Study of the morphology of copper hydroxynitrate nanoplatelets obtained by controlled double jet precipitation and urea hydrolysis

in Journal of Crystal Growth (2003), 254(1-2), 176-187

A copper hydroxynitrate of stoichiometry Cu-2(OH)(3)NO3, analogous to the layered double hydroxide family, was synthesized by the so-called controlled double jet precipitation technique, and by hydrolysis ... [more ▼]

A copper hydroxynitrate of stoichiometry Cu-2(OH)(3)NO3, analogous to the layered double hydroxide family, was synthesized by the so-called controlled double jet precipitation technique, and by hydrolysis of urea in the presence of copper nitrate. Special attention has been focused on the size, morphology and agglomeration tendency of the particles. The aim of this work is to define the optimum precipitation conditions in terms of quality and dispersability of the recovered product. Such platelet-like particles Can be used as anisotropic fillers in nanocomposite materials. Several reaction parameters such as flow and concentration of the reactant solutions, design of the reactor and addition of a growth modifier were studied. (C) 2003 Elsevier -Science B.V. All rights reserved. [less ▲]

Experimental and theoretical investigations of the electrical properties of undoped and magnesium-doped GaN layers

in Journal of Crystal Growth (2001), 230

The ac characteristics of GaN : Mg and undoped GaN layers, grown by MOVPE on sapphire substrates, are measured for a wide range of temperature and bias conditions, in order to investigate the effect of ... [more ▼]

The ac characteristics of GaN : Mg and undoped GaN layers, grown by MOVPE on sapphire substrates, are measured for a wide range of temperature and bias conditions, in order to investigate the effect of the magnesium-related level on the transport properties. Two peaks, whose height and position depend on the measurement temperature, are observed in the admittance curves (G/ω versus frequency) of the Mg-doped samples, whereas only one peak appears in undoped samples. The study of the frequency dependence of the impedance, with a model including the two metallic Au/GaN junctions, the GaN layer itself, shows that, besides the effect of the differential resistance of the layer which plays a role in both sample types, the presence of a Mg-related deep level contributes to the observed variations of the peaks in the admittance curves of the p-doped samples. Results of a theoretical steady-state and small-signal analysis based on numerical modelling of the Au/GaN/Au heterostructure complete our analysis. [less ▲]

PCAM: a multi-user facility-based protein crystallization apparatus for microgravity

in Journal of Crystal Growth (1999), 196

A facility-based protein crystallization apparatus for microgravity (PCAM) has been constructed and flown on a series of Space Shuttle Missions. The hardware development was undertaken largely because of ... [more ▼]

A facility-based protein crystallization apparatus for microgravity (PCAM) has been constructed and flown on a series of Space Shuttle Missions. The hardware development was undertaken largely because of the many important examples of quality improvements gained from crystal growth in the diffusion-limited environment in space. The concept was based on the adaptation for microgravity of a commonly available crystallization tray to increase sample density, to facilitate co-investigator participation and to improve flight logistics and handling. A co-investigator group representing scientists from industry, academia, and government laboratories has been established. Microgravity applications of the hardware have produced improvements in a number of structure-based crystallographic studies and include examples of enabling research. Additionally, the facility has been used to support fundamental research in protein crystal growth which has delineated factors contributing to the effect of microgravity on the growth and quality of protein crystals. [less ▲]

Fractal grain boundaries in growth competition

in Journal of Crystal Growth (1996), 169(1), 79-82

Evidence of fractal grain boundaries is found in polycrystals when radial patterns are found to result from a geometrical competition between different grains that have nucleated on the same center. A ... [more ▼]

Evidence of fractal grain boundaries is found in polycrystals when radial patterns are found to result from a geometrical competition between different grains that have nucleated on the same center. A simple statistical model presented herein predicts the universal fractal dimension 2.33 of such natural patterns grown through internal competition. The application to material patterns having constrained front propagation involving many components is obvious. [less ▲]

Optical and magnetic properties of Fe2+ and Cr2+ in II-VI semiconductors: The Jahn-Teller effect

in Journal of Crystal Growth (1996), 159(1-4), 875-878

The D-5 terms of Fe2+ and Cr2+ in the tetrahedral potential at cation sites in II-VI compounds split into orbital doublet and triplet states. While in Cr2+ the orbital tripler has lower energy than the ... [more ▼]

The D-5 terms of Fe2+ and Cr2+ in the tetrahedral potential at cation sites in II-VI compounds split into orbital doublet and triplet states. While in Cr2+ the orbital tripler has lower energy than the doublet the opposite is the case in Fe2+. Both ions have singlet ground states after the spin-orbit interaction is taken into account and, hence, both are Van Vleck paramagnets. The optical absorption spectra of Fe and Cr based materials differ and are explained on the basis of a dynamic Jahn-Teller effect in the former and a static one in the latter. These considerations permit us to explain the optical as well as the magnetic properties observed in these materials. [less ▲]