National Institute Of Materials Physics - Romania

High-temperature superconducting objects of Bi2Sr2CaCu2O8 and YBa2Cu3O7 highly curved in the ab-plane, such as curved/ kinked whiskers, bows and ring-like structures, were obtained within a solid-liquid-solid (SLS) grass-like growth mechanism. As-grown objects are crystals with three-dimensional epitaxy similar to conventional single crystals: they can be viewed as crystal parts 'cut' from a conventional rectangular crystal. Between our curved objects and conventional crystals, whiskers or thin films there are some differences in the superconducting properties induced only by the shape factors and no new physics is observed. Some details of the growth mechanism are discussed, emphasizing curved-line formation.

Mesoporous carbon frameworks were synthesized using the soft-template method. Ca(BH4)(2) was incorporated into activated mesoporous carbon by the incipient wetness method. The activation of mesoporous carbon was necessary to optimize the surface area and pore size. Thermal programmed absorption measurements showed that the confinement of this borohydride into carbon nanoscaffolds improved its reversible capacity (relative to the reactive portion) and performance of hydrogen storage compared to unsupported borohydride. Hydrogen release from the supported hydride started at a temperature as low as 100 degrees C and the dehydrogenation rate was fast compared to the bulk borohydride. In addition, the hydrogen pressure necessary to regenerate the borohydride from the dehydrogenation products was reduced.

Intermetallic Fe-Sn and nanocrystalline metallic Sn nanoparticles have been successfully synthesized from organic precursors using the laser pyrolysis technique with ethylene as sensitizer. Nano-structured Sn (single phase) was prepared by the pyrolysis of Sn(CH3)(4) (TMT) vapors. Controlled Fe/Sn atomic ratios, ranging from 0.69 to 1.64 were obtained for the prepared Fe-Sn nanopowders by the control of Fe(CO)(5) and TMT flows, respectively. XRD studies evidence three main phases: the tetragonal metallic Sn phase and the intermetallic FeSn2 phase and, to a much lesser extent, the cubic ternary carbide Fe3SnC. Complex core-shell structural characteristics were found by HRTEM analysis. More complete information about the Fe phase distributions in the new intermetallic Fe-Sn nanomaterial is provided by temperature dependent Fe-57 Mossbauer spectroscopy. (c) 2012 Elsevier B.V. All rights reserved.

We investigate the role of chromium and phosphorous oxides on the growth and magnetic properties of crystallized Fe-containing borosilicate glasses as well as their evolution under different thermal processing. The glasses have the ratio between SiO2 and Fe2O3 in the range 1.49 to 2.68 and Al2O3/MgO as intermediate/modifier oxides. X-ray diffraction and Mossbauer spectroscopy data revealed the presence of magnetite as the major crystalline phase in all as-prepared samples but additional amounts of hematite and of Fe-rich paramagnetic phases are also visible in some samples. The magnetic response is correlated with the disorder observed in the different sites (tetrahedral and octahedral) of the magnetite.

Unconventional in situ synthesis of MgB2 undoped and doped with fullerene (C-60) was performed by using a single-mode microwave (MW) furnace. Structural properties suggested that C substitutes for B in the crystal lattice of MgB2, while the J(c) showed typical enhancement at high magnetic fields when C chemical doping occurred. This behavior was similar to published results for another field-activated unconventional method of processing, i.e., spark plasma sintering, and it was opposite to the conventional powder-in-tube method for which chemical substitution was not realized when using addition of C-60. Certain morphologically unique features such as the occurrence of needlelike grains were revealed in the MW samples. Our MW samples showed higher relative density (<= 83%) than previously reported data. Magnetic relaxation experiments suggested the presence of microscopic flux jumps in the MW samples, while macroscopic flux jumps were not observed.

A simple and reproducible fabrication process for the synthesis of FeSe superconductors has been developed. The properties were characterized by X-ray diffraction, scanning electron microscope, superconductivity quantum interference device, and physical property measurement system. It showed a zero resistance superconducting critical temperature of similar to 7.9 K and an onset critical temperature of similar to 13.9 K. The upper critical magnetic field at zero temperature was estimated to be 31 T. The growth mechanism of FeSe flakes was also discussed.

In this work zinc oxide, ZnO, nanopowders were synthesized by co-precipitation method from a zinc acetate, Zn(CH3COO)(2), solution. The obtained nanoparticles were morphologically and structurally characterized by XRD diffraction, SEM, IR and UV-Vis spectroscopy. The new obtained material was investigated as an additive in coatings for improving wet scrub resistance, self-cleaning properties etc. The XRD pattern of the ZnO nanoparticles highlights the wurtzite hexagonal structure and also proves its purity. The correlations made based on their crystallization planes identified ZnO as the sole product. ZnO nanoparticles obtained by co-precipitation method have been introduced by strong shaking in the coating system; this has positively influenced the surface properties of the films.

A methodology is proposed to investigate in detail shrinkage kinetics under isothermal spark plasma sintering (SPS) conditions applied to ceramic nano powders such as Y2O3 stabilized ZrO2. To do so, mild SPS conditions were used (low temperatures and pressure, long dwell times). The extracted experimental activation energy has the value of 246 +/- 37 kJ mol(-1) and the slope of the curves on the intense densification stage is around 0.33. Results are in agreement with densification by a grain-boundary diffusion mechanism as for conventional sintering and the contribution from the specific pressure-assisted mechanisms as for hot pressing is insignificant. This result suggests that exploration of mild SPS might prove rewarding in separation and control of the sintering mechanisms leading to production of specific ceramic with new or improved functionality. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Superconducting bulks of MgB2 were obtained by an ex-situ two-temperature route applied to spark plasma sintering (SPS). Processing of samples was performed at lower temperatures than previously reported. Samples produced by the two-temperature route show a higher morphological uniformity, a higher density (above 98%), a higher Vickers hardness, and undesirable stronger microscale flux jumps, as indicated by magnetic relaxation measurements when compared to a sample obtained by the one-temperature route (95.3% relative density). At the same time, all sintered samples show approximately constant crystallite size, critical current density, irreversibility field, critical temperature, weight fraction of impurity phases (MgB4 and MgO), and the amount of carbon accidentally introduced during SPS processing. (C) 2012 Elsevier B.V. All rights reserved.