Publications

Microstructure and the sintering behaviour of zirconia with twinned stabiliser ions: Ca2+ + Mg-2 in oxidative and in inert environment respectively obtained by solid state reactions method were studied. The specimens were cured at 1550 degrees C and 1750 degrees C in air in a kiln with methane gas. Samples with the same composition were sintered at 1550 degrees C in Argon in electrical furnace with (graphite electrodes). Their sinterability, microstructure as well as stability was studied The properties were compared as function of the processing route, the compositions stabilised in argon protective environment at 1550 degrees C or in air at 1750 degrees C, a totally stabilisation process was effective.

The ability of the Fe-57 Mossbauer technique to reveal the spin structures in exchange coupled magnetic phases containing iron is emphasized. Experimental results obtained on layered exchange-bias systems are presented. The influence of the ferromagnetic top layer on the out-of-plane spin component of the pinning antiferromagnetic-like layer, in both Fe/FeSn2 and Fe/Fe-Gd-B exchange bias systems, is discussed.

Recently, a double perovskite oxide, Sr2FeMoO6, has been reported to show a very sharp magnetoresistance (MR) response at relatively small-applied fields and at high temperatures. The half-metallic state of this compound ensures a theoretical magnetic moment of 4 mu(B) per formula unit. Ordering control is expected to be influenced by the synthesis route. In this regard, in this article, perovskite samples were obtained from the liquid phase, using oxalic acid as complexing agent. Application of a field above 0.08 T increases the zero-field-cooled (ZFC) magnetization. Below this field, for the ZFC-curves measured from 2 K up to 400 K, magnetization increases, reaching a peak at T-cups(ZFC), before decreasing cups gradually to a paramagnetic state. Saturation moment is relatively low (2.1-2.2 mu(B)), probably due to anti-site defects, as suggested in the literature.

GaAs-based Gunn diodes with graded AlGaAs hot electron injector heterostructures have been developed under the special needs in automotive applications. The fabrication of the Gunn diode chips was based on total substrate removal and processing of integrated Au heat sinks. Especially, the thermal and RF behavior of the diodes have been analyzed by DC, impedance and S-parameter measurements. The electrical investigations have revealed the functionality of the hot electron injector. An optimized layer structure could fulfill the requirements in adaptive cruise control (ACC) systems at 77 GHz with typical output power between 50 and 90 mW.

In the present study manganese doped alumina and silica gels and thin films deposited on different substrates (refractory metals, silicon wafers) were obtained by the alkoxide and aqueous routes of the sol-gel method. The characterization of the films was realized by X-ray diffraction (XRD), and spectroellipsometry (SE) methods. The un-supported gels (obtained by the gelation of the solutions used for the film deposition) were characterized by DTA/TG analysis, IR spectroscopy, transmission electron-microscopy and electron diffraction (TEM and SAED) and BET method (adsorption of krypton at temperature of liquid nitrogen) for surface area and pores volume determination. The sol-gel materials were tested as catalysts for the ozone decomposition.

Positron annihilation lifetime spectroscopy (PALS) is tested for use as an alternative experimental probe of free volume concepts in network chalcogenide glasses. PALS measurements are carried out within binary As-Se and ternary GeAs(Sb)-S systems on the examples of g-As2Se3 and samples from stoichiometric As(Sb)(2)S-3-GeS2 and nonstoichiometric As(Sb)(2)S-3-Ge2S3 cross sections. The effect of high energy gamma-irradiation on PALS parameters is studied for ternary glass compositions. The role of induced defect related positron traps in annihilation processes is illustrated. The correlations between nanovoid sizes obtained from PALS, Monte Carlo simulation and first sharp diffraction peak (FSDP) measurements in the framework of Elliott's model are established on the example of g-As2Se3, in order to study the nanovoid topology in the real glass structure. It is shown that nanovoids with average radius of 2.9 angstrom, being effective traps for positrons with 0.37 ns defect related lifetime, are also detected within nanovoid distribution data obtained from Monte Carlo simulation. Suggesting that this group of nanovoids is responsible for the void based origin of FSDP, the analytical relationship between FSDP position and nanovoid diameter is proposed for layer like As2X3-type structures.

