Publications

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.

Very compact planar resonators were developed for the 1950 MHz frequency band. Resonators are of the stepped-impedance (SIR) type and they are folded, in order to occupy a very small surface area. On a 6.635 mm height commercial substrate with a 10.8 dielectric constant, a resonator can be fitted into a square with an edge size of 5.8 m. Small-size filters were designed by cross-coupling the compact resonators. The four-pole filters have an improved response due to the cross-couplings, which introduces transmission zeros on each side of the pass-band. The filters were manufactured and characterized. The measurement data are in good agreement with the electromagnetic field simulations. The developed filters are small-size and cost-effective and their technology is simple,. without any via holes, air-bridges or lumped elements.

Wireless communications systems require new materials with special properties in specific frequency bands. The investigations on ZST type ceramics, (Zr0.8Sn0.2)TiO4, presented in this paper, recommend this materials for applications in microwaves and millimeter waves. The ZST materials were prepared using a standard solid-state reaction technology. The samples morphology, phase-composition and microstructure investigations were performed by using the scanning electron microscopy (SEM), and energy-disperse X-ray spectrometry (EDX). The crystalline phases were identified by X-ray diffractometry (XRD). The electromagnetic properties were investigated on ZST resonators by using a Computer Aided Measurement (CAM) in microwaves, combining a HP 8757C network analyzer and a HP 8350B sweep oscillator. The dielectric characteristics at millimeter waves were analyzed by investigation of the Whispering Gallery Modes on ZST disks. The low level NiO doping provides ZST materials with temperature coefficient tau(f) in the range (-2 divided by +4) ppm/degrees C and decreases the dielectric loss. Materials with high values of the Q f product up to 50,000 and a dielectric constant about 36 at microwave frequencies were obtained. ZST dielectric resonators and substrates for hybrid integrated circuits with dimensions 1" x 1" and thickness in the range 0.6 divided by 1 mm were manufactured.

Surface-enhanced Raman scattering (SERS) was used to investigate C-60 self-assembling in solvents like pyrrolidine (Py) and Nmethyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C-60 aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C-60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at similar to342cm(-1). In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I-h C-60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800cm(-1) is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C-60 in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C-60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original H-g(l) band at similar to269cm(-1) with two others at similar to255 and similar to246cm(-1). In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118cm(-1), which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C60 clusters of different size. (C) 2004 Published by Elsevier Ltd.