George H. Beall

CHAIN SILICATE GLASS-CERAMICS

Abstract Glass-ceramics based on chain silicates: enstatite (MgSiO 3 ), potassium fluorrichterite (KNaCaMg 5 Si 8 O 22 F 2 ), and fluorcanasite (K 2 Na 4 Ca 5 Si 12 O 30 F 4 ), display a combination of high flexural strength ( > 200 MPa) and high fracture toughness (> 3 MPa rmm 1 2 ). The relationships between composition, microstrcuture, and properties are discussed. Recent and potential applications are described.

Full‐color photosensitive glass

New polychromatic (full‐color photosensitive) glasses are described in which the full color spectrum is developed by photochemical precipitation of subcolloidal silver too small to scatter light. The brilliant colors are shown to be due to selective light absorption by randomly oriented silver‐containing particles having controlled anisotropic shapes. The color photographic process and its chemistry are outlined. Controlled anisotropy is due to photo‐silver‐nucleated growth of elongated NaF⋅ (Na,Ag) X pyramidal crystallites, concentration of AgX at the apex, and subsequent photoprecipitation of this silver. Resultant colors are primarily a function of exposure flux (intensity × time) employing ultraviolet light (300 nm). The colors can be semiquantitatively described from electromagnetic theory assuming ellipsoidal metal particles suspended in a transparent dielectric medium. An absorption band moves across the visible spectrum as a function of particle eccentricity. Polychromatic glasses can be made havi...

Nanocrystalline non-alkali glass-ceramics

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Nanocrystalline glass-ceramics with high elastic modulus and moderate strength and toughness can be produced from a wide area in the SiO 2 -Al 2 O 3 -MgO-ZnO-TiO 2 system. Phase assemblages include those based primarily on spinel crystals as well as those based on enstatite and spinel; common accessory phases include Mg-petalite, Mg,Al-titanate solid solutions, quartz solid solutions and rutile. These glass-ceramics can have ultra-fine grain sizes, with spinel crystals less than 25 nm and enstatite crystals less than 100 nm in size. Such nanocrystalline microstructures are unique in the case of an enstatite glass-ceramic and provide extremely smooth polished surfaces, with average roughness of about 5-10 A (0.5-1.0 nm). These materials provide strength and toughness values higher than might otherwise be expected from such a fine-grained microstructure, due in part to the toughening effect of the lamellar twinning in the enstatite phase. The morphology of the initial phase separation provides a template for the ultimate crystalline microstructure, essentially locking in its nanocrystalline nature such that regardless of heat treatment crystals grow no larger than 200 nm.

Structure of zinc polyphosphate glasses

Abstract The technique of high-performance liquid chromatography (HPLC) has been used to determine the distributions of phosphate-anion chains and rings in a variety of zinc-phosphate-based glasses. The phosphate-anion distributions in simple binary zinc phosphate glasses are compared to those found in multi-cation alkali-zinc phosphate glasses. Multi-cation zinc phosphate glasses are chemically durable and can be tailored to exhibit transition temperatures sufficiently low for co-processing with a variety of plastics to form novel organic–inorganic composite materials. The intermediate range order in the multi-cation glasses is found to be similar to that measured in the binary zinc-phosphate glasses. Scanning calorimetry measurements indicate, however, that the multi-cation glasses are more resistant to crystallization than the binary glasses. The fraction of phosphorus sites with 2 (Q2), 1 (Q1) and 0 (Q0) bridging oxygen, as well as the ratio of bridging to non-bridging oxygen (BO/NBO), are obtained from the chromatograms. Structural data for the zinc phosphate glasses obtained using HPLC are compared to results reported in the literature that were deduced using more-traditional solid-state techniques such as NMR, XPS, and Raman spectroscopy.

Transition element-doped crystals in glass

Transition metals are well known as optically active dopants in crystalline hosts because they fluoresce broadly in the near-IR; examples include Ti3+:sapphire (Al2O3) and Cr4+:forsterite (Mg2SiO4). Because transition element ions yield stronger optical activity with those of doped crystal. The glass-ceramic process can, in many cases, also allow the growth in glass of crystals which are difficult or impossible to obtain in single crystal form. Material properties and spectroscopic data are presented for transparent glass-ceramics based on transition element-doped forsterite, willemite, mixed Li-Zn-Mg- orthosilicates, and spinel crystals. The transition element ions include Cr4+ in tetrahedral coordination and Ni2+ in octahedral coordination. The optical property measurements of the transparent glass-ceramics, including absorbance, fluorescence, and fluorescence lifetimes, duplicate those reported in the literature for single crystal or powders. Cr4+- and Ni2+-doped glass-ceramics exhibit broad emission across the telecommunications wavelength range of 1100-1700 nm.

