Publications

A series of Bi-ZSM-5 containing 4.26, 5.54, and 6.64 wt% bismuth oxide was prepared by introducing Bi during ZSM-5 synthesis. For comparison. 5 wt% Bi (expressed as bismuth oxide)-impregnated ZSM-5 samples were also prepared. These samples were characterized by N-2 adsorption-desorption isotherms at 77 K, XRD, XPS, IR spectroscopy in the lattice vibration range and in the presence of chemisorbed pyridine and CO2 and EXAFS. The characterization data confirmed the occlusion of bismuth oxide clusters in the zeolite pores. An intrinsic acidity due to bismuth oxide clusters was evidenced. This consisted, on one hand, in weak Lewis acid sites, ascribed to coordinatively unsaturated bismuth cations, and, on the other hand, in strong Bronsted acid sites, attributed to boundary Bi (. . .) O-Si linkages. These catalysts were tested in the liquid-phase oxidation of benzene, toluene, and cyclohexane using hydrogen peroxide as oxidizing agent. The results showed that Bi-ZSM-5 catalysts are more active than the reported titanium silicalites, but the efficiency of H2O2 use was quite small, an important part of this being nonselectively decomposed. A good selectivity was observed in cyclohexane oxidation. (C) 2003 Elsevier Inc. All rights reserved.

Using surface-enhanced Raman scattering (SERS) and Fourier transform infrared (FTIR) spectroscopy, we show that composites based on polyaniline (PANI) and single-walled carbon nanotubes (SWNTs) are different when they are prepared by two different methods: (1) by adding dispersed SWNTs to the polymer solutions and (2) by chemical polymerization of aniline in the presence of SWNTs. The difference originates from the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs into fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FTIR spectra of the composites PANI/SWNTs and PANI/C-60 chemically prepared. All compounds exhibit an absorption band at 1144 cm(-1) in their FTIR spectra, increasing with the carbon nanoparticules content, as a signature of a charge transfer between the constituents. Besides, the FTIR spectrum of the compounds obtained by adding SWNTs to the polymer solution display an intense absorption band with two components at 773 and 755 cm(-1), which are associated with the vibration of deformation of the benzene and the quinoid ring, respectively. This indicates a strong hindrance effect produced by the binding on the polymer chain of voluminous carbon particles as nanotubes and large fragments of nanotubes.

The presence of magnetic moments in uranium atoms in some of U2TSi3, UTSi and U2T3Si (T is a 3d, 4d or 5d transition element) compounds is discussed in terms of the hybridization of s, p, d and f atomic orbitals. The hybridization strength in these ternary series of isostructural compounds was calculated using the semiquantitative band structure approach of Straub and Harrison. The prediction criterion for the presence of magnetic ordering, comparing the band width with a critical energy, seems to hold. The dominant influence of fd hybridization effects in the change of magnetic properties inside an isostructural group is in evidence.

Capacitance Deep Level Transient Spectroscopy (C-DLTS) measurements have been performed on standard and oxygen-doped silicon detectors manufactured from high-resistivity n-type float zone material with and orientation. Three different oxygen concentrations were achieved by the so-called diffusion oxygenated float zone (DOFZ) process initiated by the CERN-RD48 (ROSE) collaboration. Before the irradiation a material characterization has been performed. In contrast to radiation damage by neutrons or high-energy charged hadrons, were the bulk damage is dominated by a mixture of clusters and point defects, the bulk damage caused by Co-60-gamma-radiation is only due to the introduction of point defects. The dominant electrically active defects which have been detected after Co-60-gamma-irradiation by C-DLTS are the electron traps VOi, CiCs, V-2(=/-), V-2(-/0) and the hole trap CiOi. The main difference between standard and oxygenated silicon at low dose values can be seen in the introduction rate of CiCs compared to CiOi. For highly oxygenated silicon the introduction of CiCs is fully suppressed, while the sum of the introduction rates g(CiCs)+g(CiOi) is independent on the oxygen concentration. (C) 2003 Elsevier B.V. All rights reserved.

We present results obtained on carbon nanotube systems by means of Raman scattering and surface enhanced Raman scattering (SERS). Such systems consist of single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), or polymer/nanotubes composites. In particular, we have focused our studies on the interactions taking place in these compounds which lead to an upshift of the radial breathing mode (RBM) in the case of SWNTs, either when they are arranged in bundles or embedded in polymers. Similar calculations has allowed us to interpret the low frequency Raman modes observed in purified MWNTs. We show that they come from the RBM of individual tubes. From intensity calculations performed to simulate Raman spectra, we demonstrate the importance of two experimental parameters, namely the diameter of internal tubes and the number of shells which constitute the MWNTs, such parameters being extracted experimentally from high resolution transmission electron microscopy images. (C) 2003 Elsevier Science B.V. All rights reserved.

The glassy composition As25Te35Si40 was investigated by microhardness, differential scanning calorimetry, X-ray diffraction, electrical conductivity and Raman spectroscopy measurements. A continuous random network model of the glass structure has been developed. (C) 2003 Elsevier B.V. All rights reserved.

Polymerization-depolymerization processes induced by bandgap light irradiation with alternating high and low intensity light control the nano-scale extension of disordered layer-like clusters in arsenic rich chalcogenides. These processes resulting in a fractal morphology of the clusters were simulated in the frame of a Monte-Carlo-Metropolis method. The fractal model for photo-darkening and photo-bleaching phenomena in arsenic chalcogenides is discussed. (C) 2003 Elsevier B.V. All rights reserved.

The effect of rare earth (Dy and Sm) and transition metal (Mn) luminescent impurities on the optical properties of As2S3 glasses is studied in a wide spectral region. The Raman, IR, band-to-band and edge absorption spectroscopy are employed to obtain information about the incorporation of impurity ions in the host glass structure and the corresponding changes in intrinsic optical characteristics. The effect of metal impurity on both extended and localized electronic states is revealed. (C) 2003 Elsevier B.V. All rights reserved.

From ac susceptibility measurements of the [Bi(Pb)](2)Ca2Sr2Cu3Oz (Bi-2223) samples doped with LiCl, we have found and report for the first time anomalously suppressed superconductivity (T, is anomalously decreasing) in the slightly under-doped region. For our BPSCCO/(LiCl)(y) samples this region is situated around y = 0.07. Hole concentration (p) per number of Cu-O planes of the Bi-2223 unit cell, determined from room temperature thermopower (S-300 K) measurements, is approximately 0.15 when y is within anomalous region. Literature data shows that for the La-based cuprates, similar suppression of superconductivity was observed at p similar to 0.12 (so-called "1/8 problem"). This discrepancy would appear because charge ordering phenomenon (probably induced by Li known as a pair breaker as e.g. Zn) may be accompanied by changes in oxygen content or oxygen ordering effects. (C) 2003 Elsevier B.V. All rights reserved.