Publications

Models are presented which are compatible with the Rietveld method for the anisotropic broadening of the diffracted peaks produced by crystallite size and microstrain. The models, derived from general crystallographic considerations, are different for different Laue groups.

Polyethylene thin films of different thickness have been obtained by evaporation method, We have produced the treeing discharge by electron beam irradiation. The films have been examined by transmission electron microscopy (TEM) and electron diffraction (ED), before and after irradiation. It is observed that: (1) the polyethylene thin films have a structure of the fine spherulites before irradiation; (2) structural damages (channels, treeing figures and zones in which the material was melted and resolidified) accompanying the space charge produced in thin polyethylene films. (C) 1998 Elsevier Science S.A.

In this paper we have considered a Gaussian polymer chain of length L as an intrinsic object enclosed on a surface embedded in the Euclidean space. When the surfaces are the sphere SD-1 in D dimensions and the cylinder, the cone and the curved torus in R-3. We have calculated analytically and numerically (using the diffusion equation and the path-integral approach) the probability distribution function G(R\R';L) of the end-to-end vector R-R' and the mean-square end-to-end distance ((R-R')(2)) of the polymer chain. Our findings are that the curvature of the surfaces induces a geometrical localization area; at short scales (L-->0) the polymer is locally flat and the mean-square end-to-end distance is just the Gaussian value Ll (l = Kuhn length), independent of the metric properties of the surface; at large scales (L-->infinity), ((R-R')(2)) tends to a constant value in the sphere case, and it is linear in L for the cylinder and reaches different constant values las a function of the geometry of the surface) for the curved torus. In the case of the cone, contraction of the chain is induced at all length scales by the presence of the vertex, as a function of the opening angle 2 alpha and the end position R' of the chain. Explicit crossover formulas are derived for G(R\R';L) and ((R-R')(2)).

The effect of thermal and epithermal neutron irradiation on the superconducting critical temperature and critical current density of some Li-doped YBa2Cu3O7-x, samples was studied. The critical temperature exhibits a peak and the critical current density a valley in their dependence on neutron fluence, for moderate dose. A simple model, based on the Van Hove scenario and the kinetics of the defect production, is used to describe both phenomena.

The systems Li1-xMn2-xO4-delta acid Li1-xNi1+xO2 were studied as oxidic cathode materials for 4 V rechargeable Li-ion cells. From a comparison of the results it follows that the spinel system can be subdivided into three regions: in region I (0 less than or equal to x < 0.05) the electrochemical performance is unsatisfying due to a large volume reduction of about 7.5%, inducing stress, multiphase materials and capacity fading. Region II (0.05 less than or similar to x less than or similar to 0.2) contains the best cathode material with a composition near x = 0.1. The spinel framework is stable against Li extraction. In region III (0.2 less than or equal to x less than or equal to 1/3) the capacity is too low for an application in the 4 V region. In the system Li1-xNi1+xO2 the highest capacity is observed for LiNiO2 with x = 0. Experimental difficulties during material synthesis were overcome by suppression of the decomposition of LiNiO2 into Li1-xNi1+xO2, Li2O and O-2. (C) 1998 Elsevier Science S.A. All rights reserved.

Iron-containing pyrochlores, having ions with various valencies on the B-sites, were investigated by magnetic measurements and Mossbauer spectroscopy. The influence of the ionic radius and of the valence state of the different cations accommodated on A or B sites on the super-exchange interactions and local distortions was emphasized. Three local configurations of iron were given as evidence in agreement with a model based on a random distribution of ions over the B-sites. (C) 1998 Elsevier Science S.A.

