Model Of Glass Structure Research Articles

Numerical simulation of outgassing characteristics of microchannel plate glass

Outgassing of internal materials is one of the main factors that cause vacuum deterioration of low-light-level image intensifiers and shorten their lifetime (especially storage lifetime). To perceive the attenuation mechanism of the vacuum degree of image intensifiers from the microscopic view, the outgassing characteristics of lead silicate glass used for the fabrication of microchannel plates (MCPs), which are the core components of image intensifiers, were numerically simulated by giant canonical Monte Carlo and molecular dynamics methods. A glass structure model of the third-generation MCPs was constructed, and the diffusion coefficients and molecular motion trajectories of hydrogen, carbon dioxide, carbon monoxide, water vapor, oxygen, and methane gases, together with their outgassing rates and cumulative outgassing amounts, were calculated based on the condensed-phase optimized molecular potentials for atomistic simulation studies force field. The calculation results show that the rising temperature of MCP glass augments the diffusion coefficients of these gases, which makes the outgassing rates increase in the initial stage but then decrease relatively quickly. Among these six kinds of gas molecules, hydrogen molecules have the largest skip distance and the highest diffusion coefficient because MCP glass can supply more effective diffusion paths for the gas molecules with a relatively small size. When an MCP glass lies in vacuum, first, the cumulative outgassing amounts of various gases from it increase rapidly and then gradually reach stable values, and the cumulative outgassing amount of hydrogen tends to reach a stable value faster than that of other gases due to its highest diffusion coefficient.

Structural role of titanium on slag properties

AbstractThe presence of titanium in ground granulated blast‐furnace slags (GGBS) has been suspected to modify cement properties. This study provides the first evidence of a relation between the TiO2 content of slags and the mechanical properties of mortars based on slag cements. It is observed that only the slags containing less than 1%TiO2 show a compressive strength at 28 days that remains within the 52.5 MPa norm with CEM III cements complying with the European Standard NF EN 197‐1. The structural origin of this chemical dependence of the performance of cements is investigated by determining directly the titanium speciation in various European slags by spectroscopic methods. Electron paramagnetic resonance indicates that about 76% of Ti in slag occurs as Ti4+. The atomic structure around Ti was determined by Ti K‐edge X‐ray absorption near edge structure, which shows that Ti is mainly five‐fold coordinated in square‐based pyramid geometry. Five coordinated Ti acts as network‐stabilizer of the silicate network as it increases the polymerization. Requiring Ca2+ for charge‐compensation of the titanyl bond, it reduces the availability of Ca2+ during glass alteration in a modified random model of glass structure, where Ca2+ atoms are clustered in percolating cationic domains. As a consequence, the presence of five‐coordinated Ti results in a slower dissolution of the slag. These peculiar structural properties of titanium may explain the detrimental role of Ti above a 1% concentration, for many physical and chemical slag properties. This work provides a scientific ground for the technological acceptability of the upper limit of the Ti‐content of GGBS.

Ionic glasses: Structure, properties and classification

Due to the absence of microstructure and virtually infinite compositional versatility, glassy materials are perfect candidates for rational methods to predict structure-property relationships. In the most common models of glass structure, a three-dimensional network backbone and less-localized interstitial species are taken as the fundamental constituents of a glass. Such theoretical frameworks break down for the wide range of glass-forming compositions in which covalent bonding does not percolate and, therefore, there is no network in the classical sense. Most prominently, this applies to the class of ionic glasses, presently emerging as a separate class of amorphous materials, which includes classical invert glasses, geometrically frustrated compounds, mixtures of simple salts and ionic liquids, and organic–inorganic hybrids. Here, we will critically review ionic glasses as a distinct group of materials in which structural predictions are complicated by the dominance of long-range bonding interactions which lack directionality. First, we will reassess Zachariasen’s and Hägg’s rules for conventional glass formation in an attempt to broaden the current understanding of glass formation to include ionic glasses. By reviewing the compositions and properties of many simple and complex glass-forming salts, we discern the commonalities shared by ionic glasses and their unique characteristics as compared to other classes of glass. Specifically, we discuss the importance of non-percolating topology to dynamic properties such as mechanical behavior, ionic conductivity, relaxation processes and crystallization. We also demonstrate the relevance of the adjacent fields of ionic liquids and amorphous metal organic frameworks, and highlight possibilities for fruitful interdisciplinary exchange. The structure of ionic glasses, being dominated by Coulombic interactions, lends itself to high compositional flexibility and therefore, creates significant opportunities in the search for optical materials with enhanced transmission windows, fast ion conductors, phase-change materials, or, e.g ., adapted solders, hermetic seals and dielectric insulators. • The field of ionic glasses is reviewed with a focus on structure and properties. • Classification criteria are proposed. • Invert, halide, oxoanionic, halide-oxide and superionic glasses are included. • Links are drawn to the adjacent fields of ionic liquids and metal organic framework glasses.

