Se Glasses Research Articles

Is the configurational entropic model able to predict the final equilibrium state reached by Se glasses after very long ageing durations?

This work compares the equilibrium states reached by glassy selenium (g-Se) after several ageing durations at temperatures lower than the glass transition temperature T g, with the forecasts given by the configurational entropic model. The comparison is carried out through experimental data collected both on glassy samples after short-term ageing and on glassy samples older than 20 years, kept at room temperature. It is shown that the configurational entropic model is not able to describe the behaviour of g-Se, both at short- and long-ageing terms with the same set of fitting parameters. For short-term ageing, the forecasts given by the entropic model are in good agreement with the experimental data; the hypothesis that the heat capacities corresponding to the equilibrium state and to the state extrapolated from the liquid differ by an amount δ is therefore properly justified. For long-term ageing, the amount δ vanishes and the assumption does not hold any longer. Indeed, experimental results support the idea that at least two separate mechanisms of relaxation coexist in the physical ageing of g-Se, one with very fast kinetics and the other with much slower kinetics.

Bond-changing structural rearrangement in glassy As3Se7 associated with long-term physical aging

Ab initio quantum chemical calculations with RHF/6-311G⁎ basis set are performed to justify destruction-polymerization transformations in glassy g-As3Se7 possible during long-term natural physical aging. It is shown that switching of two AsSeSeAs bridges into short AsSeAs and long AsSeSeSeAs fragments is energetically favorable, testifying thermodynamic possibility for such structural changes. This finding is in good agreement with previous experimental observations for g-As3Se7 performed with Raman scattering, NMR and high-resolution XPS measurements. Simulation data suggest that physical aging in Se-rich As–Se glasses can be treated as a unique cooperative relaxation process accompanied by atomic rearrangement within homopolar SeSe bonds incorporated between two neighboring AsSe3/2 pyramids.

All-solid all-chalcogenide microstructured optical fiber

The realization of an all-solid microstructured optical fiber based on chalcogenide glasses was achieved. The fiber presents As(2)S(3) inclusions selected as low refractive index material (n = 2.4) embedded in a As(38)Se(62) glass matrix (n = 2.8). The single mode regime of the fiber was demonstrated both theoretically by multipole method calculations and experimentally by near field measurements. Optical transmission measurements of the microstructured fiber and single index fibers made of the initial glasses reveal an excess of losses as high as 6-7 dB/m. This excess is not due to the guide geometry but can be explained by the presence of defects in the glass interface regions.

Ag–Ge–Se glasses: a vibrational spectroscopy study

Agx(Ge0.25Se0.75)100−x glasses with x varying from 0 to 25 have been shown to exhibit a conductivity threshold around x ~8–10. In this work, the structural changes induced by introduction of Ag in Ge25Se75 glass have been investigated using Raman and infra‐red spectroscopies. In ambient conditions, changes are observed in position, width and intensity for vibrations assigned to Ge–Se bonds, showing that the tetrahedral network is relaxed and gains flexibility as Ag is introduced. High pressure experiments on two glasses containing 5 and 25 at.% Ag confirm that Ag‐rich (25%) glasses clearly exhibit higher compressibility than Ag‐poor (5%) glasses. Copyright © 2013 John Wiley & Sons, Ltd.

In situ Raman observation of laser-induced formation of TlInSe2 crystallites in Tl–In–As–Se glass

Tl–In–As–Se glasses are obtained by co-melting of As2Se3 and TlInSe2. Laser-induced crystallization of TlInSe2 in the glass due to local laser beam heating is observed in the course of micro-Raman measurements. Dependence of the crystallization process on the laser power is studied. Simultaneously a photoplastic effect in the glass is observed leading to the mass transfer from the laser spot.