We report the synthesis and characterization of new hybrid materials based on conducting polypyrrole (PPY) and multi wall carbon nanotubes (MWCNTs). MWCNTs were prepared by spray-pyrolysis of liquid hydrocarbon-ferrocene solution in an Ar atmosphere. The composites (PPY-CNT) were obtained by in-situ chemical oxidative polymerization of pyrrole in aqueous solution containing MWCNTs. The addition of a water based Fe3O4 nanofluid to the polymerization solution results in the formation of a new hybrid nanostructure of MWCNTs coated with PPY containing magnetic nanoparticles. The properties of PPY composites were investigated by TEM, SEM, FTIR spectroscopy and X-ray diffraction (XRD) measurements. MWCNTs covered with PPY have a rough surface, containing some globular forms typical for the polymer. Significant differences between IR spectra for PPY and PPY-CNT nanocomposites appear for the bands ascribed to pyrrole ring vibrations, suggesting that an interaction between the polymer and CNT occurs and this could change the polymer conformation. XRD analysis shows that the crystalline structure of MWCNTs doesn't change significantly by the association with conducting PPY.

The nature of the out-of-plane dissipation was investigated in underdoped Y0.54Pr0.46Ba2Cu3O7-delta single crystals at temperatures close to the critical temperature. For this goal, temperature- and angle-dependent out-of-plane resistivity measurements were carried out both below and above the critical temperature. We found that the Ambegaokar-Halperin relationship [V. Ambegaokar and B. I. Halperin, Phys. Rev. Lett. 22, 1364 (1969)] depicts very well the angular magnetoresistivity in the investigated range of field and temperature. The main finding is that the in-plane phase fluctuations decouple the layers above the critical temperature and the charge transport is governed only by the quasiparticles. We also have calculated the interlayer Josephson critical current density, which was found to be much smaller than the one predicted by the theory of layered superconductors. This discrepancy could be a result of the d-wave symmetry of the order parameter and/or of the non-BCS temperature dependence of the c-axis penetration length.

Lanthanum and niobium doped PZT with composition (Pb0.93La0.07)[(Zr0.60Ti0.40)](0.9825)Nb0.0175O3 (PZTLN) was prepared by the gel-combustion method. A precursor sol was obtained from lead nitrate, zirconyl nitrate, lanthanum oxide, peroxo-citrato-niobium and a peroxo-citrate complex of titanium isopropoxide as starting precursors. Various molar ratios of citrate/nitrate (CA/NO3- = 1.3, 0.36 and 0.09) were used to prepare very fine powders of PZTLN. The gels resulting from these sols were transformed into powders by an auto-combustion process at,< 400 degrees C. The powders consisted of rhombohedral PZT (PbZr0.60Ti0.40O3), pyrochlore (Pb2Ti2O6) and lead carbonate ((Pb2OCO3)-C-.) phases. The pure rhombohedral phase is found in PZTLN pellets sintered at 1100 degrees C for all citrate/nitrate ratios. Titanium and niobium precursors were modified with peroxo radicals. During the gel-combustion reaction, the temperature of the gel increases, leading to lead evaporation. The loss of lead as well as the particle size increases as the CA/NO3- ratio decreases. The smallest grained powder (about 50 nm) was obtained with the ratio CA/NO3- equal to 0.09.

The molecular dynamics of 4-n-octyl-4'-cyanobiphenyl (8CB) confined to the nanopores of new SBA-type molecular sieves was investigated in a wide temperature range using broadband dielectric spectroscopy (10(-2)-10(9) Hz). One molecular sieve has a hexagonal structure of the pores while the other is a cellular nanoporous material. To explore the extent of surface interaction effects a high and a low filling degree were considered. For the molecular sieves with a high filling degree two relaxation regions were observed: a bulk-like relaxation process related to molecules, which behave as mesophase, located in the centre of the pores. The second relaxation process has a much lower relaxation rate than the former and is assigned to molecules located in a surface layer. The temperature dependence of its relaxation rates follows the Vogel-Fulcher-Tammann law, characteristic for glassy dynamics. For samples with a low filling degree only one relaxation process due to the surface layer was observed. Moreover, especially at the temperatures lower than the melting point of bulk 8CB, its relaxation rate is situated between the characteristic frequencies of the two relaxation processes observed for the pores with a high filling degree. This behaviour gives a measure of the extension of the influence of the wall on the neighbouring 8CB molecules. In addition, the differences revealed by the molecule dynamics inside the two types of nanoporous materials are related to both surface interactions and geometrical constraints. (c) 2005 Elsevier Inc. All rights reserved.