Alexis G. Clare

Laboratory preparation of highly pure As2Se3 glass

Abstract Arsenic selenide glass samples have been prepared using a combination of heat treatment and distillation techniques to remove impurities. Transmission spectra of the samples in the infrared region (2.5–20 μm) were measured. The spectra indicate that the majority of absorbing oxide species can be removed by a simple heat treatment. Chemical analysis shows that distilled samples contain less silicon impurity, indicating that Si is most likely incorporated during tube sealing procedures. Increased transmission for the purified samples in the 10–11 μm range shows the necessity of purification to obtain glasses with good transparency for CO 2 laser applications. The highest transmission values at 10.6 μm are greater than those suggested by simple theoretical relations. The high refractive index at this wavelength results in additional contributions to the transmitted intensity, most probably due to sample lensing effects arising from non-parallel sample faces or misalignment of the sample in the spectrophotometer beam.

A neutron diffraction investigation of the structure of vitreous As2O3

Abstract A brief review of the structural chemistry of the AsO system indicates that virtually nothing is known about the amorphous phases other than vitreous As2O3. Neutron diffraction results are reported for vitreous As2O3 and it is concluded that the best structural model is a random network of corner-sharing AsO3 pyramids. The neutron data confirm the presence in the glass of 3-membered rings of AsO3 units similar to those found in the As4O6 molecule, but not the presence of claudetite-like regions. The AsO bond length is (1.775 ± 0.003) A and the mean OAsO angle is 97.3°.

Neutron scattering studies of network glasses

Abstract An account is presented of the use of neutron scattering techniques in structural investigations of network glasses. The differences between X-ray and neutron scattering are discussed and the complementary nature of X-ray and neutron diffraction, which arises from the different atomic number variation of the relevant atomic scattering amplitudes, is emphasised. The types of neutron diffractometer employed for amorphous materials are outlined, together with the special features of neutron scattering such as isotopic substitutions. Examples are given of applications of neutron techniques in the study of multicomponent glasses to reflect the interest in such systems elsewhere in this volume.

Reactive fibre reinforced glass ionomer cements

The mechanical properties of glass ionomer cements used in restorative dentistry reinforced by chopped glass fibres were investigated. Reactive glass fibres with a composition in the system SiO(2)-Al(2)O(3)-CaF(2)-Na(3)AlF(6) and a thickness of 26 microm were drawn by a bushing process. The manufacturing parameters were optimized with respect to maximum strength of the glass fibre reinforced ionomer cements. Powder to liquid ratio, pre-treatment of the glass, grain size distribution and fibre volume fraction were varied. Glass fibre and cement were characterized by X-ray diffraction, transmission electron microscopy and energy dispersive spectroscopy techniques, respectively. The highest flexural strength of the reinforced cement (15.6 MPa) was found by compounding 20 vol% reactive fibres and extending the initial dry gelation period up to 30 min. Microscopic examination of the fractured cements indicated a distinct reactive layer at the fibre surface. A pronounced fibre pull out mode gives rise to an additional work-of-fracture contributed by pulling the fibres out of the fracture surface.

A thermodynamic, molecular dynamics and neutron diffraction investigation of the distribution of tetrahedral {;Si(n)} species and the network modifying cation environment in alkali silicate glasses

Abstract Previous molecular dynamics (MD) simulations of alkali silicate glasses are extended to keep a separate record of the bridging and non-bridging oxygen atoms and the SiO and NaO first neighbour distance distributions are compared with those extracted from neutron diffraction data. The model of ideal associated solutions is used to predict the distributions of Si ( n ) tetrahedral species for the glasses studied and these are in good agreement with nuclear magnetic resonance data. However, the corresponding distributions for the MD simulations indicate a much higher fictive temperature than for the real glasses. The spatial distribution of the Si (4) species for the MD simulations is more uniform than would pertain if the various Si ( n ) species were interconnected randomly, which has implications for the modified random network theory.

A neutron diffraction and molecular dynamics investigation of the structure of vitreous beryllium fluoride

Abstract A combined neutron diffraction and molecular dynamics study is reported for vitreous beryllium fluoride. The structure of vitreous BeF 2 is remarkably similar to that of vitreous silica and may be envusaged as a random network of corner-sharing BeF 4 tetrahedra, with a BeF bond length of (1.553 ± 0.005) A. In contrast to the predictions of the molecular dynamics simulation using simple pair potentials, the BeF 4 tetrahedra in the glass are extremely regular and network disorder arises from a wide distribution of BeFBe and bond torsion angles. It is this last aspect of the glass structure which eliminates structural models based on the crystalline BeF 2 polymorphs.

Chemical durability of commercial silicate glasses. I. Material characterization

Six commercial silicate glasses; silica, sodalime silicate, two fiberglass compositions and two Bioglass® compositions were subjected to three solutions: distilled water, Dulbeccos phosphate buffered saline solution and Hams F-12 1x cell culture media under the exact experimental conditions that would be encountered during a cell culture study. For companion, a binary sodium silicate glass was also exposed. Weight loss, diffuse reflectance infrared spectroscopy (DRIFTS) and potentiometric titration were used to determine the chemical evolution of the substrates during a typical period for cell culturing. The silica, sodalime silicate and high-silica fiberglass material showed only small changes in all cases except for differences in OH active site concentration. The bioglass compositions and the low-silica fiberglass exhibited solution-dependent dynamic surface chemistry. The sodium silicate was too dynamic for even the most aggressive buffering system. The purpose of this study was to elucidate cell behavior to be reported in a later paper.

Optical properties of cerium and europium doped fluoroaluminate glasses

Abstract Fluoroaluminate glasses with the composition 36.5 AlF3:13.5 YF3:20 CaF2:10 MgF2:10 SrF2:10 BaF2 were doped with cerium and europium under various melting conditions. Depending on the melting conditions, different oxidation states of cerium and europium were obtained in the glasses. Using UV-visible absorption and fluorescence spectroscopy, these glasses were characterized. Cerium doped fluoroaluminate glasses had broad UV emission but exhibited apparent concentration quenching after concentrations of less than 1 mol% CeF4. Europium doped glasses exhibited a broad UV/blue emission due to Eu2+ and while concentration quenching was not observed with increased levels of EuF3, the ratio of Eu2+/Eu3+ decreased with increasing total europium content. The apparent obeyance of the tarnishing model indicates that the Eu2+/Eu3+ ratio might be increased by increased exposure to H2.

Points of contact between theory and experiment: a comparison of molecular dynamics and neutron diffraction results

Abstract The wealth of detail available from molecular dynamics (MD) simulations of silicate glasses makes it necessary to show that, where points of contact exist between modelling and experiment, there is sufficient agreement to substantiate other conclusions drawn from the computer simulations that cannot be probed directly by experiment. The present paper focusses on the structural properties which can be studied by neutron diffraction, using recent data on glasses in the system Na 2 OK 2 OSiO 2 as an example, and demonstrates the powerful role which MD simulations can play in the detailed interpretation of diffraction data.

Differences between surface and bulk properties of glass melts I. Compositional differences and influence of volatilization on composition and other physical properties

The influence of volatilization on the composition, density and surface tension of potassium silicate and soda lime silica melts at 1400°C was examined using the sessile and pendant drop arrangements, diffuse reflectance Fourier transform spectrometry and weight loss measurements. Volatilization of alkali from the melts was modeled as a combined mechanism that included diffusion of volatile species from bulk to surface and a chemical decomposition reaction of alkali oxide on the surface. It was also found that the volatilization of alkali could lead to differences in composition and structure of bulk and surface of the same melt depending upon the rate of diffusion of the alkali through the melt; fast diffusion of alkali results in a surface tension that remains unchanged for long periods whereas blocked diffusion results in a surface tension that changes with time initially rapidly leveling off to an equilibrium surface profile.