Elastic transition thresholds in Ge–As(Sb)–Se glasses

We prepared several groups of GexAsySe1−x−y and Gex Sby Se1−x−y glasses with a fixed y at 10 at%, 15 at% and 20 at%, respectively, and measured their density and elastic moduli with the aim of understanding the correlation between the chemical compositions, the mean coordination numbers (MCNs) and the physical properties. It was found that whilst the glass density on average showed a maximum around MCN≈2.45 and a minimum at MCN≈2.6, varying the proportion of the chemical elements could modify the density and smear out these transition thresholds. The elastic moduli of the glasses were estimated from the shear and compressional wave velocities measured by ultrasonic pulse interferometry. We simultaneously observed two elastic transition thresholds: the first one at MCN≈2.45 and the second one at MCN≈2.6, which appear to be closely correlated with changes in the glass microstructure. However, differences were observed in the elastic moduli of groups of samples with the same MCN = 2.4 and 2.6 but different compositions suggesting that the chemical compositions also influence the elastic properties especially in the Ge–Sb–Se glasses where the elements have larger differences in mass and atomic radius.

Investigation of thermal properties of the Ge–As–Se glasses by differential scanning calorimetry with heat flow harmonic modulation

Thermal properties of the GexAsySe100−x−y glasses have been studied using differential scanning calorimetry with harmonic temperature modulation. The temperature changes of the reverse and non-reverse components of the heat flow in these materials have been analyzed. The concentration dependences of the enthalpy of relaxation, activation energy of viscous softening and the increase of heat capacity in the softening range with average coordination numbers Z=2.12–2.72 have been obtained. It has been shown that the essential changes of thermodynamic parameters of glasses under study are caused by their structural transition. Correlations between the nature of changes of thermal characteristics of the GexAsySe100−x−y glasses and the features of their mechanical relaxation have been determined.

Incorporation of Ga into the structure of Ge–Se glasses

The structure of Ga-containing Ge–Se glasses is studied by high-resolution X-ray photoelectron and extended X-ray absorption fine-structure spectroscopies. The results show that in GaxGeySe100–x–y glasses both Ga and Ge atoms are 4-fold coordinated, whereas Se atoms are 2-fold coordinated. For Se-deficit samples with Se content less than 67 at.%, Ge–Ge(Ga) bonds appear. First, Ge–Ge bonds are formed indicating preference for the formation of (Se)2 > Ga < (Se)2 units over the regular GeSe4/2 tetrahedra. Metal-Ga bonds appear only in the most Se-deficit sample. Addition of Ga facilitates more uniform Se distribution in comparison to binary Ge–Se or ternary As(Sb)–Ge–Se systems. The observed structural features are correlated with some basic thermo-mechanical properties of the investigated glasses.

Observation of a Continuous Random Network Structure in GexSe100–x Glasses: Results from High-Resolution 77Se MATPASS/CPMG NMR Spectroscopy

The coordination environments of Se atoms in binary Ge(x)Se(100-x) glasses with 5 ≤ x ≤ 30 are investigated using a novel, two-dimensional (77)Se nuclear magnetic resonance (NMR) spectroscopic technique. The high-resolution isotropic (77)Se NMR spectra allow for the identification of up to four distinct Se sites in these glasses. The chemical shift tensor parameters for these sites offer unique insights into their local site symmetries and nearest- and next-nearest-neighbor coordination environments. The structural results, when taken together, provide direct evidence in favor of the existence of a randomly connected network of GeSe(4) tetrahedra and Se-Se chain fragments in these glasses.

Structural relaxation in Se-rich As–Se glasses

Enthalpic structural relaxation in the AsySe(100-y) glassy system was studied by means of differential scanning calorimetry (DSC). Glasses in the compositional range 0≤y≤15 were examined. Based on the curve-fitting results, the description of relaxation behavior was obtained in terms of the Tool–Narayanaswamy–Moynihan model. These results were further confirmed by a novel simulation-comparative non-fitting method. Evolution of TNM parameters with changing arsenic content was interpreted in terms of joint movements of occasionally linked Se polymeric chains continuously transiting towards rather independent positional changes within inter-linked segments or clusters. The interpretation represents a novel approach associating a phenomenological description with factual changes in the molecular structure of the material.

Influence of Bi on topological self-organization in arsenic and germanium selenide networks

The influence of Bi on the network structure of Ge20Se80−yBiy and As40−ySe60Biy glasses is investigated by high-resolution X-ray photoelectron spectroscopy (XPS). The results show that Bi enters the glass network of both Ge–Se and As–Se glass systems in the form of BiSe3/2 pyramids, with a significant increase in the concentration of homopolar Ge–Ge and As–As bonds, respectively, compared to Bi-free samples. In these compositions, topological self-organization is found to play a minor role in comparison to phase separation tendencies at the nanoscale caused by Bi addition. It is suggested that BiSe3/2 rich regions occur as partially ordered nano-thin sheets, which may account for the percolative-type response of electrical conductivity observed in similar Bi containing systems.

