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.     

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.     

Nanoindentation of glass wool fibers

The nanoindentation technique is used to analyze the depth dependence of the hardness and the reduced elastic modulus of bulk glasses and glass wool fibers (4-12 μm in diameter) of calcium aluminosilicate composition. In spite of the fiber geometry and the delicate sample mounting-technique, nanoindentation proves to be a relatively accurate method that provides reproducible data for both hardness (H) and reduced elastic modulus (E_r) of thin glass fibers. It is found that H and E_r are generally lower for the fiber than for the bulk sample. Within a given fiber, both H and E_r are approximately constant with increasing indentation depth. However, both of these parameters decrease with diminishing fiber diameter. This trend is attributed to an increase of the free volume of the fibers with decreasing fiber diameter, i.e. to an increase of the fictive 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.     

Indentation response of glass with temperature

The mechanical response of different glasses to a Vickers indentor has been investigated between room temperature and T_g+50 °C. The permanent deformation, from which hardness is estimated, as well as the brittle fracture characteristics, allowing for an evaluation of the fracture toughness, were measured and analysed. Comparison between a standard float glass and advanced glasses such as chalcogenide (with mainly covalent bonding) and metallic glasses was made to get a more general insight into high temperature indentation behaviour. As temperature increases, the glass response becomes more and more time-dependent, and in the vicinity of T_g the permanent deformation was observed to increase rapidly for all glasses. Further, while the standard float glass showed an enhanced apparent toughness at elevated temperatures due to a brittle to ductile transition, almost no change in apparent toughness was revealed in the GeAsSe glass emphasizing the time-dependent response of glass at elevated temperature.     

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.     

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.     

Point defect formation and annihilation in silica glass by heat-treatment: Role of water and stress

It was discovered that E′ centers were created by heat-treatment when silica glass contains water and has residual stress. Silica glass samples were heat-treated at 1000 °C for various lengths of time in 355 torr (47 000 Pa) water vapor pressure and dry nitrogen gas atmospheres. The electron paramagnetic resonance (EPR) signal of E′ centers increased initially with heat-treatment time in both atmospheres but then decreased afterwards in the wet atmosphere. It is known that water molecules eliminate paramagnetic defects, such as E′ centers and non-bridging oxygen hole centers (NBOHCs) by reacting with these defects in the glass, transforming them to non-paramagnetic species such as Si-OH or Si-H. The present study indicates that water molecules are also capable of initially creating paramagnetic defects in the glass structure by breaking the silica network structure in the presence of stress. The present observation may be relevant to mechanical strength reduction of silica glasses, which is commonly observed in the presence of water and stress.     

Effect of organic acid vapors on the alteration of soda silicate glass

Organic acids were previously shown to be involved in the alteration of historic soda silicate glasses in humid atmospheres under museum storage conditions. The present study investigates the role of these pollutants on the visual, compositional and structural modification of soda silicate glasses. Replica glasses aged in humid or humid/acidic atmospheres under accelerated conditions were examined and compared using light microscopy, electron microprobe and Raman spectroscopy. The characteristic modifications induced by each atmosphere are described. In the acid polluted atmosphere the leaching process created a layer that retained the transparency of the glass but with a chemical structure hydrated and more polymerized following the loss of alkali and the associated non-bridging oxygens, and the formation of new bridging bonds. In an unpolluted humid atmosphere, the dissolution process caused disruption of the silicate network at the glass surface and formation of an opaque gel layer. The hydrated silicate species and the cations in this gel layer subsequently polymerized to form a new amorphous material, hydrated and more depolymerized than the original glass. This investigation confirms that organic acid pollutants are responsible for the modifications observed on altered historic soda silicate glasses in the collections of the National Museums of Scotland.     

