Publications

Multiharmonic ac-magnetic susceptibility chi(1) chi(2), chi(3), of neutron irradiated Li-doped YBa2CU3O7-x, has revealed a nonmonotonic dependence of all harmonics on the neutron fluence. The irradiation has a strongly depressive influence on the intergrain connection suggesting an increase of the effective thickness of the intergranular Josephson junction at a neutron fluence of 0.98 x 10(17) cm(-2). Less damaged are the intragrain properties. A spectacular enhancement of the superconducting intragranular properties reflected in the characteristics of all harmonics was observed at highest fluence phi = 9.98 X 10(17) cm(-2). We assume that this effect results from the development of a space inhomogeneous distribution with alternating defectless and defect-rich regions.

The sintering-sublimation technics was conceived for increasing the reliability of the photoconductive devices, by decreasing the concentration of mobiles donors, which are utilised for photoconductive material sensibilisation. The technics combines three different technics: the vapor phase deposition; the vacuum vaporising; the ceramic sinterisation. This special technics gives stoichiometric layers. The technics avoids dissociation (the flash sublimation technic), sublimates the compounds from two different sources, on heated support, or sublimates the compound, with Cd excess. It was investigated, by mass spectrometry and optical absorption studies, the presence, in gaseous state, of the A(II)B(VI) molecules. The condensation on the substrate of the vapors is difficult because the Cd, Zn and Hg elements have difficulties in the condensation process. The Frenkel model, of the condensation, can be successfully used for describing the process. This model uses the observations concerning the difficulty of the condensation on supports of those elements, at higher temperatures than 90 - 195 K. It is supposed that the support surface is a potential surface, with holes and potential walls, whose dimensions can not overcome the interatomic distances, for polished surfaces.

The structure of a glassy sample of Ge30As4S66 has been measured by means of synchrotron radiation. X - ray diffraction data were processed and the atomo-electronic radial function (AERDF) was calculated. The high resolution AERDF curve allowed for an accurate determination of the first order distances between the atoms and the coordination numbers. A model of spatial structure, based on the concept of raft configuration was tested. An other model of the structure based on the continuous random network concept (CRN) was tried. The general conclusion drawn from this study supports the idea that raft and disordered configurations interlinked so that the dangling bonds in the structure be minimized, is the best model for such type of glass.

Detailed stiffness and internal friction (Q(-1)) versus temperature curves were obtained for liquid-phase-sintered silicon carbides using advanced resonant beam analysis up to 1400 degrees C. As-sintered materials display a stable Q(-1)-peak near 1100 degrees C, superimposed on an increasing background. The change of stiffness associated with the damping peak is quantitatively related to the amount of matter in pockets of the amorphous intergranular phase in which the refractory SiC matrix grains are embedded. The successful removal of the amorphous pockets by annealing at 1900 degrees C is deduced from the disappearance of the damping peak and confirmed with transmission electron microscopy.

Using the transport and magnetization measurements, the influence of neutron irradiation at a fluence of 5 x 10(17) n cm(-2) on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz has been investigated. The neutron irradiation was found to decrease critical temperature and transport critical current density, increase the residual and normal state resistivity, and improve the intragranular critical current density with 1.6 x 10(5) A/cm(2) (at 77.3 K and in the applied field up to 160 kA m) and Delta M-irr/Delta M-nonirr ratio (up to factor of 3) at highest field used for investigation. The field dependence of this ratio, which is below the unity at very low field but higher than I at high fields, correlated with the shape of the hysteretic loops as well as with the change of the transport parameters after irradiation suggests the role of the irradiation-induced effects on the grain edges. We discuss these effects in the framework of the Bean-Livingstone surface barriers and geometrical barriers.

Correlated studies of Surface Enhanced Raman Scattering (SERS) spectroscopy, photoluminescence (PL) and viscosity on aggregation of C-60 fullerene in N-methyl-2-pyrrolidinone (NMP) solutions were performed. The aggregation is a result of storing of the solutions at the room temperature for different periods of time. The modified phonon spectrum in the aggregate phase is a consequence of the intermolecular interactions that lead to a large reduction in parent I-h C-60 symmetry. The Raman range below 800 cm(-1) is the most diagnostic for aggregate assignment. Substitution of the original of H, (1) band from similar to 269 cm(-1) with a new one at similar to 255 cm(-1) relates the formation of (C-60, NMP) complexes. In the second stage of aggregation, this band shifts at similar to 246 cm(-1) and a new band, associate to the interball interaction, is formed at similar to 94 cm(-1). PL emission of the C-60 solutes in NMP presents two components, one in red (1.92-2.00 eV) and another in green (2.18-2.3 eV), which originate in two successive processes. Their weights depend on C-60 concentration and on storage time. In the beginning are formed quickly molecular complexes (C-60, NMP) by charge transfer, which on associate slowly in stable aggregates. A dependence of specific viscosity on C-60 solution concentration is observed too.

Alignment properties of rubbed films of polyvinyl imidazole were studied by observing with the polarizing microscope the features of the nematic liquid crystal (LC) droplets onto these layers. It was found that the observing of the droplets can reveal the direction of easy axis on the alignment substrate not only in the case of LC having the director normal to the LC/air interface but also in the case of the director tangent to this interface, when no disclination line could be seen. We observed that some of the LCs are oriented parallel to the rubbing direction, but the other LCs are perpendicularly oriented. This peculiar behavior is at variance to that found on rubbed layers of polyvinyl alcohol. The role of the end-groups in the structure of the LCs on the alignment properties is discussed.

Zinc oxide thin films were obtained by laser ablation of a Zn target in oxygen reactive atmosphere, the oxygen being supplied either by a standard gas inlet valve or from a radio-frequency (rf) oxygen plasma. Pt-coated silicon and MgO were used as substrates. The influence of the deposition parameters as laser wavelength (266, 355, 1064 nm), laser fluence (1.5-20 J/cm(2)) and oxygen pressure (1-60 Pa) was studied. The influence of the rf plasma beam addition on the morphological proprieties of zinc oxide films was particularly investigated, simultaneously with several configurations of the direction of the ablation plasma, the rf plasma beam and the substrate. The obtained films, with thicknesses in the range of 50 nm to 1 mu m have been characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM). (c) 2005 Elsevier B.V. All rights reserved.

This work presents pulsed laser deposition of cobalt-based Hensler thin films Co2MnX (X = Si, Ga, Ge, Sn, SbSn) on different substrates (Si, GaAs, InAs). The deposition processes developed in vacuum (about 10(-5) Pa) to avoid oxidation of the films and targets. The temperature of the substrates during the depositions was kept below 500 K to minimise interface interdiffusion. From X-ray diffraction, we found that the films are crystalline and slightly oriented. The stoichiometric composition of the films was further checked by EDS, while the size and density of droplets were determined by SEM. The magnetic properties of the films are consistent with those of the bulk material used as target. (c) 2005 Elsevier B.V. All rights reserved.

Thin films of zirconium tin titanate, (Zr0.8Sn0.2)TiO4 (ZST) were deposited using a pulsed electron beam source based on a channel-spark discharge for target ablation. An advanced degree of crystallization was obtained for the films deposited on alumina substrate post-annealed at 1000 degrees C. The crystalline lattice constants of the films are very close to those of the target material, which confirms the same stoichiometry in ZST films and in the bulk. (c) 2005 Elsevier B.V. All rights reserved.