Aging in chalcohalide glasses: Origin and consequences

The application of chalcogenide and chalcohalide glasses is limited by their uncontrolled drift in properties over time due to aging processes. In the present work, we perform aging experiments on some chalcohalide glasses in oxidizing, inert and reducing atmospheres and afterwards we measure the elemental concentration depth profiles in the surface layer of the glasses by using secondary neutral mass spectroscopy. The results show that anionic diffusion processes occur in the glasses during aging. The aging process leads to a decrease in microhardness of the studied glasses, which is attributed to both physical aging (i.e., structural relaxation) and chemical aging (i.e., compositional change of the surface layer).     

Aging,glass transition,supercooled and equilibrium liquid states of alkali germanate glasses studied by Brillouin scattering

The elastic properties of alkali germanate glasses, xR₂O?(100 ? x)GeO₂ (R = Li, Na, K, Rb, Cs ; x = 14, 28), have been studied by Brillouin scattering in the wide temperature range up to 1200 °C. The remarkable aging effect of Brillouin shift Δν_L has been observed below a glass transition temperature T_g ≈ 500 °C. The temperature dependence of longitudinal sound velocity V_L of well annealed glasses shows the gradual decrease below T_g, while on further heating the remarkable decrease is observed above T_g. The scaled temperature dependence of V_L is nearly independent on alkali metals below the melting temperature T_m. While on further heating above T_m, the drastic decrease of V_L and increase of α_L show the remarkable alkali dependence. It may be attributed to the appearance of dynamic process related to ionic hopping of alkali metals released from glass network above T_m.     

Halide glasses

Halide glasses comprise a relatively uncommon class of inorganic glass systems which have heretofore been considered mainly as laboratory novelties. Serious attempts to develop these systems have generally been lacking owing to both difficulties in fabrication as well as because of the toxicity and chemical instability of some of these glasses. While halide glasses based upon beryllium fluoride and zinc chloride are well-known, the glass forming ability of ZrF₄, AlF₃, HfF₄, and more recently PbF₂ have also been demonstrated. The purpose of this review is to assess the technical and scientific importance of these glass systems, particularly with respect to advances within the past ten years.     

Halide glasses

The reasons for the formation of vitreous materials in halide systems are discussed in relation to the nature of the chemical bond. A review of fluoride, chloride, bromide and iodide glasses is presented and discussed in terms of stability and technical interest. The most interesting physical properties, especially optical, include the potential of ultra-low loss fiber for power and telecommunications applications. Recent structural investigations by X-ray, computer simulation, etc, allow the presentation of structural models explaining the glass formation by references to the corresponding crystalline halide materials. A structural systematic is described and compared with the traditional oxide glasses.     

Glassy states: Concentration glasses and temperature glasses compared

The behavior of glass-forming systems in the equilibrium state above the glass temperature is still a heavily investigated field. Surprisingly, the behavior of the glass itself is less widely investigated. Even less investigated is the behavior of glass-forming materials in which composition is changed. Here we look at the behavior of glasses after temperature-jumps and compare that behavior with that of glasses subjected to concentration-jumps. Moisture and carbon dioxide are used as the plasticizing environments. Surprisingly, the glass created by jumping (down) to a given final condition via a change in concentration is more stable than that formed by a change in temperature – this in spite of the external condition of temperature and chemical activity (RH or carbon dioxide pressure) being the same. Furthermore, the concentration glass under such conditions has a higher excess volume than the temperature glass and its response does not ‘merge’ with that of the temperature glass, hence, the concentration glass is not the same as a temperature hyperquenched glass.     

