National Institute Of Materials Physics - Romania

Graphene nanopowder (G) with average thickness particle size of about 6-8 nm was added to MgB2 commercial powder. Starting composition was (MgB2)((1-x))(G)(x),x = 0.0125, 0.025, 0.05. Processing was performed by Spark Plasma Sintering (SPS) technique. All added samples have high density (above 95%). The critical temperature (T-c) and the lattice parameter a (c-axis lattice parameter is constant) show a small variation suggesting that carbon substitution for boron is low. TEM observations show the presence of un-reacted graphene plates supporting the T-c and structural results. It also indicates that G-addition does not modify the MgB2 microstructure. Despite this, there is an optimum doped sample (MgB2)(0.9875)(G)(0.0125) for which the critical current density at temperatures below 25 K is slightly higher at high magnetic fields than for the pristine sample. The addition of G is found as one of the least effective C-source additions enhancing J(c). We discuss results as being strongly related to variation of the residual stress. (C) 2016 Elsevier Ltd. All rights reserved.

The electrochemical deposition of ZnxTey thin films was studied in an ionic liquid consisted in choline chloride - ethylene glycol eutectic mixture (Ethaline). ZnCl2 and TeO2 precursors were dissolved in 5-24 millimolar concentrations and the temperature was maintained constant at 60 degrees C. Cyclic voltammograms on Pt from electrolytes with both Te4+ and Zn2+ ions displayed three cathodic processes as limiting currents or peaks: Te underpotential deposition, codeposition of ZnTe compound, and deposition of Zn-rich binary telluride. The corresponding anodic waves or peaks were also identified. The shape of Nyquist electrochemical impedance plots recorded on various applied polarization showed that the electrode first covers with a Te film, and then the more cathodic polarization produces a co-reduction of Te4+ with Zn2+ as ZnTe compound. The formation of Zn metal-rich ZnxTey film at excessive negative polarization is indicated by the lowest value of maximum phase angle in Bode plots. The Zn/Te ratio (apparent stoichiometry) in the films deposited on Cu substrate is controlled by carrying out constant potential electrolyses with different ionic ratios in Ethaline bath. Film morphology, elemental analysis, and crystalline structure were evidenced by SEM microscopy, EDX analysis and X-ray diffraction, respectively.

We investigate theoretically the spectral and dynamical effects of the short-range exchange interaction between a single manganese (Mn) atom hosted by cylindrical CdTe quantum dots and its light-hole excitons or biexcitons. Our approach is based on the Kohn-Luttinger k . p theory and configuration interaction method, the dynamics of the system in the presence of intraband relaxation being derived from the von Neumann-Lindblad equation. The complex structure of the light-hole exciton absorption spectrum reveals the exchange-induced exciton mixing and depends strongly on the Mn position. In particular, if the Mn atom is closer to the edges of the cylinder, the bright and dark light-hole excitons are mixed by the hole-Mn exchange alone. Consequently, their populations exhibit exchange-induced Rabi oscillations which can be viewed as optical signatures of light-hole spin reversal. Similar results are obtained for mixed biexcitons, in this case the exchange-induced Rabi oscillations being damped by the intraband hole relaxation processes. The effect of light-hole heavy-hole mixing is also discussed.

We report for the first time the microwave characterization of 0.92(Bi0.5Na0.5)TiO(3-)0.08BaTiO(3) (BNT-BT0.08) ferroelectric thin films fabricated by the sol-gel method and integrated in both planar and out-of-plane tunable capacitors for agile high-frequency applications and particularly on the WiFi frequency band from 2.4 GHz to 2.49 GHz. The permittivity and loss tangent of the realized BNT-BT0.08 layers have been first measured by a resonant cavity method working at 12.5 GHz. Then, we integrated the ferroelectric material in planar inter-digitated capacitors (IDC) and in out-of-plane metal-insulator-metal (MIM) devices and investigated their specific properties (dielectric tunability and losses) on the whole 100 MHz-15 GHz frequency domain. The 3D finite-elements electromagnetic simulations of the IDC capacitances are fitting very well with their measured responses and confirm the dielectric properties determined with the cavity method. While IDCs are not exhibiting an optimal tunability, the MIM capacitor devices with optimized Ir/MgO(100) bottom electrodes demonstrate a high dielectric tunability, of 30% at 2.45 GHz under applied voltages as low as 10 V, and it is reaching 50% under 20V voltage bias at the same frequency. These high-frequency properties of the MIM devices integrating the BNT-BT0.08 films, combining a high tunability under low applied voltages indicate a wide integration potential for tunable devices in the microwave domain and particularly at 2.45 GHz, corresponding to the widely used industrial, scientific, and medical frequency band. (C) 2016 AIP Publishing LLC.

