Publications

Superconducting whiskers of Bi-2212 have been grown by an airtight crucible method in elevated magnetic fields, up to 10 T. Growth behavior with and without application of a magnetic field, as well as the SEM detailed study of the growth defects of the whiskers allow to emphasize some aspects of the growth mechanism. The conclusion is that whiskers are growing mainly through continuous crystallization at the base-end and from the so-called 'micro-crucible' formed by the glassy or powdered substrates. The supply of cations by a vapor transfer is very probable, but is taking place to the micro-crucible reaction site rather than to the droplet, as in the classic vapor-liquid-solid (VLS) growth mechanism. Therefore, whiskers grow by a modified VLS-microcrucible mechanism (MVLS-MC). Application of magnetic fields during growth has proven to be a powerful tool to investigate and influence some processes. Observation of stoichiometric Bi(Pb)-2212, CuO and Ca2CuO3 whiskers is also reported. (C) 2004 Elsevier B.V. All rights reserved.

Stable CoFe(2)O(4) nanoparticles have been obtained by co-precipitation using a microwave heating system. Transmission electron microscopy images analysis shows an agglomeration of particles with an average size of about 5 nm, and X-ray diffraction reveals the presence of a pure ferrite nanocrystalline phase. X-ray photoelectron spectroscopy and thermal gravimetric analysis show the presence of organic matter in the range of about 16 wt%. The magnetic response in DC fields is typical for an assembly of single-domain particles. The measured saturation magnetization is slightly larger than the bulk value, probably due to the presence of small amounts of Co and Fe. AC magnetization data indicate the presence of magnetic interactions between the nanoparticles. Crown Copyright (C) 2004 Published by Elsevier Inc. All rights reserved.

In this work, it is shown that the analysis of thermally stimulated currents (TSC) and current transients (CT) at constant temperature can be a suitable tool to study the influence of deep levels on space charge density N(T) in irradiated silicon diodes. In particular, the occurrence of space charge sign inversion (SCSI) can be related to signal discontinuities in TSC and CT measurements. This approach has been adopted in this work to study devices made of standard Float Zone (FZ) and Diffusion Oxygenated Float Zone silicon, irradiated by gamma-rays up to a dose of 300 Mrad. Our study shows that all the samples are inverted at 50 K after a low-temperature excitation. Several space charge sign inversions, from positive to negative and vice versa, have been observed between cryogenic and room temperature, and have been related to carriers emission from dominating deep traps. Only standard FZ silicon remains inverted at room temperature after a dose of 300 Mrad. (C) 2004 Elsevier B.V. All rights reserved.

High frequency (95 GHz) ESR measurements have been performed down to 5 K, on three cubic boron nitride (c-BN) single crystals doped with beryllium. The measured samples exhibit at low temperature different ESR spectra, which are sensitive to low temperature in situ illumination using a series of Kr+- and Ar+-laser lines. The analysis of the ESR spectra resulted in the identification of several native paramagnetic centers responsible for the observed component spectra lines. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Impedance spectroscopy studies on polyaniline doped by various amounts of HCl are reported. The measurements were performed at room temperature, over a wide range of frequencies [1000 Hz to 45 MHz]. The experimental data shows that the resistance dominates the AC behavior of polyaniline confirming that the charge transport occurs through an one-dimensional hopping process.

Dopant yttrium at 150 p.p.m. level (Y to Al atomic ratio) in ultra -high purity polycrystalline alumina has been found to be, together with the coefficient of porosity, the reason of grain growth : with abnormal grain growth (AGG) or with uniform grains. The powder, green bodies and sintering samples were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and packing models. In this paper we studied by mathematical modeling the correlation between structure of powder and green bodies in correlation with the porosity in sintering compacts. A formula, which describes the packing of the spherical-cylindrical homogeneous particles, has been obtained. The porosity of the low Y doped alpha-Al2O3 was investigated.

Modelling of several large molecular clusters possible to be formed in non-crystalline arsenic chalcogenides (As2S3, As2Se3 or mixed compositions) during cooling down of the melts has been performed. The structure of minimum free energy was calculated in the frame of valence force field theory using a Monte-Carlo-Metropolis method. The structural characteristics of the clusters were computed after energy relaxation. There was demonstrated that specific cluster configurations of the type nano-hat, nano-pie and nanotube are crystallo-chemically compatible with the non-crystalline structure of the arsenic chalcogenides.

The possibility to enhance the self-nonlinear emission characteristics in Nd-activated nonlinear crystals by direct pumping into the emitting level F-4(3/2) is discussed. It is inferred that the reduction of threshold and the increase of the slope efficiency for the intracavity emission at the fundamental frequency caused by the reduction of quantum defect between the pump and the emitted quanta in the case of direct pumping are strongly amplified in the self-nonlinear processes. Moreover, the diminished heat generation could contribute to a more stable emission and to the scaling to a higher power. The possibility to use the direct pumping is determined by the spectroscopic properties of the laser material. This is illustrated for Nd3+ in Ca4GdO(BO3)(3), where, for direct pumping, the thermally activated transition I-4(9/2)(Z(2)) --> F-4(3/2)(R-1) at 887 nm is suitable. Pumping in this band induces an enhancement of the self-doubled emission power at 530 nm by similar to2.5 times for an absorbed power four to five times above threshold as compared with the 811-nm pumping into the level F-4(1/2) of Nd3+. (C) 2004 Optical Society of America.

The atomic structure of dislocations in cubic boron nitride has been investigated by high resolution transmission electron microscopy. Most of the perfect dislocations, screw and 60degrees edge, are dissociated. A 60degrees dislocation which was undissociated has been analysed. Computer simulation is performed in an attempt to characterise the core structure. Twinning dislocations and dislocations resulting from the intersection of stacking faults are also revealed. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

New magnetic composite materials, ribbon samples, with nominal compositions FexCu1-x (x=0.2; 0.5; 0.8) were obtained by the melt spinning technique. Various information was provided via X-ray diffraction, Mossbauer and magnetic measurements. The structural aspects: phase composition, size and texture effects, have been corroborated with magnetic properties. Finally, the possibility to choose the adequate composition and the suitable processing in order to obtain desired anisotropic magnetic materials were presented.