Silica under hydrostatic pressure: A non continuous medium behavior

The homogeneous/inhomogeneous structure of glasses is still a debated question. Hydrostatic high pressure experiments allow us to determine if a glass behaves as an elastic continuous random network or if a nanometer scale heterogeneity has to be taken into account. In order to get information on the homogeneous/inhomogeneous structure of glasses, in situ high pressure Raman experiments are performed on silica in the elastic domain up to 4.7 GPa. A strong decrease of the Boson peak intensity is observed between 1 bar and 3 GPa. We show that this decrease does not correspond quantitatively to the effect of pressure on a homogeneous elastic medium. From the interpretation of the narrowing of the main Raman band width under pressure as a narrowing of the θ inter-tetrahedral Si–O–Si angle distribution it is shown that the decrease of the Boson peak intensity is correlated to the decrease of the intrinsic inhomogeneity of the silica glass. These results confirm the occurrence of an intrinsic inhomogeneity at a nanometer scale even in a single component glass like SiO₂ which is very important for the interpretation of the optical or mechanical properties of the glasses.     

Inelastic neutron and low-frequency Raman scattering in a niobium-phosphate glass for Raman gain applications

We present measurements of the vibrational spectrum of a binary niobium-phosphate glass in the THz frequency range using inelastic neutron and Raman scattering. The spectra of these glasses show a low-frequency enhancement of the vibrational density of states (“boson peak”). Using a recently developed theory of vibrational excitations in disordered solids we are able to reconcile the measured neutron and Raman spectra using fluctuating elastic and Pockels constants as a model concept. As the spontaneous Raman susceptibility is a key parameter for Raman amplification our results suggest a significant gain profile for application of niobium-phosphate glasses in Raman amplifiers.     

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.     

Low-frequency Raman scattering and small-angle X-ray scattering of glasses inclined to phase decomposition

Low-frequency (<1000 cm⁻¹) Raman scattering of lithium aluminosilicate (12Li₂O : 15Al₂O₃ : 73SiO₂ with 4 mol% TiO2) glasses with addition of titanium dioxide has been studied. With a heat treatment at temperatures 660°C, 700°C, 720°C and 820°C and for various times and sequences of temperature, our samples decompose into nanometer sized dispersed aluminotitanate particles. In Raman spectra of these glasses an evolution of a boson peak was observed. The width of the relatively broad boson band decreases as does the frequency of the band. From small-angle X-ray scattering data we conclude that the boson peak is connected with elastic vibrations of amorphous or crystalline regions of inhomogeneity with a dimension of ∼1.7 nm in initial glasses or larger depending on the heat treatment sequences.     

Third-order elastic constants determination in soda–lime–silica glass by Brillouin scattering

Brillouin light scattering allows the measurement of sound velocity and elastic moduli in transparent materials. The ability to select a small scattering volume and to use specific scattering configurations gives important information about the gradient and anisotropy of mechanical properties. Brillouin experiments are often used to measure the second-order elastic constants. When a high accuracy in frequency measurements is achieved, Brillouin scattering may allow the determination of third-order elastic constants in pre-stressed media. Hence, Brillouin scattering provides in principle, a method for the analysis of stress fields in tempered glasses. In order to validate the technique, samples of float soda-lime–silica glass submitted to controlled stresses by four-point flexion were investigated. The results show the expected profile for the velocity of sound waves propagating in directions parallel and perpendicular to the surface of the samples, respectively. They allow the determination of several third-order elastic constants of the investigated glass. This technique was applied to several samples of tempered glass corresponding to different values of the surface stress. The main result is the observation of the expected general trend, namely, through the thickness of the sample, a parabolic variation of the sound velocity whose amplitude increases with the magnitude of the surface stress.     

Oxynitride glasses, their properties and crystallisation - a review

Oxynitride glasses were first discovered as intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of such oxynitride glass phases determine the properties of the material. In particular they have been shown to control high temperature mechanical properties. The desire to understand the nature of these grain boundary phases has resulted in a number of investigations on oxynitride glass formation and properties which have shown oxynitride glasses to possess refractory behavior and higher, elastic modulus, viscosity and hardness compared to the corresponding oxide glasses. This paper provides a review of the preparation and characterisation of oxynitride glasses and outlines the effect of composition, especially nitrogen content, on properties such as glass transition temperature, hardness, Young’s modulus and viscosity.     

Bulk metallic glasses based on binary rare earth elements

Bulk metallic glasses (BMGs) are usually based on single element such as Zr, Cu, and Fe. In this work, we report the formation of a series of BMGs based on arbitrarily selected binary rare earth elements. Compared with single-base BMGs, the binary-base BMGs have unique characteristics of excellent glass-forming ability, tunable physical, chemical and mechanical properties. The binary-base BMGs might be of significance in scientific studies and have potential applications, and the approach for formation of metallic glasses has implications in the search for new BMG systems.     

