shifted the IR peak wavenumber, making the fictive temperature appear higher than the bulk. During the second heat-treatment at 950 °C, the sample with the as-heat-treated surface and uniform fictive temperature of 1200 °C developed non-uniform fictive temperature distribution with the bulk fictive temperature becoming lower than the surface fictive temperature. Usually, surface structural relaxation is faster than bulk structural relaxation and the surface fictive temperature becomes lower than the bulk fictive temperature when heat-treated at a lower temperature than the initial fictive temperature. The observed anomalous feature was attributed to chlorine volatilization from the glass surface layer creating a high viscosity surface layer. This conclusion was supported by the diffusion data of chlorine in the glass available in the literature.     

Sub-critical crack growth rate of soda-lime-silicate glass and less brittle glass as a function of fictive temperature

Sub-critical crack growth rates of soda-lime-silicate glass and less brittle glass with different fictive temperatures were compared using the DCDC method under both dry and humid atmospheres in order to investigate the origin of the unique mechanical features of the less brittle glass developed by Ito and his collaborators. In both dry and humid atmospheres, the crack velocity of the soda-lime-silicate glass was slower than that of the less brittle glass. For both glasses, the glass sample with higher fictive temperature showed a slower crack growth rate under both dry and humid atmospheres. These observations can be explained by the tendency for the plastic flow at the crack tip; the soda-lime-silicate glass is expected to show easier plastic flow under tension than the less brittle glass, and also the samples with higher fictive temperatures are expected to show easier plastic flow, leading to greater fracture toughness, K_IC, and slower crack growth rate.     

Exponential structural relaxation of a high purity silica glass

While most other glasses exhibit non-exponential structural relaxation characteristics even when the change of fictive temperature is small, a high purity silica glass exhibited exponential structural relaxation. This was demonstrated by showing that the non-exponential exponent or β value of the KWW function of the high purity silica glass approaches unity when the change of the fictive temperature approaches zero both from higher and lower temperature sides of the heat-treatment temperature. The non-exponentiality of the structural relaxation of this glass when fictive temperature change is finite is due to the change of relaxation time during the structural relaxation.     

Preparation of silica and soda-silica glasses by the sol-gel process

Samples of silica glass and soda-silica glasses (containing 1 wt% and 13 wt% Na₂O) were prepared by hydrolysis of alkoxides to form gels, followed by heating the gels at low temperatures, below the glass transformation. The water necessary for hydrolysis was either taken from the atmosphere or added to the starting solution to accelerate the process. In the latter case the gelation time exhibited a minimum for a certain quantity of water added. The heat treatment process to form a glass from a gel was studied by DTA and TGA. At low temperatures (below 100°C) an endothermic DTA peak was observed due to the loss of organic volatiles and water. A large exothermic peak was also found at 276°C for the silica and at 365–395°C for the soda-silica gels. At higher temperatures the 13Na₂O87SiO₂ (wt%) composition gave a DTA trace very similar to the same composition conventionally melted which had been rapidly quenched from well above the transformation range (i.e. with a high fictive temperature). The preparation of bulk samples of the glasses is discussed.     

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.     

Fictive temperature of GeO2 glass: Its determination by IR method and its effects on density and refractive index

A simple IR method of determining the fictive temperature of GeO₂ glass, both surface and bulk, was developed using the same technique developed earlier for silica glass. Specifically, IR absorption and reflection peak wavenumbers of GeO₂ structural bands were found to be correlated with the fictive temperature of the glass. Using this method structural relaxation kinetics can be investigated. Density and refractive index of GeO₂ glass were also measured as a function of fictive temperature.     

Effects of aluminum impurity on the structural relaxation in silica glass

To elucidate effects of Al impurity on the glass-forming process in silica glass, the structural relaxation in Al-containing silica glass, with alkali ions of only trace levels, was investigated by observing the fictive temperature. The fictive temperature was determined by infrared (IR) absorption analysis. Al, even at concentrations lower than 10 wtppm, increases the relaxation time and the activation energy of the -relaxation. It also suppresses the sub-relaxational process due to OH ions. These results indicated that Al should have other effects on structural relaxation than alkali–aluminate complex formation, as has been thought to be the cause for an increase in the -relaxation time and thus the viscosity of silica glass. Furthermore, the structural relaxation does not merely depend on the number of non-bridging oxygen atoms in the glass network.     

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.     

Oxygen and silver diffusion into float glass

In the float glass process, molten glass is floated on a molten metallic tin bath, such that tin penetrates the glass surface. Consequently, the glass has distinctly two different faces; the tin-penetrated face (bottom face) and the opposite face (top face). In this paper, the effects of tin on oxygen and silver diffusion into the top and bottom faces of a soda–lime–silica float glass are reported. It was revealed that oxygen diffusion from the atmosphere into the bottom face at temperatures above glass transition temperature was extremely suppressed near the surface region of the glass. This was not observed for the top face. This effect was ascribed to chemical reactions between the diffused oxygen and Sn²⁺ near the surface of the glass. Silver diffusion was also influenced by the tin due to chemical reactions of Ag⁺ ions with Sn²⁺, leading to the precipitation of nanometer-sized Ag crystals. As a result, the coloration due to the surface plasmon resonance of the Ag crystals was significantly different between the top and bottom faces because of differences in the nucleation and growth processes associated with the Ag crystals.     

Thermal expansion of synthetic fused silica as a function of OH content and fictive temperature

A matrix of synthetic fused silica samples with OH contents from 30 to 1300 ppm and of a fictive temperature from 1000 to 1300 °C has been characterized regarding their thermal expansion with high precision. The thermal expansion increases with fictive temperature and drops with OH content. Although fictive temperature and OH are coupled due to the influence of OH on the relaxation of the network, an independent influence of the OH content on thermal expansion has been observed. This may provide a deeper insight into the impact of impurities incorporated into the fused silica network.     

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.