Water absorption and moisture resistance of As-S-Se-I glasses

It is known that chalcogenide glasses are resistant to the action of nitric, hydrochloric, hydrofluoric and other corrosive acids, but it is not stability in water. The chemical resistance of glass is important for optical element application in a wet environment. In the present work it is shown that the introduction of selenium in As-S-I glass reduces the glass solubility. An increase in selenium content up to 10 mol % leads to insolubility in water, but water is adsorbed in the glass. The glass structure model and the mechanism of dissolution of glass were proposed.

Glass formation and characterization in the 3Al2O3·2SiO2-LaPO4 system

Rare earth oxide–aluminate–phosphate–silicate (REAPS) glasses are useful precursors for ceramic-matrix-composites (CMCs) with important thermal and mechanical properties. It is important to determine the glass structure, relaxation and crystallization properties for designing and controlling CMC formation. Transparent 3Al2O3·2SiO2-LaPO4 glasses containing 25–80mol% mullite (3Al2O3·2SiO2) component were prepared by quenching from high temperature melts using a containerless technique. Glass transformation and crystallization behavior were examined by differential scanning calorimetry and X-ray diffraction. The glass transition onset increased from 845 to 906°C with mullite content. The temperature interval between Tg and crystallization was maximized at 200°C for 50mol% mullite glass. Below 40mol% mullite, successive appearance of LaPO4 (monazite) and mullite gave rise to two crystallization peaks, while at higher mullite content, only one combined exotherm was observed. A glass structure model constructed from 27Al, 31P and 29Si magic angle spinning (MAS) NMR and Raman spectroscopy results indicated that Si4+ and P5+ remained tetrahedrally bonded while Al3+ ions were predominantly in four-fold coordination with some five-coordinated sites. The presence of La2O3 component resulted in an increased proportion of AlO4 tetrahedra. The PO4 polymerization state varied from Q3 to Q2 with increasing LaPO4 content. The SiO4, AlO4, and PO4 units form a continuous network with PO4 tetrahedra attached to aluminosilicate framework through two or three P–O–Al linkages. SiO4 tetrahedra crosslink with AlO4 tetrahedra to form Q4(4Al) and Q4(3Al) units. The glass structure model helps explain the crystallization sequence as a function of mullite content and the formation of different CMC textures.

Estimating accuracy of 17O NMR measurements in oxide glasses: Constraints and evidence from crystalline and glassy calcium and barium silicates

The use of 17O nuclear magnetic resonance (NMR) spectroscopy to measure site populations in silicate and aluminosilicate glasses has provided insights and challenges to conventional models of glass structure. In order to better understand the level of accuracy and precision achievable, we have synthesized crystalline barium metasilicate (BaSiO3), barium orthosilicate (Ba2SiO4), tricalcium silicate (Ca3SiO5), a barium silicate glass ((BaO)0.45(SiO2)0.55), and a calcium silicate glass ((CaO)0.56(SiO2)0.44), and report 17O NMR spectra for all of these. After correcting the observed intensities for quadrupolar effects, we measure an NBO content of 66.7%±0.6% for the BaSiO3, compared to the known value of 66.7%. Applying the same techniques for the glasses gives an NBO content of 58.8%±0.8% (vs. the expected 55.5%±1.4% from stoichiometry) for the barium silicate and 76.9%±1.2% (vs. 78.6%±1.4%) for the calcium silicate. Within our uncertainties, we find no evidence for deviation from conventional models of glass structure for the glasses studied here. We also see no NMR signal (detection limit of about 0.5%) at the expected position for “free” oxide ions (bonded only to Ca2+), as newly constrained by our data for crystalline Ca3SiO5, which contains this species.

