National Institute Of Materials Physics - Romania

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.

Microspherical lenses (50 divided by 800 mu m in diameter) based on glassy As(2)S(3), have been produced by a special flame melting technique. The microlenses have been used for focusing the red - infrared laser light at the end of an optical fiber. Microlens arrays have been produced and applied in coupling optical fibers to a light remitting diode (LED), in order to increase the intensity. A double ball lens coupling scheme has been designed and manufactured.

Phase transition and field driven hysteresis evolution of a two-dimensional Ising grid consisting of ferroelectric-antiferroelectric multilayers that take into account the long range dipolar interactions were simulated by a Monte-Carlo method. Simulations were carried out for a 1 + 1 bilayer and a 5 + 5 superlattice. Phase stabilities of components comprising the structures with an electrostatic-like coupling term were also studied. An electrostatic-like coupling, in the absence of an applied field, can drive the ferroelectric layers toward 180A degrees domains with very flat domain interfaces mainly due to the competition between this term and the dipole-dipole interaction. The antiferroelectric layers do not undergo an antiferroelectric-to-ferroelectric transition under the influence of an electrostatic-like coupling between layers as the ferroelectric layer splits into periodic domains at the expense of the domain wall energy. The long-range interactions become significant near the interfaces. For high periodicity structures with several interfaces, the interlayer long-range interactions substantially impact the configuration of the ferroelectric layers while the antiferroelectric layers remain quite stable unless these layers are near the Neel temperature. In systems investigated with several interfaces, the hysteresis loops do not exhibit a clear presence of antiferroelectricity that could be expected in the presence of anti-parallel dipoles, i.e., the switching takes place abruptly. Some recent experimental observations in ferroelectric-antiferroelectric multilayers are discussed where we conclude that the different electrical properties of bilayers and superlattices are not only due to strain effects alone but also due to long-range interactions. The latter manifests itself particularly in superlattices where layers are periodically exposed to each other at the interfaces.