Publications

A series of TiO2 photocatalysts dispersed on SiO2, modified with metals (Mn, Cr, V) or nonmetals (N or S) or covered with Au nanoparticles were prepared by incipient wetness impregnation, impregnation with urea or thiourea, and deposition-precipitation, respectively. The photocatalysts where characterized by chemical analysis, X-ray diffraction (XRD), diffuse reflectance UV-Vis spectroscopy (DR-UV-Vis) and Fourier transform infrared spectroscopy (FTIR). The photocatalytic activity was checked in the photo-generation of hydrogen from sugar solutions. The nature of the modifying species was found to influence the photoactivity and N and S led to more effective catalysts than metals. This behavior was attributed to the fact the N and S are less effective in forming recombination centers compared with the metals. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

In this paper we present electron paramagnetic resonance (EPR) and Mossbauer (transmission method, TMS and measurements in the scattering method, CEMS) experiments on Fe-57 (0.11at.%) doped nanocrystalline anatase-type TiO2, synthesized by a hydrothermal method. Different Fe3+ and Fe2+ ions positions, with various hyperfine interactions evidenced in Mossbauer spectra (MS), confirm a partial magnetic ordering at room temperature. The magnetic hyperfine fields in the TMS spectra, better resolved at lower temperatures, do not change essentially with temperature. The first CEMS measurements, carried out on iron-doped TiO2 nanoparticles, reveal a larger disorder in the surface particles layer. The temperature dependence of the double integral EPR spectral intensity, proportional with the sample susceptibility, shows an anomalous behaviour. It suggests the bound magnetic polaron (BMP) mechanism for the magnetic ordering.

Networks of dots made of As2S3 chalcogenide has been obtained by Vacuum Evaporation and Pulsed Laser Deposition from bulk glassy As2S3 by the adapted method of SCEV (screen evaporation method). A very thin layer of Eu2O3 was deposited from a different target in the final deposition process. After a heat treatment at 180 degrees C for 90 minutes in nitrogen atmosphere a structural transformation of the amorphous As2S3 to realgar (AsS) occurred in the PLD deposited dots and the network of dots develops a luminescence effect. (Received November 25, 2011; accepted December 2, 2011)

Thin (380-510 nm) films of a low silica content bioglass with MgO, B2O3, and CaF2 as additives were deposited at low-temperature (150A degrees C) by radio-frequency magnetron sputtering onto titanium substrates. The influence of sputtering conditions on morphology, structure, composition, bonding strength and in vitro bioactivity of sputtered bioglass films was investigated. Excellent pull-out adherence (similar to 73 MPa) was obtained when using a 0.3 Pa argon sputtering pressure (BG-a). The adherence declined (similar to 46 MPa) upon increasing the working pressure to 0.4 Pa (BG-b) or when using a reactive gas mixture (similar to 50 MPa). The SBF tests clearly demonstrated strong biomineralization features for all bioglass sputtered films. The biomineralization rate increased from BG-a to BG-b, and yet more for BG-c. A well-crystallized calcium hydrogen phosphate-like phase was observed after 3 and 15 days of immersion in SBF in all bioglass layers, which transformed monotonously into hydroxyapatite under prolonged SBF immersion. Alkali and alkali-earth salts (NaCl, KCl and CaCO3) were also found at the surface of samples soaked in SBF for 30 days. The study indicated that features such as composition, structure, adherence and bioactivity of bioglass films can be tailored simply by altering the magnetron sputtering working conditions, proving that this less explored technique is a promising alternative for preparing implant-type coatings.

