National Institute Of Materials Physics - Romania

We investigated the transport properties of a series of composites made of La2/3Sr1/3MnO3 and different copolymers. The temperature dependence of the electrical resistance of the samples is strongly dependent on the nature of the polymers. We found four types of temperature dependences of the electrical resistance: i) simple current dependence of resistivity for poly(methyl-methacrylate-co-styrene) copolymers, ii) single peaked characteristics for poly(acryl amide-co-vinyl acetate), iii) multipeaked dependence found in poly(methyl-methacrylate-co-butadiene) and star type polysiloxane-g-styrene, and iv) current independent characteristics visible in the samples made with linear polysiloxane-based composites.

For power applications of superconducting films. the critical current density (J(c)) and the thickness of the film (d) should be as high as possible. Since J(c) decreases with both thickness and magnetic field, artificial pinning centres in addition to natural ones are required to keep J(c) high. The earliest cost-effective method used for introducing artificial pinning centres was the so-called substrate decoration, i.e., growing nanoscale islands (nano-dots) of certain materials on the substrate prior to the deposition of the superconducting thin film. Later on another version of this approach proved to be successful: building up a layered distribution of a second phase using a multilayer deposition (quasi-superlattices). Several materials have been used for the Creation of artificial pinning centres. Here we report on the artificial pinning centres induced in YBCO thick films by substrate decoration and quasi-superlattice approaches using nano-dots of Pd and non-superconducting YBCO. The cross-sectional AFM images show evidence of c-axis correlated columnar defects. These defects significantly contribute to the pinning of magnetic flux and increase critical current in the films. We observed an important shift of the position of the maximum in the thickness dependence of J(c)(B) towards higher thicknesses compared with pure YBCO films by both approaches. A high J(c)(B) in our quite thick films provides a very high total critical current per cm of the film width. Critical current as high as 800 A/cm width was achieved in a 2.4 mu m thick quasi-superlattice film with non-superconducting YBCO nano-dots. (C) 2009 Elsevier B.V. All rights reserved.

Sr2FeTaO6 and Sr2Fe0.7Co0.3TaO6 double perovskites, unsupported and supported on gamma-Al2O3 were prepared, characterized and studied in the catalytic combustion of methane, as a test reaction for volatile organic compounds (VOCs) destruction. These mixed oxides, with double perovskite structure, present good catalytic activities, and after their dispersion - 5 % (wt.) - on gamma-Al2O3 support, the reaction rates increased compared with the unsupported samples.

Superconducting SiC and B4C-doped MgB2 samples containing up to three layers of pure Fe have been prepared by Field-Assisted-Sintering Technique (FAST). The adhesion between the Fe and doped-MgB2 layers is excellent due to the enhanced local interdiffusion and increased solubility of Fe into MgB2 grains that are typical for the non-equilibrium processes. Magnetic measurements showed the largest irreversibility, hence, the largest critical current density in the B4C-doped sandwich with one Fe foil. Additionally, the drawback of flux jumps at low temperatures is almost absent for one Fe-layer sandwiches. However, the irreversibility is smaller than in pure MgB2 and SiC-doped MgB2 samples prepared by the same technique, but without the insertion of Fe foil. When the number of Fe foils is increased to three and an excess of 5 % wt. Mg is added, the SiC-doped sandwiches show almost similar irreversibility values and field dependence (at 5 K and at 20 K) as the pristine MgB2 samples without Fe. But, the flux jump is recovered at the same level found for the pristine MgB2 without Fe. An explanation in terms of doping dependence of the ratio between the thermal and magnetic diffusion constants is proposed.

A novel method for the synthesis of nanostructured films produced by depositing gas-phase magnetic nanoparticles is presented and the properties of the films are reported. The technique mixes metal vapour and small argon clusters produced in a supersonic expansion. The condensed clusters are subsequently deposited in situ onto copper grids. The cluster size is controlled by the vapour pressure of the metal inside the pick-up chamber. Detailed analysis of the transmission electron micrographs of the Fe clusters shows that there is a simple linear relationship between the average metal cluster diameter and the metal vapour pressure during deposition. Furthermore, the nanoparticles show a relatively narrow size distribution for a given set of experimental conditions. Structural and magnetic investigations have been performed on Fe cluster samples, and the influence of the metal vapour pressure has been studied. Detailed analysis of the magnetic and structural data has been performed and valuable information such as cluster size distributions, strength of the interparticle dipolar interactions and average magnetic moment per cluster are derived. It is shown that, at room temperature, the magnetic behaviour of the films is consistent with nanoparticle supermoments interacting via dipolar interactions.

We measured directly the depth-dependent Fe spin rotation upon magnetization reversal in exchange-coupled Fe/MnF2 bilayers using nuclear resonant scattering of synchrotron radiation from an Fe-57-probe layer buried at different depths within the Fe film. Our results show that the exchange-biased ferromagnetic layer develops a noncollinear spin structure along the film normal direction, reminiscent of a partial domain wall parallel to the Fe/MnF2 interface. This is contrary to most theoretical models of exchange bias which assume a collinear spin structure in the ferromagnetic layer.

The relaxation of the irreversible magnetization of MgB2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H alpha T-2 in the low T range, which appears to be 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 easily be explained through the occurrence of micro flux jumps, leading to a finite magnetization relaxation rate in the low-T limit. (C) 2008 Elsevier B.V. All rights reserved.

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.

Bulks and tapes of MgB2 with 5 wt.% SiC were prepared from Mg, B and SiC. Mixing of the raw powders was performed in Ar atmosphere by high energy milling using a planetary mill. Number of balls (10 or 20), size (10 mm or 7 mm, respectively), rotation speed (200 rpm or 400 rpm) and milling time (up to 3 h) were varied. Powders were compared through XRD and SEM. From the as-prepared powders, bulk and tape samples were prepared and investigated by magnetic (SQUID) and transport (four-probe method) measurements. Results suggest that through variation of the milling conditions such as rotation speed, size and number of balls it is possible to control the value of the critical current density, J(c) and the slope of the J(c)(B) curve, For different heat treatment conditions milling with 400 rpm and using 20 balls of 7 mm always resulted in lower J(c)(B) than for the tapes or bulks produced from hand-milled mixture. Use of 10 balls of 10 mm keeping all the other conditions constant changed the slope of the J(c)(B) curve possibly improving J(c) above a crossover at 4 T. Mild milling with a rotation speed of 200 rpm (20 balls x 7 mm) resulted in enhancement of J(c) in the entire investigated region B <= 14 T, but the slope was steeper than for the best sample produced from the hand-milled mixture and a crossover is likely to occur for B > 14 T leading to lower J(c)-values. The best samples had at 4.2 K J(c) = 3.6 x 10(-2) A center dot cm(-2) at 14 T and J(c) = 5 x 10(3) A center dot cm(-2) at 10 T.