National Institute Of Materials Physics - Romania

Neutron irradiation is a promising method for a spectacular enhancement of the critical current density of the high temperature superconductors. The suppression of the mechanisms responsible for superconductivity under irradiation are analyzed and put side by side with the effects on the flux line lattice dynamics. Additionally, the evolution of the creation/generation balance of the defects of the crystal structure is connected to the evolution of the response of the vortex system with an emphasis on the ceramic materials.

The template method was used for preparing Zn1-xCoxO nanowires with x ranging from 0.01 to 0.05. Thus, electrochemical deposition was employed for filling up the pores of polycarbonate ion track nanoporous membranes with the desired material. The method allows a good control over the morphology and composition of the deposited nanowires, as evidenced by scanning electron microscopy (SEM) and energy dispersive X ray analysis (EDX). Measurements of the magnetic properties showed a paramagnetic behavior of the nanowire arrays for the whole set of temperatures and Co concentrations.

Despite many investigations already performed with respect to sintering stability and grain growth vs. annealing conditions in low yttrium doped alpha- Al2O3 (150 ppm) - some fundamental questions are still open. These questions concern mainly the influence of small level of impurities at grain boundary in the sintering behavior and grains growth of the materials, i.e. the microstructure function of the annealing time at annealing temperature of 1650 degrees C. The low yttrium doped alpha- Al2O3 (150 ppm) samples were sintered and annealed at high temperatures. Besides conventional micro structural studies, such as grain diameter and grain size functions of distribution of the annealing temperature were studied by electron microscopy investigations and statistical measurement. The implications of the different microstructures with respect to micro structural stability have discussed and compared to observations described in the literature.

PZT type films were prepared using the thermionic vacuum arc (TVA) method; a crucible filled PZT type powder (PbO/ZrO/TiO) was heated by a thermo-electron beam emitted by the circular cathode of the TVA gun. Applying high voltage potential (3000 +/- 200V) on the anode from a regulated HEINZINGER high voltage power supply the thermo-electrons were accelerated toward the anode crucible, evaporating the anode materials and initiating a plasma discharge. The morphology and the structure of the prepared films analyzed by transmission electron microscopy, (TEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy, showed nanostructured, smooth thin films with specific PZT stoichiometry.

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.