Step-wise kinetics of natural physical ageing in arsenic selenide glasses

The long-term kinetics of physical ageing at ambient temperature is studied in Se-rich As–Se glasses using the conventional differential scanning calorimetry technique. It is analysed through the changes in the structural relaxation parameters occurring during the glass-to-supercooled liquid transition in the heating mode. Along with the time dependences of the glass transition temperature (Tg) and partial area (A) under the endothermic relaxation peak, the enthalpy losses (ΔH) and calculated fictive temperature (TF) are analysed as key parameters, characterizing the kinetics of physical ageing. The latter is shown to have step-wise character, revealing some kinds of subsequent plateaus and steep regions. A phenomenological description of physical ageing in the investigated glasses is proposed on the basis of an alignment–shrinkage mechanism and first-order kinetic equations.

The Influence of Dysprosium Addition on the Crystallization Behavior of a Chalcogenide Selenide Glass Close to the Fiber Drawing Temperature

A series of chalcogenide glasses based on Ge16.5Ga3As16Se64.5 (at.%), doped nominally with 0, 1000, 3000, and 6000 ppmw (by weight) of Dy foil, is prepared by using conventional method: melting inside a sealed silica glass ampoule, which is rocked to homogenize the melt, followed by melt quenching and annealing. Examination of the as‐prepared glasses, using scanning electron microscopy with energy dispersive X‐ray spectroscopy (SEM‐EDX), reveals that dysprosium‐rich areas of devitrification have a tendency to form at the surfaces of the prepared chalcogenide glasses which have been adjacent the silica ampoule walls during the final stages of melting, quenching, and annealing; such dysprosium‐rich areas of devitrification are not observed using SEM‐EDX at the noncontacting‐silica‐ampoule chalcogenide glass surface nor in the interior of the chalcogenide glass bulk. Samples taken from the interior of the prepared glass boules exhibit an increase in glass transition of up to 9 ± 2°C, and rise in other isoviscous temperatures, with increasing dysprosium content. Extended isothermal heat treatment at the estimated fiber‐drawing temperature, followed by quenching to room temperature and analysis using SEM‐EDX and X‐ray diffraction (XRD), is carried out to investigate the influence of dysprosium addition on crystallization behavior on reheating the chalcogenide glass. Increasing the dysprosium level, surprisingly, appears on the one hand to help to restrain crystallization of the bulk glass on glass reheating, whereas on the other hand, and at the same time, to exacerbate the surface devitrification on glass reheating. The bulk crystallizing phase on reheating the glasses is distorted face‐centered cubic α‐Ga2Se3. The same phase was found to grow in the bulk glass during melt cooling of Dy‐doped Ge–As–Ga–Se glasses in our earlier work.

Crystallization kinetics and thermal stability in Ge–Sb–Se glasses

AbstractThe crystallization kinetics of Ge–Sb–Se chalcogenide glasses prepared by melt‐quenching method was investigated using differential scanning calorimetry under non‐isothermal conditions with several different heating rates. Kissinger's equation and Matusita model were employed to analyze kinetic crystallization behavior of the glasses. The crystallization parameters were calculated and the crystallization mechanism was studied. The results indicate that the crystallization activation energy increases rapidly at the glass with a mean coordination number (MCN) of 2.4, and reaches its maximum at MCN of 2.65. These two transition thresholds correspond to the structural phase transition in the glassy network. The evolution of the glass transition temperature (Tg), activation energy for crystallization (Ec), glass forming ability and thermal ability as functions of MCN and chemical composition might have substantial implication on screening the best glass for the application in photonics.

Structural investigations of Ge5AsxSe95−x and Ge15AsxSe85−x glasses using x-ray diffraction and extended x-ray fine structure spectroscopy

The structure of Ge5AsxSe95−x (x = 10, 20, 30, 38 at.%) and Ge15AsxSe85−x (x = 10, 25, 34 at.%) glasses has been investigated by high-energy x-ray diffraction and extended x-ray absorption fine structure measurements. The experimental datasets have been modelled using the reverse Monte Carlo simulation technique. The model atomic configurations have been analysed in detail. It has been found that the homonuclear Ge–Ge, As–As, Se–Se and heteronuclear Ge–As bonds play an important role in the structure formation of the Ge–As–Se glasses. The total number of these bonds decreases quite slowly with the mean coordination number similarly to the nonlinear refractive index.

