National Institute Of Materials Physics - Romania

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.

In this paper, surface enhanced Raman scattering (SERS) studies on the chemical polymerization of 3,4-ethylene dioxythiophene (EDOT) in the presence of single-walled carbon nanotubes (SWNTs) are reported. Both a non-covalent and covalent functionalization of SWNTs with poly(3,4-ethylene dioxythiophene) (PEDOT) is invoked as a result of the following variations induced in the SERS spectra of PEDOT and SWNTs: (i) the appearance of a Raman line at 140 cm(-1) indicating the formation of a new covalent bond between PEDOT and SWNTs; (ii) an increase in the intensity of the Raman line at 705 cm(-1), associated with the deformation vibration mode of C-S-C bond, the result of a steric hindrance effect induced by the bonding of PEDOT on SWNTs; and (iii) the enhancement of the Raman band with maximum at 1540 cm(-1) (G(-) component) in SWNTs when the PEDOT weight in the PEDOT/SWNTs composite increases. Using the PEDOT/SWNTs composite as a positive electrode and an electrolytic solution containing LiPF6, the charge-discharge characteristics of the rechargeable lithium cells are determined. High specific discharge capacity are reported for the PEDOT/SWNTs composite (ca. 218 mAh g(-1)) in comparison with PEDOT doped with FeCl4- ions (ca. 25 mAh g(-1)).

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J (c)), pinning force (F (p)) and pinning potential in thick (1.3-1.6 mu m) YBa2Cu3O7-delta (YBCO) films grown by Pulsed Laser Deposition on SrTiO3 substrates decorated with LaNiO3 nanodots deposited by a few (5-15) laser pulses, in comparison with those of a 1 mu m thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO3 target, which have, at 77.3 K, a J (c) of 40-125% higher than in pure YBCO in fields between 1 and 2 T, and F (p) increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.

Experimental evidence that nuclear magnetic resonance (NMR) can detect structural changes of piezoelectric La3Ga5SiO14 induced by dilute paramagnetic ions is presented. Gd3+ and Eu3+ cations have been incorporated into La3Ga5SiO14 monocrystals. As expected, the line-width of the tetrahedral Si-29 magic angle spinning (MAS) NMR spectra as well as the inverse of the T-2 relaxation time of Ga-71 increases with the concentration of the paramagnetic ions. A surprising result is shown by Ga-71 multiple quantum (MQ) MAS NMR spectrum, which changes with the concentration of paramagnetic ions. The changes in the Ga-71 MQMAS spectra can be explained by a more ordonated distribution of Ga ions inside the oxygen tetrahedra. The Ga-71 MQMAS NMR spectra allow identification of the one octahedral and two tetrahedral Ga sites. (C) 2009 Elsevier Inc. All rights reserved.