Publications

Nanostructured powders processed by ball milling of a mixture of Fe and Fe3O4 at room temperature are shown to undergo an incomplete redox reaction with formation of FeO during the milling process. This reaction is favored by the high energy introduced during the mechano-alloying process. Concurrent effects of milling such as grain refinement down to the nanometre scale lead at the end of the milling processes to a mixed multiphase powder of nanograins, with Fe and Fe oxide grains inter-dispersed. We show that in the as-milled Fe/Fe3O4 powder, during milling process, wustite (FeO) is formed as a consequence of the redox reaction. Moreover, with increasing temperature, the system undergoes an inverse phase transformation towards the initial Fe and Fe3O4 phases until about 450 degrees C. Above this temperature the reduction reaction Fe + Fe3O4 = 4FeO is reinitiated, resulting in sharp decrease of Fe and Fe3O4 content from about 550 degrees C and almost complete disappearance of these phases at about 900 degrees C. This transformation was investigated via an energy-dispersive in situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray patterns after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. The structural and magnetic characterizations of the nanograin powders, as-milled and annealed at several temperatures, are studied using XRD, SEM and magnetic measurements. Such ferromagnetic-antiferromagnetic composites are extensively studied as they exhibit exchange bias effect, with a large impact in technological applications. The magnetic behaviour and intrinsic mechanisms leading to the occurrence of exchange bias effects are discussed and related to the samples microstructural features. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanograin powder. (C) 2011 Elsevier B.V. All rights reserved.

SrZrO3 powders are obtained by solid state reaction from SrCO3 and ZrO2 precursors, without involving intermediate calcination and grinding steps. The resulted powders are essentially within a single phase, with sub-micron average crystallite size. Pellets of these powders show a relatively poor sintering behavior, when fired up to 1600 degrees C. Alternatively, spark plasma sintering technique is used in order to obtain nearly 100% dense samples at the expense of excessive grain coarsening (i.e., up to 5 mu m in diameter). Crystalline structure, composition and morphology of the specimens obtained in this work are investigated by X-ray diffraction, scanning and transmission electron microscopy together with energy dispersive X-ray spectroscopy. (C) 2011 Elsevier B.V. All rights reserved.

Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn2+ impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed. (C) 2011 Elsevier Inc. All rights reserved.

Hydrogen adsorption in Metal-Organic Frameworks is improved by using platinum and palladium on different carbon support: activated carbon or carbon nanofibres. Although, hydrogen adsorption is directly dependent of the specific surface area of the material, it is not the only factor contributing to the hydrogen adsorption, as presented in this work. Direct synthesis of metal-organic frameworks on supported Pt and Pd catalysts, reduces the surface area significantly, but the presence of the catalysts overcomes this drawback by improving hydrogen adsorption by a factor of 2.2, in case of platinum and by a factor of 1.4 in the case of palladium, compared to the metal-organic framework without catalysts.

Superconducting Y123 thin films with (001), (100/010) and (103/110) orientations were obtained by molecular-organic- source MBE (MOMBE) for optimum growth temperatures of 600, 515 and 600 degrees C, respectively, using a Ba(DPM)(2)(phen)2 source material and ozone atmosphere. Superconductivity was obtained without post processing. According to the literature, use of a Ba(DPM)(2)(phen)(2) precursor was expected to yield a lower optimum growth temperature, but there were no significant differences between the use of this source and a Ba(DPM)(2) precursor. For our c-axis oriented Y123 film, the optimum growth temperature was higher than the reported record-low growth temperature (500 degrees C by MOCVD using Ba(DPM)2(phen)2) but it was lower than the values for other growth techniques. This can have a positive impact on fabrication of layered devices or integration through the decrease of inter-diffusion between the layers. Results also suggest that processes are complex and other details than the source type should be carefully considered.

We report on an original technique for nanoengineered pinning centers that combine three previously reported approaches: substrate decoration, quasi-multilayers and targets with secondary phase nanoinclusions. We have used a 4% BZO-doped YBCO target, and Ag nanodots, all grown by PLD. Such an approach gave interesting results in terms of pinning landscape, as proved by TEM studies. Angle-dependent measurements of critical current I-c showed that, for smaller fields, the absolute maximum in I-c occur for fields perpendicular to the a-b planes, while at larger fields the absolute maximum in I-c occurs for fields parallel to the a-b planes, in both cases with a clear second, local maxima. Measurements also showed a smooth change in the character of pinning with magnetic field, from the out-of-plane to in-plane-dominant pinning. For the out-of-plane magnetic field, the highest Ic-w (critical current per cm width) obtained so far, at 77.3 K, occurred in a 5.8 mu m Ag nanodots / BZO-doped YBCO trilayer: 782 A/cm-w in self-field, 167 A/cm-w in 1 T and 18 A/cm-w in 3 T.

