Publications

Sr2FeMoO6 was synthesizes by soft chemistry route using citric and/or oxalic acid as complexing agents. This method provides good compositional control with promising results as a base for further improvements. The material shows complex temperature- and field(magnitude and orientation) dependent electro-magnetic behavior as probed by magnetic and transport measurements. The zero-field cooling curves of magnetization versus temperature taken at different magnetic fields, show two temperature regions separated by a transition temperature T-cups(ZFC). When H increases, T-cups(ZFC) values decrease. At 5 K and 5 T the saturation moment is 3.55 JIB per formula unit, and the coercitive force is 0.021 T. The relative value of the magnetoresistive effect (i.e. 100 (rho(H)-rho(0))/rho(H), %, with rho being resistivity) at 5 K is 68% and 36% when applied field H is parallel and perpendicular the measuring current I, respectively. The results are discussed in relation to magnetic sublattices and the interaction that produces anti-site defects and spin-dependent electron anisotropy. (C) 2006 Elsevier B.V. All rights reserved.

In this paper a study of some microwave microstrip bandpass filters (BPF) using defected ground structures (DGS) is presented. This technique allows designs of tight couplings without the necessity of using very narrow coupling gaps. Based on the results of the study, four-pole cross-coupled planar microwave bandpass filters with a single and with two ground slots are designed, fabricated and measured. The performances of these novel structures indicate some technological advantages, compared to classical microstrip filters, without defected ground.

The thermal behaviour of five polynuclear coordination compounds containing tartaric anion as ligand, namely (NH(4))(3)[LnFe(C(4)O(6)H(4))(3)(OH)(3)] (Ln=La and Eu), (NH(4))(2)[PrFe(C(4)O(6)H(4))(3)(OH)(2)] and (NH(4))[LnFe(C(4)O(6)H(4))(3)(OH)]center dot 3H(2)O (Ln=Nd and Gd) was investigated. The reaction progress was studied by TG/DTA and FTIR measurements. Oxalates and oxocarbonates were identified as intermediates. In the case of Ln=La, Nd, Pr, Eu and Gd, pure LnFeO(3) was obtained as final decomposition product. The thermal decomposition of Eu-Fe compound, leads to a mixture of mixed (ortho-ferrite (EuFeO(3)) and garnet (Eu(3)Fe(5)O(12))) and simple oxides (Eu(2)O(3) and alpha-Fe(2)O(3)).

The critical temperature, T-c, for (Bi,Pb)Sr(Ca2-x, Y-x)CuO/2223, with x between 0 and 0.45, polycrystalline samples have been investigated. T-c of (Bi,Pb)(2)Sr2Ca2-xYxCu3O10+delta phase, 2223, found in samples with x 0.05, displays a parabolic dependence with the increase of Y content. The change of T-c for 2223 originates mainly from the increase of x. The variation of both x and oxygen content, delta, influences the T-c of 2212. The doping state of 2212 and 2223 in Y substituted samples suggests that, generally, in the samples consisting of both superconducting phases, 2212 is in over-doped state whereas 2223 is in under-doped state. The last affirmation suggests that 2212 could bring a significant contribution in the transport parameters of (Bi,Pb)SrCaCuO/2223 samples.

Pure TiO2 and S-doped TiO2 sol-gel nanopowders were prepared by controlled hydrolysis-condensation of titanium alkoxides. The influence of different Ti-alkoxides (tetraethyl-, tetraisopropyl- and tetrabutyl-orthotitanate) used in obtaining TiO2 porous materials in similar conditions (water/alkoxide ratio, solvent/alkoxide ratio, pH and temperature of reaction) has been investigated. The relationship between the synthesis conditions and the properties of titania nanosized powders, such as thermal stability, phase composition, crystallinity, morphology and size of particles, BET surface area and the influence of dopant was investigated. The nature of the alkyl group strongly influences the main characteristics of the obtained oxide powders, fact which is pointed out by thermal analysis, X-ray diffraction, TEM and BET surface area measurements.