Kinetics of relaxation and crystallization of sodium metaphosphate glass

Kinetics of structural relaxation and crystallization of NaPO₃ glassforming melt is studied by means of thermal analyses. It is demonstrated that activation energy depends strongly on fictive temperature T_f. The dependence of the onset temperature T_o of the glass transition interval on the heating rate q⁺ is investigated for samples that were previously cooled down at a rate q⁻ of about 850 K/min. The dimensionless fragility F is a measure of the dependence of the activation energy for spatial rearrangement on the changes of structure. According to the present results F is large for NaPO₃ (i.e. phosphates are ‘fragile’ substances).     

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.     

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.     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

Non-isothermal crystallization kinetics of ZnO-BaO-B2O3-SiO2 glass

Glass of composition 40SiO₂-30BaO-20ZnO-10B₂O₃ (mol%) was made by conventional melting and casting process. Crystallization kinetics of above glass has been investigated under non-isothermal conditions, using the formal theory of transformations for heterogeneous nucleation. The procedure is applied to the experimental data obtained by differential thermal analysis (DTA), using continuous-heating techniques. The crystallization results are analyzed, and both the activation energy of crystallization process as well as the crystallization mechanism is characterized. Dilatometric measurement of this glass was also done and data obtained was used to calculate the viscosity of the formed glass. Development of crystalline phases on thermal treatments of the glass at various temperatures has been analyzed by X-ray diffraction. Microstructure of the crystalline phases was investigated by scanning electron microscopy (SEM) and the development of various structural features with variation in heat treatment cycle was observed. The nucleation and growth of these phases in the matrix of glass has been described and discussed.     

Roughness of glass surfaces formed by sub-critical crack growth

This paper presents a study on the roughness of glass fracture surfaces formed as a consequence of sub-critical crack growth. Double-cantilever-beam specimens were used in these studies to form fracture surfaces with areas under well-defined crack velocities and stress intensity factors. Roughness depends on crack velocity: the slower the velocity, the rougher the surface. Ranging from approximately 1 × 10⁻¹⁰ m/s to approximately 10 m/s, the velocities were typical of those responsible for the formation of fracture mirrors in glass. Roughness measurements were made using atomic force microscopy on two glass compositions: silica glass and soda lime silica glass. For silica glass, the RMS roughness, R_q, decreased from about 0.5 nm at a velocity of 1 × 10⁻¹⁰ m/s to about 0.35 nm at a velocity of 10 m/s. For soda lime silica glass, the roughness decreased from about 2 nm to about 0.7 nm in a highly non-linear fashion over the same velocity range. We attributed the roughness and the change in roughness to microscopic stresses associated with nanometer scale compositional and structural variations within the glass microstructure. A theory developed to explain these results is in agreement with the data collected in the current paper. The RMS roughness of glass also depends on the area used to measure the roughness. As noted in other studies, fracture surfaces in glass exhibit a self-affine behavior. Over the velocities studied, the roughness exponent, ζ, was approximately 0.3 for silica glass and varied from 0.18 to 0.28 for soda lime silica glass. The area used for these measurements ranged from (0.5 μm)² to (5.0 μm)². These values of the roughness exponent are consistent with values obtained when the scale of the measurement tool exceeds a critical size, as reported earlier in the literature.     

The glass transition temperature versus the fictive temperature

A comparison of the values of the glass transition temperature (T_g) measured on cooling and the limiting fictive temperature measured on heating as a function of cooling rate is performed for a polystyrene sample using both capillary dilatometry and differential scanning calorimetry (DSC). The results from both techniques indicate that is systematically lower than T_g presumably due to the breadth of the relaxation on cooling. The Tool-Narayanaswamy-Moynihan (TNM) model is used to fit the experimental data from dilatometry and DSC in order to ascertain the origins of the higher value of T_g compared to .     

