Publications

Superconducting (SC) MgB2 thin films were prepared in situ in one step by vacuum co-deposition at extremely low growth temperature (T-s similar to 110-150 degrees C) and at very low Mg deposition rate, far off from the MgB2 stability regime of the thermodynamic phase diagram. As-grown films are nanocrystalline and exhibit (002) texture, moderate surface roughness and SC onset up to T-c(on) = 18 K. This is the highest critical temperature ever observed in low-T-s as-grown MgB2 films. Our preparation method at very low T-s is favorable for the fabrication of layered SC hybrid electronic devices, where deposition and post-annealing at high temperature are detrimental to device functioning. Simple post-annealing of our films in ultrahigh vacuum enhances T-c(on) up to 22 K due to the improvement in structural properties.

The synthesis of a new garnet, Er3Li0.275Nb0.275Ga4.45O12 (ErLNGG), by solid-state reaction, is reported. A comparison of the fluorescence spectra of Er3+ in ErLNGG, Er-doped (0.1 at.%) calcium lithium niobium gallium garnet (CLNGG), and Er(50 at.%):CLNGG is presented. The narrowing of luminescence lines suggests an ordering of the partially disordered CLNGG with the increase of erbium concentration.

Output performances of passively Q-switched, composite Nd:YAG/Cr4+:YAG lasers that consisted of bonded, all-poly-crystalline ceramics Nd:YAG and Cr4+:YAG are reported. Laser pulses at 1.06 mu m with 2.5-mJ energy and 1.9-MW peak power are obtained from a 1.1-at % Nd:YAG/Cr4+:YAG ceramics that was quasi-continuous-wave (quasi-CW) pumped with a diode laser. Single-pass frequency doubling with LiB3O5 (LBO) nonlinear crystal at room temperature yielded green laser pulses at 532 nm of 0.36-mJ energy and 0.3-MW peak power, with a conversion efficiency of 0.27.

In this paper we present some comparative studies between the optical and electrical properties of the films prepared by vacuum evaporation from amidic monomeric compounds obtained from maleic anhydride and aniline derivative with different functional groups [CN; NO2], and the composite films prepared by spin-coating from a polycarbonate of bisphenol A matrix embedding clusters of these amidic monomers. We have emphasised the effect of the polymeric matrix on the properties of the synthesised monomers with the purpose to change the physical properties of the films (optical transmission, luminescence, electrical conduction, optical non-linear phenomena). The correlation between the morphology of the films and the optical and electrical properties has also been investigated. (C) 2011 Elsevier B.V. All rights reserved.

This paper discusses the properties of two arylenevinylene oligomers thin films deposited by vacuum evaporation on different substrates. The morphology of the thin films was investigated by scanning electron microscopy and atomic force microscopy, and the optical properties by UV-Vis and fluorescence spectroscopy. The optical nonlinear phenomenon of two-photon absorption induced fluorescence has been evidenced in the thin films of these compounds. We have also investigated the electrical properties of semiconductor/insulator/semiconductor (ITO/oligomer/Si) heterostructures in correlation with the contact energetic barriers, morphological and structural properties of the layers. (C) 2011 Elsevier B.V. All rights reserved.

Pentylcyanobiphenyl was used successfully as a testmolecule to show the acid sites upon some commercial nanoscaled titanium dioxide materials. The surface interactions were investigated in detail using methods sensitive to the surface or to the bulk material, e. g., X-ray photoelectron spectroscopy (XPS), infrared spectroscopy, and thermal analysis. Different oxidation states of surface Ti ions were revealed. Several species were found: the majority were bonded through the nitrogen lone pair of the cyano group, but bonding to the surface OH groups by benzene pi electrons might also appear. The N 1s lines of adsorbed 5CB were able to distinguish between Lewis acid and relatively weak Bronsted acid sites.

In this contribution we report results of our investigations on the ferromagnetic behavior of pristine and chemically treatedcarbon nanotubes, synthesized by AC arc discharge method using iron powder as catalyst. The magnetic properties of the carbon nanotubes samples before and after chemical treatment were measured at room temperature in a vibrating - sample magnetometer. We observed significant changes of the ferromagnetic behavior of carbon nanotubes after the purification and concluded that the magnetic properties of carbon nanotubes synthesized using ferromagnetic catalyst are governed by the ferromagnetic catalyst nanoparticles attached to their walls during synthesis.

A new HA/Y2O3:alpha Al2O3 system was obtained by the deposition of a hydroxyapatite thin layer (similar to 150 nm), by magnetron sputtering, onto sintered substrates of low yttria (150 ppm) doped alumina. The yttria doped alumina was synthesized from very pure nanopowders, by mechanical homogenization, followed of pressing, sintering and different annealing regimes, in order to obtain bodies with various degree of nano-micro-structuration. TEM/HRTEM, SEM and EDS morpho-compositional analysis techniques were used to characterize the yttria doped alumina substrate and the hydroxyapatite superficial layer. The interfacial bonding strength between the sintered substrate and the hydroxyapatite thin film was analyzed by SEM. The evolution of the grains growth of the Y2O3:alpha Al2O3 substrate samples, with and without the hydroxyapatite covering layer is observed and discussed. A strong structural interdependence between the covering layer and the sintered substrate was emphasized. The grains' growth behaviour of the studied samples showed a linear evolution. A mathematical model for the grain growth process description was built, the grain size being interpreted as a random variable having a log-normal or Weibull probability density function.

We present an investigation of the field dependence of the pinning force of dense MgB2-based superconducting composites as obtained by spark plasma sintering with different ingredients which are designated to enhance the pinning force F. Generally, the latter quantity, scaled to the maximum pinning force F-max, obeys a scaling law as a function of the scaled field h when the irreversibility field B-irr is considered as scaling field. The scaled function is described in terms of a generalized scaled function h(p)(1-h)(q). However, in our samples the scaling is absent. In addition, the peak of the scaled function shifts to higher reduced fields h when the temperature increases. Depending on the level of doping and the nature of the nanosized particles used to built the superconducting composites, we found that the scaled force can be depicted either with a combination of generalized scaled functions with different exponents or with a single function but multiplied with an envelope, usually an exponential factor. The former dependence is present in the samples with a high amount of nanoparticles and mirrors two types of pinning of close weights. The presence of the exponential factor in the latter dependence is attributed to the combined effect of the intrinsic anisotropy of MgB2 and the random orientation of the grains relative to the magnetic field.

In this work we design some atomic scale simulation methods as investigative tools in the study of the formation of compounds for the reversible storage of hydrogen in bulk materials. It was verified that the reaction between the LiBH4 and MgH2 is energetically favored for temperatures above 280 K and that this system can be used in the hydrogen storage and the fuel cell application. To identify the reaction mechanism at the interface of LiBH4, MgH2 and carbon layers we performed some Molecular Dynamics simulations and QM/MM calculations. The results show that the layers of ions formed at the interface with the graphite may assure the right arrangement of the atoms to start the formation of the crystals. Moreover, the presence of the hexagonal layers of graphite may play a role as a pattern template for the layers of boron atoms in the MgB2 lattice.