Publications

The influence of Pb2+ doping on the optical properties of LiF crystals before and after irradiation with X-ray was studied by using optical absorption and photoluminescence measurements. After irradiation it was observed an increase of the absorption due to primary and aggregate electronic defects together with a higher photoluminescence signals associated with the F-2 and F-3(+) colour centres in doped samples compared with the undoped ones.

In this paper a bandpass planar filter, with cross-couplings and with a pair of attenuation poles, is investigated. The equi-ripple in-band response and the location of the poles on the frequency axis can be precisely controlled by using a new technique which combines accurate electromagnetic field simulations and fast linear circuit optimization, allowing the design of bandpass filters with improved performances. To illustrate the procedure, a microstrip bandpass filter was designed, verified by em-field simulation, fabricated and tested. The response of the designed filter is in good agreement with the specification, confirming the possibilities of realizing microwave bandpass filters with rigorously controlled characteristics, with reduced design time and non-prohibitive computational effort.

Nanometric powders in the TiO2-CeO2 System were synthesized using sol-gel methods aiming at corrosion inhibitor preparation. The precursors employed were Ti(OiPr)(4) as TiO2 source and Ce(NO3)(3)center dot 6H(2)O or Ce(CH3CO2)(3)center dot xH(2)O as CeO2 source. Several parameters were varied, like the cerium precursor (cerium nitrate or cerium acetate) and the TiO2:CeO2 molar ratios. For both cerium sources binary powders with TiO2:CeO2 = 4:1 or 1:1 molar ratios were prepared. The obtained materials were dried 12 h in an oven at 100 degrees C and then thermally treated for 1 h at 400 degrees C with a heating rate of 1 degrees C.min(-1) in order to eliminate the water and organic residues. In all cases, nanometric - predominantly amorphous - powders have been obtained which crystallized after the I h thermal treatment at 400 degrees C. The only exception was recorded for the TiO2:CeO2 = 4:1 composition prepared from the Ce(NO3)(3)-6H(2)O precursor which remained amorphous after the thermal treatment. Even though the primary particle sizes were similar to 4-5 nm, the material is constituted from aggregates larger than 50 nm. The desired presence of cerium in two valence states, necessary in active corrosion protection, was achieved and it was put into evidence by XPS and DTA/TGA for the TiO2:CeO2 = 4:1 binary compositions. Future studies have to be performed in order to reduce the agglomeration of the material.

Superconducting SiC and B4C-doped MgB2 samples containing up to three layers of pure Fe have been prepared by Field-Assisted-Sintering Technique (FAST). The adhesion between the Fe and doped-MgB2 layers is excellent due to the enhanced local interdiffusion and increased solubility of Fe into MgB2 grains that are typical for the non-equilibrium processes. Magnetic measurements showed the largest irreversibility, hence, the largest critical current density in the B4C-doped sandwich with one Fe foil. Additionally, the drawback of flux jumps at low temperatures is almost absent for one Fe-layer sandwiches. However, the irreversibility is smaller than in pure MgB2 and SiC-doped MgB2 samples prepared by the same technique, but without the insertion of Fe foil. When the number of Fe foils is increased to three and an excess of 5 % wt. Mg is added, the SiC-doped sandwiches show almost similar irreversibility values and field dependence (at 5 K and at 20 K) as the pristine MgB2 samples without Fe. But, the flux jump is recovered at the same level found for the pristine MgB2 without Fe. An explanation in terms of doping dependence of the ratio between the thermal and magnetic diffusion constants is proposed.

