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.     

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.     

Low-temperature heat capacity of GeO2 and B2O3 glasses: thermophysical and structural implications

The heat capacities of rapidly quenched B₂O₃ and GeO₂ glasses have been measured by adiabatic calorimetry from less than 10 to 350 K. The residual entropies at 0 K obtained from these new results and ancillary data are 11.2±0.8 and 6.6±1.1 J/mol K for B₂O₃ and GeO₂ glasses with fictive temperatures of 980 and 543 K, respectively. On a g atom basis, both of these entropies are similar to the low value found previously for SiO₂ glass. For B₂O₃, this is likely due to the small contribution of boroxol groups to the topological entropy of the glass. The importance of the Boson peak increases in the order SiO₂, GeO₂, B₂O₃ which corresponds to the order of decreasing glass transition temperatures. Some implications about short- and medium-range order are finally drawn from these results and the differences between the glass and crystal phases of the three oxides.     

Glass formation in and structural investigation of Li2S + GeS2 + GeO2 composition using Raman and IR spectroscopy

Li₂S + GeS₂ + GeO₂ ternary glasses have been prepared and a wide glass-forming range was obtained. The glass transition temperatures increase with the GeO₂ concentration in the glasses. The vibrational modes of both bridging (Ge–S–Ge) and non-bridging (Ge–S⁻) sulfurs are observed in Raman and IR spectra of binary Li₂S + GeS₂ glasses. Additions of GeO₂ to this binary glass increase the bridging oxygen band (Ge–O–Ge) at the expense of decreasing the bridging sulfur band (Ge–S–Ge), whereas the bands associated with the non-bridging sulfurs (Ge–S⁻) remain constant in intensity up to high GeO₂ concentrations. At higher concentrations of GeO₂ (⩾60%), the non-bridging oxygen band, which is not observed at low and intermediate GeO₂ concentrations, appears and grows stronger. From these observations, it is suggested that the added lithium ions favor the non-bridging sulfur sites over the oxygen sites to form non-bridging sulfurs, whereas the added oxygen prefers the higher field strength Ge⁴⁺ cation to form bridging Ge–O–Ge bonds. The structural groups in the Li₂S + GeS₂ + GeO₂ glasses that are consistent with results of Raman and IR spectra are described and are used to develop a structural model of these glasses.     

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.     

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.     

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.     

Raman spectra and thermal analysis of a new lead–tellurium–germanate glass system

Differential scanning calorimetry (DSC) and Raman scattering studies of a new glass system, lead–tellurium–germanate glasses in the form of (90−x)GeO₂·xTeO₂·27PbO·10CaO with x=0, 10, 20, 30, and 40, are reported. The glass samples were fabricated using a conventional melt-quenching method. The Raman spectra and possible glass structures are discussed for different TeO₂ contents. The results indicate that increasing TeO₂ content up to 40 mol% in the glass system decreases the glass transition temperature and melting temperature, and suppresses the crystallization tendency in the fiber pulling temperature range. The lead–tellurium–germanate glass, GTPC, possesses a larger refractive index and a smaller maximum phonon energy than that of a lead–germanate glass, 63GeO₂·27PbO·10CaO, and shows a better thermal stability compared to a tellurite glass, 75TeO₂·20ZnO·5Na₂O (TZN). These improved properties could be beneficial for fabricating rare-earth doped fiber devices.     

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.     

Structure and properties of lanthanum galliogermanate glasses

Lanthanum galliogermanate glasses were prepared. Raman spectra, molar volumes, glass transition temperatures and activation energies for glass transition and crystallization were obtained. For glasses having the same La₂O₃/GeO₂ ratio, the molar volumes increase with the Ga₂O₃ content, and the glass transition temperatures, activation energies for glass transition and crystallization, increase initially then decrease as the ratio of Ga₂O₃/GeO₂ increases. For glasses having the same Ga₂O₃/GeO₂ ratio, the molar volumes increase with La₂O₃ content, and the glass transition temperatures increase as the ratio of La₂O₃/GeO₂ increases. The change of glass structure and its properties with composition is correlated with the concentration of lanthanum ion.     

The structure of SiO2-GeO2 glasses: A spectroscopic study

Four glasses of the SiO₂-GeO₂ binary system have been synthesized via a sol-gel route followed by a heat treatment and a quench. Glass structure has been determined by Ge K-edge X-ray absorption spectroscopy (XAS) at low temperature and Raman spectroscopy. These mixed glasses present a continuous random network of interconnected GeO₄ and SiO₄ tetrahedra, with GeO₄ tetrahedra similar to the GeO₄ units in GeO₂ glass and continuous compositional variations from GeO₂-rich regions to SiO₂-rich regions. Such a random mixture is consistent with physical properties of these binary glasses as well as with the chemical dependence of their polyamorphism at high pressure. This EXAFS-derived mean Ge-O-Si angles are close to the Ge-O-Ge mean angle in GeO₂ glass, 134° and 130°, respectively. This misfit with the Si-O-Si angles might explain the ease of formation of isolated and pair defects centers, which are suspected to be at the origin of photo-induced modifications of optical properties in Ge-bearing SiO₂ glasses.     

Considerations on the structure and physical properties of B2O3-SiO2 and GeO2-SiO2 glasses

Data of density, refractive index and thermal expansion coefficient for B₂O₃-SiO₂ and GeO_2-SiO₂ glasses have been analyzed. The volumes of the structural units are the same found for the vitreous B₂O₃, GeO₂ and SiO₂. The volume of any structural unit is constant over the entire composition region of the glass system. The same has been found for the differential refraction and unit refraction of the structural units in these glasses. Different features are observed for the differential expansion of the structural units. There is a considerable change with composition in the differential expansion of BO₃, GeO₄ and SiO₄ units. The effect is attributed to a change in the asymmetry of vibrations with the number of Si-O-B or Si-O-Ge linkages in the matrix. The thermal expansion coefficient is mainly determined by the contribution of B₂O₃ or GeO₂ in the concerned glasses.     

