National Institute Of Materials Physics - Romania

A new fluorine-free chemical solution deposition route has been developed for YBa2Cu3O7-x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.

The output performances of a diode-pumped Nd:YAG laser passively Q-switched by Cr4+:YAG saturable absorber crystal were investigated function of temperature. Increase of the temperature from 25 to 150A degrees C increased slightly the laser pulse energy, and did not change the pulse duration. Furthermore, an increased absorbed energy of the pump radiation was necessary at temperatures higher than 25A degrees C in order to maintain Q-switch operation. Measurements concluded that Cr4+:YAG transmission did not vary when temperature increased to 150A degrees C. The decrease of Nd:YAG emission cross section with temperature and the changes of resonator configuration due to thermal effects were considered as main reasons for this behavior. The results of this work are valuable for designing a laser-ignition system for industrial gas engine or automotive industry.

Electronic equipment ran be effectively shielded by the use of a conductive barrier placed between the source of electromagnetic waves and the equipment to be shielded. This paper describes a work which covers some considerations on the materials used for shielding and aspects of the design optimization from the electromagnetic compatibility point of view of a shield with application to a shielding box for a portable computer.

We investigate theoretically the transport properties of a mesoscopic system driven by a sequence of rectangular pulses applied at the contact to the input (left) lead. The characteristics of the current which would be measured in the output (right) lead are discussed in relation with the spectral properties of the sample. The time-dependent currents are calculated via a generalized non-Markovian master equation scheme. We study the transient response of a quantum dot and of a narrow quantum wire. We show that the output response depends not only on the lead-sample coupling and on the length of the pulse but also on the states that propagate the input signal. We find that by increasing the bias window the new states available for transport induce additional structure in the relaxation current due to different dynamical tunneling processes. The delay of the output signal with respect to the input current in the case of the narrow quantum wire is associated to the transient time through the wire.

The transition from the crystalline state to amorphous state and back has been studied in the particular case of the GeSb2Te4 phase-change material by a computer simulation procedure. Modelling at the nanoscale indicates specific structural characteristics, especially the multiplicity of the amorphous phase as opposite to the uniqueness of the crystalline phase. in the particular case of the Si(12)Ge(10)AS(30)Te(48) switching glass two types of ordering have been pointed out and characterized. (C) 2009 Elsevier B.V. All rights reserved.

Glass-ceramics of the compositions Bi(4)Ge(3)O(12) (BGO) and Bi(2)O(3)-GeO(2) have been obtained by pouring molten materials onto graphite plates heated below and above the glass transition temperatures. Glass transitions temperatures have been measured by DSC. T(g) is lower in the case of BGO than with the Bi(2)O(3)-GeO(2) glass ceramics material. Crystallization is evaluated by X-ray diffraction and microscopic observations. The XRD measurements confirm the amorphous structure of the samples cast below T(g). The Hruby factor, which measures the stability of the glass-ceramics, is about 0.21 in the case of BGO glass ceramics suggesting a rather stable glass. The fluorescence under U.V. excitation is larger by one order of magnitude in the case of BGO glass-ceramics by comparison to the Bi(2)O(3)-GeO(2) glasses, suggesting it is a good candidate for scintillation materials. The images obtained under U.V. excitation shows structured luminescent centers in the case of BGO glass-ceramics. (C) 2009 Elsevier B.V. All rights reserved.

Multilayered TiO2 films were obtained by sol-gel and dipping deposition on quartz substrate followed by thermal treatment under NH3 atmosphere. In an attempt to understand the close relationship between microstructural characteristics and the synthesis parameters, a systematic research of the structure and the morphology of NH3 modified TiO2 sol-gel films by XRD and Atomic Force Microscopy is reported. The surface morphology has been evaluated in terms of grains size, fractal dimension and surface roughness. For each surface, it was found a self-similar behavior (with mean fractal dimension in the range of 2.67-3.00) related to an optimum morphology favorable to maintain a nano-size distribution of the grains. The root mean square (RMS) roughness of the samples was found to be in the range of 0.72-6.02 nm. (C) 2009 Elsevier B.V. All rights reserved.

Transparent oxyfluoride glass-ceramic in the system SiO2-Al2O3-CaF2-EuF2 containing Eu-doped CaF2 nanocrystals were produced by using the controlled crystallization of melt-quenched glass. X-ray diffraction and transmission electron microscopy data have revealed the formation of the CaF2 nanocrystals of about 65 nm size. Photoluminescence spectra have shown an increase of the splitting of the luminescences associated to the Eu3+ ion along with annealing time which is consistent with the Eu3+ environment evolving from a glassy to a crystalline state. (C) 2009 Elsevier B.V. All rights reserved.

A new model is proposed for the structure of low atomic coordination chalcogenide glasses. The closed clusters model is based on the various types of clusters, packed by van der Waals forces in a molecular-type packing. Arguments are given to support the new model, taken as example the typical chalcogenide glass As2S3. (C) 2009 Elsevier B.V. All rights reserved.

Electrochemical deposition was performed in order to prepare CdS nanowires. The method employed for preparation of such high aspect ratio nanostructures was template replication. Ion track polycarbonate membranes were used as templates. The nanowires were studied by scanning electron microscopy (morphology characterization), energy dispersive X ray analysis (composition) and optical spectroscopy. Optical reflection spectroscopy was performed in order to determine the band gap value while photoluminescence spectroscopy was used for getting information regarding the point defects in the material.