This work is focusing on the investigation of those radiation induced defects causing degradation effects of Silicon detector performance. Comparative studies of the defects induced by irradiation with Co-60- gamma rays, 23 GeV protons and I MeV equivalent reactor neutrons revealed the existence of some point defects and cluster related centers having a strong impact to damage properties of Si diodes. The detailed relation between the "microscopic" reasons as based on defect analysis and their "macroscopic" consequences for detector performance are presented. In particular, it is shown that the changes in the Si device properties (depletion voltage and leakage current) after exposing to high levels of Co-60- gamma irradiation can be completely understood by the microscopically investigated formation of two point defects: i) a defect formed via a second order process (I-p) that can be associated with the long searched for V2O complex or with a Carbon related center and is the cause for the observed type inversion effect in Oxygen lean material; ii) a bistable donor (BD) created during irradiation that is strongly generated in Oxygen rich material, associated with one of the earlier thermal donors in Si. It is the cause for the observed positive space charge induced by irradiation in oxygenated Si diodes. Specific for hadron irradiation are the annealing effects which decrease resp. increase the originally observed damage effects as seen by the changes of the depletion voltage (effects known as "beneficial" and "reverse" annealing, respectively). A group of four cluster related defects proved to be responsible for these annealing effects. Their formation is not affected by the Oxygen content or Si growth procedure suggesting that they are complexes of multivacancies located inside extended disordered regions.

The corrosion and behavior of carbon steel in 2M HCl in the presence of N-(2-hydroxybenzilidene) thiosemicarbazide (HBTC) was investigated using weight loss and electrochemical studies. The morphology of carbon steel surface was investigated using scanning electron microscopy (SEM) and Mossbauer spectrometry. The corrosion current was determined using Tafel polarization. The inhibition efficiency increased with HBTC concentration; the experimental results suggest that the presence of HBTC in the solution increases the surface coverage (); a decrease in the corrosion spot with the increase of the HBTC concentration indicates good adsorbability of HBTC on the metal surface. The adsorption of this compound on the metal surface is found to obey Langmuir's adsorption isotherm. Mossbauer spectroscopy showed at this stage the main product of corrosion is a non-stoichiometric amorphous Fe3+ oxyhydroxide, consisting of a mixture of , , and -FeOOH, where -FeOOH is the main phase.

We study ester-type liquid crystal, 4'-cyanophenyl 4-n-hexyl benzoate and its homologue 4'-cyanophenyl 4-n-pentyl benzoate. We used the Thermally Stimulated Depolarization Currents method and found information about phase transition temperature, and activation energies. The temperatures of phase transitions were compared to those obtained by classical DSC/DTA measurements. Simultaneously with the TSDC, we measured the optical transmission; we found a decrease of the optical transmission when it was measured after a greater pre-applied polarizing electric field.

We report the fabrication of composites made of La2/3Sr1/3MnO3 and two comb-type polysiloxane-based copolymers, polysiloxane-g-styrene and polysiloxane-g-(styrene-co-butyl-acrylate), in an attempt to improve the elastic properties of the polymer-based composites. The composites were obtained by the co-precipitation of a polymer solution mixed with the lanthanum manganite followed by sintering at appropriate temperatures. The rheological properties of the composites before as well as the magnetic and transport properties after sintering process are presented.

We study theoretically the electronic transport in parallel few-level quantum dots in the presence of both intradot and interdot long-range Coulomb interaction. Each dot is connected to two leads and the steady-state currents are calculated within the Keldysh formalism using the random-phase approximation for the interacting Green functions. Due to the momentum transfer mechanism between the two systems it is possible to get a nonvanishing current through an unbiased Coulomb-blockaded dot if the other dot is set in the nonlinear transport regime. The transitions between the levels of the passive dot reduce the drag current and lead to negative differential conductance.

A novel method for the synthesis of nanostructured films produced by depositing gas-phase magnetic nanoparticles is presented and the properties of the films are reported. The technique mixes metal vapour and small argon clusters produced in a supersonic expansion. The condensed clusters are subsequently deposited in situ onto copper grids. The cluster size is controlled by the vapour pressure of the metal inside the pick-up chamber. Detailed analysis of the transmission electron micrographs of the Fe clusters shows that there is a simple linear relationship between the average metal cluster diameter and the metal vapour pressure during deposition. Furthermore, the nanoparticles show a relatively narrow size distribution for a given set of experimental conditions. Structural and magnetic investigations have been performed on Fe cluster samples, and the influence of the metal vapour pressure has been studied. Detailed analysis of the magnetic and structural data has been performed and valuable information such as cluster size distributions, strength of the interparticle dipolar interactions and average magnetic moment per cluster are derived. It is shown that, at room temperature, the magnetic behaviour of the films is consistent with nanoparticle supermoments interacting via dipolar interactions.