Ultralow load indentation techniques can be used to obtain time-dependent mechanical properties, termed indentation creep, of materials. However, the comparison of indentation creep data to that obtained during conventional creep testing is difficult, mainly due to the determination of the strain rate experienced by the material during indentation. Using the power-law creep equation and the equation for Newtonian viscosity as a function of stress and strain rate, a relationship between indentation strain rate, , and the effective strain rate occurring during the indentation creep process is obtained. Indentation creep measurements on amorphous selenium in the Newtonian viscous flow regime above the glass transition temperature were obtained. The data were then used to determine that the coefficient relating indentation strain rate to the effective strain rate is equal to 0.09, or .
Calculations were performed of the crystal growth rates in lithium disilicate glass in the low-temperature regime where homogeneous nucleation is observed. The computations were executed using the gain-loss (Becker-Doring) equations that form the framework of Classical Nucleation Theory (CNT). The growth rates were obtained in several different ways, using various choices for the kinetic model, the generalized diffusion coefficient, and the physical input data. The results of these calculations are compared with recently obtained experimental values of the growth rates. To cite this article: M.C. Weinberg et al., C. R. Chimie 5 (2002) 765-771 © 2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS lithium disilicate glass / crystal growth / computations / Classical Nucleation Theory / kinetic modelRésumé -Des calculs de vitesse de croissance cristalline ont été réalisés pour un verre de disilicate de lithium, en régime de basse température, où une nucléation homogène est observée. Les calculs sur ordinateur ont été exécutés en utilisant les équations de perte de gain (Becker-Doring), qui forment le canevas de la théorie classique de la nucléation (TCN). Les vitesses de croissance ont été obtenues de différentes manières, selon divers choix de modèle cinétique, de coefficient de diffusion généralisée et de données physiques introduites. Les résultats de ces calculs sont comparés avec des valeurs de vitesse de croissance récemment obtenues.
The effect of the oxidation state of iron on the phase separation of xNa 2 O⅐(100 -x)SiO 2 glasses, x ؍ 18.56 and 13, containing 0.5 mol% Fe 2 O 3 was studied. The oxidation state of iron in the glasses was varied by changing the melting conditions, such as melting temperature and melting atmosphere. The oxidation states of the iron ion were determined using colorimetric and UV-VIS-NIR spectrophotometric methods, and a comparison was made between the results obtained using these two methods. Immiscibility temperatures of the glasses were determined using opalescence and clearing methods. The immiscibility temperature of the sodium silicate binary glasses decreased ϳ25°C with the addition of 0.5 mol% Fe 2 O 3 . The immiscibility temperature of the doped glasses increased slightly with increased concentration of Fe 2؉ ion in the glass. The prediction of immiscibility tendency on the addition of a minor amount of third component was made using models proposed by Tomozawa and Obara and Nakagawa and Izumitani. The Tomozawa and Obara model showed good agreement with measured immiscibility values.
A 50:50 vol% MgO-Y 2 O 3 nanocomposite with~150 nm grain size was prepared in an attempt to make 3-5 lm infraredtransmitting windows with increased durability and thermal shock resistance. Flexure strength of the composite at 21°C is 679 MPa for 0.88 cm 2 under load. Hardness is consistent with that of the constituents with similar grain size. For 3-mm-thick material at 4.85 lm, the total scatter loss is 1.5%, forward scatter is 0.2%, and absorptance is 1.8%. Optical scatter below 2 lm is 100%. Variable intensity OH absorption (~6% absorptance) is observed near 3 lm. The refractive index is 0.4% below the volume-fraction-weighted average of those of the constituents. Thermal expansion is equal to the volumefraction-weighted average of expansion of the constituents. Specific heat capacity is equal to the mass-fraction-weighted average of heat capacities of the constituents. Thermal conductivity lies between those of the constituents up to 1200 K. Elastic constants lie between those of the constituents. The Hasselman mild thermal shock resistance parameter for the composite is twice as great as that of common 3-5 lm window materials, but half as great as that of c-plane sapphire.
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