Sodium tracer diffusion in glasses of the type (Na2O)0.2(B2O3)y(SiO2)0.8 − y

Sodium tracer diffusion coefficients, D^?_Na, have been measured in sodium borosilicate glasses of the type (Na₂O)_0.2(B₂O₃)_y(SiO₂)_0.8−y as a function of temperature and the composition parameter y. At constant temperature, the tracer diffusion coefficient of sodium decreases as y increases. The activation enthalpy derived from sodium tracer diffusion data for temperatures up to about 350 °C increases about linearly with increasing values of y from about 70 to 100 kJ/mol. The pre-exponential factor of the sodium tracer diffusion coefficient as a function of y varies by about one order of magnitude and has a minimum at near y = 0.3.     

Thermodynamical miscibility in 0.8[xB2O3-(1 − x)P2O5]-0.2K2O glasses

The thermodynamical miscibility in 0.8[xB₂O₃-(1 − x)P₂O₅]-0.2K₂O glasses has been studied by measuring the glass transition temperature and the mixing enthalpy. Measurements have been performed by differential scanning calorimeter and hydrofluoric acid solution calorimetry at 298 K. The enthalpies of mixing are significantly negative over the whole range of composition. The results are discussed in terms of intermolecular associations.     

Sodium tracer diffusion in glasses of the type (CaO·Al2O3)x(2 SiO2)1−x

Tracer diffusion coefficients of the radioactive isotope Na-22 were measured in glasses of the type (CaO·Al₂O₃)_x(2 SiO₂)_1−x to study the diffusion of sodium as a function of glass composition, x, temperature and initial water content. The diffusion of Na-22 in glasses diffusion-annealed in dry air can always be well described by a single tracer diffusion coefficient, but sometimes not in samples annealed in common air. It was found that the sodium tracer diffusion coefficient decreases by about six orders of magnitude when the glass composition x changes from 0 to 0.75 at 800 °C. The temperature dependence of the diffusion of sodium seems to decrease as the silica content increases. Variations of the initial water content in some of the glasses investigated did not very significantly influence the rate of the tracer diffusion of sodium.     

Combined in-situ SAXS/WAXS and HRTEM study on crystallization of (Cu60Co40)1 − xZrx metallic glasses

CuZr as well as CoZr are well known metallic glass-formers in a wide compositional range. Since the binary Cu-Co system exhibits a metastable liquid-liquid miscibility gap, i.e. Cu and Co tend to separate from each other, the ternary Cu-Co-Zr system is a promising candidate to form phase separated glass-glass composites. In this work (Cu₆₀Co₄₀)_1 − xZr_x metallic glasses with relatively low Zr contents of x = 37 and x = 32 were prepared by melt spinning and investigated by in-situ small-angle and wide-angle X-ray scattering (SAXS/WAXS) and differential scanning calorimetry (DSC). Certain heat treated samples were additionally investigated by high-resolution transmission electron microscopy (HRTEM). Even for x = 32 there are no indications for any kind of phase separation in the as-quenched state within experimental resolution, i.e. the critical temperature T_c for a liquid-liquid phase separation has already decreased from 1556 K for binary Cu₆₀Co₄₀ to a temperature below the glass transition temperature T_g = 762(5)K found for (Cu₆₀Co₄₀)₆₈Zr₃₂. Combined in-situ SAXS/WAXS and HRTEM investigations reveal that thermal annealing also does not induce an amorphous-amorphous phase separation. Instead the formation of nano crystallites of a so far unknown Cu-rich/Zr-poor phase with relatively low activation energy for crystallization E_a = 116(7) kJ/mol at temperatures far below the crystallization temperature deduced from DSC measurements is observed.     

Ionic conduction, diffusion and glass transition in 0.2[XNa2O · (1−X)Rb2O] · 0.8B2O3