Glass structure of aluminosilicate containing phosphate with low chemical stability and complex component--Development of glass structure model of aluminosilicate containing phosphate and manufacture of glass fertilizer

Glass structure of aluminosilicate containing phosphate with low chemical stability and complex component--Development of glass structure model of aluminosilicate containing phosphate and manufacture of glass fertilizer

X-ray photoelectron spectroscopy (XPS) and magnetic susceptibility studies of vanadium phosphate glasses

Vanadium phosphate glasses with the nominal chemical composition [(V 2O 5) x (P 2O 5) 1− x ], where x = 0.30, 0.40, 0.50, and 0.60, have been prepared and investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. Asymmetries found in the O 1s, P 2p, and V 2p core level spectra indicate the presence of primarily P–O–P, P–O–V, and V–O–V structural bonds, a spin–orbit splitting of the P 2p core level, and more than one valence state of V ions being present. The magnetic susceptibility data for these glasses follow a Curie–Weiss behavior which also indicates the presence of some V ions existing in a magnetic state, i.e., a valence state other than that of the non-magnetic V 5+. From qualitative comparisons of the abundance of the bridging oxygen or P–O–P sites as determined from the areas under the various O 1s peaks with the abundances of differing phosphate structural groups associated with the presence of different valence states of the vanadium ions, a glass structure model consisting of a mixture of vanadate phosphate phases is proposed for these glass samples. These include V 2O 5, VOPO 4, (VO) 2P 2O 7, VO(PO 3), and V(PO 3) 3 with the abundance of orthophosphate (PO 4) 3− units increasing with increasing vanadium content.

Development of a system for measuring the complex impedance of borosilicate glasses at high pressures and temperatures: Application to the study of Li- and Na-doped borosilicate glasses

An apparatus for measuring the complex impedance of samples with high impedances is described. Complex impedance spectra were collected from a range of borosilicate glasses of composition (B 2O 3) 4(Li 2O)(LiBr) x (NaBr) 1− x at pressures and temperatures ranging from 1 to 5 GPa and 350 to 450 °C, respectively. These data were used to determine AC conductivities and activation energies in order to test the Modified Random Network model of glass structure. Our results are in line with the predictions of this theory.

Structure and the extent of disorder in quaternary (Ca-Mg and Ca-Na) aluminosilicate glasses and melts

The structure of multi-component silicate melts and glasses (e.g., Ca-Mg and Ca-Na aluminosilicates) can provide insight into the properties of natural silicate melts and has implications for relevant magmatic processes. In spite of its importance, the atomic and molecular structure of most multi-components (e.g., quaternary) melts and glasses has not been fully described, primarily because of insufficient resolution obtained with conventional spectroscopic and scattering methods; the information obtained by these methods is compromised by severe inhomogeneous peak broadening due to structural complexity. Here we report the first 1 7 O and 2 7 Al 3QMAS NMR spectra for quaternary, Ca-Mg and Ca-Na peralkaline aluminosilicate glasses (i.e., M/Al > 1, M is one monovalent or one-half a divalent cation). These data reveal new details into the molecular structure of multi-component aluminosilicate melts, which include the presence of a substantial fraction of V Al in the Ca-Mg aluminosilicate glasses and I V Al-O- I V Al in both glasses at 1 atm. Traditional models of glass structure do not support the presence of such species given these high-silica, peralkaline compositions. These results suggest that Al avoidance is violated in the multi-component peralkaline aluminosilicate glasses, and that the presence of Mg 2 + in the melts increases the extent of disorder in the melts (compared with Ca 2 + and Na + ). These factors lead to an increase in configurational entropy and the activity coefficients of the oxides, and may provide an explanation for the decrease in viscosity of these complex melts.

Prediction of chloride solubility in CaO–Al2O3–SiO2 glass systems

It is known that fly ash from municipal waste incinerators is particularly problematic because it contains a significant concentration of heavy metals and chloride. One of the methods for treating chloride is to entrap it in a glassy slag, which is produced by melting bottom ash with fly ash. The main components of the glassy slag are SiO2, Al2O3 and CaO. However, only a limited amount of chloride can be retained in the glass, and the relationship between glass structure and chloride solubility is still unclear. The present study was intended to examine the solubility of chloride in slag-like CaO–Al2O3–SiO2 glass systems in which CaCl2 was substituted for CaO. Glass compositions with varying non-bridging oxygen (NBO) content were investigated. It was found that NBO in glass has a significant influence on glass properties. The solubility of chloride was also found to strongly relate to the NBO content. It was proposed that chloride ions occupy the site around NBO and the chloride solubility should be determined by the NBO content. The maximum chloride content was found to agree with this glass structure model. This study could allow us to predict the maximum chloride content that can be dissolved in the slag.

