Isokom Temperatures Research Articles

Structural relaxation mechanisms in hydrous sodium borosilicate glasses

Borosilicate glasses (16Na2O–10B2O3–74SiO2, NBS) with water contents up to 22 mol% H2O were prepared to study the effect of water on structural relaxation using DTA, viscometry and internal friction measurements. The results show that the glass transition temperature Tg of DTA and the isokom temperature T12, of viscometry are in excellent agreement, confirming the equivalence of enthalpy and viscous relaxation for NBS glass. Combining Tg data with water speciation data demonstrates that OH groups are mainly responsible for the decrease of Tg with increasing hydration, while molecular water plays only a minor role. Internal friction spectra at 7.125 Hz confirm the decisive influence of water on mechanical relaxation. The temperature range of α-relaxation (glass transition) strongly decreases while two β-relaxation peaks (sub-Tg) progressively appear with increasing water content. A high temperature β-relaxation peak, attributed to the presence of OH groups, shifts from 670 to 450 K as total water content increases from 0.01 to 5 wt%. A low temperature β-relaxation peak, attributed to molecular water, appears at 380 K and 330 K in glasses containing 3 and 5 wt% H2O, respectively. These findings suggest that relaxation mechanism of different hydrous species at low temperature may contribute to fatigue of stressed glasses.

Mixed intermediate effect on mechanical and rheological performances in Zn[sbnd]Mg silicate glasses

In contrast to the mixed alkali or mixed alkaline earth effect, the mixed intermediate effect has received little attention in glass community. Here, we focus on the mixed intermediate effects, i.e. mixed MgZn effects, by partial substitution of MgO for ZnO in sodium silicate glassy series. Opposite to the previously reported “mixed alkaline earth effects”, here the compositional dependences of microhardness and isokom temperature following the gradual substitution of Mg by Zn show a positive deviation from the linear tendency between the two end-members, which is maybe due to the competition between cationic potential energy mismatch and network polymerization. Through the structural identification by Raman scattering technique, we give a microscopic explanation for the observed mixed MgZn effects. Finally, we identified the mixed MgZn effects on viscosity and configurational entropy, which indicates that the Zn and Mg have a similar dynamic role at glass transition temperature but play a more different role at lower viscosity.

Mixed alkaline‐earth effect on the mechanical and rheological properties of Ca–Mg silicate glasses

AbstractIn this article, we investigate the mixed alkaline‐earth effect in a silicate glass series with varying the molar ratio of [MgO]/([CaO]+[MgO]). This effect manifests itself as a minimum in Vickers microhardness (HV), coefficient of thermal expansion (CTE), and isokom temperatures at 1012(Tg) and 102 Pa·s, and as a maximum in liquid fragility. To probe the structural origin of the mixed alkaline‐earth effect in CTE and Hv, we conducted the Raman measurements. In contrast to the aluminosilicate glasses, the present glass series exhibit a negative deviation of shift of peak position at ~1100 cm−1 from a linear additivity, indicating the role of the aluminum speciation in affecting the vibration modes. By fitting the Vogel–Fulcher–Tamann equation to the high‐temperature viscosity data, we found a near‐linear increase of the fractional free volume with the gradual substitution of Ca by Mg, confirming the dynamic structural mismatch model describing the mixed modifier effect. This work gives insight into the mixed modifier effect in glassy systems.

The viscosity window of the silicate glass foam production

The production of silicate glass foam allows diverse resources and waste materials to be used in the production. Testing of such large palette of materials complicates and prolongs the optimisation process. Therefore, it is crucial to find a universal criterion for foaming silicate glass melts which can offer a practical starting point for the optimisation procedure. The melt viscosity might be the most important parameter for controlling the foaming process and the glass foam density. In this work, we attempt to define a viscosity range in which foaming of different glasses results in a maximum of foam expansion. The expansion maximum is obtained for different glasses (labware, E-glass, CRT panel, soda-lime-silica) by foaming with CaCO3 at isokom temperature and from literature data. In general, the viscosity window was found to be within 104–106Pas when foaming with MnO2 or metal carbonates (CaCO3, Na2CO3, MgCO3, SrCO3, dolomite) whereas SiC requires higher temperatures and correspondingly lower viscosities (103.3–104.0Pas). These findings can help assessing the implementation of new resources in the glass foam production.