Electroluminescence of strained Si0.80Ge0.20Si(001) pin diodes has been investigated experimentally and by quantitative modeling. The key aspect of this investigation was that by selective epitaxial growth the experimental critical thickness for plastic relaxation (80 nm at T-epi=700 degrees C and large areas) could be increased in finite pads. SiGe layers with thickness of 60, 72 or 370 nm have been grown within the intrinsic i region of pin structures. Samples free of misfit dislocations revealed electroluminescence with the SiGe no-phonon peak and its transversal optical-phonon replica corresponding to interband transitions. It was found that by increasing the thickness of the SiGe layer the drop in the electroluminescence with increasing temperature could be shifted to higher temperature, so that for the 370 nm thick SiGe sample the emission was observed to persist still at 300 K. Modeling based on drift-diffusion and carrier recombination equations was used to simulate the current-voltage characteristics of the pin diodes and their band gap electroluminescence. It was found that the modeling results can account for the temperature and thickness dependence of the electroluminescence. Hole and electron Shockley-Read-Hall recombination times could be evaluated. (C) 1998 American Institute of Physics.

Carbon nitride (CNx) films have been prepared by UV (at 248 nm) laser-induced chemical vapor deposition using different materials (alumina, laser-activated alumina, pre-deposited Ti layers on alumina, sapphire and quartz) as deposition substrates. A mixture of ethylene. nitrous oxide and ammonia was chosen as the gas-phase precursor. The changes induced in the gas-phase composition by the irradiation in different experimental conditions were determined by IR transmission measurements. The film composition and morphology were studied by X-ray photoerectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron diffraction (TED). The degree of chemical content modification, especially its dependence on the nature of the substrate, is described for the first time. Depending on the substrate nature, the specific nucleation and growth morphology of crystallites were observed. The electron diffraction data agree well with experimental results obtained in previous works and theoretical data referring to crystalline alpha- and B-C3N4 phases. (C) 1998 Elsevier Science S.A.

Two aspects of the selective epitaxial growth of Si and Si(1-x)Ge(x) will be discussed. First the facet formation as dependent on the oxide wall orientation and the lateral size of oxide openings. Besides the often cited {111}, {113} and {110} planes additional planes were observed, the {119} and the {0 1 12} planes. Second, the reduction of misfit dislocation density by reducing the area of the pads allows strained Si(1-x)Ge(x) layers to grow much thicker than the critical thickness. As an application for the latter the electroluminescence of forward biased PIN diodes with strained Si(0.80)Ge(0.20)/Si(001) will be discussed as being dependent on the thickness of the SiGe layer. It was found that in thicker strained samples the band edge electroluminescence persists up to room temperature. Quantitative modelling of the electroluminescence could explain the temperature and SiGe thickness dependence of the electroluminescence. (C) 1998 Elsevier Science S.A. All rights reserved.

The supercurrent transport properties of epitaxial Bi2Sr2CaCu2O8+delta films in zero applied magnetic field were investigated in a temperature interval of approximate to 20 K below the mean-held critical temperature T-c0. The modification of the shape of the I-V curves observed by varying the temperature was explained in terms of vortex-fluctuation-induced layer decoupling and vortex-antivortex unbinding, revealing a strong probing-length dependence. The change of the effective dimensionality of thermally excited vortices involved in the dissipation process leads to the appearance of a few characteristic regions in the current-temperature diagram. Above a temperature value T*<T-c0, the superconducting layers are decoupled, as predicted by Monte Carlo simulations and renormalization-group analyses. In this region, the resistivity exhibits two-dimensional (2D) behavior corresponding to the superconducting (CuO2)(2) layers (2D-layer behavior. However, the resistive transition seems to be mainly related to the 2D behavior at the film level. In the sensitivity window of our measurements, finite resistance in the limit of small transport currents was detected to occur above a temperature value T-c. <T*, through the dissociation of vortex-string-antivortex-string pairs. By decreasing the temperature and/or by increasing the transport current, the I-V curves in the double logarithmic plot show a clear downward curvature. This can be described in terms of current-induced quasi-2D vortex pair unbinding, with a nonzero critical-current density resulting from the interlayer Josephson coupling. At even lower temperatures and/or higher transport currents, the I-V curves exhibit a crossover from quasi-2D to 2D-layer behavior, due to the decrease of the probing length below the Josephson length, where the interlayer Josephson coupling becomes irrelevant. The temperature dependence of the 2D I-V exponent is in good agreement with recent Langevin simulations of the Coulomb gas model, revealing an anomalous diffusion of vortex fluctuations.