We have investigated the ionic transport in the 0.2[XNa₂O · (1−X)Rb₂O] · 0.8B₂O₃ mixed-alkali system with X=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 in the glassy and the undercooled-liquid state by means of impedance spectroscopy and tracer diffusion experiments. The calorimetric glass-transition temperature T_g obtained by differential scanning calorimetry shows a minimum with composition. The composition dependence of the electrical conductivity below T_g exhibits a minimum, as well. These deviations from an ‘ideal’ linear mixing rule are usually denoted as mixed-alkali effect. The dc conductivities times temperature σ_dc×T follow the Arrhenius law in the range below and above T_g, respectively. The glass transition appears as a kink in the Arrhenius presentation of σ_dc×T. Below the glass-transition temperature the onset frequency ν_on of the conductivity dispersion has an Arrhenius-like temperature dependence. According to ‘Summerfield scaling’ the activation enthalpies of σ_dc×T and ν_on are expected to be the same. This is indeed observed but only for the single-alkali compositions. The activation enthalpies of σ_dc×T as a function of composition show a classical mixed-alkali maximum, however the activation enthalpies of the onset frequencies as a function of composition exhibit a nearly constant behavior in contrast to the expectation from Summerfield scaling. The tracer diffusion measurements reveal a major difference in diffusion of ⁸⁶Rb and ²²Na in mixed-alkali glasses. A diffusivity crossover of tracer diffusion coefficients of ²²Na and ⁸⁶Rb occurs near X=0.2. By comparison of tracer and conductivity diffusivities the Haven ratio is deduced which shows a maximum near the conductivity minimum composition.     

Crystallization and glass transition kinetics in Cu ion substituted Cux-Ag1 − xI-Ag2O-V2O5 superionic glasses

Crystallization kinetics and thermal properties in superionic glasses Cu_xAg_1 − xI-Ag₂O-V₂O₅ for x = 0.1-0.3 have been thoroughly investigated. X-ray diffraction and differential scanning calorimetry measurements confirm the precipitation of at least three compounds during crystallization, viz. AgI, Ag₄V₂O₇ and Ag₈I₄V₂O₇. The activation energies for structural relaxation (E_s) and crystallization (E_c) obtained using Moynihan and Kissinger formulation exhibit interesting trends with CuI substitution. The E_s value decreases with CuI substitution in the system. Further, the E_c values corresponding to precipitation of Ag₄V₂O₇ and Ag₈I₄V₂O₇ exhibit increasing trend, whereas, for that of AgI precipitation a decreasing trend with CuI content. The Avrami parameter calculated from Augis-Bennett method and Ozawa equation suggests predominantly surface crystallization in the glassy system. The electrical conductivity-temperature (σ-T) cycles interestingly demonstrate increasing precipitation of AgI with CuI content in the glass matrix.     

Structure of partially reduced xPbO (1 − x)SiO2 glasses: combined EXAFS and MD study

We have studied the structure of partially reduced lead-silicate glasses using combined EXAFS (extended X-ray absorption fine structure) and MD (molecular dynamics) methods. The analysis was performed for glasses of x[(1 − p)Pb pPbO] (1 − x)SiO₂ composition, x = 0.3, 0.5, 0.7, where parameter (1 − p) describes the degree of reduction, i.e. the content of the granular metallic phase, appearing as the result of the reduction process (e.g. annealing in hydrogen atmosphere). In the EXAFS experiment (1 − p) was expressed via the time of reduction realized at 400 °C (1.5 h, 24 h, 70 h), whereas in the MD simulations it was determined precisely by using proper numbers of particles (corresponding to (1 − p) = 0.0, 0.25, 0.5, 0.75 and 1.0). In the paper we describe in detail the local structure around lead atoms and its changes in the function of glass composition and reduction degree. The tendency for agglomeration of Pb⁰ into clusters, the formation of the granular metallic phase, and continuity of silica and lead oxide subnetworks are discussed. A good agreement between EXAFS-extractcd and MD-extracted parameters of the short-range structure encouraged us to preform a medium-range order analysis, based on the MD simulations only. Moreover, combining the EXAFS and MD methods we could correlate the reduction time (technological parameter) with the degree of reduction (1 − p) and the actual state of the granular structure. The latter relation may be useful for controlled production of reduced glasses of pre-requcstcd physical properties.     

The effect of composition on UV-vis-NIR spectra of iron doped glasses in the systems Na2O/MgO/SiO2 and Na2O/MgO/Al2O3/SiO2

Glasses with the compositions xNa₂O · 10MgO · (90 − x)SiO₂, 10Na₂O · xMgO · (90 − x)SiO₂, 5Na₂O · 15MgO · xAl₂O₃ · (80 − x)SiO₂, xNa₂O · 10MgO · 10Al₂O₃ · (80 − x)SiO₂, 10Na₂O · 10MgO · xAl₂O₃ · (80 − x)SiO₂, 10Na₂O · 5MgO · 10Al₂O₃ · (80 − x)SiO₂ were melted and studied using UV-vis-NIR spectroscopy in the wavenumber range from 5000 to 30 000 cm⁻¹. At [Al₂O₃] > [Na₂O], the UV-cut off is strongly shifted to smaller wavenumbers and the NIR peak at around 10 000 cm⁻¹ attributed to Fe²⁺ in sixfold coordination gets narrower. Furthermore, the intensity of the NIR peak at 5500 cm⁻¹ increases. This is explained by the incorporation of iron in the respective glass structures.     