Water, the other network modifier in borate glasses

In the present study we have investigated whether the effect of water on properties of borate glasses resembles that of alkali oxide. Soda–lime–borate glasses with nominal compositions of x Na2O, 10 CaO, (90-x) B2O3 (x=5, 15 and 25mol%) were doped with up to 8wt.% H2O by processing glass powder+distilled water in platinum capsules in an internally heated gas pressure vessel at 1523K and 500MPa. The water content of hydrous glasses was determined by Karl-Fischer titration and near-infrared spectroscopy. The glass transition temperature Tg. was derived from DTA and micropenetration experiments for which the effect of water loss at the surface of the hydrous glasses was studied. Heating glass samples at 10Kmin−1 in the DTA resulted in Tg values which are close to T12 isokom temperatures confirming the equivalence of enthalpy relaxation and viscous relaxation for borate glasses. For all three glass series it is shown that Tg strongly decreases whereas the liquid fragility strongly increases upon the addition of water. These findings reveal that H2O primarily causes breaking of B–O–B bonds rather than supporting 4-fold coordinated boron as it is well-known for alkali oxides in this concentration range.

Impact of ZnO on the structure and properties of sodium aluminosilicate glasses: Comparison with alkaline earth oxides

The structural role of ZnO in aluminosilicate glasses is not yet well understood. However, this understanding is requisite for explaining the impact of ZnO on the macroscopic properties of these glasses. In this work, we present a quantitative analysis of the impact of ZnO on the structure and properties of sodium aluminosilicate glass. The properties under study include density, molar volume, refractive index, coefficient of thermal expansion, elastic moduli, isokom temperatures, and liquid fragility. The impact of ZnO is compared with that of four alkaline earth oxides (MgO, CaO, SrO, and BaO) in the same family of aluminosilicate glasses. Our study is performed on a series of glasses with varying Si/Al ratios to quantify the impact of each divalent cation oxide on aluminum speciation and the resulting impact on macroscopic properties. Our results reveal pronounced changes in the scaling of most measured properties around the compositions with [Al2O3]=[Na2O], particularly for the glass series with MgO or ZnO. Based on 27Al MAS and 3QMAS NMR experiments at high field, the structural origin of this change is ascribed to the change in aluminum speciation. We thus demonstrate that the impact of Zn2+ on aluminosilicate glass structure and properties is similar to that of Mg2+, following their similarity in ionic radius, despite their differences in electronic structure.

Viscosity and Fragility of Alkaline‐Earth Sodium Boroaluminosilicate Liquids

Understanding the composition and temperature dependence of viscosity of silicate liquids is of the highest importance not only for geological processes but also for production of industrial glass. In this work, we have determined the temperature dependence of equilibrium liquid viscosity of 36 alkaline‐earth sodium boroaluminosilicate liquids as a function of the Si/Al ratio and the type of alkaline‐earth oxide (MgO, CaO, SrO, or BaO). We demonstrate that the isokom temperature at 1012 Pa s (i.e., the glass transition temperature) generally increases with increasing Si/Al ratio, whereas the isokom temperatures at 104 and 101.5 Pa s exhibit a decrease with increasing [Al2O3] in the peraluminous regime. The isokom temperatures decrease with increasing alkaline‐earth size in the peralkaline regime, whereas they increase with increasing alkaline‐earth size in the peraluminous regime. The liquid fragility index m exhibits a minimum value at an intermediate Si/Al ratio, with the position of the minimum increasing to a higher value of [Al2O3] with increasing alkaline‐earth size. We have discussed our findings in terms of the underlying structural and topological changes as a function of composition and temperature.

