Publications

TiO2 anatase doped with 0.5 at%Fe powders were prepared by sol-gel technique. Titanium (IV) butoxide and iron (III) acetylacetonate were used as starting precursors, n-butanol as solvent, acetylacetone is added as a chelating agent and acetic acid to decrease the kinetics of the hydrolysis and condensation of Ti(O-Bu-n)(4). The evolution of the network bonds and the structural characterization of the gel were studied by thermogravimetric analysis (TG), differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), Fourier transform infrared spectroscopy(FTIR) scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques, respectively. A gelprecusor nanopowder with the mean grain size of about 15-20 nm was obtained. In order to prepare a Fe-cloped TiO2 material with the structure of the anatase, a thermal treatment at 450 degrees C, 3 h in air, was fund enough.

Hydroxyapatite (HAP) thin films (0.2 pm, 0.3 pm and 1.2 mu m thickness) were grown by RF magnetron sputtering in argon atmosphere onto silicon substrates at high deposition rates (0.6 mu m/h). Crystalline HAP films were obtained using a low temperature (100 degrees C) followed by post-deposition annealing at 300 degrees C, 450 degrees C, 500 degrees C and 550 degrees C in environmental air for 1 hour. An important influence of the films thickness upon the crystallization degree was noticed at intermediate annealing temperatures, as obtained from XRD measurements. For low and high temperatures similar values were obtained with a better crystallization degree for the thinner films. FTIR absorption led to the same conclusion considering the shape of stretching and bending PO4 lines. This suggests that the crystallization process has a diffusion component besides usual thermal activation process.

Relaxation of the thermal residual strain in Nb3Sn wires is realized through cycles of multiple torsion loadings ('pre-torsion') at room temperature. As a consequence, the critical current, I-c, is enhanced. This effect is stronger than previously reported mechanical treatments of multiple bending ('pre-bending'). The maximum I-c is attained for complex mechanical treatments of pre-torsion and pre-bending. Complex treatments allow efficient and relatively independent control of residual strain relaxation over all three directions of the wire, resulting in enhancement of I-c towards the theoretical limit of the material. Our findings have an immediate positive impact on the performance of Nb3Sn wires and, hence, on their applications (e.g. react-and-wind coils).

Fluorinated hydroxyapatite [Ca-10(PO4)(6)(OH)(x)F1-x] implantologic structures could constitute an alternative to classic hydroxyapatite [Ca-10(PO4)6(OH)21 ones in medical applications due to their bioactivity and low solubility. Bioactive apatites are usually used as coatings onto surface of biocompatible metals, leading to implants which take advantage of the coating's bioactivity and good mechanical properties of metals. In this work, powders and apatite type films were prepared on Ti6Al4V medical grade alloy using three sol-gel chemical routes by mixing different calcium and phosphorous precursors Ca(NO3)2, P2O5 and C6H15PO3, and two fluoride reagents: HPF6 and NH4F. The resulting calcium phosphates and fluorapatite/hydroxyapatite solid solution (FHA) compounds were investigated by SEM, XRD and evaluated in vitro in SBF Kokubo solution. An efficient preparation algorithm was designed for the Ca(NO3)(2+) C6H15PO3+ NH4F chemical route which produced monophasic powders with a high crystallinity degree. This chemical route led to FHA sol-gel coatings with bioactive potential.

The paper presents the phototransport and photoluminescence investigations on nanocrystalline porous silicon. Spectral dependence curves of the phototransport and photoluminescence were taken at room temperature. A typical phototransport curve presents several maxima and shoulders, while the photoluminescence curves present only one maximum. The temperature dependence of the phototransport was measured in the 85 - 250 K range for different wavelengths. All these curves present only one activation energy on the whole temperature interval. The experimental results are interpreted taking into account a quantum confinement model. Previous microstructure investigations proved that the studied nanocrystalline porous silicon is formed by a nanowire network. Then, the electron Hamiltonian can be split into the sum of a 1D longitudinal Bloch part and a 2D transversal part, the nanowire wall acting like a potential well. Thus, a number of discrete energy levels will appear into the band gap. Both phototransport and photoluminescence maxima are due to the transitions between these levels, corresponding to a mean nanowire diameter of 3.2 nm, in good agreement with the previous microstructure and electrical transport investigations. On the contrary, the temperature dependence of the phototransport is determined by the surface states located on the internal surface and/or the interface between nanocrystallites.

We investigate nonstationary electronic transport in noninteracting nanostructures driven by a finite bias and time-dependent signals applied at their contacts to the leads. The systems are modeled by a tight-binding Hamiltonian, and the transient currents are computed from the nonequilibrium Green-Keldysh formalism. The numerical implementation is not restricted to weak coupling to the leads and does not imply the wideband limit assumption for the spectral width of the leads. As an application of the method we study in detail the transient behavior and the charge dynamics in single and double quantum dots connected to leads by a steplike potential, but the method allows as well consideration of nonperiodic potentials or short pulses. We show that when the higher-energy levels of the isolated system are located within the bias window of the leads, the transient current approaches the steady state in a nonoscillatory smooth fashion. At moderate coupling to the leads and fixed bias the transient acquires a steplike structure, the length of the steps increasing with system size. The number of levels inside a finite bias window can be tuned by a constant gate potential. We find also that the transient behavior depends on the specific way of coupling the leads to the mesoscopic system.

We have demonstrated an electron pump with in-plane gate (IPG) transistors fabricated by focused ion beam (FIB) implantation on GaAs-A1(x)Ga(1-x)As modulation-doped heterostructure with a two-dimensional electron gas (2DEG). The maskless fabrication process does not require any alignment between the circuit elements including the sources, the drains and the gates of the IPG transistors. Although, the device is operating at low frequencies (tens of kHz), it is capable to produce a current more than one order of magnitude higher than other electron pumps based on Coulomb blockade and single electron tunneling effects. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Comparative experiments regarding electrodeposition of zinc oxide either as films on Pt and Au substrates or as microwires grown in microporous polycarbonate membranes were carried out using nitrate aqueous solutions as electrolyte. Linear sweep voltammetry and electrochemical impedance spectroscopy together with SEM and XRD techniques were used for investigation. The equivalent electrical circuits for fitting experimental results were similar to those for characterisation of metal/polymer coating systems.

The electronic and optical properties of self-assembled InN/GaN quantum dots (QDs) are investigated by means of a tight-binding model combined with configuration-interaction calculations. Tight-binding single-particle wave functions are used as a basis for computing Coulomb and dipole matrix elements. Within this framework, we analyze multiexciton emission spectra for two different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure. The impact of the symmetry of the involved electron and hole one-particle states on the optical spectra is discussed in detail. Furthermore we show how the characteristic features of the spectra can be interpreted using a simplified Hamiltonian which provides analytical results for the interacting multiexciton complexes. We predict a vanishing exciton and biexciton ground-state emission for small lens-shaped InN/GaN QDs. For larger systems we report a bright ground-state emission but with drastically reduced oscillator strengths caused by the quantum confined Stark effect.

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field H-c2 and irreversibility field H-irr when compared with pristine MgB2. The reason for better H-c2 and H-irr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of H-c2 and H-irr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T >= 24 K, there is mixed pinning (H <= 5 T).