Threshold switching in niobate glass — thick-film devices

Thick-film, niobate-glass threshold switches have been made in reducing gas atmospheres at high temperatures (1100–1400°C) using SiO₂ and B₂O₃ as glass formers. Their temperature characteristics are far superior to those of similar vanadate switches previously reported [1,2], and they are stable at low ON currents (< mA) if they are protected from atmospheric oxidation. They do not require forming.The process consists in pre-firing the printed, powdered glass in air and then reducing in argon/hydrogen gas. While the reducing conditions are not critical and virtually full reduction to tetragonal NbO₂ occurs, the pre-firing conditions in air are critical if high-resistance devices are to be obtained. Many devices can be made on a single alumina chip and the results of a statistical analysis are given.The electrical characteristics — measured, chiefly, using a double-pulse technique to examine the change in delay time with pulse separation, ON current, pulse width, etc. — are similar to those previously reported for the vanadate switches [2]. These measurements, together with the results of a microscope and electron-microprobe examination, indicate that a thermal mechanism of switching with filament or channel formation is the most likely. A steady-state computer simulation based on the thermal model indicates the temperatures to be expected during switching, and these are calculated to be less than the NbO₂ semiconductor-metal transition temperature of roughly 800°C, for all the cases examined.     

Indentation-induced microhardness changes in glasses: Possible fictive temperature increase caused by plastic deformation

The microhardness around a large indentation was measured for different types of glasses. In soda-lime silicate glass, a typical normal glass, the region in the immediate vicinity of the indentation was found to exhibit a lower hardness than the region far removed from the indentation. In silica glass, a typical anomalous glass, the region in the immediate vicinity of a large indentation was found to exhibit a higher hardness than the region far removed from the indentation. Asahi less brittle glass, an intermediate glass between normal and anomalous glasses, was found to exhibit little change in hardness in the vicinity of the large indentation. These findings can be explained by a deformation-induced fictive temperature increase leading to a lower hardness for soda-lime silicate glass and a higher hardness for silica glass.     

An electrospray technique for hyperquenched glass calorimetry studies: Propylene glycol and di-n-butyl phthalate

We describe an electrospray technique for in situ preparation, for differential scanning calorimetry study, of samples of molecular liquids quenched into the glassy state on extremely short time scales (hyperquenched). We study the cases of a hydrogen-bonded liquid, propylene glycol, PG and a Van der Waals liquid, di-n-butyl phthalate DBP. Using a fictive temperature method of obtaining the temperature dependence of enthalpy relaxation, we show that the electrospray method yields quenching rates of ∼10⁵ K/s, while the more common method, dropping a sealed pan of sample into liquid nitrogen, yields only 120 K/s. These hyperquenched samples start to relax, exothermically, far below the glass temperature, at a temperature (0.75T_g) where the thermal energy permits escape from the shallow traps in which the system becomes localized during hyperquenching. This permits estimation of the trap depths, which are then compared with the activation energy estimated from the fictive temperature of the glass and the relaxation time at the fictive temperature. The trap depth in molar energy units is compared with the ‘height of the landscape’ for PG, the quasi-lattice energy of the liquid based on the enthalpy of vaporization, and the single molecule activation energy for diffusion in crystals. The findings are consistent with the mechanism of relaxation invoked in a current model of relaxation in glassforming liquids. In the case of di-n-butyl phthalate we investigate the additional question of sub-T_g annealing effects. We find the ‘shadow’ glass transition, (an annealing prepeak) seen previously only in multicomponent mineral and metallic glasses. The phenomenon is important for understanding microheterogeneities in viscous liquid structures.     

Waveguide produced by fiber on glass method using Er-doped tellurite glass

We report the fabrication of waveguides using the fiber on glass (FOG) method. Taking advantage of a Thermal Mechanical Analyzer (Shimadzu TMA-50), we were able to produce a new type of waveguide by coupling an erbium doped fiber core onto a planar glass substrate. Both optical fiber core and substrate were fabricated from tellurite glass. Important thermal characteristics of the substrate and fiber like the transition temperature T_g, the temperature for the crystallization onset T_x and the maximum crystallization temperature T_c were determined by Differential Thermal Analysis (DTA). The thermal expansion coefficient of the tellurite glass was determined by Thermal Mechanical Analysis (TMA).