Publications

Substituted ferrites with the chemical composition MxFe1-xFe2O4-nH(2)O (M: Mn(II), x: 0.35, n: 0.4; M: Ni(II), x: 0.38, n: 1; M: Zn(II), x: 0.45, n: 0.35) have been prepared by a soft chemical method using as starting materials magnetite and suitable acetates. These compounds have been characterised by elemental chemical analysis, thermal analysis, IR spectroscopy, X-ray diffraction on powder, transmission electron microscopy studies as well as Mossbauer spectroscopy and magnetic measurements at room temperature. The X-ray diffraction study indicates that the substituted ferrites adopt a cubic structure and the size of particles determined from the diffraction peak is fairly close to that indicated for agglomerates by transmission electron microscopy. The magnetic methods agree to the substitution of Fe(II) in the octahedral positions preserving the inversed spinel structure. (C) 2003 Elsevier B.V. All rights reserved.

Critical current density J(c) of Na0.35CoO2.1.3H(2)O superconductor (T-c=4.4 K) has been estimated from magnetization measurements: J(c)(1.9 K,0 T)=5x10(4) A/cm(2), J(c) (1.9 K,0.4 T)=4x10(2) A/cm(2) and J(c) (3.4 K,0 T)=10(3) A/cm(2). J(c) versus applied magnetic field H cannot be fitted, as for high-T-c superconductors (HTS), by a power law for any temperature. The temperature dependence for a given H is not of the linear type as for MgB2, but is close to quasiexponential (associated with the thermally activated flux creep) as for HTS. Irreversibility field data points H-irr versus (1-T/T-c) obey a power law with power exponent n=3.4. The irreversibility line is located below, but in vicinity of, the line for Bi-2223. Details of morphology are also presented. (C) 2004 American Institute of Physics.

Precursor powders of Ba/Zr = 1:1 chloride have been prepared by freeze drying (FD) and conventional mixing. A simple, laboratory-made apparatus for sublimation of the frozen water from the cryo-particles is presented. Both powders have the same phase formation behavior. Up to 500-600 degreesC, Zr chloride transforms into ZrO2. At high temperatures, BaCl2 starts melting and decomposing and two subsequent processes take place: (a) chlorination of the Zr atoms from the oxide and (b) formation of BaZrO3 phase, These processes depend on treatment atmosphere and reactivity of the starting powders. Freeze-dried powders with higher reactivity and/or O-2 atmosphere are suitable to obtain high content of BaZrO3 phase. In the freeze-dried powder, decomposed in O-2 atmosphere at 1190 degreesC for 20 h, 90% of BaZrO3 phase content was attained. No new phases were observed during decomposition of the freeze-dried powder. In the as-synthesized freeze-dried powder, some XRD lines remained unidentified. (C) 2003 Elsevier B.V. All rights reserved.

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.