Development of empirical potentials for sodium borosilicate glass systems

New parameter values are proposed for the empirical potentials used to describe SiO₂-B₂O₃-Na₂O alkali borosilicate glass systems. They are based on Buckingham potentials, but include dependence between the fitting parameters and the glass chemical composition to improve the representation of the complex environment around the boron atoms. In particular, the boron anomaly (observed when the [Na₂O]/[B₂O₃] ratio varies) is correctly reproduced. The structural and mechanical properties of a wide range of glass compositions and of reedmergnerite crystals are correctly simulated: bond distances, mean angles, densities, elastic moduli. The deviations from the experimental values are small.     

Hidden slow time scale of correlated motions in supercooled liquids: Multi-time correlation function approach

A three-time correlation function of particle displacements is introduced and numerically calculated by performing molecular dynamics simulations of binary soft-sphere supercooled systems. It is found that the two-dimensional representations of the three-time correlation function reveal couplings of particle motions that exist over a wide range of time scales. Furthermore, it is demonstrated that the systematic change in the second time interval in the three-time correlation function enables us to analyze how the correlations in mobility decay with time. From this analysis, the characteristic time scale of dynamical heterogeneity is quantified. We find that the dynamical heterogeneity time scale becomes much slower than the α-relaxation time as the temperature decreased.     

Multifractal nature of heterogeneous dynamics and structures in glass forming ionic liquids

Complex dynamics and structures of ionic liquids exemplified in 1-ethyl-3-methylimidazolium nitrate (EMIM-NO₃) are examined by molecular dynamics simulations. Correlation length of the radial charge distribution function and density distribution function show different temperature dependence. Density profiles are obtained from the accumulated positions visited by ions during the MD runs. The profile originating from the coexistence of layered structures of density (density wave) and those of charges (charge density wave), shows complicated heterogeneity, which is proven to be multifractal in nature. Thus, present is more than one characteristic length scale together with their mixing. The multifractal density profiles are formed by the multifractal walks with fast and slow ions. Generally, the coexistence of different length scales due to the different species or the different local structures can be the mechanism to form similar multifractal dynamics and structures.     

Phase Transitions in Chlorobenzene-Cis-Decalin Mixtures: A Nuclear Magnetic Resonance,Thermal Analysis and Phase Diagram Study

Using nuclear magnetic resonance and differential scanning calorimetry we have been able to observe the molecular rotation properties of chlorobenzene-cis-decalin mixtures in their glassy, amorphous and crystalline phases. The results indicate: that in “rapidly” cooled samples the behavior of the host dominates the properties of the mixtures; that reorientation of the guest molecule is less restricted in the amorphous phase than in the glassy phase; that when the material crystallizes from the amorphous phase on warming, reorientation again becomes severely restricted. The temperatures at which these phenomena occur agree with the phase diagram that has been determined for these materials. Similar experiments on t butyl chloride-cis-decalin mixtures support the above conclusions. These conclusions are in agreement with the previous dielectric studies.

Our magnetic resonance and thermal analysis experiments support the argument that the behavior of the CD host dominates the behavior of guest polar molecules in rapidly cooled mixtures. At the lowest temperature (110 K) CD forms a glassy phase unaccompanied by any heat of crystallization. On warming this glassy phase transforms near 150 K to an amorphous phase where considerable motion of both host and guest molecules occurs. Above 160 K the material crystallizes and reorientation is once again severely restricted up to the eutectic melting of 210 K. There appears to be no time dependence to these transformations and, as long as one stays below the crystallization temperature, the glassy-amorphous transition is reversible.     

Structural and transport properties of quaternary glass system: LiF-Li2O-SrO-Bi2O3

Bismuth based glasses containing LiF, Li₂O and SrO were investigated by different physical, spectroscopic and transport techniques. The results show that density of the glass system increases whereas glass transition temperature decreases with increase in LiF content. The decrease in glass transition temperature is attributed to the entry of the fluoride ions into the glass network mainly substitutionally in place of oxygen ion. The increase in dc electrical conductivity in the present glasses with increase in the fluorine ions is due to the mixed contribution of the positively charged lithium cations throughout the glass network and the negatively charged fluorine, which may act as impurity and/or as terminal non-bridging halide ion. Infrared and Raman spectroscopic results indicate that the glass network consists of BiO₆ octahedral and BiO₃ pyramidal units.     