RDF analysis, positron annihilation and Raman spectroscopy of xTiO2-(60 − x)SiO2-40Na2O non-linear optical glasses: III. Non-bridging oxygen bonds tracing and structure analysis

xTiO₂-(60 − x)SiO₂-40Na₂O glasses have proven an interesting linear and non-linear optical properties [M. Abdel-Baki, F. Abdel Wahab, F. El-Diasty, Mater. Chem. Phys. 96 (2006) 201]. The investigated glasses show one order of magnitude enhancement for the second-order index of refraction and third-order optical susceptibility over some TiO₂ silicate glasses. In this work, we continue studying these glasses using three different techniques to analyze the glass structures seeking to provide a deep insight for the relation between structure, compositions and the optical characteristics of these glasses. Radial Distribution Function analysis (RDF) combined Raman spectroscopy are used to study these glasses. Positron annihilation lifetime spectroscopy and Doppler broadening measurements are carried out to investigate the change in the glass structure as the incorporation of TiO₂ concentration into glass. The origin of the non-bridging oxygen (NBO) bonds has been traced to correlate their existence with the measured non-linear optical properties of the investigated glasses.     

Bridging, non-bridging and free (O) oxygen in Na2O-SiO2 glasses: An X-ray Photoelectron Spectroscopic (XPS) and Nuclear Magnetic Resonance (NMR) study

High resolution Na 1s, O 1s and Si 2p core level XPS spectra of six Na₂O-SiO₂ glasses ranging in composition from 100 to 45 mol % SiO₂ have been collected using the Kratos Ultra Axis instrument with its unique charge compensation system. The O 1s spectra for the glasses are well resolved so that bridging oxygen (BO, Si-O-Si) and non-bridging oxygen (NBO, Na-O-Si) signals can be accurately fitted and quantified without resorting to constraints or assumptions. The same samples were analysed by ²⁹Si MAS NMR to obtain Q-species abundances from which BO and NBO proportions were calculated. Similar BO:NBO ratios were obtained by both methods over the entire compositional range studied. They are also consistent with most previous XPS and NMR results for glasses containing more than ~ 65 mol % SiO₂. Our XPS and NMR experimental results, however, differ somewhat from previously published XPS and NMR results for glasses containing less than about 65 mol % SiO₂.Na is mobile in the X-ray beam and mobility causes BO:NBO ratios to increase with time of exposure. Na mobility here has been circumvented to yield reliable BO:NBO ratios of the glasses. The ratios are lower than previously reported in XPS studies and are similar to ratios obtained from our ²⁹Si MAS NMR results on the same glasses. The XPS and ²⁹Si MAS NMR results also indicate the presence of a third oxygen species in sodic glasses. As has been proposed for CaSiO₃ glass and for sodic and potassic glasses containing La, we suggest that O²⁻ is present in sodic glasses at small concentrations. The O²⁻ content correlates with increased soda content and may be associated with, and instrumental in development of, three dimensional percolation channels in the glasses. The XPS O 1s line width of the BO peak is broader than the NBO peak, indicating more than one contribution to the BO peak. As observed in crystalline Na metasilicate and Na disilicate, BO of Na-silicate glasses may be of two types, one arising from BO bridging two Si atoms, and the second BO signal arising from BO bonded not only to two Si atoms but also to Na.     

Brillouin spectra of mixed alkali glasses: xCs2O(1−x)Na2O5SiO2

The Brillouin spectra of a series of mixed alkali glasses, of the composition _xCs₂O(1?x)Na₂O5SiO₂, have been measured and analyzed. Significant nonlinearities are found for the variation of the isothermal compressibility [K_T,0(T_f)] and Landau-Placzeck ratio, with a minimum observed in each as a function of x, while the longitudinal and transverse sound velocities and related elastic constants, C₁₁, C₄₄ and Young's modulus, E, vary monotonically (nearly linearly) with x. The results are interpreted in connection with related ultrasonic data and the effect of varying composition on the mechanical properties of mixed alkali glasses.     

