National Institute Of Materials Physics - Romania

We present the preparation, structural and magnetic properties of nanosized magnetite obtained by the crystallization of a series of Fe-containing borosilicate glasses. Several compositions with the ratio Fe2O3/SiO2 spanning from 0.37 to 0.67 were investigated as a function of two nucleators Cr2O3 and P2O5, respectively, and modifiers and intermediates (Al2O3 and MgO). Mossbauer spectroscopy revealed the degree, the type and the location of disorder induced by a specific composition and nucleators. In addition to magnetite, it was also revealed the presence of large amounts of Fe-rich paramagnetic phases. The magnetic response is analysed in relation with the amount of Fe ions which remain dispersed in the glassy matrix as noninteracting (paramagnetic) ions. We discuss the role of the nucleators on the disorder in both tetrahedral and octahedral sites of the magnetite.

The complementary investigation techniques, Mossbauer spectroscopy, transmission electron microscopy with selected area electron diffraction (TEM/SAED), X-ray diffraction (XRD) have been used to investigate the fate of the Valea lui Stan, Romania, gold-ore nanoscale-minerals during the long time of residence in the waste dumps. The preliminary investigations showed such waste dumps to contain significant amount of metals which cannot be identified by conventional methods. An intense research activity started up in order to evaluate the possibilities to recycle Valea lui Stan waste dumps and to recover metals by chemical or phytoextraction procedures. The waste dumps naturally show different mineral constituents with clay minerals as major phases, observed by XRD-technique. Although the waste dumps materials have whitish-yellowish colours, MOSSBAUER technique evidences the presence of the finely dispersed iron bearing minerals. The authors are focusing to inspect and analyze Fe-compounds in the samples collected from Valea lui Stan's waste dumps in order to identify the magnetic phases by Mossbauer technique.

Iron doped (0.25-7.5% molar) hydrothermal nano-SnO2 was characterized by electron magnetic resonance (EMR) and Mossbauer spectroscopies. Only a small fraction of transition metal ions are in magnetic ordered state, contrary to the similar crystallographic compound TiO2. Temperature dependences of spectra suggest that by increasing iron concentration, or annealing temperature, iron ions migrate to nanoparticles surfaces forming disordered iron oxides.

The influence of Au nanoparticles dimension on the photocatalytic performances of the TiO2 aerogels-Au composites was evaluated. Structural and morphological peculiarities of the TiO2 aerogel, unloaded and loaded with 5 and 22 nm Au nanoparticles, were studied by transmission electron microscopy technique, X-ray diffraction and N-2-sorption measurements. UV-Vis diffuse reflectance measurements were performed to determine the band gap energies. The photocatalytic activity was evaluated by monitoring the salicylic acid photodegradation. It was found that the Au nanoparticles promote the anatase phase crystallization and produce the decrease of the specific surface area and band gap energy values. The porous composite with the smallest Au nanoparticles dimension exhibits the best photocatalytic performances.

The synthesis of a new garnet, Er3Li0.275Nb0.275Ga4.45O12 (ErLNGG), by solid-state reaction, is reported. A comparison of the fluorescence spectra of Er3+ in ErLNGG, Er-doped (0.1 at.%) calcium lithium niobium gallium garnet (CLNGG), and Er(50 at.%):CLNGG is presented. The narrowing of luminescence lines suggests an ordering of the partially disordered CLNGG with the increase of erbium concentration.

A new HA/Y2O3:alpha Al2O3 system was obtained by the deposition of a hydroxyapatite thin layer (similar to 150 nm), by magnetron sputtering, onto sintered substrates of low yttria (150 ppm) doped alumina. The yttria doped alumina was synthesized from very pure nanopowders, by mechanical homogenization, followed of pressing, sintering and different annealing regimes, in order to obtain bodies with various degree of nano-micro-structuration. TEM/HRTEM, SEM and EDS morpho-compositional analysis techniques were used to characterize the yttria doped alumina substrate and the hydroxyapatite superficial layer. The interfacial bonding strength between the sintered substrate and the hydroxyapatite thin film was analyzed by SEM. The evolution of the grains growth of the Y2O3:alpha Al2O3 substrate samples, with and without the hydroxyapatite covering layer is observed and discussed. A strong structural interdependence between the covering layer and the sintered substrate was emphasized. The grains' growth behaviour of the studied samples showed a linear evolution. A mathematical model for the grain growth process description was built, the grain size being interpreted as a random variable having a log-normal or Weibull probability density function.

We report on the synthesis of electrode layers by Pulsed Laser Deposition technique. We selected Nickel Oxide-8Yttria-Stabilized Zirconia as anode, La0.6Sr0.4Co0.2Fe0.8O3-delta perovskite material as cathode, and Yttria doped (Ceria-alpha Alumina) as electrolyte. We studied electrode structure (average grain size, porosity, and grain shape), the deposition uniformity and electrode-electrolyte interfaces. The electrodes were investigated in terms of morphology (Scanning Electron Microscopy) and composition (Energy Dispersive X-ray Spectroscopy). The initial powder and the synthesized cathodes have been investigated superficially and in bulk by Mossbauer spectroscopy, in order to detect and analyse local properties. We identified the conditions for electrode layer synthesis in respect with porosity, composition, and mean grain size.

N-type P-doped silicon single crystals with resistivity higher than 8000 Omega cm were irradiated with I-127(6+) ions of 28 MeV kinetic energy. The penetration of the ions through the target and the processes of energy loss were simulated using the CTRIM Monte Carlo code, and point defect production was calculated in the frame of our diffusion-reaction model. Trapping phenomena were investigated using the method of thermally stimulated currents without applied bias. The modeling of the current-temperature curves takes into consideration both point defects and stress-type trapping centers, produced by the ions stopped into the crystal.

This paper reports on the conduction mechanisms in amorphous SiO2 films with embedded Ge nanoparticles. For this, measurements of current-temperature and current-voltage were employed and correlated with the microstructure results obtained from transmission electron microscopy (TEM). TEM images reveal that our films contain big Ge nanoparticles with low density and small Ge nanoparticles with high density, the last ones being the only responsible for the electrical transport. Two transport mechanisms were found at low and high temperature respectively, namely hopping on localized states in a band near Fermi level and charge excitation to the extended states at mobility edge.

In this paper we continue the previous investigations on nanostructured GexSi1-x films. The films were deposited by magnetron sputtering and annealed in N-2 atmosphere at 700 degrees C. Their structure was investigated and correlated with the electrical behavior. For this, conventional and high-resolution transmission electron microscopy together with selected area electron diffraction was used. Electrical measurements of current-voltage and current-temperature curves were made. The majority of crystallites that forms the films have the composition Ge50Si50 and 15 - 30 nm size. The I - T characteristics have Arrhenius dependence, with two activation energies interpreted as transitions between quantum confinement levels.