Phase competition of Cu64Zr36 and its effect on glass forming ability of the alloy

In the present paper, crystallization behavior and glass forming ability (GFA) of Cu₆₄Zr₃₆ bulk metallic glass (BMG) were studied based on the crystal phase competition. Electrical resistivity and X‐ray diffraction results indicate that Cu₆₄Zr₃₆ glass underwent a two‐stage crystallization process, during which Cu₁₀Zr₇ and Cu₅₁Zr₁₄ crystals precipitate at first and then there are only Cu₁₀Zr₇ and Cu₈Zr₃ phases at the end of the second stage. Intriguingly, it was found that it is the competing phase Cu₁₀Zr₇ that weakens GFA of the Cu₆₄Zr₃₆ alloy, because the Cu₁₀Zr₇ precipitate is fully restrained in the cast rod when substituting Zr with 7.5at%Ti and thereby its diameter with fully amorphous structure is enlarged from 1 mm to 2 mm.     

Rapid quenching and glass formation in chalcogenide glasses: Preparation and properties of Ge–As–Te glasses over an extended composition ranges

In Ge–As–Te system, the glass forming region determined by normal melt quenching method has two regions (GFR I and GFR II) separated by few compositions gap. With a simple laboratory built twin roller apparatus, we have succeeded in preparing Ge_7.5As_xTe_92.5−x glasses over extended composition ranges. A distinct change in T_g is observed at x = 40, exactly at which the separation of the glass forming regions occur indicating the changes in the connectivity and the rigidity of the structural network. The maximum observed in glass transition (T_g) at x = 55 corresponding to the average coordination number (Z_av) = 2.70 is an evidence for the shift of the rigidity percolation threshold (RPT) from Z_av = 2.40 as predicted by the recent theories. The glass forming tendency (K_gl) and ΔT (=T_c−T_g) is low for the glasses in the GFR I and high for the glasses in the GFR II.     

A study of the primary crystallization of a mixed anion silicate glass

This paper presents the results of a study of the primary crystallization of a multicomponent mixed anion silicate glass. The primary phase, leucite, and the secondary phase, diopside, were formed by surface crystallization, while the secondary phase, phlogopite, was formed by volume crystallization. The influence of the particle size of glass powder samples in the range 0-1 mm on the temperature of the DTA crystallization peak, T_p, the height of the peak, (δT)_p, and the parameter /(ΔT)_p was studied. The behaviors of the parameters (δT)_p and /(ΔT)_p depend only on the change in the surface-to-volume nuclei ratio, as is the case with polymorphic crystallization. However, the particle size ranges in which the surface, volume and mixed crystallization mechanism dominate are considerably narrower for this glass. The influence of the duration of a pre-DTA heat treatment on the parameters T_p, (δT)_p, and ΔT_p for fixed temperatures in the range T = 550-750 °C was investigated. The T_p of the samples thermally treated for different times, at fixed temperatures, decreased up to t = 5 h and then increased to a value close to that of an as-quenched sample, after which the value remained constant. The curves of (δT)_p, and ΔT_p as a function of time for fixed temperatures show a maximum. The influence of the temperature of the pre-DTA heat treatment on the parameters T_p, (δT)_p, and ΔT_p for fixed times of t = 1−5 h was also investigated. On increasing the pre-DTA heat treatment time, the curves changed significantly. The curves for 3 and 5 h in the temperature range 580-660 °C were similar to the nucleation curve, which indicated that the volume nucleation process proceeded in this temperature range. The behavior of these parameters, as a result of the simultaneous action of different nucleation mechanisms and crystal growth differ from those previously reported for the case of polymorphic crystallization.     

Conversion of batch to molten glass, I: Volume expansion

Batches designed to simulate nuclear high-level waste glass were compressed into pellets that were heated at 5 K/min and photographed. Three types of batches were prepared, each with different amounts of nitrates and carbonates. The all-nitrate batches were prepared with varying amounts of sucrose. The mixed nitrate-carbonate batches were prepared with silica particles ranging in size from 5 to 195 μm. One batch containing only carbonates was also tested. Sucrose addition had little effect on expansion, while the size of silica was very influential. Sucrose addition reduced primary foam for batches containing 5-μm silica, but had no effect on batches containing larger particles. Excessive amounts of sucrose increased secondary foam. The 5-μm grains had the strongest effect, causing both primary and secondary foam to be generated, whereas only secondary foam was produced in batches with grains of 45 μm and larger. We suggest that the viscosity of the melt and the amount of gas evolved are the main influences on foam production. As more gas is produced in the melt and as the glass becomes less viscous, gas bubbles coalesce into larger cavities until the glass can no longer contain the bubbles and they burst, causing the foam to collapse.     