Decoupling between Enthalpy Relaxation and Viscous Flow and Its Structural Origin in Fragile Oxide Glass‐Forming Liquids

The structural relaxation kinetics at the glass transition in tellurium oxide (TeO2)‐based glasses has been examined from viscosity and heat‐capacity measurements to clarify the features of the structural relaxation in fragile oxide glass‐forming liquids. A large decoupling between enthalpy relaxation and viscous flow, i.e., a large discrepancy between the activation energies for the enthalpy relaxation (recovery), ΔH, and viscous flow, Eη, has been demonstrated in TeO2‐based glasses. The values in xK2O·xMgO·(100 − 2x)TeO2 glasses, for example, are ∼919–1051 kJ/mol for ΔH and ∼ 577–701 kJ/mol for Eη, given the ratio of ΔH/Eη≈ 1.44–1.59. Some viscosity and heat‐capacity data (all data have been reported previously) obtained from similar experiments in Sb2O3–B2O3 glasses belonging to the category of strong glass‐forming liquids have been reanalyzed in this paper for comparison; a strong coupling was found to exist between ΔH and Eη, i.e., ΔH/Eη≈ 0.98–1.18. An origin of decoupling between ΔH and Eη in fragile glass‐forming systems such as TeO2‐based glasses has been discussed by considering the glass structure model for fragile glasses; strongly bonded correlated (highly constrained) regions are surrounded or connected by weakly bonded noncorrelated (unconstrained) parts.

Direct observation of multiple oxygen sites in oxide glasses: recent advances from triple-quantum magic-angle spinning nuclear magnetic resonance

Triple-quantum, magic-angle spinning nuclear magnetic resonance (3QMAS NMR) on 17O has demonstrated remarkable improvements in resolution among signals from different oxide species in glasses. As many as four separate types of oxygen anion environments can be observed simultaneously in some compositions; Si–O–Si, Al–O–Al, B–O–B, Si–O–Al, Si–O–B, and Al–O–B bridging oxygens can be distinguished and at least roughly quantified; non-bridging oxygens bonded to Si vs. B can in some cases be separated, and there are hints of oxygen tricluster species. These types of data are already providing important new constraints and tests of models of glass structure and properties.

Multiple‐Quantum Magic‐Angle Spinning 17O NMR Studies of Borate, Borosilicate, and Boroaluminate Glasses

Multiple‐quantum magic‐angle spinning (MQMAS) 17O NMR spectroscopy has been applied to study boron oxide, sodium borate, sodium potassium borosilicate, and sodium boroaluminate glasses. Up to eight distinct oxygen sites are identified, with the chemical shift and quadrupolar coupling parameters determined for each. Previous assignment of the B‐O‐B and Si‐O‐B resonances has been found to be incorrect. In contrast to standard models of glass structure, three‐coordinated boron mixes with the silicon oxide network to a great extent. Sodium borosilicate glasses with low‐sodium content are likely to be phase‐separated on the nanoscale. Those with intermediate sodium content form homogeneous glasses with boron atoms distributed evenly in the SiO2 network. A boron avoidance rule analogous to the aluminum avoidance rule may apply in this region, when BO4 groups are abundant. Adding excess sodium cations to the system may again lead to compositional heterogeneity. The existence of diborate group as the basic building unit in glasses with appropriate sodium content is not well supported. The network‐forming cations in sodium boroaluminate glasses are well mixed, with no Al‐O‐Al resonance observed by MQMAS. The effect of cation type and thermal history on glass structures is also discussed.