Viscosity and kinetic fragility of alkaline earth zinc phosphate glasses

Kinetic fragility indices m and F_1/2 as well as glass transition temperature T_g of alkaline earth zinc phosphate melts of molar composition 20 MO–30 ZnO–50 P₂O₅ (with M = Ba, Sr, Ca, Mg) were determined using viscometry and differential scanning calorimetry (DSC). Beam bending and concentric cylinder experiments were performed to measure the flow resistance in temperature ranges above glass transition and close to liquidus, respectively. Different upscan rates of DSC runs through the glass transition were used to correlate changes of the fictive temperature with kinetic fragility. Both methods revealed that glass transition temperature correlates negatively and kinetic fragility positively with the size of M. Metal cation mixing (M + Zn) led to a negative deviation from linearity for T_g, while exchanging M resulted in a linear dependence of T_g, if scaled with averaged charge-to-distance ratio. The fictive temperature method overestimated the compositional dependence of m by a ratio up to 1.9.     

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.     

Thermally modulated differential scanning calorimetry of a glass of composition 45Na2O-40B2O3-10Al2O3-5In2O3

The kinetics of structural relaxation and of glass transition of the 45Na₂O-40B₂O₃-10Al₂O₃-5In₂O₃ glassforming melt is studied by means of standard DSC and of temperature modulated DSC. In this way the dependence of the fictive temperature on cooling rate is determined simultaneously with the determination of the dependence of the dynamic glass transition temperature on modulation frequency. Both sets of data are fitted together in terms of the equation of Ritland-Bartenev. It was found that the activation energy of the structural relaxation exhibits a moderate dependence on temperature with the dimensionless fragility parameter α=3.3 (for strong systems, α is about 1 and increases to about 8 for some very fragile polymeric systems).     

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.     

Structural and optical properties of the gadolinium-lead-germanate glasses

Glasses in the system xGd₂O₃(100 − x)[7GeO₂·3PbO] with 0 ≤ x ≤ 40 mol% have been prepared from melt quenching method. The influence of gadolinium ions on structural behavior in lead-germanate glasses has been investigated using FTIR, UV-VIS and EPR spectroscopy. The structural changes have been analyzed with increasing rare earth concentration.FTIR data suggest that the glass network modifications has taken place mainly in the germanate part whereas the lead part remained unmodified and its network consists mainly from the [GeO₄], [GeO₆], [Ge₂O₇] structural units and with interconnected through Ge-O-Ge bridges in [GeO₄] structural units. The changes in amplitude and bandwidth of the UV-VIS bands ranging from 200 nm to 350 nm depend on the content of Gd₂O₃.By increasing the Gd₂O₃ content in the glass matrix, the optical band gap energy increases, indicating changes of the lattice parameters and that no non-bridging-oxygens form upon the addition of gadolinium oxide. The decreasing trend has been observed both in optical gap band energy and refractive index of oxide glasses at x = 10 mol% Gd₂O₃ indicating breaks up the [GeO₄] tetrahedral units bonds and create of non-bridging oxygen atoms. For sample with x ≥ 20 mol%, the gadolinium ions having a behavior of network formers (g ≈ 4.8) will coordinate more with the excess of oxygen. Accordingly, the gadolinium ions are generally suspected to improve their environment of network formers.     

Sub-critical crack growth in sodium germanate glasses

Sub-critical crack growth in binary sodium germanate glass was investigated over a wide range of the crack velocities, 10⁻⁷-10⁻² m s⁻¹, by using small-size specimens with double cleavage drilled compression configuration. For evaluating the intrinsic sub-critical crack growth, crack initiation and subsequent propagation of the crack were performed in heptane. With increasing Na₂O content in sodium germanate glass, sub-critical crack growth curve shifted toward higher stress intensity factors first up to 10 mol% Na₂O, but more addition of Na₂O caused the curve to shift to lower stress intensity factor regions. In other words, fracture toughness shows a maximum at the composition of 10Na₂O·90GeO₂, whose value is 1.07 MPa m^1/2. This compositional dependence of fracture toughness originates from the so-called germanate anomaly. On the other hand, the slope of sub-critical crack growth curve for the glass containing >10 mol% Na₂O was much shallower than that for soda-lime glass. These glasses are very fatigable even in inert condition. It is considered that this fatigue behavior can be caused by the microscopic structural variation, which is the presence of GeO₆ units in GeO₂ glass network, and that these units can be the fatigue crack path.     

Thermal expansion of glasses in the As2Se3–AsI3 system

Thermal expansion coefficients (α) of glasses in the As₂Se₃–AsI₃ system are measured in the glass transition region and temperature dependence of the fictive temperature is calculated on the basis of relaxation model. It is found that the increase of AsI₃ content results in: an increase of α, decrease of the glass transition temperature (T_g), increase of the α change at T_g, an effect of quenching rate on α, and also changes in the structural relaxation times spectrum. The data are discussed within the framework of the assumption that the addition of AsI₃ to As₂Se₃ results in: (1) destruction of the As₂Se₃ glass network, (2) structural inhomogeneity of the glasses increase, (3) the temperature dependence of chemical–structural equilibria occurring in the liquid state increases.     

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.