Conversion of batch to molten glass, II: Dissolution of quartz particles

Quartz dissolution during the batch-to-glass conversion influences the melt viscosity and ultimately the temperature at which the glass forms. Batches to make a high-alumina borosilicate glass (formulated for the vitrification of nuclear waste) were heated at 5 K min^− 1 and quenched from temperatures of 400 to 1200 °C at 100 K intervals. The batches contained quartz as a silica source, with particles ranging from 5 to 195 μm in diameter. The content of unreacted quartz in the samples was determined with X-ray diffraction. Most of the fine quartz dissolved during the early batch reactions (at temperatures < 800 °C), whereas coarser quartz dissolved mostly in a continuous glass phase via diffusion. The mass-transfer coefficients were assessed from the data as functions of the initial particle sizes and the temperature. A series of batches were also tested that contained nitrated components and additions of sucrose, known to accelerate melting. While sucrose addition had no discernible impact on quartz dissolution, nitrate batches melted somewhat more slowly than batches containing carbonates and hydroxides in addition to nitrates.     

Isobaric and isothermal glass transition of PMMA: pressure-volume-temperature experiments and modelling predictions

The scaling law for relaxation times, recently proposed by Casalini and Roland, is utilized in the framework of KAHR (Kovacs, Aklonis, Hutchinson, and Ramos) phenomenological theory. With this approach it is shown that the pressure, volume, and temperature (PVT) data obtained on Poly(methyl-methacrylate) (PMMA) can be reliably predicted, in the region of the alpha-relaxation, by using only two fitting parameters, namely: the relaxation time in the reference state, τ_g, and the fractional exponent, β, that describes the dispersion of the alpha-relaxation.     

The review of possible interrelations between ionic conductivity, internal friction and the viscosity of glasses and glass forming melts within the framework of Maxwell equations

The review contains the results of application of Maxwell equations for mechanical relaxation and electrical conductivity, to the systematization of large amount of experimental data related to mechanical, viscous, and electrical properties of inorganic glasses and glass forming melts. The generalization of internal friction results shows the existence of characteristic values for the ratios of temperatures, responsible for α-, β- and ionic relaxations; they are independent on the frequency and chemical composition. This is the evidence for the main role of elastic deformations at various corpuscular processes and the existence of characteristic scale of activation barriers predetermined by local volumes of deformation. It is shown the possibility of very precise calculation of the temperature of “ionic” internal friction maximum for one-alkali oxide glasses directly from Maxwell equation and d.c. conductivity experimental data. The volumes of particles overcoming the potential barrier at viscous flow practically coincide with the results of direct structural determinations. The existence of universal relation between d.c. conductivity and viscosity for the extremely wide temperature intervals (Littleton relation) is proved for silicate and phosphate melts. The theory of this dependence is proposed. The results show the effectiveness of the attempts to unite the continual and discrete approaches within the framework of Maxwell equations to obtain the simplest understanding the mechanisms of different types of relaxation. The review comprises many Russian papers unknown in English scientific literature.     

Structural relaxation of Ti40Zr25Ni8Cu9Be18 bulk metallic glass

Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass has a unique quenched-in nuclei/amorphous matrix structure. The crystallization of quenched-in nuclei, when the experimental isothermal annealing time is within its incubation time, may not disturb the enthalpy relaxation, which makes it have the accordingly common enthalpy relaxation behavior with amorphous materials. The alloy's annealing time dependence of recovery enthalpy follows a stretched exponential function with the mean relaxation time obeying an Arrhenius law. The equilibrium recovery enthalpy ΔH_T^eq, mean relaxation time τ and stretching exponent β are all dependent on the annealing temperature, and generally, a higher annealing temperature comes with a lower value of ΔH_T^eq, τ and a higher value of β. Two parameters, β_g and τ_g, representing the stretching exponent and the mean structural relaxation time at the calorimetric glass transition temperature, respectively, are correlated with glass forming ability and thermal stability, respectively. For Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG, the high value of β_g, which is much higher than 0.84 and approaches unity, reveals its good glass forming ability, while, on the other hand, the low value of τ_g indicates a worse thermal stability compared with typical BMGs.     