Lutetium and thulium based rare earth bulk metallic glasses

We report the formation of lutetium and thulium based bulk metallic glasses based on the correlations between the thermodynamic, kinetic, elastic and other properties of metallic glasses. The two novel rare earth based bulk metallic glasses (REBMGs) exhibit excellent glass formation ability, high elastic moduli, considerable smaller Poisson’s ratio, high thermal stability, and even higher mechanical strength than that of typical high strength Zr-based BMGs. The reasons for the properties of the Lu- and Tm-based and other REBMGs are discussed.     

Relationship between the imperfection of Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> garnet samples of different crystallographic orientations and their physical properties

The kinetic, optical, and dielectric properties of Y₃Fe₅O₁₂ garnet single crystals of different structural quality have been investigated. It is established that the ratio of the intensities of coherent and incoherent scattering characterizes the conduction electron concentration in inhomogeneous magnetic samples. A correlation between the physical properties of the samples and the character of their structural inhomogeneity is revealed.     

Raman spectroscopic study of SbxSe100−x phase-separated bulk glasses

The structure of Sb_xSe_100−x bulk glasses is investigated with the aid of Raman scattering over a wide composition range. The Raman spectra of the glasses exhibit unusual features when compared with other structurally similar binary glasses, owing to the phase separation of the present glasses in a certain composition range. The evolution of the Raman spectra and the depolarization ratio of the polarized and depolarized Raman intensities are consistent with the phase separation effect. The present findings are discussed alongside with calorimetric data from the literature that have been used up to now to extract structural information in an indirect way. The capability of Raman scattering as a tool for investigating phase separation in homogeneous media is demonstrated.     

Formation and properties of RE55Al25Co20 (RE = Y,Ce, La,Pr, Nd,Gd, Tb,Dy, Ho and Er) bulk metallic glasses

We report that a series of ternary RE₅₅Al₂₅Co₂₀ (RE = Y, Ce, La, Pr, Nd, Gd, Tb, Dy, Ho and Er) alloys can be readily cast into bulk glasses by a conventional casting method. The characteristics and properties of these new bulk metallic glasses (BMGs) are studied and compared. Due to the chemical comparability and well-regulated variety in atomic size, properties and elastic constants of these rare earth elements, the RE₅₅Al₂₅Co₂₀ BMGs could be regarded as a model system to investigate the glass-forming ability, thermal stability, glass transition, crystallization behavior, liquid fragility, elastic and mechanical properties as well as their relationships. An attempt is made to highlight commonality and contrasts of the effects of various factors on the metallic glasses formation and properties.     

Structural development in Ge-rich Ge–S glasses

The Raman spectra of Ge–S glasses in the Ge-rich region from Ge 33% to 46% have been investigated in order to know the structural development of the network glasses. From the detailed curve fits, we have found that there is an unassigned peak at 410 cm⁻¹ and it becomes larger with increasing Ge composition. To clarify the structural origin of the peak, we virtually constructed the atomic arrangement of the glassy state starting from the crystalline state through the liquid state and changed the composition gradually depleting the medium in sulfur. From the consideration of the structural modeling and the atomic orbital theory, we suggest that single Ge–S chain is a probable structural origin of the peak.     

Influence of superheat before quench on the structure and stability of NiP metallic glasses studied by neutron scattering techniques

The temperature dependent structure of Ni₈₁P₁₉ metallic glasses, rapidly quenched from melts which have undergone different heat treatments, has been investigated by small angle neutron scattering and neutron diffraction. All samples contain in the as-quenched state particles or clusters of a very wide range of sizes. These are amorphous regions of different compositions in a large composition range and/or crystallites of another Ni composition than the matrix, such as Ni₃P, Ni₅P₂ or Ni₁₂P₅, or even “voids”. The results can be interpreted within the concept of a heterogeneous structure existing in eutectic melts in a certain temperature range. Comparison is made with results from the literature on samples from the same ribbons investigated by calorimetry as well as by fractography and Weibull analysis of mechanical properties.     

X-ray scattering studies of the short and medium range ordering in AsxTe1−x glasses

The structures of As_xTe_1−x(x = 0.2, 0.4, and 0.5) glasses are studied using the differential X-ray anomalous scattering technique. The partial distribution functions have also been obtained for the stoichiometric As₂Te₃ glass, giving information on the medium range ordering. All the glasses show chemical disorder to differing extents, the As₂Te₃ glass being the most disordered. The Te coordination number undergoes a change at x=0.4 where it is 2.5 compared with 2 for AsTe. This change indicates the existence of about 50% of threefold Te sites in the stoichiometric glass, as well as in the Te-rich glasses. Some of the physical properties of the glasses may be explained based on these results.     