First-order branched tree structures of MoO3 belts on sillimanite fibers (refractor wool) were grown by a vapor transport method. At longer growth times, MoO3 micro-belts on micro-belts form a flower-like structure. Most belts are of alpha-MoO3, and they grow in the two opposite directions along the c-axis. They show a high level of crystal quality, clean surfaces, sharp edges, and a triangular-shape tip with certain angles. The most frequent are angles of 47 degrees and 94 degrees. The growth mechanism is of vapor-solid type. In the initial stages of growth, elements specific for spontaneous spread and island formation on the surface of the sillimanite fibers described in the literature are observed. Significant enhancement of the thickness suggests that a layer-by-layer 2D growth is also probable. It is thought that responsible for such growth is direct vapor deposition of the Mo-O vapors on the already formed belts, this being similar to thin film growth. Curved surfaces of the substrate (in our case of the sillimanite fibers), on which the MoO3 belts are growing, are important and can be used for the directional growth control of the belts and formation of hierarchical structures. The vapor transport method allows the formation of beta-MoO3 belts that grow in the b-axis direction.

Thermal behaviour of CuS (covellite) obtained from the Cu(CH3COO)(2)center dot H2O and Na2S2O3 center dot 5H(2)O system, working at different molar ratio (1:6 and 1:4) in presence/absence of NH4VO3, was studied. It was established that the presence of vanadium in the system induces a densification of CuS nodules, but do not change the hexagonal CuS structure. It has an important influence in thermal behaviour of copper sulfide CuS obtained also. The morphological characteristics of CuS play an important role in the thermal stability and the stoichiometry of the thermal decompositions. Also, the possibility to obtain copper sulfides with greater cooper content was investigated.

The superconducting properties of iodine-intercalated high-temperature superconducting Bi2Sr2Ca2Cu3O10+x phase (Bi-2223) were systematically studied. It was found that for samples containing a significant amount of Bi2Sr2CaCu2O8+x , iodine intercalation results in the dramatic decrease of the inter-granular critical current density, as well as a significant decrease of the critical temperature (T-c), the critical current density in the grains (J(cg)), and of the amount of Bi-2223. For samples with a large amount of Bi-2223, T-c changes insignificantly, whereas J(cg) can even increase. We argue that the different behavior of the superconducting parameters is the result of various oxygen concentrations, and we explain the effect of iodine intercalation based on the parabolic dependence between T-c and the number of holes per CuO2 layer. The H(T) curves (determined from the peak position in the loss signal of ac susceptibility) for intercalated samples deviate significantly from the quasi 2D-like behavior, pointing toward an enhancement of the 3D fluctuations of vortices. For the change in the values and dimensionality of the flux pinning in the process of the intercalation, we attempted a qualitative explanation based on the models proposed in literature.

In this contribution, an extensive analysis of the primary defects in silicon: vacancy, interstitial and Si-FFCD is performed. Irradiation studies are a useful tool to study production, characteristics and annealing of defects. The experimental results obtained after high proton fluence irradiation of silicon detectors are used in this paper to understand aspects related to the existence and proprieties of primary defects. Investigations on possible differences induced by irradiation in the lattice of silicon, using transmission electron microscopy analysis, have been started and some first preliminary results are presented.

We present a study of the effect of the crystallites composition and size on the optical properties of vitreous aluminophosphate matrix doped with CdSxSe1-x crystallites. The influences of the glass matrix composition and, heat treatment temperature and duration on the stoichiometry of the crystalline phase have been investigated by optical methods. We have studied the effect of the sodium oxide contained in the matrix, annealing temperature variation from 425 degrees C to 475 degrees C and duration variation from 2 h to 6 h on the light absorption process, evaluating an optical band gap energy between E-g=1.74 eV and E-g=2.29 eV associated with changes in the composition of the crystallites. The blue shift of the absorption edge observed in samples containing Na2O annealed at 425 degrees C for 4 h or at 450 degrees C for 2 h was attributed to a variation in the crystallites' composition because their size is not favourable to the quantum confinement. We have also investigated the effect of individual CdS and CdSe clusters on the absorption properties of the aluminophosphate glass and on the process of zinc incorporation in the crystalline phase. We have made some assumptions concerning the effect of the aluminophosphate glass matrix on the direct band to band light absorption mechanism characterising the II-IV bulk semiconductor.