Thermal dissolution of Ag(Cu) in amorphous Ge(Sx Se1 − x)2 system

Thermally induced solid state reaction of Ag(Cu) into thin Ge(S_x Se_1 − x)₂ films with x = 0, 0.1, 0.4 and 1.0 was investigated using a step by step technique in order to design films with exact Ag(Cu) concentrations for applications in integrated IR optical devices. A thin film of Ag(Cu) was deposited on top of the host Ge(S_x Se_1 − x)₂ films followed by annealing in vacuum at constant temperature, which resulted in homogeneous films of good optical quality. The variation in Ag(Cu) concentration in the films ranged between 5 and 35 at.%. The kinetics of the diffusion and dissolution of metal in the host films was measured by optically monitoring the change in thickness of doped chalcogenide during consecutive thermal annealing steps. The kinetics studies revealed that the thermal dissolution rate of the Cu is greater than that of Ag. Optical UV-VIS transmission spectra of chalcogenide glass layers, undoped and thermal doped by Ag(Cu), were measured to establish the optical properties of the films. The spectra were analyzed using the technique proposed by Swanepoel and the results show that the addition of metal increases the absorption coefficient in the power-law regime and consequently the optical gap decreases and the refractive index increases. The amorphous character of the films was checked by X-ray diffraction which confirmed the amorphous structure of all Ag(Cu)GeSSe thin films.     

Electric conductivity of glasses in the system Na2O/CaO/SiO2/Fe2O3

The addition of polyvalent transition metal ions to the usually insulating traditional soda-lime-silica glasses can lead to semiconducting properties. We report on synthesis of glasses and glass-ceramics in a soda-lime-silicate based system containing Fe₂O₃ in the concentration range from 5 to 30 mol%. Two sub-systems were considered, in one of them the ratio [Na₂O]/[Fe₂O₃] was varied while in the other one, the ratio [SiO₂]/[Fe₂O₃] was changed. The phase composition of the synthesized products was characterized by X-ray diffraction and energy dispersive X-ray analysis, while the electrical properties were studied by impedance spectroscopy. Partially crystallized non-reduced samples are semiconducting even at room temperature while the glassy samples (both reduced and non-reduced) exhibit semiconducting properties at temperatures equal or larger than 100 °C. An attempt is done to predict the physical approximation explaining the conduction process in the glasses.     

Studies of some mechanical and optical properties of: (70 − x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Specimens of the glassy system: (70 − x)TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by the melt-quenching. An ultrasonic pulse-echo technique was employed, at 5 MHz, for measuring: the ultrasonic attenuation, longitudinal and shear wave velocities, elastic moduli, Poisson ratio, Debye temperature and hardness of the present glasses. It is found that the gradual replacement of TeO₂ by Li₂O in the glass matrix up to 30 mol% leads to decrease the average crosslink density and rigidity of prepared samples which affects the properties, i.e., the hardness, ultrasonic wave velocities and elastic moduli are decreased, while the Poisson ratio and the ultrasonic attenuation are increased. Also, optical absorption spectra were recorded in the range, 200-800 nm for these glasses. The obtained results showed that a gradual shift in the fundamental absorption edge toward longer wavelengths occurred. Values of both of the optical energy gap, E_opt, and width tails, ΔE, are determined. It is observed that E_opt is decreased and ΔE increased with the increase of Li₂O in the glass matrix up to 30 mol%. The compositional dependences of the above properties are discussed and correlated to the structure of tested glasses.     

Transfer of Amide and 2-Methoxyethoxy Groups and Sodium Encapsulation in the Reaction of TaCl3[N(TMS)2]2 with Sodium Bis(2-methoxyethoxy)aluminum Hydride: X-ray Structure of [NaAl{N(TMS)2}(OCH2CH2OMe)3]2