Effect of tin-oxide on the physical properties of soda-lime–silica glass

The effect of tin-oxide on the physical properties of soda-lime–silica glasses was investigated. Glasses containing tin-oxide concentrations ranging from 0 to 3 mol% were synthesized in the laboratory. In some of the glasses, an attempt was made to control the ratio of Sn²⁺ to Sn⁴⁺ present in the glass. Dilatometry, beam-bending viscosity, and sonic resonance experiments were performed on the glasses to determine the role of tin on the thermal expansion, glass transition temperature, dilatometric softening temperature, annealing temperature, strain temperature, and elastic modulus of the glass. Mössbauer spectroscopy was used to determine the presence and relative amount of Sn²⁺ and Sn⁴⁺ in the glasses. The data suggest that the substitution of relatively small amounts of tin for a modifier species in the glass composition results in an increase in the network connectivity. In addition, the results suggest that this increase in network connectivity is more apparent in glasses containing higher relative amounts of Sn⁴⁺ compared to Sn²⁺.     

Effect of polymerization on the boson peak, from liquid to glass

Reactive epoxy-amine mixtures during isothermal polymerization have been characterized by Raman scattering measurements in order to follow the modification of the vibrational density of states. In the initially liquid solutions an increasing number of Van der Waals bonds are replaced by stiffer covalent bonds. During the chemical reaction, the molecular diffusion slows down and ultimately the systems are frozen in a glassy structure. The transformation from reactive liquids to chemical glasses is accompanied by microscopic structural changes driven by the bonding process. During the reaction the samples density and the sound velocity increase, both of them contribute to significantly change the Debye level. By combining Raman with Brillouin inelastic X-ray scattering measurements, we compare the relative variations of the boson peak with that of the Debye level. We find that the shift and intensity variation of the boson peak are fully explained by the modification of the elastic properties of the medium and a boson peak master curve is obtained.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

Rare earth based bulk metallic glasses

Recently, the rare earth based bulk metallic glasses (REBMGs) have attracted increasing interest due to their unique properties and potential applications as functional glassy materials. These REBMGs display many fascinating properties such as heavy fermion behavior, thermoplastic properties near room temperature, excellent magnetocaloric effect, hard magnetism, and polyamorphism, all of which are of interest not only for basic research but also for metallurgy and technology. These characteristics and properties are ascribed to the unique electronic, magnetic and atomic structures of the REBMGs. In this review paper, the fabrication, glass-forming ability, polyamorphism, elastic, thermal, and physical properties are summarized and discussed. Owing to the unique electronic structure of rare earth elements, the electric and magnetic properties of the REBMGs are especially addressed. The works have implications for seeking novel metallic glasses with controllable properties and for understanding the nature of glass formation. The development of REBMGs as functional materials might promote and extend the commercial applications of metallic glasses.     

Inhomogeneous nature of UV absorption bands of bulk and surface oxygen-deficient centers in silica glasses

Photochemical and photoluminescence studies of oxygen-deficient centers stabilized in the bulk and on the surface of silica glasses clearly demonstrate the inhomogeneous nature of the absorption and luminescence spectra of oxygen-deficient centers. The conclusion is drawn that the inhomogeneity of the absorption spectra is due to the dispersion of the energy of the S₀-S₁ transition, while the inhomogeneity of the luminescence spectra is due to the dispersion of the energy barrier of intersystem crossing. The inhomogeneous nature of the oxygen-deficient centers in silica glasses is assumed to be caused by a small dispersion in the geometrical parameters of different groups of centers with similar chemical properties.     

Thermal and mechanical properties of rare earth aluminate and low-silica aluminosilicate optical glasses

Aluminate glasses containing 45–71.5 mol% alumina, 10–40 mol% rare earth oxide, and 0–30 mol% silica were synthesized from precursor oxides. The glass transition and crystallization temperatures were determined by differential scanning calorimetry; the structural and mechanical properties were investigated by Raman and Brillouin spectroscopy. The range of the supercooled liquid region varies from ∼40 °C to 200 °C, providing a useful working range for compositions with 5–30 mol% silica. Raman scattering showed the presence of isolated SiO₄ species that strengthen the network-forming structure, enhance glass formation, and stabilize the glass even when they are present at fairly low concentrations. Sound velocities were measured by Brillouin scattering. From these and other values, various elastic moduli were calculated. The moduli increased with both aluminum and rare earth content, as did the hardness of the glasses. Young’s modulus was in the range 118–169 GPa, 60–130% larger than that for pure silica glass.