Crystallization of monoclinic WO3 in tungstate fluorophosphate glasses

Tungstate fluorophosphate glass compositions with high WO₃ concentration were prepared in the ternary system (80−0.8x)NaPO₃-(20−0.2x)BaF₂-xWO₃ with x = 40,50 and 60 mol%. Transparency decreases as WO₃ concentration increases but can be improved by addition of oxidizing systems such as CeO₂ or Sb₂O₃/NaNO₃. Characterizations by thermal analysis (DSC) point out that an increase in the amount of WO₃ results in a higher glass transition temperature. In addition, such compositions are very stable against devitrification since samples containing 40% and 50% of WO₃ donot even exhibit the expected crystallization event. In these samples, the stability against crystallization decreases with the WO₃ content and vitreous sample containing 60% of WO₃ exhibits an endothermic event around 620 °C due to crystallization of monoclinic WO₃ phase. In these glasses, it was shown that the nucleation stage can be induced by thermal-treatment when external nucleating agents such as Ti or Sb are used. Finally, gold-doped samples exhibit a higher crystallization tendency and monoclinic WO₃ phase can be grown in such glasses.     

Structural characterization of phosphate glasses doped with silver

The present paper reports the influence of silver oxide addition on the local structure of 2P₂O₅ · CaO · 0.05ZnO glass matrix. The glass samples were investigated through several methods: X-ray powder diffraction (XRD), scanning electron microscopy (SEM), infrared absorption (FT-IR) and Raman scattering. The X-ray diffraction patterns confirm the vitreous character of these samples over the explored compositional range and the SEM pictures confirm this information. The phosphate structural units of the network former are assessed from FT-IR and Raman spectra as ultra-, meta-, pyro- and orthophosphate units. A slight structural depolymerization process of these phosphate-based glasses was evidenced for higher silver oxide content. In vitro behavior of the bulk glass sample with the highest silver oxide content was tested by immersion in simulated body fluid (SBF). X-ray diffraction and SEM measurements made on the SBF treated sample revealed growth of a crystalline phase on the surface sample.     

Stretched exponential relaxation in the Coulomb glass

The relaxation of the specific heat and the entropy to their equilibrium values is investigated numerically for the three-dimensional Coulomb glass at very low temperatures. The long time relaxation follows a stretched exponential function, f (t) = f ₀exp - (t/τ)^β , with the exponent β increasing with the temperature. The relaxation time diverges as an Arrhenius law when T→ 0. Received 24 May 2001 and Received in final form 12 September 2001     

Microexplosions in tellurite glasses

Femtosecond laser pulses were used to produce localized damage in the bulk and near the surface of baseline, Al₂O₃-doped and La₂O₃-doped sodium tellurite glasses. Single or multiple laser pulses were non-linearly absorbed in the focal volume by the glass, leading to permanent changes in the material in the focal volume. These changes were caused by an explosive expansion of the ionized material in the focal volume into the surrounding material, i.e. a microexplosion. The writing of simple structures (periodic array of voxels, as well as lines) was demonstrated. The regions of microexplosion and writing were subsequently characterized using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and atomic force microscopy (AFM). Fingerprints of microexplosions (concentric lines within the region and a concentric ring outside the region), due to the shock wave generated during microexplosions, were evident. In the case of the baseline glass, no chemistry change was observed within the region of the microexplosion. However, Al₂O₃-doped and La₂O₃-doped glasses showed depletion of the dopant from the edge to the center of the region of the microexplosions, indicating a chemistry gradient within the regions. Interrogation of the bulk- and laser-treated regions using micro-Raman spectroscopy revealed no structural change due to the microexplosions and writing within these glasses. Received: 27 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. +1-509/376-3108, E-mail: sk.sundaram@pnl.gov     

Effect of ZnS shell on the Raman spectra from CdSe nanorods

We investigated the influence of an epitaxially grown ZnS shell on the phonon spectra of CdSe nanorods of different sizes. The CdSe related Raman peaks shift with addition of a ZnS shell. The longitudinal optical phonon shifts slightly due to strain and the low‐energy shoulder shifts stronger, which can be explained within a model for surface optical phonons. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The ionic transition and the glass transition in ion-conducting glasses

In an ion-conducting glass, there may be the ionic transition which characterizes ‘melting’ of the modifying ions in a similar way as the glass transition characterizes ‘melting’ of the glass network former. The ionic transition can be observed, electrically, by the thermally stimulated depolarization current technique. It is demonstrated that the relaxation time for the ionic transition is the same (near 100 s) with that for the glass transition in so far as the heating/cooling rate lies within, say, 10⁻¹-10⁻⁵ K/s as adopted in our routine works.     

Cross-section and rate coefficient calculation for electron impact excitation, ionisation and dissociation of H2 and OH molecules

The weighted total cross-section (WTCS) theory is used to calculate electron impact excitation, ionisation and dissociation cross-sections and rate coefficients of OH, H₂, OH⁺, H₂ ⁺, OH^- and H₂ ^- diatomic molecules in the temperature range 1500–15000 K. Calculations are performed for H₂(X, B, C), OH(X, A, B), H₂ ⁺(X), OH⁺(X, a, A, b, c), H₂ ^-(X) and OH^-(X) electronic states for which Dunham coefficients are available. Rate coefficients are calculated from WTCS assuming Maxwellian energy distribution functions for electrons and heavy particles. One and two temperature (θ_e and θ_g respectively for electron and heavy particles kinetic temperatures) results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θ^b)exp (-c/θ).     

The superconducting gap and an unidentified 110–190 K phase of YBa2Cu3O7 observed by far-infrared ellipsometry

We have measured the complex dielectric function of pure and Fe-doped YBa₂Cu₃O₇ ceramics, between 10 and 150 cm^–1 and 20 and 300 K, with a far-infrared, rotating-analyzer laser ellipsometer. The superconducting phase transition and the gap energy are clearly observable. The undoped ceramic exhibits remarkable dielectric hysteresis between 110 K and 190 K. This points to a still unidentified phase which we observe to be sensitive to a weak magnetic field.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed.