Journal of the American Ceramic Society

Cormier, L.; Majérus, O.; Neuville, D. R.; Calas, G.

doi: 10.1111/j.1551-2916.2005.00657.xpmid: N/A

High‐temperature neutron diffraction and Raman spectra have been obtained on M2O–2B2O3 (M=Li, Na, K) glasses and melts. Both techniques indicate a coordination change of boron atoms: the tetrahedral boron sites present in the glasses are converted into triangular boron sites. These changes of the borate network yield modifications of the alkali environment, as assessed for Li using the isotopic substitution technique. We observe that Li atoms are in a charge‐compensating position in the glass and in a modifying position in the liquid. These structural modifications have important implications toward understanding the physical properties of borate melts.

Jones, Robert L.

doi: 10.1111/j.1551-2916.2005.00659.xpmid: N/A

Corrosion of E‐glass fibers in aqueous organic acids has been investigated. Oxalic acid is particularly severe and causes almost total strength loss after exposure for only a few days. Of particular importance is that oxalic acid is a degradation product of the plastic matrix of glass reinforced plastic materials. Other organic acids, such as glyoxylic and malonic acids, also corrode the glass fibers, and these too are likely degradation products of plastic. The proposed mechanism of corrosion is an ion exchange reaction in which metal ions on the glass surface are replaced by hydrogen ions from the acid. This is enhanced by complex formation of the leached cation with the anion associated with the acid. Some cations are leached more extensively in some acids compared with others, and boron has been implicated in the corrosion by organic acids.

Beerkens, Ruud G. C.; Schaaf, John

doi: 10.1111/j.1551-2916.2005.00691.xpmid: N/A

A method for the prediction of gas evolution from a glass melt during fining processes has been described. This procedure is based on the assumption of thermodynamic equilibrium conditions between the species in the glass melt and co‐existing gas phases. The method has been applied to estimate (a) the onset temperature of fining and (b) the evolution of different gases from glass melts. Both are strongly dependent on water concentration in the melt, the temperature, and the content of fining agent (sulfates). Expressions for foam bubble lifetimes have been presented, and a combination of these equations with gas evolution rates has been used in a foam formation model. This model calculates the number of bubble layers on top of glass melts during secondary foaming (foam formation during fining). A key parameter for foam stability and the foam layer thickness is the mobility of the surfaces of the lamellae of the bubbles in the foam layers. The water content in the melt, sulfate concentration levels, heating rates, furnace atmospheres, and bubble sizes govern the glass melt foaming behavior.

Corrales, L. René; Du, Jincheng

doi: 10.1111/j.1551-2916.2005.00695.xpmid: N/A

The distribution of cation and anion components of sodium calcium aluminosilicate was studied by classical molecular dynamics simulations in a high‐temperature melt in the bulk and at the vacuum–melt interface. A significant redistribution of the sodium and non‐bridging oxygen (NBO) ions was observed. Subsequently, a sodium depleted calcium aluminosilicate melt was simulated to determine the sensitivity of the redistribution of ions near the vacuum–melt interface to the presence of sodium ions. It is found that the thermodynamic equilibrium condition near the melt surface favors the enrichment of NBO ions that is closely associated with enrichment of the sodium ions.

Toutois, Pascaline; Miele, Philippe; Jacques, Sylvain; Cornu, David; Bernard, Samuel

doi: 10.1111/j.1551-2916.2005.00685.xpmid: N/A

A series of boron nitride fibers were prepared by varying the tension applied on a same lot of poly(methylamino)borazine fibers during their pyrolytic conversion at 1800°C. Their microstructure/microtexture was studied by X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy investigations. Such analyses showed that ceramic fibers presented good crystallinity, but crystallites oriented along the fiber axis with fiber stress. It is most interesting that stretching was essentially effective during the pyrolysis to 1000°C, preventing fiber crimping, and decreasing the fiber diameter. Therefore, the fiber strength increased, while the fiber modulus also increased because of an improvement of the basal layer orientation along the fiber axis.

Glaesemann, G. Scott; Winningham, Michael J.; Clark, Donald A.; Coon, Jeffrey; DeMartino, Steven E.; Logunov, Stephan L.; Chien, Ching‐Kee

doi: 10.1111/j.1551-2916.2005.00716.xpmid: N/A

As optical fiber penetrates further into the communications infrastructure and comes closer to the home or business, higher optical power levels are expected. Several studies have shown that sharply bent optical fiber will fail prematurely when exposed to high optical power levels. In an extreme case, where the fiber is bent to a maximum bend stress on the order of 2 GPa and subjected to a power level of 1–2 W in the near‐infrared wavelength window, optical fiber will fail in minutes. Time to failure decreases with increasing bend stress and optical power. A recent report suggests that power levels in the range of a few hundred milliwatts may be enough to induce delayed failure in bent fiber. This study explores the progression of events leading to failure. Light that escapes the core of bent fiber passes into the coating, where a small amount is absorbed and converted to heat. The coating heats to a stable temperature and visually darkens with time. This is followed by an abrupt rise in temperature, which occurs as the coating transforms to a highly absorptive material, consistent with thermal runaway. The abrupt rise in coating temperature stimulates viscoelastic deformation of the glass. Glass deformation is explained in terms of the ability of highly quenched glass to experience viscous flow at temperatures well below the glass transition range (i.e. sub‐Tg aging or relaxation). As the glass portion of the fiber moves toward a “kinked” configuration, it concentrates more power on a smaller region of coating, resulting in further temperature increase. There is no evidence of the fiber fuse effect in the lower viscosity glass core. The final kinked configuration of the glass fiber leads to complete attenuation of the light and failure is complete. Coating decomposition is self‐limiting with no visible flame. A coating with a refractive index near or below that of silica was found to virtually eliminate this failure mode.

Kiczenski, T. J.; Stebbins, Jonathan F.

doi: 10.1111/j.1551-2916.2005.00677.xpmid: N/A

We have examined temperature‐induced changes in the local structure of fluorine in silicate and aluminosilicate glasses through the application of 19F magic‐angle spinning nuclear magnetic resonance to glasses synthesized with a wide range of fictive temperatures. We have observed an increase in Si–F bonding in binary silicates with increasing temperature, as well as a change in the degree of ordering in mixed modifier silicates. We have also observed an increase in F–Al bonding in aluminosilicates, although the degree of this change appears to be composition dependent. These results could help explain the significant changes in properties of melts on the addition of fluorine.

Saito, K.; Sekiya, E. H.; Ikushima, A. J.; Ohsono, K.; Kurosawa, Y.

doi: 10.1111/j.1551-2916.2005.00726.xpmid: N/A

Limit of the Rayleigh scattering loss in Ge‐doped silica core fiber was estimated from the results of structural relaxation and the Rayleigh scattering measurements, and the most suitable fiber‐drawing condition to reduce the Rayleigh scattering loss was determined. An annealing furnace, which could be attached to a fiber‐drawing tower, was developed for realizing the optimum fiber‐drawing condition. Drawing tests with this furnace confirmed our estimation of the loss.