Photoluminescence (PL) and Raman spectroscopy were used to characterize the tin dioxide (SnO(2)) powder and the SnO(2)/single-walled carbon-nanotube (SWCNT) composites. The SnO(2) powder shows a strong PL broad band centered at 1.95 eV (635 nm) under excitation at 280 nm (4.43 eV) and four PL bands peaking at: 1.95 eV (635 nm), 2.67 eV (464 nm), 2.82 eV (439 nm), and 2.98 eV (416 nm) under excitation at 350 nm (3.54 eV). PL decay-time measurements of the SnO(2) powder indicates a PL lifetime of about 1.53 ns. In the SnO(2)/SWCNT composites, prepared mechanicochemically, a quenching of PL is observed as the concentration of SWCNT increases. The Raman spectra were recorded at two excitation wavelengths: 514.5 and 676.4 nm. At the excitation wavelength of 676.4 nm (1.83 eV), the Raman spectrum shows a narrowed asymmetric profile of the G band (similar to 1585 cm similar to 1), which indicates that in the SnO(2)/SWCNT composite there is an interaction between SnO(2) and metallic SWCNTs. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present the fabrication and the superconducting properties of MgB2 ceramic samples doped with carbon via preceramic polymers: linear and cyclic polysiloxane-co-styrene and linear polysiloxane-co-vinyl-ferrocene. The samples were produced using the spark plasma sintering technique. The use of polysiloxane copolymers was suggested by the appropriate content of carbon, silicon, and oxygen which are necessary to increase the upper critical field and to create pinning centers. The short processing time limits the diffusion length of the chemical elements from the polymer into the superconducting grains. Therefore, in addition to doping with carbon, the rest of the components create pinning centers along the polymeric chain, able to better pin the flux lines. The copolymerization allowed us to obtain both linear and cyclic copolymers as well as to control the content of chemical elements, including the presence of small amounts of iron oxides. The latter are responsible for magnetic pinning.

In this paper, we demonstrate that, by continuous single beam excitation, one can generate an abnormal anti-Stokes Raman emission (AASRE) whose properties are similar to a coherent anti-Stokes Raman scattering (CARS). The effect has been observed in materials which possess intrinsically nonlinear properties (LiNbO(3) and CdS), which have the electric susceptibility of third order different from zero, chi((3)) not equal 0, as well as in materials that become nonlinear under resonant optical excitation. In the latter case, we used poly-3,4-ethylendioxythiophene (PEDOT) in its undoped state deposited electrochemically on Au support. Raman studies corroborated with images of optical microscopy demonstrate that the production of AASRE is conditioned by the existence of a particular morphology of the sample able to ensure efficient transport of the light inside the sample through a multiple light scattering mechanism. In this context, it was found that LiNbO3 and CdS in powder form as well as the PEDOT films layered on a rough Au substrate are suitable morphological forms. We explain AASRE as resulting from a wave-mixing mechanism of the incident laser light omega(l) with a Stokes-shifted Raman light omega(S) produced by a spontaneous Raman light scattering process, both strongly scattered inside the sample. As a CARS process, AASRE is conditioned by the achievement of phase-matching requirements, which makes the difference between the wave vectors of mixing light close to zero, Delta k = /2k(l) - k(S) - k(CARS) /approximate to 0. In condensed media, the small dispersion of the refractive index makes Delta k approximate to 0 so that the formation of a favourable phase-matching geometry may be accomplished even at a crossing angle. of travelling scattered light omega(l) and omega(S). For tightly focused beams, the requirement of phase matching relaxes; it is no longer sensitive to the Raman shift, so that a wide intense anti-Stokes Raman spectrum is observed at an angle larger than the Stokes Raman spectrum.

A passively Q-switched Nd:YAG/Cr4+:YAG micro-laser with three-beam output was realized. A single active laser source made of a composite, all-ceramics Nd: YAG/Cr4+:YAG monolithic cavity was pumped by three independent lines. At 5 Hz repetition rate, each line delivered laser pulses with similar to 2.4 mJ energy and 2.8-MW peak power. The M-2 factor of a laser beam was 3.7, and stable air breakdowns were realized. The increase of pump repetition rate up to 100 Hz improved the laser pulse energy by 6% and required similar to 6% increase of the pump pulse energy. Pulse timing of the laser-array beams can by adjusted by less than 5% tuning of an individual line pump energy, and therefore simultaneous multi-point ignition is possible. This kind of laser can be used for multi-point ignition of an automobile engine. (C) 2011 Optical Society of America