National Institute Of Materials Physics - Romania

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.

The carrier transport mechanisms in as-grown LaAlO3 and La2Hf2O7 high-k insulator layers deposited on n- and p-Si were deduced from temperature dependent C-V and I-V characteristics correlated with photoelectric measurements. Large, parallel shifts in the high frequency C-V curves are explained by the presence of a large density of interface states and an approximate analytical formula relating the density of states to the C-V shift is deduced. The space charge limited current is explained by the existence of impurity channels situated energetically near the conduction or valence band of silicon.

Fatigue is investigated in epitaxial Pb(Zr,Ti)O-3 films grown on SrRuO3/SrTiO3 substrates with Pt or SrRuO3 (SRO) top electrodes. It was experimentally determined that fatigue occurs irrespective of whether the top electrode is Pt or SRO. The fatigue behavior is strongly dependent on the frequency. A polarization recovery was observed for both types of top electrodes, but the recovery is almost complete for a SRO top electrode and only about 40% from the initial polarization value for Pt top electrodes. The results are tentatively explained by the frequency response of the deep traps and by migration of oxygen vacancies.