Influence of the polymer architecture on the high temperature behavior of SiCO glasses: A comparison between linear- and cyclic-derived precursors

Two series of cross-linked polysiloxanes, precursors for silicon oxycarbide glasses, have been synthesized from a linear and a cyclic Si-H-containing siloxane having the same chemical formula (SiCOH₄). The crosslinking has been achieved by hydrosilylation reaction with various amounts of divinylbenzene (DVB). A detailed structural characterization has been performed by ²⁹Si and ¹³C MAS NMR, FT-IR and chemical analysis. As a result, two different structural models have been proposed for the two series of resins. The two resins have been pyrolyzed at 1400 °C and the resulting SiCO ceramics characterized by X-ray diffraction. It has been shown that the stability of the amorphous silica phase present in the SiCO ceramics is strongly influenced by the molecular organization of the starting precursors. The presence of siloxane rings in the cyclic-derived polysiloxane decreases the stability of the amorphous SiO₂ and promotes the crystallization of cristobalite.     

Lattice vibrations of glasses

Measurements of the thermal conductivity of glasses have led to a model of their lattice vibrations. They are not described as waves, as they are in crystals, but as uncorrelated harmonic motions of individual atoms. The lattice vibrations of fully cerammed, crystalline glass ceramics have been found to be either waves, as in crystals, or vibrations of individual atoms, as in glasses. The understanding of the cause for this different behavior has been aided by studies of systematically doped crystals.     

Crystallization of gel-derived glasses

The crystallization behaviors of gels, gel-derived glasses and melted glasses are reviewed, and comparisons are effected among the observed behaviors. These comparisons are related to the behavior expected on the basis of models for the nucleation frequency and crystal growth rate, as well as the anticipated differences in material characteristics among gels and the two types of glasses. Different temperature regimes are identified where different comparisons between melted and gel-derived materials are expected. It is concluded that a need exists for more definitive, detailed data on the crystallization behavior of gel-derived materials.     

Optical spectra of glasses

This presentation focuses on several ways in which structural and dynamical information can be obtained from optical spectroscopy by examining results of infrared and Raman studies of selected oxide glasses. Though these studies represent only a small fraction of the work on most of the glasses mentioned, and many other important forms of optical spectroscopy are not treated here, they illustrate useful approaches to the study of amorphous materials by optical methods.     

Lanthanum fluorozirconate glasses

Investigations in the ZrF₄-BaF₂-LaF₃ ternary system show the existence of a vitreous area in the range: ZrF₄: 56–67%, BaF₂:25–36%, LaF₃:3–13%. For a standard glass of molar composition 0.62 ZrF₄, 0.30 BaF₂, 0.08 LaF₃, the refractive index is 1.5248 with the glassy transition occuring at 310°C and fusion occuring at 545°C.Quaternary glasses have been obtained by including a MF fluoride (M = Na or Li) with the previous ternary glass according to the formula: (0.62 – 0.4x)ZrF₄(0.30 –0.5x)BaF₂(0.088 – 0.1x)LaF₃xMF. Evolution of glassy transition, refractive index, density and molar refractivity is discussed in relation to the structural role of the Na⁺ and Li⁺ ions. In these zirconium fluoride glasses, lithium could act as a network former at low concentration and as a network modifier at higher concentration.     

Electrical properties of phosphate glasses

The electrical properties of glasses in the Na₂OP₂O₅, Ag₂OP₂O₅ and (1?x)Na₂OxAg₂OP₂O₅ systems have been measured over a range of temperature and composition.The properties of the Na₂OP₂O₅ and Ag₂OP₂O₅ glasses have been compared within the phosphate system as well as with silicate glasses. The silver-containing glasses show higher conductivity and lower temperature coefficients when compared with the sodium-containing glasses. A maximum in the room temperature resistivity of the (1?x)Na₂O?xAg₂O?P₂O₅ system was found around the mole ratio of 0.16:0.84 Ag₂O:Na₂O, indicating a mixed-alkali effect. A similar effect was seen in the tan δ, but not in the T_g-against-composition plots. A linear relationship was noted for the tan δ-versus-log₁₀ (resistivity) plot, as has been seen in other glass-forming systems.     

Stress-strain-acoustic emission response of glasses

Using the response of stress-strain-acoustic emission the microcrack behavior of ordinary and tempered glasses has been studied. As a result the authors propose two new concepts, the degree of microfracture and the damage ratio, which can be used as indications of material strength.     

