Publications

The magnetic properties of FINEMET-type nanocrystalline alloys and isolated ferromagnetic AgCo nanoparticles are investigated both experimentally and numerically. Theoretical models of spins that simulate ideal nanocrystalline alloys and isolated nanoparticles are considered while their magnetic properties are derived from Monte Carlo simulation of low-temperature spin ordering. Interesting features such as magnetic polarization of the matrix due to penetrating fields arising from nanograins and the role played by the crystalline fraction in the overall L magnetic behaviour, in the case of nanocrystalline alloys are investigated. For isolated nanoparticles it is shown that the competition between surface and bulk anisotropy gives rise to surface spin disorder that, together with finite-size effects, is responsible for the experimentally observed lack of saturation of the magnetization in high applied fields. These simulation results are confirmed by experimental data obtained on FINEMET nanocrystalline alloys and isolated ferromagnetic AgCo colloidal nanoparticles.

The influence of preliminary treatment in hydrogen plasma on elimination of radiation defects and formation of thermal donors has been studied in detector structures made of standard and oxygenated float zone silicon has been studied. A new type of thermal donors has been found in as-treated diodes. These thermal donors are unstable and can be eliminated by heat-treatment at 200-250 degrees C. After irradiation with 3.5 MeV electrons the detectors had been annealed at temperatures of 50-350 degrees C. It has been found that preliminary hydrogenation at 300 degrees C leads to disappearance of main vacancy-type radiation defects at lower annealing temperatures, The annealing of hydrogenated and irradiated crystals is accompanied by hydrogen redistribution and formation of hydrogen-related donors. Preliminary irradiation influences on both these processes.

Hardening laser surface offers new possibilities for the enhancement of the mechanical resistance of superficial layers. Essential for the laser treatment processes is the efficiency at which the incident laser power is coupled into the work piece. The aim of the work was to study the effect of some factors such as spot shape, beam angle of incidence and surface coating on the steel surface absorptivity. Samples made of carbon steel were laser processed using a CW CO2 - 1,2 kW laser unit. Different process parameters (beam power, spot dimension and traverse speed), were used. The microstructure of laser hardened layers which have been investigated by optical microscopy. The mechanical characterization of the layers has been done by hardness measurements. Correlation has been established between the structure of the laser-processed layers and the process parameters.

Effect of Bi additives on the piezoelectric properties of PT ceramics having the formula (Pb1-3x/2Bix(Ti0.98Mn0.02)O3, with x = 0,04, 0.06, 0.08 was investigated. The samples were prepared by conventional ceramic technique, using p.a. purity raw materials. The Mixed powders were sintered at temperatures between 1050-1300 degrees C. Poling was done in fields of about 70 kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degrees C. A Curie point of 450 degrees C, a low dielectric constant (< 140) and a large electromechanical anisotropy), (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4) were found. These enhanced properties make such ceramics very attractive for high temperature and high frequency transducers applications.

Raman spectroscopy and cyclic voltammetry (CV) are used for the investigation of the oxidation-reduction processes of single-wall carbon nanotubes (SWNTs) films in an HCl 0.5 M solution. In the potential ranges (+100; +800) and (0; +1500) mV vs. saturated calomel electrode (SCE), the oxidation-reduction reactions of SWNT films, in both aqueous and semi-aqueous HCl 0.5 M solutions, are reversible and irreversible, respectively. At potentials higher than +1000 mV vs. SCE, SWNTs break in fragments of different sizes. A post-treatment with an alkaline solution results in a partial restoration of the SWNTs. The functionalization of SWNTs with polydipherylamine (PDPA) is presented as an application of the reversible oxidation-reduction reactions of nanotubes in an HCl 0.5 M solution. In order to differentiate between a deposition of successive layers of the polymer and a functionalization process, a spectroelectrochemical study of the deposition of polypyrrole (PPY) on SWNT films was carried out, too. (c) 2004 Elsevier B.V. All rights reserved.

Designing the high-frequenqy devices by using artificial neural network's (ANN) emphasize the ANN propensity for storing knowledge, for generalizing patterns in data and for offering computational efficiency. Two neural models are discussed in this paper. The model of a band-pass forward-coupled filter uses the ARMA coefficients, which describe the FDTD signal at the output port of the filter in time-domain. On the contrary, the second model, which was developed for a tunable planar resonator, uses directly the device response in frequency-domain.

This paper presents the results of the vibration modes measurements by X-ray topography in SC-cut quartz and Y-cut unpolished langasite resonators. A comparison of these results with X-ray diffraction topography images on AT-cut quartz resonators and Y-cut polished langasite resonators pointed out the behavior of mass-loading effect with plate orientation angle and with the surface state of the piezoelectric substrate. The results of the X-ray topography investigations are compared with electrical measurements performed on the same resonators. 5MHz Sawyer, plan parallel SC-cut quartz and Y-cut unpolished langasite resonators, with 14mm plate diameter and various electrode thickness and diameters have been investigated by Xray topography on fundamental, third and fifth overtones. The measurements were performed by conventional transmission Laue setting using white beam synchrotron radiation at LURE/DCI, Orsay, France. The study by X-ray topography on SC-cut quartz resonators and unpolished Y-cut langasite resonators pointed out a good agreement with the results obtained on the same resonators by electrical measurements. The conclusion is that the SC-cut quartz resonator characteristics present a similarly harmonic dependence with those of the Y-cut langasite resonators thus revealing the stress-compensated feature of the Y-cut in langasite crystal.

The paper describes the investigations of Ba1-xSrxTiO3 ceramic materials prepared by solid-state reaction techniques. The addition of MgO and MnO2 improves sintering process and decreases the dielectric loss. The materials exhibit a dielectric constant epsilon(r) similar to 1000 and low loss tan delta similar to 10(-3) at microwave frequencies.

Band-pass filters with dielectric resonators of (Zr-0.8, Sn-0.2,)TiO4 ceramic materials (ZST) were developed for X-band applications. The ZST materials exhibit a dielectric constant of about 36.8 and low loss in microwave range. A Qxf product higher than 50,000 was achieved for the ZST samples with 0.2% wt NiO sintering addition. The external coupling was performed Using microstrip lines on a substrate of 9.22 dielectric constant and 0.635 mm height. Filters with minimum insertion loss between 0.46 dB and 1.74 dB, the bandwidth between 40 MHz and 170 MHz and with the central frequency around 10 GHz were manufactured.

New cathode material for Direct Methanol Fuel Cells (DMFC) based on multi-wall carbon nanotubes (MWCNT), aiming to improving the power density of the cell was analysed. Commercially available MWCNTs were oxidized in a mixture of HNO3 and H2SO4. Pt catalyst was electroless deposited using a chloroplatinic acid solution, based on H-2 (PtCl6) center dot 6H(2)O. The morphology of the deposits was analysed by TEM and SEM. Uniformly distributed nano particles of Pt with diameters of approximately 5 nm along the MWCNTs surface, agglomerates of thick Pt deposits, as well as complete uncoated nanotubes were observed. An optimisation of the size and distribution of the Pt particles could be obtained by varying the process parameters (dispersion time and solution concentration). The MWCNTs + Pt were used to produced a paste, which was screen-printed on Nation foil. The membrane electrode assembly (MEA) was tested at 40 degrees C with air and 3% methanol in water. The performances were improved as compared with the standard DMFC electrodes.