We report on the characterization of metallurgical phases and their magnetism at the interfaces of nanoscale MgB2/Fe layered structures. MgB2/Fe-57 multilayers with varying layer thicknesses were prepared by vacuum deposition and investigated, before and after annealing by electrical resistance measurements, x-ray diffraction and Fe-57 conversion-electron Mossbauer spectroscopy (CEMS) down to 5 K. Interfacial Fe-B phases, such as Fe2B, were identified by CEMS. A superparamagnetic-to-ferromagnetic transition is observed with increasing Fe-57 film thickness. Ultrahigh vacuum annealing at 500 degrees C of the multilayers leads to strong diffusion of Fe atoms into the boundary regions of the MgB2 layers. MgB2 in the as-grown multilayers is non-superconducting. Structural disorder and the effect of Fe interdiffusion contribute to the suppression of superconductivity in the MgB2 films of all the as-grown multilayers and the thinner annealed multilayers. However, an annealed MgB2/Fe-57/MgB2 trilayer with thicker (500 angstrom) MgB2 layers is observed to be superconducting with an onset temperature of 25 K. At 5 K, the annealed trilayer can be conceived as being strongly chemically modulated, consisting of two partially Fe-doped superconducting MgB2 layers separated by an interdiffused weakly magnetic Fe-B interlayer, which is characterized by a low hyperfine magnetic field B-hf of similar to 11 T. This chemically modulated layer structure of the trilayer after annealing was verified by Rutherford backscattering.

The problem of the two-level laser is studied analytically. The steady-state solution is expressed as a continued fraction and allows for accurate approximation by rational functions. Moreover, we show that the abrupt change observed in the pump dependence of the steady-state population is directly connected to the transition to the lasing regime. The condition for a sharp transition to Poissonian statistics is expressed as a scaling limit of vanishing cavity loss and light-matter coupling, kappa -> 0, g -> 0, such that g(2)/kappa stays finite and g(2)/kappa > 2 gamma, where gamma is the rate of nonradiative losses. The same scaling procedure is also shown to describe a similar change to the Poisson distribution in the Scully-Lamb laser model, suggesting that the low-kappa, low-g asymptotics is of more general significance for the laser transition.

Thin films of arsenic sulphide have been obtained by Pulsed Laser Deposition from bulk As(2)S(3). A very thin layer of Eu(2)O(3) was deposited by PLD from a different target in the next deposition process. After a heat treatment at 180 degrees C for 45 minutes in inert atmosphere a structural transformation of the amorphous As(2)S(3) to realgar (As(2)S(2)) occurred and the film develops a strong luminescence effect.

A comparative study of ignition realized by a classical spark plug and by a Nd:YAG laser in 12% methane-air mixture was performed. Various parameters of the ignition, such as the peak pressure, the pressure building time, or the speed propagation of the flame front were recorded at various initial filling pressure of the combustion chamber, or function of the laser pulse energy. Advantages of ignition by laser in comparison with classical ignition by an electrical spark plug are discussed.

We report here our results in the preparation of ZnO films with high UV band to band characteristic luminescence emission by potentiostatic electrodeposition. Zinc nitrate aqueous baths with different concentration and additives were employed for the preparation of the films on platinum substrates. We focused our research in determining how the electrodeposition bath composition, i.e. zinc nitrate concentration and addition of KCI or polyvinyl pyrolidone and applied overpotential influence the morphological and optical properties of the oxide films. Scanning electron microscopy was employed for characterizing the films in terms of morphology. Optical reflection, photoluminescence spectroscopy and cathodoluminescence were used for determining the optical characteristics of the samples. The morphology of the deposit varies from hexagonal prisms to platelets as a function of the deposition rate. This experimental parameter also influences the luminescence properties. We found that at low deposition rates high UV luminescent material is obtained. (C) 2011 Elsevier Ltd. All rights reserved.

We have used scanning Hall probe microscopy to investigate vortex structures and vortex dynamics in Bi2Sr2CaCu2O8+delta thin films in very low perpendicular magnetic fields. After nominally zero field cooling in the Earth's field we find that the vortices appear to be in a stable glassy state in our highly disordered samples. After applying a cancellation field of a few Oersted at low temperature, however, the system enters a new regime at very low magnetic induction when the only image contrast is due to vortices that are intermittently trapped on strong pinning centres. This state shares many of the signatures of the re-entrant vortex liquid phase that has been theoretically predicted in these highly anisotropic materials at very low vortex densities. Analysing the trapping times for vortices in the fluctuating state we estimate that the pinning potential of typical strong pinning centres is about 900 K under our experimental conditions. To our knowledge, this is the first direct experimental evidence for the existence of a dynamic liquid-like vortex state in this highly anisotropic material at very low magnetic induction.