EPR study of molybdenum-lead-phosphate glasses

The local symmetry and interaction between paramagnetic ions in xMoO₃(1 ? x)[2 P₂O₅PbO] glasses with 0.5 ? x ? 50 mol% are investigated by EPR spectroscopy. For x ? 10 mol% the isolated Mo⁵⁺ ions surrounded by five oxygen ligands in a square-pyramidal form (C_4v symmetry) prevail. The short range disorder in the environment of Mo⁵⁺ ions is not significantly (ΔR/R ≈ 2%). At high molybdenum content (x > 20 mol%) the dipole–dipole and superexchange coupled Mo⁵⁺ ions appear and their number increases with the MoO₃ content. These two aspects are also correlated with the network modifier and former role of molybdenum oxide in function of its concentration. Thus a strong depolymerization of the phosphate structure and the formation of P–O–Mo or Mo–O–Mo bonds in studied glasses appear.     

Structural analysis of bioactive glasses

In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Qⁿ. ²⁹Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q² state. The chemical shift of the ³¹P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The ²⁹Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q² and Q³ were calculated, showing that all glasses studied are predominantly Q² in structure, i.e. silica chains which readily dissolve. The changes in the chemical shifts of the Q² and Q³ species with composition have been interpreted as resulting from the preferential association of Na⁺ with Q³ and Ca²⁺ with Q².     

Structure analyses of Ba-silicate glasses

The structures of four different Ba-silicate glasses were analysed applying a broad spectrum of diffraction and spectroscopic methods, atomic force microscopy and computer simulations. This collaborative study offers new insights into the influence of the network modifier BaO. 3 at.% carbon were incorporated into two of the glasses with the intention to obtain additional information about the effect of partial substitution of carbon for oxygen on the considered structures. A structure model for Ba-silicate glasses with compositions close to BaSi₂O₅ is discussed that differs with respect to other models proposed in previous studies. We propose that [SiO₄] tetrahedra do not exclusively form six-membered rings, but also considerable numbers of smaller rings arranged in (slightly) folded layers which alternate with barium layers and/or barium chains. Additionally, the occurrence of tetrahedral chains, possibly isolated rings and of regions in which the [SiO₄] tetrahedra form a silica-like network is assumed.     

Internal friction of alkaline-earth-iron-metaphosphate glasses

The internal frictions of Mg, Ca, Ba iron-metaphosphate glasses were studied from 120 to 700 K using the low-frequency torsion pendulum technique. The internal friction as a function of temperature revealed two distinct peaks. The low-temperature peak was absent in glass without iron oxide. Its activation energy compared favourably with that for dielectric loss and dc conductivity in similar glasses. This suggests the same mechanism for all three processes, probably connected with electron transfer between iron ions of different valence.     

Infrared conductivity of chalcogenide glasses

The general features of the conductivity spectra of chalcogenide glasses in the frequency range 10⁹ to 10¹³ Hz are briefly reviewed.     

Optical properties of selenite glasses

The aim of this work is to obtain multicomponent selenite glasses containing other non-traditional glass formers such as V₂O₅, TeO₂ and MoO₃ and to verify their optical properties in the visible spectral region. Glasses containing MoO₃ and TeO₂ are transparent in the visible range and near IR region from 400 to 2300 nm. Transparent coloured glasses were obtained due to the electron transfer charge processes. Using IR spectroscopy it was determined the main building units of the amorphous network. It was found the presence of TeO₄, SeO₃ and MoO₄ units.     

Optical properties of chalcogenide glasses

Over the past two decades chalcogenide glasses have been researched in order to assess their suitability as passive bulk optical component materials for 3–5μm and 8–12μm infrared applications. This research has led to a greater understanding of the physical properties of these materials, and the present paper concentrates on the optical properties and applications of these bulk chalcogenide glasses. The various factors influencing the intrinsic and extrinsic optical loss mechanisms in materials are discussed, numerical data on the basic optical properties of chalcogenide glasses is presented and applications are discussed.