Linda R. Pinckney

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.

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.

Record Voc-Values for Thin-Film Polysilicon Solar Cells on Foreign Substrates using a Heterojunction Emitter

Thin-film polysilicon solar cells on foreign substrates are often considered as a promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon-crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon layers with a heterojunction emitter. This is the highest open-circuit voltage obtained for polysilicon solar cells with a p-n structure on foreign substrates

Cr/sup 4+/:forsterite nanocrystalline glass-ceramic fiber

Summary form only given. Glass-ceramics are two-phase systems consisting of a base glass within which crystals are grown by heat treatment, a process known as ceramming. Recent progress on fiberization of these materials led to the demonstration of a rare earth doped fiber laser and amplifier with an efficiency uncompromised by the presence of the crystals embedded within the core of the single mode fiber. Transitions metal ions are notoriously inefficient when incorporated into a glass host, due to the difficulty of controlling the local environment, to which they are very sensitive. However to date, the properties of Cr/sup 4+/-doped glass-ceramic systems have received little attention, despite the apparent advantages of crystal site control. This paper investigates the fluorescence properties of Cr/sup 4+/-doped forsterite glass-ceramics and shows that many of the important characteristics of the single crystal are reproduced in the glass-ceramic system. Furthermore, many of the spectroscopic parameters are also observed in glass-ceramic fiber, highlighting the potential for a new class of fiber laser and amplifier made from transition metal-doped glass-ceramics.

Nickel-doped nano-crystalline glass-ceramic fiber

The spectroscopic properties of Ni(2+)- doped nanocrystalline glass-ceramic fibers are reported. The cerammed fibers show strong fluorescence with peak wavelength at 1250 nm, 3-dB bandwidth at ~250nm, measured lifetimes at >1ms, and low-fluorescence saturation powers (~35mW) for 980-nm diode pumping. Current diode-pumped output powers are ~100microW .