Abstract  The reaction between the tantalum compound TaCl₃[N(TMS)₂]₂ and the hydridic reducing agent sodium bis(2-methoxyethoxy)aluminum hydride (Vitride) has been investigated in toluene solution at room temperature and found to afford the dimeric aluminate complex [NaAl{N(TMS)₂}(OCH₂CH₂OMe)₃]₂ as the sole isolable product. The molecular structure of the product establishes the existence of a four-coordinate aluminum atom and the formal transfer of the 2-methoxyethoxy and bis(trimethylsilyl)amide groups to the aluminate product. The aggregation of two NaAl{N(TMS)₂}(OCH₂CH₂OMe)₃ units serves to bind the two sodium cations in a crown-ether fashion through six ancillary oxygen atoms. Index Abstract  The reaction between TaCl₃[N(TMS)₂]₂ and the reducing agent Vitride has been examined and found to furnish the dimeric aluminate complex [NaAl{N(TMS)₂}(OCH₂CH₂OMe)₃]₂ in low yields. X-ray diffraction analysis confirms the attendant transfer of the 2-methoxyethoxy and bis(trimethylsilyl)amide groups to the aluminate product, which exists as a dimer that encapsulates two sodium counter ions via the 2-methoxyethoxy appendages.

Three-dimensional structure of fast ion conducting 0.5Li2S + 0.5[(1 − x)GeS2 + xGeO2] glasses from high-energy X-ray diffraction and reverse Monte Carlo simulations

A high-energy X-ray diffraction study has been carried out on a series of 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses with x = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8. Structure factors were measured to wave vectors as high as 30 Å⁻¹ resulting in atomic pair distribution functions with high real space resolution. The three dimensional atomic-scale structure of the glasses was modeled by reverse Monte Carlo simulations based on the diffraction data. Results from the simulations show that at the atomic-scale 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses may be viewed as an assembly of independent chains of (Li^+-S)₂GeS_2/2 and (Li^+-O)₂GeO_2/2 tetrahedra as repeat units, where the Li ions occupy the open space between the chains. The new structure data may help understand the reasons for the sharp maximum in the Li⁺ ion conductivity at x ∼ 0.2.     

Si(0 0 1) surface oxidation by N2O

Ultra-thin homogeneous oxynitride films are prepared on Si(0 0 1). The Si surface is cleaned in UHV by heating (flashing) and is exposed to different pressures of N₂O at altered temperatures. Thus oxynitride layers of different thickness and different properties are grown depending on the N₂O pressure and the Si temperature. This is illustrated by a schematic diagram. The properties of the different oxynitride layers were studied by a combined photoemission electron microscopy (PEEM) and photoelectron spectroscopy (PES) investigation using highly monochromatic synchrotron radiation. The amount of oxygen and nitrogen incorporated in the oxynitride layers is determined from the PES measurements. The typical surface morphology for different preparation conditions is shown in PEEM images.     

The glass transition temperature and infrared absorption spectra of: (70−x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Glasses of the system: (70−x) TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by melt quench technique. Dependencies of their glass transition temperatures (T_g) and infrared (IR) absorption spectra on composition were investigated. It is found that the gradual replacement of oxides, TeO₂ by Li₂O, decreases the glass transition temperature and increases the fragility of the glasses. Also, IR spectra revealed broad weak and strong absorption bands in the investigated range of wave numbers from 4000 to 400 cm⁻¹. These bands were assigned to their corresponding bond modes of vibration with relation to the glass structure.     

Synthesis and X-ray diffraction study of complex neptunium(VI) potassium chromate K2[(NpO2)2(CrO4)3(H2O)] · 3H2O

The crystal structure of the K₂[(NpO₂)₂(CrO₄)₃(H₂O)] · 3H₂O compound was established. The structure consists of anionic layers [(NpO₂)₂(CrO₄)₃(H₂O)] _n ²ⁿ , between which K+ ions and crystallization water molecules are located. The coordination polyhedra of Np atoms are distorted pentagonal bipyramids. All chromate ions are bound in a tridentate-bridging fashion. __________ Translated from Kristallografiya, Vol. 49, No. 4, 2004, pp. 676–680. Original Russian Text Copyright ? 2004 by Grigor’ev, Fedoseev, Budantseva, Bessonov, Krupa.     

Structural study of (As2S3)0.6(GeS2)0.4 glass

(As₂S₃)_0.6(GeS₂)_0.4 glass in non-irradiated and γ-irradiated states has been studied by using high-energy synchrotron X-ray diffraction, extended X-ray absorption fine structure spectroscopy, and positron annihilation lifetime spectroscopy. The experimental results are explained by the local changes around As and Ge atoms upon irradiation. These changes are suggested to involve chemical bonds distortion, formation of defective bonds with wrong coordination, rotation of structural units and appearance of additional free volume in the glass network.