We report on the fabrication of n+3C/n-4H SiC heterojunction diodes (HJDs) potentially promising the ultimate thermal stability of the junction. The diodes were systematically analyzed by TEM, X-ray diffraction, AFM, and secondary ion mass spectroscopy, indicating the formation of epitaxial 3C-SiC crystal on top of 4H-SiC substrate with continuous interface, low surface roughness, and up to similar to 7 x 10(17) cm(-3) dopant impurity concentration. The conduction band off-set is about 1 V as extracted from CV measurements, while the valence bands of both SiC polytypes are aligned. The HJDs feature opening voltage of 1.65 V, consistent with the barrier height of about 1.5 eV extracted from CV measurement. We finally compare the electrical results of the n+3C/n-4H SiC heterojunction diodes with those featuring Si and Ge doped anodes in order to evaluate current challenges involved in the fabrication of such devices. (C) 2016 AIP Publishing LLC.

Mesoporous carbon aerogels (CAs) impregnated with (Pt-Ru) nanoparticles were prepared, incorporated into carbon paste electrodes (CPEs) and investigated as electrocatalysts for CH3OH electro-oxidation. The sol gel method, followed by supercritical drying with liquid CO2 and thermal pyrolysis in an inert atmosphere, was used to obtain high mesoporous CAs. (Pt-Ru)/CAs nanocomposites with various (Pt-Ru) loading were prepared by using Ru(AcAc)(3) and H2PtCl6 as metal precursors and the impregnation method. The morpho-structural peculiarities of the so prepared (Pt-Ru)/CAs electrocatalysts were examined by using elemental analysis, N2 adsorption -desorption isotherms, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray (EDX) and selected area electron diffraction (SAED). Cyclic voltammetry measurements, carried out at (Pt-Ru)/CA-CPEs incorporating nanocomposites with various Pt-Ru loading and different specific surface areas, showed that CA with the highest specific surface area (843 m(2)/g) and impregnated with 6% (w/w) (Pt Ru) nanoparticles exhibit the best CH3OH electro-oxidation efficiency. The Michaelis Menten formalism was used to describe the dependence of the oxidation peak current on the CH3OH concentration, allowing the estimation of the modified electrodes sensitivities. Thus, for (Pt-Ru, 10%)/CA(535)-CPE was observed the highest sensitivity (12.5 +/- 0.8 mA/M) and, at the same time, the highest maximum current density ever reported (153.1 mA/cm(2) for 2 M CH3OH and an applied potential of 600 mV vs. SHE). (C) 2016 Elsevier B.V. All rights reserved.

This study investigated the mechanism of iron monosulfide (FeS) oxidation by dissolved oxygen (O-2(aq)). Synthetic FeS was reacted with O-2(aq) for 6 days and at 25 degrees C. We have characterized the initial and reacted FeS surface using Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM/EDX) analysis, Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). It was found that during the aqueous oxidation of FeS new solid phases (disulfide, polysulfide, elemental sulfur, ferric oxyhydroxides and Fe3O4) develop on the mineral surface. The results of potentiodynamic polarization experiments show that after 2 days of FeS electrode immersion in oxygen bearing solution (OBS) at initial pH 5.1 and 25 degrees C the modulus of cathodic Tafel slopes dramatically decreases, from 393 mV/dec to 86 mV/dec. This decrease is ascribed to the change of the mechanism of electron transfer from cathodic sites to O-2 (mechanism of cathodic process). The oxidation current densities (j(ox)) indicate that mineral oxidative dissolution is not inhibited by pH increase up to 6.7. Another conclusion, which emerges from the analysis of j(ox), is that the dissolved Fe3+ does not intermediate the aqueous oxidation of FeS. The results of electrochemical impedance spectroscopy (EIS) show that after 2 days of contact between electrode and OBS the properties of FeS/water interface change. From the analysis of the EIS, FTIR spectroscopy, Raman spectroscopy and SEM/EDX data we can conclude that the change of FeS/water interface properties accompanies the formation of new solid phases on the mineral surface. The new characteristics of the surface layer and FeS/water interface do not cause the inhibition of mineral oxidation. (C) 2016 Elsevier Inc. All rights reserved.

In melt-spun FePtB-based ribbons, the addition of Ag has been proven to decrease the temperature of phase transformation from the A1 fcc FePt phase to the hard magnetic tetragonal L1(0) phase. Alloys with 6 and 9 at.% Ag added to the initial FePtB have been synthesized by rapid solidification from the melt. The samples have been laser irradiated and submitted to nitriding procedure. This procedure has been proven beneficial for inducing complete transformation of A1 to L1(0) phase and a strong (001) texturing. Ag segregation combined to mechanisms of creation of vacancies and diffusion of N give rise to the formation of an intergranular disordered region and due to an improved interfacial coupling between FePt grains, enhanced coercivity and two-phase magnetic behavior is obtained.

Thin films of Bi2Sr2CaCu2O8 (Bi-2212) with the non-c-axis (117) orientation were grown by MOCVD on (110) LaAlO3 single crystal substrate. XRD theta - 2 theta scans show that films contain also (119) and (011) Bi-2212 impurity grains. We propose and report characterization of the non-c-axis films by XRD phi-psi scans. By this approach, the assumed theoretical film-substrate relationship of the (117) Bi-2212 grains is demonstrated experimentally. The result also confirms twinning in the span rooflike (117) Bi-2212 grains. The impurity (119) and (011) Bi-2212 grains are also rooflike and in-plane aligned according to film-substrate relation and AFM images so that the film can be considered in-plane epitaxial.

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.