Determination of specific heat in multi-component chalcogenide glasses of Se–Te–Sn–Pb system using modulated differential scanning calorimetry

In the recent past, glass physicists have focused their attention on the element lead as a modifier due to observation of carrier-type reversal in chalcogenide glasses for a particular composition of certain glassy systems after the incorporation of lead. Keeping this fact in mind, some new quaternary glasses have been synthesized using Pb as modifier in the ternary Se78Te20Sn2 alloy parent glass. Specific heat studies have been made in glassy Se78−xTe20Sn2Pbx(0≤x≤6) alloys using the modulated differential scanning calorimetry (MDSC) technique. An extremely large increase in the specific heat values has been observed at the glass transition temperature. The composition dependence of specific heat values below glass transition temperature (Cpe) and after glass transition (Cpg) has been also explained.

Diffusion of Ag ions under random potential barriers in silver-containing chalcogenide glasses

We show that a random walk model of diffused ions interprets well the experimentally deduced Nyquist plots in ionic chalcogenide glasses. Fitting of the model to the experimental data in the Nyquist plots of Ag–As–S(Se) glasses taken as case examples yields important physical parameters such as number of mobile Ag+ ions, hopping relaxation time and diffusion coefficient. It was found that the number of freely moving Ag+ ions is almost independent of Ag content in Agx (As0.33S(Se)0.67)100−x glasses, while the maximum hopping relaxation time is highly dependent on Ag concentration.

Nonisothermal crystallisation kinetics of amorphous selenium prepared by high-energy ball milling: A comparison with the melt-quenching and thin-film techniques

Differential scanning calorimetry (DSC) studies were performed under nonisothermal conditions at various heating rates for glassy Se made by high-energy ball milling. Comparisons were made between the ball-milling technique and the melt-quenching and thin-film techniques. Well-defined endothermic and exothermic peaks were observed at the glass-transition temperature, Tg, and the onset temperature of crystallisation, Tc. The isoconversional method of Vyazovkin was used to determine the variation in the activation energy for crystallisation with temperature, Eα(T). The value of Eα(T) was dependent on the sample preparation method. The thermal stability of the Se glasses was evaluated by calculation of the temperature difference (Tc−Tg) and the S-parameter. In addition, the glass-forming ability was estimated by the criteria of reduced glass-transition temperature, Trg, and the Hruby number, HR. The structures of the Se samples that resulted from the DSC analysis were identified using an X-ray diffractometer. The glasses formed using the thin film technique were the most stable.

Near-zero anomalous dispersion Ge_115As_24Se_645 glass nanowires for correlated photon pair generation: design and analysis

We show that highly nonlinear chalcogenide glass nanowire waveguides with near-zero anomalous dispersion should be capable of generating correlated photon-pairs by spontaneous four-wave mixing at frequencies detuned by over 17 THz from the pump where Raman noise is absent. In this region we predict a photon pair correlation of >100, a figure of merit >10 and brightness of ~8×10(8) pairs/s over a bandwidth of >15 THz in nanowires with group velocity dispersion of <5 ps∙km(-1) nm(-1). We present designs for double-clad Ge(11.5)As(24)Se(64.5) glass nanowires with realistic tolerance to fabrication errors that achieve near-zero anomalous dispersion at a 1420 nm pump wavelength. This structure has a fabrication tolerance of 80-170 nm in the waveguide width and utilizes a SiO(2)/Al(2)O(3) layer deposited by atomic layer deposition to compensate the fabrication errors in the film thickness.

Compositional dependences of average positron lifetime in binary As–S/Se glasses

Compositional dependence of average positron lifetime is studied systematically in typical representatives of binary As–S and As–Se glasses. This dependence is shown to be in opposite with molar volume evolution. The origin of this anomaly is discussed in terms of bond free solid angle concept applied to different types of structurally-intrinsic nanovoids in a glass.