Role of Interface Structure on Mechanical Properties of Fluorine-Doped Si<sub>3</sub>N<sub>4</sub>-SiC Ceramics

Changes in interface chemistry and, accordingly, in interface structure of a Si3N4-SiC ceramic, due to the presence of fluorine, was found to markedly affect the mechanical response of the material. Owing to a lowered cohesive interface strength, as a consequence of fluorine segregation at grain and phase boundaries, an increased fraction of intergranular fracture was monitored upon crack propagation. Moreover, in comparison to undoped specimens, fluorine-doped samples revealed markedly higher creep rates, which are related to a decrease in the apparent grain-boundary viscosity. The experimental results allow a correlation between the macroscopic mechanical properties and the atomic interfacial glass structure. A glass structure model is presented, which emphasizes the influence of broken (non-bridging) bonds on the bulk mechanical performance of Si3N4-SiC-based composites.

NMR studies of phosphorus in SiPTe and GePTe glasses

Glass formation and structure in the systems PGeTe and PSiTe has been studied by nuclear magnetic resonance (NMR) including 31P magic-angle spinning, spin echo decay spectroscopy, and in situ molten-state investigations. Despite similar regions of glass formation, NMR results illustrate dramatic structural differences between the two systems. In P-Ge-Te glasses homopolar P-P bonding appears to be predominant, while the P-Si-Te glass system shows a much greater tendency towards heteropolar bonding, qualitatively consistent with a randomized chemical bonding. Constraints posed by these NMR experiments upon possible models of glass structure are discussed.

Glass viscosity and structural relaxation by parallel plate rheometry using a thermo-mechanical analyser

Glass viscosity from 10^5 poises and up to an extended 10^11 poises has been successfully measured by parallel plate rheometry using a commercial thermo-mechanical analyser (TMA), Perkin-Elmer TMA-7. The viscosity data measured on a standard reference soda-lime silicate glass, NIST 710a, are in excellent agreement with the standard viscosity data of the glass supplied by the National Institute of Standards and Technology (NIST). The technique is also found very effective in monitoring the structural relaxation process in glasses. Subsequently, the relationship between glass structure and its relaxation mechanism is studied. It is found that the relaxation mechanism in the glass can be adequately represented by a double exponential function with a rapid and a slow component. The rapid and slow relaxation components are attributed to the relaxations originating from the modifier-rich and glass-former-rich regions in the glass respectively, following the modified-random-network (MRN) model of glass structure.

Volumetric effect of topology in chalcogenide glass systems

Topological models of glass structure predict features in the property versus the average coordination number, Z, plots at Z ⋍ 2.40 and at Z ⋍ 2.67 . Topological and chemical ordering effects coexist in chalcogenide network glasses and therefore features due to topological effects are not clearly seen in many systems. The volumetric effect, V t, of topological origin has been determined by subtracting from the measured mean atomic volume, V m, the volume, V c, calculated by taking into account the effects of chemical ordering. The V m versus Z and V t versus Z plots have been examined for several binary and ternary systems. While the V m versus Z plots of these systems have different appearances, their V t versus Z plots exhibit similar features, with a minimum in V t at Z ⋍ 2.40 and a local maximum in V t at Z ⋍ 2.67 .

Host-structure-dependent non-Lorentzian persistent-hole shapes in organic glasses.

Nonexponential photon-echo decay curves and non-Lorentzian persistent-hole shapes of chromophores in organic glasses have been observed experimentally, and explained by considering fractal structures of the organic-glass hosts. The photon-echo decay curves are well reproduced by the nonexponentially decaying function of \ensuremath{\tau}, exp(-cT${\mathrm{\ensuremath{\tau}}}^{\mathit{D}/3}$), where c is a constant, T is the temperature, \ensuremath{\tau} is the delay time between the first and second excitation pulses, and D is the fractal dimension of the glass host. The persistent-hole shapes are also well reproduced by the Fourier transform of this function. The fractal dimensions D obtained by fitting the theoretical curves to the observed hole shapes were characteristic of the hosts, and almost independent of the guest chromophore. The non-Lorentzian persistent-hole shapes were observed in nonionic as well as in ionic guest chromophores. We also obtained consistent values of the host fractal dimension from the photon-echo decay curves. A simple model of glass structure is presented which can consistently explain the differences between polymeric glass host and monomeric glass host with regard to the measured hole shapes, echo decay curves, and the interrelations between the hole widths and the inhomogeneous widths.

Log-normal spectrum of low-energy vibrational excitations in glasses

It is shown that the spectrum of excess low-energy ( E∼2–10 meV) density of vibrational states in glasses found by inelastic neutron scattering is described by a log-normal law with a universal standard deviation of log E. This regularity is explained within the framework of the cluster model of glass structure.