Publications

Nickel ferrite was obtained by the thermal decomposition of three malates coordination compounds: [Fe2Ni(C4H4O5)(2.5)(OH)(2)]NO3.5H(2)O, [Fe2Ni(C4H8N2O3)(4)](NO3)(8).24H(2)O and (NH4)[Fe2Ni(C4H4O5)(3)(OH)(3)].3H(2)O. The solid state transformations were investigated by means of TG/DTA analysis, IR, X-ray diffraction, Mossbauer, magnetic and BET investigations. The solid state mechanisms, as well as the characteristics of the final products are strongly influenced by the nature of the precursors, especially by their homogeneity at atomic scale and composition.

A PZT-type material with the composition near the morphotropic phase boundary and the chemical formula of PbZr0.51Ti0.465W0.015Li0.01O3 is studied. in this material, both tetragonal and rhombohedral phase are present. The proportion of the two phases in material depends on the sintering temperature. The nondestructive microanalysis is performed with an electron probe microanalyser (EPMA), equipped with wavelength dispersive spectrometers and energy dispersive X-ray analyzer. The X-ray elemental line scans are carried out on the following chemical elements: Pb, Zr, Ti and W. The electron and X-ray images provide maps with the elemental distribution in the lead matrix, in phases, inclusions and grain boundaries. Based on microanalysis results, dielectric and piezoelectric properties are discussed.

Oxide films grow in atmosphere as a result of oxidation reaction and mass transport of the element through the oxide film. In most III-V compounds (as GaAs) semiconductor oxides of III-element and V-element are formed individually. The AlGaAs/GaAs laser diodes were exposed to facet oxidation at mirror surfaces in normal environmental conditions for a storage process. It was observed a decrease of optical output power in the early stage of experiment together with a constancy of the ratio P/P(0) for the long time term (month). We suggest the development at the mirror laser facet of a natural oxide film as Ga2O3 and GaAsO.. We present a SEM (Scanning Electron Microscopy) image of a natural oxide film grown in atmosphere at the surface of AlGaAs as laser facet, as well as a picture of a facet oxidation of a laser device. The EDS (Electron Dispersion Spectrum) of a natural oxide put into evidence the O (Kalpha) signal. Dielectric coating using Al2O3 and a-Si layers protected a part of laser devices, and at the mirror surface a reflectivity of 70% in the emission range of the laser (890nm) was measured. The emitted power for dielectric coated devices was constant, so the rapid degradation process was slowed down.

The Fe spin configurations in both the soft- and hard- magnetic phases of nanocomposite Fe/RE2Fe14B exchange-spring ribbons have been analysed by Mossbauer spectroscopy. New criteria for evaluating the inter-phase exchange-coupling strength are proposed. The coupling strength is discussed in relation to the annealing conditions and the relative content of the soft magnetic phase.

Lead titanate (PT) is well known as a good piezoelectric material with high Curie temperature (similar to 490 degrees C) and low dielectric constant (similar to 200). Pure lead titanate cannot be sintered as ceramic bodies, due to its high crystalline anisotropy (c/a=1.064), but dense ceramic bodies can be obtained if small additions of other elements are used. In this regard we investigated the effect of Y addition on the structure and piezoelectric properties of PT ceramics, with general formula (Pb1-3x/2Yx)(Ti0.98Mn0.02)O-3, and x=0.04, 0.06, 0.08 and 0.10. Density and coupling factors were determined as a function of sintering temperatures, and doping level. Temperature dependence of the electromechanical behaviour was also investigated for temperatures as high as 500 degrees C. The Y modified PT ceramics have low dielectric constant ( 450 degrees C), and large electromechanical anisotropy.

Our fluorescence study gave information on the presence of chemical defects and irregularities in crystal structure, offering a good tool for quality evaluation of organic crystalline materials. We have analysed the correlation between the bulk or thin film samples' quality determined by experimental parameters (thermal gradient, moving speed of the growth interface, crucible configuration, slow or fast solidification rates) and the fluorescence characteristics for pure meta-dinitrobenzene and meta-dinitrobenzene doped with 8 hydroxyquinoline or 1,3 dihydroxybenzene. The comparative study of the position and shape of the fluorescence emissions for pure and controlled doped materials nave us information on the chemical defects.

Mechanisms for orientation selection in complex oxide thin films are discussed, focusing on two examples: (1) development of a preferential texture in sol-gel prepared polycrystalline PZT films on Pt electrode layers and (2) selection of oxides for epitaxial growth on Si (111) and (100). In sol-gel PZT, a direct link was found between the formation of a well crystallized transient fluorite phase, promoted by reducing pyrolysis conditions, and the emergence of a strongly preferred (111) texture after crystallization. Meanwhile, in the MBE growth of epitaxial oxides on silicon, matching of silicon and oxide surface lattices is not a sufficient condition to achieve layer-by-layer growth, needed for planar films. In fact, for planar growth on Si(111), fluorite-structure compounds are required, while rocksalt and perovskite structure oxides are suited for the growth of low-roughness films on Si(100). These examples illustrate the important role played by surface energy in orientation selection. (C) 2003 Elsevier Ltd. All rights reserved.

The phase composition, local structure and magnetic interactions were investigated in FexOx-SiO2 nanocomposites obtained by modern sol-gel methods. The very sensitive method of Mossbauer spectroscopy has evidenced the strong influence of the silica matrix precursor on the iron phase composition in the initial composites as well as on the size distribution of the hematite nanoparticles obtained by suitable thermal annealing treatments.

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178cm(-1) associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH4OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. (C) 2004 Elsevier Ltd. All rights reserved.