National Institute Of Materials Physics - Romania

We investigated the possibility of tuning luminescent spectra for a given Si/SiO2 nanoparticle size distribution in aqueous solution. The luminescence spectra of Si/SiO2 Suspensions in aqueous solutions of different particle/cluster concentrations were studied. The different Si/SiO2 nanoparticle/cluster concentrations were produced by both centrifugation and dilution. Depending on the centrifugal force we found a reversible shift up to 100 nm toward UV, regardless of the excitation wavelength. The shift could be reverse by sonication of the sample. A-scenario based on particle and cluster size variation is also discussed.

Ba0.6Sr0.4TiO3 (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance-voltage (C-V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.

NiCu thin films for magnetic applications have been produced by using an electrochemical method. Their structural and morphological properties have been investigated by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM and EDX) and are discussed in correlation with the growth conditions.

The most popular process of MgB2 wire fabrication is the so-called in situ powder-in-tube (PIT) process, which is characterized by filing metallic tubes with Mg and B mixed powder, drawing and rolling the material into wires and, finally, by applying a heat treatment. X-ray diffraction analyses were carried out for MgB2 cores and SiC-added wires. XRD patterns reveal that nearly single-phase MgB2 was obtained. Also, in this paper are presented the obtained magnetization curves.

Well-compacted MgB2 specimens with the density higher than 90 % of the theoretical value were obtained by electric-field assisted sintering. This method assures a good grain connectivity, which leads to the appearance of efficient pinning centres at the grain boundaries. We measured the DC magnetization curves and the relaxation of the irreversible magnetization using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. The critical current density is of the order of 1010 A/m(2) at H = 20 kOe and T = 10 K. A crossover plastic creep at high temperatures T - elastic creep at low T described by H proportional to T-2 in the low T - high H domain was observed. This is caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can be explained through the occurrence of micro flux jumps, which seem to be responsible for the finite magnetization relaxation rate in the low-T limit. The relaxation of the irreversible magnetization allowed us the precise determination of the characteristic pinning energy barrier.

It is shown that x-ray microtomography (XRT) is a powerful technique to observe on extended volumes the morphology, alignment and local bulk density in MgB2 tapes, thin films and especially bulk specimens. For the bulk samples obtained from mechanically milled powders for different conditions, several types of the 3D microstructural patterns were observed. These significant differences between the samples were not revealed by SEM. Microtomography can be a useful additional technique to characterize the samples and to obtain unique information, e. g. on local bulk density, but it cannot replace the existing methods: advantages and limitations are discussed. It is concluded that the method is promising and further developments are expected to be helpful for a better understanding of the complex processing-microstructure-properties relationship in the MgB2 superconductor.

CoFe2O4 and CoMn0.2Fe1.8O4 ferrites were synthesized using standard solid state phase ceramic method and coprecipitation and oxidation method. X-ray diffraction, Mossbauer spectroscopy, and surface electron microscopy were used to characterize the samples' microstructures. Vibrating sample measurement and strain gauge technique were used to characterize the magnetic and magnetostrictive properties of both powders and sintered samples, respectively. The specific magnetization increased from 55 emu/g for stoichiometric ferrite samples obtained by conventional method to 179 emu/g for coprecipitated ferite powders respectively. In the manganese substituted cobalt ferrite case the specific magnetization increased only twice, from 77 emu/g to 168 emu/g. The maximum magnetostrictive coefficient of coprecipitated samples of 145 ppm is smaller compared to that of solid state phase ceramic samples of 195 ppm. The most important benefit in the case of the coprecipitated and oxidized samples is the four fold increase of the maximum strain derivative compared to solid state phase ceramic samples.

Indium nitride is an attractive semiconductor material for optoelectronic applications, high-speed electronics and solar cells. We report successful deposition of polycrystalline InN thin films onto kapton polyimide flexible substrates by reactive RF magnetron sputtering method. The optical, structural and morphological characterization data are presented.

Star- and brush-polysiloxanes were synthesized via living radical polymerization using two different catalytic systems. The new compounds were characterized by H-1-NMR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) techniques. CuCl and 2,2'-bipyridyl proved to be the most efficient catalytic system. The type of catalytic system and the reaction conditions control the number of arms for star polymers or the grafted-chain density for brush polymers. The use of chloromethyl styrene as monomer or co-monomer yields homogeneous star- or brush-polysiloxanes, although chloromethyl styrene is an inimer system. The homogeneity of materials was verified by GPC and DSC (TOPEM) methods. For linear grafted polysiloxanes the molecular simulations reflect a tendency of chains to adopt globular conformation for high values of the branching density. The polysiloxanes grafted with styrene/chloromethyl styrene were modified with azobenzene in order to obtain systems able to react to the light stimuli. Rheological behavior of the polymer solutions were evaluated before and after irradiation with UV light. After the UV irradiation a significant viscosity increase was evidenced.

Magnetite nanoparticles coated with tyrosine and histidine were synthesized by in situ decomposition of the precursors-coordination compounds containing the aminoacids as ligands. All of the obtained compounds were characterized using infrared spectroscopy (FTIR), X-ray diffraction, Mossbauer spectroscopy, magnetic and thermal measurements. Data demonstrate the presence of the aminoacids chemisorbed on the surface of magnetite nanoparticles and their superparamagnetic character. (C) 2008 Elsevier B.V. All rights reserved.