On the dependence of the properties of glasses on cooling and heating rates: I. Entropy, entropy production, and glass transition temperature

The glass transition is theoretically described in terms of a generic non-equilibrium thermodynamics approach employing De Donder's structural order parameter method, appropriate expressions for the relaxation behavior of glass-forming systems and a simplified but qualitative correct model of glass-forming melts with one order parameter related to the free volume of the system. Employing this approach the behavior of a variety of thermodynamic quantities describing glass-forming systems in vitrification and devitrification processes is interpreted theoretically. The present paper is devoted to the computation of the entropy, the entropy production and the glass transition temperature in dependence on the cooling and heating rates, varying latter parameter in a broad interval. A comparison with experimental results is given and some further consequences and possible extensions are discussed briefly.     

Method to assess the homogeneity of partially crystallized glasses: Application to a photo-thermo-refractive glass

We describe a new method for the study of both optical and crystallization homogeneity of partially crystallized glasses or glass–ceramics at different spatial scales (from 100 mm to 1 μm). The method relies on the association of different techniques, such as interferometry, optical microscopy and differential scanning calorimetry. The method was tested to probe the optical and crystallization homogeneity of both as-made and UV-exposed, and pre-nucleated samples of a photo-thermo-refractive (PTR) glass. This study demonstrates that pure UV-exposure did not lead to an improvement of the crystallization homogeneity of the glass. However, the benefit of associating UV-exposure and nucleation thermal treatment was clear. These combined treatments permit to homogenize the crystal distribution in PTR glass at millimeter and micron scale. This result is of major commercial interest.     

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.     

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.     

Simulation of the optical properties of Tm:ZBLAN glass

The optical properties of Tm:ZBLAN glass have been simulated. The simulated optical spectra agreed substantially with the observed spectra. The population of the energy levels of the Tm³⁺ ion under dual-wavelength excitation for upconversion emission and amplification was evaluated by solving the rate equation with the simulated transition probabilities and spectral shapes of the transitions of which the initial levels were from the ³H₆ to ³P₂ levels. The large population of the ¹G₄ level, which is the upper level for 480 nm laser oscillation, was estimated for the conditions of laser oscillation at room temperature. We also proposed new combinations of the excitation wavelengths for the amplification of the 1470 nm signal.     

Magnetic field dependence of the magnetocaloric effect in magnetic nanoparticle systems: A Monte Carlo simulation

We study the entropy change dependence on the applied magnetic field for a dipolar interacting superparamagnetic system of fine particles. Using a Monte Carlo technique, we have obtained an enhancement of the entropy for increasing applied magnetic fields; the maximum of the entropy curves occurs at higher temperatures for larger magnetic fields. It was also observed that the blocking temperature always decreases as the magnetic field increases.     

The split network analysis for exploring composition-structure correlations in multi-component glasses: II. Multinuclear NMR studies of alumino-borosilicates and glass-wool fibers

The preceding part [M. Edén, J. Non.-Cryst. Solids, 357, (2011) 1595-1602] introduced the “split network” strategy for estimating the network polymerization degree (r_A) and mean number of bridging oxygen (BO) atoms for a network former A, given that these parameters are known for all other network builders in the multi-component oxide glass. However, as the detailed ordering of BO and non-bridging oxygen (NBO) species is often difficult to assess experimentally, we summarize some “rules of thumb” for predicting the coordination number and tendency to accept NBO ions for Al³⁺, B³⁺, Si⁴⁺ and P⁵⁺ cations: they are helpful in scenarios devoid of experimental data. Using the parameters r and , we present expressions for the BO/NBO distributions among tetrahedrally coordinated cations, as predicted from the binary and random models. Multinuclear ¹¹B, ²⁷Al and ²⁹Si solid-state NMR is exploited to derive the split network representations of a set of Na-Ca-(Al)-(B)-Si-O glasses. These results are subsequently used to gain structural insight into two commercial glass-wool fibers that constitute alumino-borosilicate networks modified by Na⁺, K⁺, Ca²⁺ and Mg²⁺ ions.