Publications

We study a first order transition of the anchoring direction imposed by PVA coating of SiO layer. Contrary to the model of anchoring competition, no vanishing of the anchoring strength was observed close to the transition. We explain this behaviour by memory effects.

The normal-incommensurate phase transition in Rb2ZnCl4 was studied by Rayleigh scattering of Mossbauer radiation. The discontinuity in resonance effect (epsilon) proves a stepwise variation of the inelastic component of the scattered radiation. The result is discussed in connection with the photon-phonon interaction. (C) 1999 Elsevier Science Ltd. All rights reserved.

Information about guest-host interaction concerning the incorporation of large complexes in molecular sieve was obtained using rhodamine 6 G as test molecule. We embedded the dye in samples of MCM-41 and EMT type molecular sieves and observed that high concentrations can be reached without undesirable dye aggregation. The excitation peak at 520 nm with shoulder at 528 nm shows that our molecular sieves behave as a hydroxylic, relatively polar environment, a little more polar than water.

Magnetic compounds of the type R2Fe17-xVx (R=Y, Gd and x = 1, 1.5, 2) were synthesized by are melting (with or without interstitial carbon) and studied by X-ray diffraction (XRD), Mossbauer spectroscopy and hysteresis loop measurements. Compounds with the structure Y2Fe17-xVx (x = 1, 1.5, 2) are hexagonal, whereas the intermetallics containing carbon introduced by melting and those of the type Gd2Fe17-xVx (x = 1, 1.5, 2) are rhombohedral. For each symmetry, the lattice parameters a/c were obtained from the XRD spectra and a considerable expansion of the unit cell volume was observed in the Y2Fe17-xVxC (x = 1, 1.5, 2) systems. Room temperature transmission Mossbauer spectra were analyzed using two methods: fit with several sextets, corresponding to the various inequivalent iron sites and fit with a hyperfine magnetic field distribution, to yield the average hyperfine magnetic field. The average hyperfine magnetic field was studied as a function of vanadium content x for all classes of compounds under investigation. The maximum hyperfine field values were obtained for the carbides. The hysteresis loops recorded at 4.2 K in an applied field of 1.5 T provided complementary information on the saturation magnetization as a function of vanadium substitution for each group of intermetallics involved. (C) 1999 Elsevier Science Ltd. All rights reserved.

Nanocrystalline CNx particles and thin films were produced in a flow reactor system by the CO2 laser pyrolysis of sensitised NH3-N2O-C2H2 reactant gas mixtures. Depending an the substrate nature (silicon or quartz) the SEM and TED analysis of the CNx thin films revealed specific nucleation and growth morphologies of crystallites, resembling the beta-C3N4 phase, in the case of C3N4 nanopowder synthesis, the relative concentration of the sensitisers (SF6/C2H4) was varied to,minimise SF6 decomposition. The majority of the data obtained from powder analysis (XRD, XPS, and ir transmission measurements) suggests the presence of CN bonded phases and the formation of alpha- and beta-C3N4 structures. (C) 1999 IoM Communications Ltd.

Superconductor-ferroelectric heterostructures were prepared by deposition of ferroelectric doped-PZT type material on the top of the superconductor YBCO thin films. Both, ferroelectric and superconductor thin films were obtained by off-axis RF magnetron sputtering method with a single target. On these heterostructures, measurements were made in order to evidence the influence of contact between the two materials on the electrical and thermal parameters. X-ray diffraction patterns of superconductor thin film show a good crystallization c-axis oriented perpendicular to the substrate plane, and a perovskite structure without traces of pyrochlore at 550 degrees C the ferroelectric thin film deposited by RF magnetron sputtering (off axis method). Important variations of the critical temperature of superconductor films have been obtained, due to the increased dielectric permittivity (epsilon).

Hematite powders with distinct particle morphology were obtained by hydrothermal synthesis, in the temperature range of 160-300 degrees C. Goethite and ferric hydroxide precursors prepared by precipitation and oxidation under different reaction conditions were used. The hydrothermal reactions were developed in aqueous neutral or alkaline suspensions. In some cases additives were used as growth shape agents. By changing and controlling the reaction parameters, oxide powders with desired particle shapes (acicular, polyhedral, platelike, spherical, hexagonal) and dimensions (0.1-30 mu m) were obtained. The characteristics of hematite powders, green bodies and sintered compacts were investigated by X-ray diffraction, electron microscopy, transmission and electron conversion Mossbauer spectroscopy. The correlation between the preparation conditions and the properties of the obtained iron oxides is discussed together with their potential applications. (C) 1999 Elsevier Science Ltd and Techna S.r.l. All rights reserved.

Photoconductive properties of SrBi2Ta2O9 thin films in the 250-400 nm wavelength range were investigated. The films were deposited on Pt/SiO2/Si substrates using the Chemical Solution Deposition and were crystallized by conventional thermal annealing at 850 degrees C. One sensitivity maximum was observed in the spectral distribution of the photoconductive signal and was attributed to band-to-band generation in the film. The wavelength corresponding to this maximum was found to be dependent on the applied voltage, on the delay time, defined as the time between the application of light on the film and the reading of the generated photocurrent, and on the wavelengths sweeping direction tug or down). The gap value was estimated to be around 3.94-4 eV. (C) 1999 Elsevier Science Limited. All rights reserved.

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a nonzero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.

High power laser diodes are of interest due to their potential use in medicine and military applications. This paper presents a systematic study of rapid degradation on our AlGaAs/GaAs large optical cavity devices. The increase of normalized threshold current vs. time was experimentally studied. An evaluation for normalized threshold current vs. absorption coefficient for different reflectivities is presented. For the optical output vs. time curve an abnormal increase of light characteristics was experimentally observed. The variation of the optical power was correlated to material parameters during operation and this behavior has been proposed as a practical criterion to select devices that are on their route to rapid degradation. The output optical power vs. electron fluence curve was measured for irradiated devices and a dislocation climb motion was assumed.