Mixed alkaline earth effect in sodium aluminosilicate glasses

While the mixed alkali effect has received significant attention in the glass literature, the mixed alkaline earth effect has not been thoroughly studied. Here, we investigate the latter effect by partial substitution of magnesium for calcium in sodium aluminosilicate glasses. We use Raman and NMR spectroscopies to obtain insights into the structural and topological features of these glasses, and hence into the mixed alkaline earth effect. We demonstrate that the mixed alkaline earth effect manifests itself as a maximum in the amount of bonded tetrahedral units and as a minimum in liquid fragility index, glass transition temperature, Vickers microhardness, and isokom temperatures (viz., the temperatures at η=1013.5 and 1012.2Pas). The observed minima in fragility, glass transition temperature, and isokom temperature are ascribed to bond weakening in the local structural environment around the network modifiers. We suggest that, since the elastic properties of the investigated system are compositionally independent, the minimum in Vickers microhardness is closely correlated to the minimum in isokom temperatures. Both of these properties are related to plastic flow and the translational motion of structural units, and hence both may be related to the same underlying topological constraints. This indicates that there might not be any significant difference in the onset of the rigid sub-Tg constraints for the investigated compositions.

Microscopic Origins of Compositional Trends in Aluminosilicate Glass Properties

Revealing and understanding the microscopic origins of the macroscopic properties of aluminosilicate glasses is important for the design of new glasses with optimized properties. In this work, we study the composition‐structure‐property relationships in 20 MgO/CaO sodium aluminosilicate glasses upon Al2O3‐for‐SiO2 and MgO‐for‐CaO substitutions. We find that some properties (density, molar volume, Young's modulus, and shear modulus) are linear through the investigated range of Al2O3 compositions, while others (refractive index, coefficient of thermal expansion, Vickers hardness, isokom temperatures, and liquid fragility index) exhibit a change in the slope around the composition with [Al2O3] = [Na2O], which is especially pronounced for the glasses containing MgO. We discuss these phenomena based on structural information obtained by NMR spectroscopy and topological considerations.

Topological Model for the Viscosity of Multicomponent Glass‐Forming Liquids

Topological constraint theory holds the key to understanding the temperature and composition dependence of the dynamics of glass‐forming liquids. Unfortunately, existing analytical models currently do not apply to multicomponent industrial glasses. Moreover, models that are strictly empirical in nature fail to provide insight into the underlying physical trends and cannot extrapolate beyond the compositional ranges used for fitting of their parameters. In this paper, we present a phenomenological model offering an improved description of the composition and temperature dependence of the shear viscosity of multicomponent liquids. The model has a clear physical foundation based on the temperature dependence of the network constraints with only two empirical fitting parameters per oxide component. The model predicts the isokom temperatures of 7141 viscosity measurements for 760 different compositions with a root‐mean‐square error of only 6.55 K.

Kinetic fragility of hydrous soda-lime-silica glasses

The effect of hydration on the kinetic fragility of soda-lime-silica glasses was investigated by viscometry in the glass transition range. Water-bearing glasses were prepared from industrial float glass (FG) and a ternary model glass (NCS = 16Na 2O 10CaO 74SiO 2 in mol%) by bubbling steam through the melt at 1480 °C and up to 7 bar. Additionally, a sodium borosilicate glass (NBS = 16Na 2O 10B 2O 3 74SiO 2 in mol%) was hydrated under equal conditions. As detected by infrared spectroscopy water dissolves in the glasses exclusively as OH-groups. The hydration resulted in a total water content C W up to ≈ 0.2 wt% for FG, NCS and NBS glasses. Kinetic fragility, expressed by the steepness index m, was determined from the temperature dependence of η at the glass transition. Viscosity data from previous studies on hydrous float glasses ( C W > 1 wt%) were surveyed together with literature data on the (H 2O)–Na 2O–CaO–SiO 2, (H 2O)–Na 2O–SiO 2 and (H 2O)–SiO 2 systems to expand the range of water concentration and bulk composition. We could demonstrate that m decreases for all glasses although water is dissolved as OH and should depolymerize the network. An empirical equation of the general type m = a − b log C W where a, b are fitting parameters, enables m to be predicted, for each glass series as function of the water content C W. The enlarged data base shows that the parameter B of the Arrhenius viscosity-temperature relation decreases much stronger than the isokom temperature at the glass transition.

