Glasses Tech – Prescription Glasses, Prescription Eyeglasses

March 8, 2010

Fragilities of liquids predicted from the random first order transition theory of glasses

Filed under: Glasses 2000 — admin @ 2:53 am

Eyeglasses

A microscopically motivated theory of glassy dynamics based on an underlying random first order transition is developed to explain the magnitude of free energy barriers for glassy relaxation. A variety of empirical correlations embodied in the concept of liquid “fragility” are shown to be quantitatively explained by such a model. The near universality of a Lindemann ratio characterizing the maximal amplitude of thermal vibrations within an amorphous minimum explains the variation of fragility with a liquid’s configurational heat capacity density. (more…)

Fluoride sites in aluminosilicate glasses: High-resolution 19F NMR results

Filed under: Glasses 2000 — admin @ 2:53 am

We present here high-resolution, 19F NMR spectra, collected with sample spinning rates to 25 kHz, for sodium and calcium silicate and aluminosilicate glasses. Several distinct fluoride ion sites are well resolved and can be assigned to various coordination environments based on clear similarities to crystalline model compounds. In aluminosilicates, fluoride with one Al and several Na or Ca neighbors predominate, but silicon-fluoride groups may also be significant. Small concentrations of the latter can also be detected in sodium silicate glasses, suggesting a possible role in reduction of viscosity. (more…)

Magnesium coordination environments in glasses and minerals: New insight from high-field magnesium-25 MAS NMR

Filed under: Glasses 2000 — admin @ 2:49 am

A comparison of 25Mg magic angle spinning (MAS) NMR spectra of crystalline and glassy diopside (CaMgSi2O6) reveals that the chemical shift in the disordered phase corresponds to that of the mineral, suggesting that sixfold coordination is essentially retained upon vitrification. Likewise, a crystalline leucite analogue (K2MgSi5O12) known to possess fourfold-coordinated Mg has the same peak position as the corresponding glass. In addition to being a sensitive probe of local structure by analogy with crystalline phases, these data may be understood in terms of the effect of competing cation field strengths: competing network modifier cations with higher field strength induce higher magnesium coordination numbers, with a consequent increase in bond length and decrease in chemical shift. (more…)

Water speciation in silicate glasses and melts: Langmuir limited site model

Filed under: Glasses 2000 — admin @ 2:48 am

The distribution of dissolved water in a haplogranitic glass between molecular water and reacted OH groups, as measured by Sowerby and Keppler (1999) at temperature and pressure, is compared to a Langmuir model. In this model the number of reactive sites in the glass is limited, so as the total water content increases, the concentration of reacted OH groups saturates. (more…)

Si CPMAS NMR investigations of silanol-group minerals and hydrous aluminosilicate glasses

Filed under: Glasses 2000 — admin @ 2:48 am

One-pulse magic angle spinning (MAS) 29Si and 1H-29Si cross-polarization (CP) MAS nuclear magnetic resonance (NMR) spectroscopy was performed on minerals with OH groups and on hydrous aluminosilicate glass samples. The silanol-group samples used were krauskopfite, rosenhahnite, thaumasite, ussingite, and KHSi2O5 with known Si to H distances and ellenbergerite, the proton positions of which are not as well defined. The Si-H distances from the minerals can be compared with the cross-polarization time constants (SiH) and the proton spin-relaxation times in the rotating frame [1(H)], and a rough correspondence exists between a mineral’s shortest Si-H distance and its SiH value. Also, fast [1(H)] values correspond to large bulk H densities. (more…)

Fivefold-coordinated aluminum in tectosilicate glasses observed by triple quantum MAS NMR

Filed under: Glasses 2000 — admin @ 2:47 am

Eight glasses with molar Mg/2Al 1 in the system MgO-Al2O3-SiO2 have been studied by magic angle spinning (MAS) NMR spectroscopy. Using triple quantum (3Q) NMR techniques we find evidence for significant concentrations of Al coordinated to five O atoms in all glasses, the proportion increasing with decreasing Mg/Al and decreasing silica content. In glasses with Mg/2Al = 1, up to 6% of the Al is estimated to be coordinated to five rather than four O atoms. (more…)

Effect of aluminum on Ti-coordination in silicate glasses: A XANES study

Filed under: Glasses 2000 — admin @ 2:38 am

The structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. (more…)

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

Filed under: Glasses 2000 — admin @ 2:37 am

Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia–Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. (more…)

Relationships between the medium-range structure of glasses and crystals

Filed under: Glasses 2000 — admin @ 2:36 am

P. H. Gaskell*
Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK

* E-mail: phg1@cam.ac.uk

The known structure of a crystalline phase is almost always useful in investigating the unknown structure of the compositionally equivalent glass. For the local environment around elements like Si, B and P, the correspondence between site geometry and symmetry can be impressively close. Beyond near neighbours, any relationship becomes less obvious – at least in real-space data. Progress in understanding the medium-range structures of glasses has been painfully slow as a result. One essential clue is given by reciprocal-space features at low Q (scattering vector) in X-ray or neutron scattering data, which are clearly related to the medium-range structure. Interpretation of these features as ‘quasi-Bragg’ scattering allows direct comparison between the structures of the glass and equivalent crystalline phases. (more…)

Water in Zr-bearing synthetic and natural glasses

Filed under: Glasses 2000 — admin @ 2:36 am

François FARGES1,* and Stéphanie ROSSANO2

1 Laboratoire des Géomatériaux, Université de Marne-la-Vallée, (and Stanford University and Institut Universitaire de France) 5 Boulevard Descartes – Champs S/Marne 77454 – Marne la Vallée Cedex, France
2 Laboratoire de Minéralogie-Cristallographie, Universités de Paris 6, 7 et IPGP, UMR CNRS 7590 – 2 place Jussieu, 75252 – Paris cedex 05, France (now at the Laboratoire des Géomatériaux, Université de Marne-la-Vallée) (more…)

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