Publications

The present paper analyzes the appearance of voltage percolation thresholds in the current - voltage characteristics measured on silicon-based nanocrystalline systems that present random space distribution of the nanocrystallites. This percolation phenomenon is explained on the basis of the probability of tunneling under applied bias and is related to the samples microstructure.

In the present paper, we intend to bring some clarifications related to the role played by primary defects in long term degradation, to the dependence of degradation on the orientation of the wafer, as well as on the spatial distribution of the degradation produced by low energy particle irradiation.

Optical and Mossbauer spectroscopy were used to put into evidence the morphological changes induced by the thermal treatments in the YAG:Eu nanocrystals synthesized by a sol-gel method. Both methods exhibit a drastic change of the monitored parameters (ratio of the areas of the electric and the magnetic dipole transitions - for optical spectroscopy and Mossbauer normalized area - for Mbssbauer spectroscopy) at the phase transition amorphous-crystalline, followed by a slow evolution of these parameters with the increase of the annealing temperature. This slow evolution is due to the increase in size of the nanocrystals.

A ceramic high temperature superconductor [HTS] of Y-Ba-Cu-O type has been investigated at 77 K by using a standard X-band Electron Paramagnetic Resonance (EPR) configuration. At very low microwave power (< 1 mW) the non-resonant or zero field signal (ZFS) was in phase with DPPH signal, pleading for an unambiguous absorption process, but it commutes to a typical superconductor signal (i.e. opposite to DPPH signal phase) with increasing the microwave power. At the same time, CU2+ signal appreciably changes its shape with increasing microwave power. These anomalous behaviors could be in part explained by a conventional SQUID response at microwave frequency by taking into account that the sample itself could be described by a collection of both Josephson and proximity junctions.

AC-susceptibility vs temperature measurements performed on (Bi,Pb)(2)Sr2Ca1-x YxCu2-yZnyOz phase present in sintered Bi1.8Pb0.4Sr2Ca2-x YxCu3-yZnyOw polycrystalline samples, 0 <= x <= 0.45 and y = 0 and 0.03. We report an anomalous suppression of superconductivity for x approximate to 0.3. For the first time in Bi-based cuprates, we report the presence of this anomaly not only in the Zn doped samples but also in the samples without Zn.

Directional solidification techniques have been applied to produce Ni based intermetallic alloys with preferentially oriented columnar crystals extended along the complete length and parallel to the solidification direction. Enhanced ductility is expected from such alloys. In this paper we present the research results concerning the application of this technique to some complex Ni3Al-Fe-B alloys obtained from compacted mixtures of elemental powders. The corresponding master alloys have been obtained in a vacuum induction furnace by the known Exo-Melt process [1]. The directional solidification of these alloys was subsequently performed on cylindrical samples, at two solidification rates, 30 and 15 mm/h. The influence of the rate and composition used on the dimensional variations, densities, microstructure, constituent phases and lattice parameters, as well as on the surface chemical composition have been documented and are presented in the paper.

Preparation of dielectric materials with optimal properties for wireless applications requires special thermal treatments due to the difficult control of the cationic ordering. The BZT samples prepared by solid-state reaction were sintered at temperatures in the range 1400-1600 degrees C for 4 h. For compositional, structural and morphological characterization, XRD, SEM and EDX were employed. The order-disorder transition of the BZT material was analyzed. With the increase of the sintering temperature, a long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed. The dielectric parameters were measured in the microwave range and were correlated with morphological and structural properties. In order to improve the microwave properties, annealing treatment at 1400 degrees C for 10 h was performed. Sintering temperatures greater than 1550 degrees C are required for undoped BZT compositions, in order to obtain low microwave loss (Qf > 100,000). (c) 2006 Elsevier Ltd. All rights reserved.

The PbS micro-crystalline and nano-crystalline films were obtained using the Chemical Bath Deposition method. We investigated comparatively the structural, electrical and photoconductive properties of these films deposited on glass. PbS/SiO2/Si heterostructures made with nanocrystalline PbS layers present interesting peculiarities. Their C-V curves show hysterezis. In a field effect arrangement it is possible to control the photoconductive short and long wavelength ratio response. The heterostructure properties were explained by taking into account the interface charged traps.

Using magnetization measurements, we report the existence of an irreversibility in the magnetization in Y0.47Pr0.53Ba2Cu3O7-delta single crystals above the critical temperature (T-c = 13 K) that is visible up to 200 K. Magnetization vs temperature displays irreversibility up to an applied field of 500 Oe. We noticed a weak irreversibility at high temperatures and a stronger one at low temperatures. In the weak irreversibility regime, both zero-field- and field-cooled magnetization follow similar Curie-Weiss (C-W) dependences, with slightly different parameters. The C-W parameters, decrease fast with increasing field. The C-W temperature is negative, suggesting that the average interaction between spins is antiferromagnetic. All these data suggest that the paramagnetic state is in fact a free spin liquid containing floating antiferromagnetic clusters as well as superconducting phase fluctuations.

Thermoluminescence (TL) of Eu2+-doped BaFCl polycrystalline powders X- ray irradiated at room temperature show several TL peaks at 364 K, 408 K, 470 K, 524 K and 646 K. Thermally activated recombination processes responsible for TL have been studied using complementary techniques as TL and its bleaching, diffuse reflectance, X-ray excited luminescence (XL) and photostimulated luminescence (PSL). Experimental results indicate that F(Cr) centres are the dominant electron trap centres, the Eu2+-trapped hole aggregate centres playing the role of recombination centres. The 470 K and 646 K TSL glow peaks are assigned to the recombination inside the triple aggregates (Eu2+ - hole centre - F(Cl-) centre). The remaining 408 K and 524 K TL peaks are tentatively assigned to recombination between F centres and double aggregates (Eu2+ - hole centre). (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.