The viscosity of trachytes, and comparison with basalts, phonolites, and rhyolites

The viscosity of natural liquids representative of the glassy portion of pumice collected from the deposits of the Campanian Ignimbrite (IGC) and Monte Nuovo (MNV) eruption of Phlegrean Fields has been measured in the temperature range from 1770 K down to the glass transition, and for a dissolved water content range from dry to nearly 4 wt.%. Measurements were performed by a combination of techniques involving concentric cylinder and micropenetration apparatuses, depending on the specific viscosity range. These measurements, together with those made on samples from the Agnano Monte Spina (AMS) eruption of Phlegrean Fields presented in a companion paper, represent the first viscosity determinations for natural trachytic liquids. Liquid viscosities have been parameterized by means of a modified VFT equation that allows the calculation of viscosity as a function of temperature and water content. Calculated viscosities are compared with those pertaining to natural liquids of phonolitic, rhyolitic, and trachybasaltic composition, showing that trachytes are intermediate between rhyolites and phonolites, consistent with the dominant eruptive style associated with the different magma compositions (mainly explosive for rhyolites and trachytes, either explosive or effusive for phonolites, mainly effusive for basalts). Compositional diversity among the analyzed trachytes corresponds to liquid viscosity differences of one to two orders of magnitude, with higher viscosities approaching that of rhyolite at the same temperature–water content conditions. All hydrous natural trachytes and phonolites become indistinguishable when isokom temperatures (i.e., temperatures corresponding to the same viscosity) are plotted versus a compositional parameter given by the molar ratio on an element basis (Si+Al)/(Na+K+H). In contrast, rhyolitic and basaltic liquids display distinct trends, with the more fragile basaltic liquids crossing the curves pertaining to all other compositions.

GeAs thiophosphate glasses: properties and NMR spectroscopy

Stoichiometric GeAsP sulphide glasses containing 50–90 mol% GeS 2 and 4–25 mol% P 2S 5 have been investigated to determine the compositional and structural dependence of physical properties. The latter vary monotonically with increasing P 2S 5 concentration: at constant GeS 2 content, the thermal expansion coefficient increases, whereas refractive index, isokom temperatures and density decrease as P 2S 5 replaces As 2S 3. The introduction of P is marked by absorption bands in the 900–1400 cm −1 range that are absent in P-free glasses. The compositional dependence of optical properties and molar volume indicates that the dominant P species in these glasses is the tetrahedral SPS 3/2 group. This interpretation of the local P environment is confirmed by 31P MAS NMR spectral data, which further indicate that the latter species tend to have AsS 3 groups as next-nearest neighbors.

Effects of Water Content on the Properties of Sodium Aluminosilicate Glasses

Sodium aluminosilicate glasses of the general formula 25Na2O.xAl2O3.(75 –x)SiO2 were prepared with a range of hydroxyl contents. Both the glass transformation and isokom temperatures decrease as the hydroxyl content increases. The magnitude of the decrease in each property is a function of the alumina content of the glass, with the largest effects occurring for the glass containing 25 mol% Al2O3. The magnitude of the effect of water as a function of alumina content reflects the decrease in the concentration of nonbridging oxygens in the glass with increasing Al2O3 content, since the effect of the terminal hydroxyl species on the connectivity of the glass is enhanced by the elimination of nonbridging oxygens. The results indicate that the large effect of water content on the properties of these glasses must be considered when discussing the role of alumina in glasses.