Excited state absorption in Cr3+-doped gahnite glass ceramics

Abstract We report on the excited state fluorescence and absorption characteristics of chromium-doped gahnite glass ceramic. The material is characterized by a very structured and wide emission spectrum despite its lowest excited state being the doublet 2 E g , usually associated with very weak electron–phonon coupling. Excited state absorption is principally from the doublet 2 E g to three higher-energy doublets though an additional contribution due to the gahnite glass ceramic host is also present during UV excitation. The chromium excited state absorption has been extracted from the overall spectrum. Utilizing data from the emission, absorption and excitation spectra, the configuration coordinate diagram describing octahedrally coordinated Cr 3+ ions in the gahnite glass ceramic has been produced. From this diagram the expected excited state absorption spectrum, including the influence of bleaching, has been modelled and a comparison made with experimental data.

Phase transitions induced by solid solution in stuffed derivatives of quartz: A powder synchrotron XRD study of the LiAlSiO4-SiO2 join

Abstract The crystal structures of stuffed derivatives of quartz within the Li1-xAl1-xSi1+xO4 system have been refined by Rietveld analysis of powder synchrotron X-ray diffraction (XRD) data. Our results reveal an Al-Si order-disorder transition at x = ~0.3 and a β-α displacive transformation at x = ~0.65. Structural variations across the series result from an interplay of three mechanisms: tetrahedral tilting associated with Al-Si order-disorder; Li positional disorder along structural channels parallel to c; and tetrahedral rotation related to the β-α transition. At both microscopic (local bonding) and macroscopic (spontaneous strain) scales, the substitution of Li+ and Al3+ for Si4+ closely mimics temperature in its effect on the quartz framework.

Excited state absorption in the gahnite glass ceramics and its parent glass doped with chromium

Abstract We report on comparison of the basic spectroscopy and first of all excited state absorption (ESA) characteristics measured for the chromium doped pink gahnite glass ceramics and its green parent glass doped with chromium. The ESA spectrum of the ceramics is rather predictable and reveals peaks which can be ascribed to doublet–doublet transitions, typical for the Cr 3+ ions in the high crystal field, and is additionally affected by the ground state absorption bleaching. Unlike the ceramics, the parent glass, being a low-field material, reveals quartet–quartet ESA, typical for the Cr 3+ ions in the low crystal field. The ESA spectrum apart of chromium-related features and bleaching of the chromium ground state absorption, is connected with other possible centers of the host. Based on basic spectroscopic data we create configuration coordinate diagrams for both cases, predict the shape of the expected ESA spectra and compare them with measured ones. A reasonable agreement has been achieved.

Glasses containing closed-shell transition metal complexes: 1. Luminescence

Abstract We report the first spectroscopic evaluation of several new glasses, containing molecular complexes of transition metal ions. Under 308 nm excitation these glasses show broad and efficient luminescence, peaking in the blue and green. We have measured the temperature dependence of the total light yield and decay time. The thermal quenching of the luminescence appears to be very small for zirconate glasses between low and room temperature.

Glasses containing closed-shell transition metal complexes: 2. Excited-state absorption

Abstract We have evaluated several new glasses, containing molecular complexes of d 0 transition metal ions, from the point of view of their suitability as possible tunable laser materials. Using as excitation source the 308 nm line of an excimer laser, we found that in spite of the relatively large quantum efficiency, these glasses have very strong excited-state absorption (ESA) of cross-sections about three order of magnitude greater than the stimulated emission cross-sections. The ESA spectra are very broad and their maxima are shifted to the violet or ultraviolet region with respect to luminescence, however in view of its high level, ESA still precludes any laser action in these materials. Nevertheless, one of these glasses (niobate-doped) suggest a direction for further development and possibly reduced ESA.