National Institute Of Materials Physics - Romania

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.

The concentration of primary radiation defects induced by charged pions, in the energy range 20 MeV-50 GeV, in the bulk of silicon, GaAs and diamond has been calculated in order to characterise the radiation resistance of these materials. The mechanisms by which pions impart energy to the lattice have been analysed and their contribution has been evaluated starting from the data on pion-nucleus interaction. The energy partition of the primary recoil nuclei between ionisation and displacements has been considered in the frame of the Lindhard theory. The energy dependence of the concentration of primary radiation defects presents two maxima: one in the region of the delta resonance, and another one around 1 GeV. The main conclusion is that diamond is hardner to pion irradiation than both silicon and GaAs in the whole energy range investigated. The pion-induced degradation in the inner detector system at LHC has been evaluated, starting from the simulated spectra, and considering different detector materials: diamond, silicon and GaAs. (C) 1998 Elsevier Science B.V. All rights reserved.

Raman measurements were carried out for sputtered amorphous SiNi thin films. Raman scattering in a-Si is not influenced by a low Ni content (less than or equal to 8.3 at.%) to any significant degree, the position of peaks and their widths are not changed. A shift to lower frequencies and a broadening of the high frequency band transverse optical (TO) (which results in a redistribution of phonon states to lower frequencies) are observed with an increase of the Ni content. These modifications are correlated with the presence of Ni in the coordination sphere of Si, as resulted from the calculated local structure in Ni-Si alloys on the basis of the diffraction patterns. (C) 1998 Elsevier Science S.A. All rights reserved.

Thin Au-C-60 films with global composition AuxC100-x (x between 0.35 and 4.50 at.%) were prepared by vacuum co-deposition and investigated by X-I ay diffraction, EDS, XTEM and electrical transport. Evidence indicated the formation of distorted nanocrystalline Au, presumably stabilized by interface electron transfer into C-60 LUMO. Electrical results are interpreted in the frame of the dominant current model, with a continuous density of localized states induced by Au in the C60 gap. There is also evidence for variable range hopping at low temperatures. (C) 1998 Elsevier Science S.A. All rights reserved.

The resistive transition R(T) and the current-voltage characteristics (I-V) in the range 10(-9)-10(-6) V and close to the critical temperature (T-c) were measured in a single-grain of melt-textured YBa2Cu3O7-delta (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the (a,b) planes and in zero external magnetic field using a SQUID picovoltmeter. In the case of the melt-textured sample, we measured the I-V curves also at higher levels of dissipation. For T < T-c, we interpreted the lower part of the I-V curves in the framework of the Jensen-Minnhagen model of current-induced unbinding of thermally-created vortex-antivortex pairs, which leads to V proportional to I(I-I-c1)(a-1). Our results confirm a recently proposed current-temperature (I-T) phase diagram by S.W. Pierson. In addition, we determined for the first time the dependence of the characteristic (unbinding) current I-c1 with t = 1 - T/T-c for temperatures very close to T-c and low transport currents. (C) 1998 Published by Elsevier Science B.V. All rights reserved.