National Institute Of Materials Physics - Romania

The structural and morphological evolution of the NiZn ferrite during mechanical activation has been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM) and chemical analysis. During the milling process an important amount of metallic phases due to the vials and balls contaminates the ferrite powder and functions as co-participant to the mechanical activation. The XRD- spectra of the as-milled powders showed the presence of the metallic contribution up to 200 hours of milling. For longer intervals of time the extralines intensities due to the presence of metallic phases decrease and only a spinel phase is observed after 400 hours of total milling. The TEM image revealed, after 200 hours, spheroidal nanometric particles in the range 30-80 nm, alongside of some needle type particles. When the milling time was increased, some big plate monocrystals appear and grow. After 400 hour milling, polyhedral crystals 250 divided by 1000 nm size could be observed in the powder.

We measure the azimuthal anchoring strength and easy axis gliding of the nematic 5CB on weakly anisotropic substrate. We study the anchoring evolution at a time scale ranging from a few minutes up to a few months. We separate approximately the elastic and the viscous response and we obtain the gliding velocity as a function of the applied torque. We discuss the observed gliding times and nonlinear gliding viscosity.

The characterization of nanocrystals embedded in alkali halide crystals concerning shape and size distribution has been performed. Optical absorption, transmission electron microscopy (TEM), x-ray diffraction (XRD) and electron spin resonance (RES) methods has been used. The experimental results are in good agreement with the theoretical considerations concerning absorption and scattering of light by the metal clusters.

Annealing studies of a proton irradiated Si test diode (1MeV- neutron equivalent fluence is phi (eq) = 1.82x10(13) cm(-2)) were performed. The macroscopic characterization shows that during the annealing experiment the type inversion effect happened. The changes in the microscopic features during the annealing treatment were investigated using Thermally Stimulated Current method.

New aspects of the influence of water vapour in the interaction of SnO2 gas sensors with different reducing gases are evidenced by an original experimental method. This consists in measuring the conductance transients during and after rapid transitions from dry into humid air containing identical concentrations of methane and CO, respectively. The results indicate that while methane and water compete in reacting with one type of oxygen ions on the surface, CO reacts mainly with another type of surface oxygen, almost insensitive to water. (C) 1999 Elsevier Science S.A. All rights reserved.

Polymer-coated quartz microbalances were produced and studied. They exhibited high sensitivities for heavy noncyclical hydrocarbons and CO2 concentration measurements. Detection limit of 1 ppm CO2 and 10 ppm Shellsol (Shell solvent containing: 28.7% C10H22; 66.4% C11H24; 5% C12H26) are obtained and reported. The time constants of the absorption and desorption processes were investigated also. A diffusion-free behaviour was observed and related to the porosity of the layer. Collateral AFM (atomic force microscopy) analysis of the polymer structure was performed. We explain big values of the absorption-desorption time constant for CO2 and small values for CnH2n+2 mixtures in connection with the peculiarities of the chemical reaction involved by the sensing mechanisms. (C) 1999 Elsevier Science S.A. All rights reserved.

The local configurations and related hyperfine interactions of the tetrahedrally coordinated Fe2+ in two natural spinels were investigated. A special fitting procedure of the Mossbauer spectra is proposed in order to cover the involved mechanisms over the whole temperature range. The behavior of the quadrupole splitting vs temperature was successfully explained in terms of the split of the electronic 3d ground state doublet under a non-cooperative Jahn-Teller effect also providing estimates of the splitting energy for each configuration.

Surface-enhanced Raman scattering (SERS) has proved to be an effective technique for studying the structural properties of conducting polymer thin films. The enhancement process has a twofold origin, electromagnetic and chemical, The electromagnetic enhancement, which is the dominant mechanism in SERS generation, consists in the excitation of localized and delocalized surface plasmons (SPs) in the metallic support of the thin film, The Raman emission of the adsorbed molecules on the metal surface (the most efficient being Ag, Au and Cu) is due to the intense evanescent electromagnetic field located at the interface between the metal and the surrounding medium. The second enhancing mechanism for SERS is of chemical origin, involving the formation of new chemical bonds between the molecules and the metal surface, with the polarizability thus becoming considerably higher than that of the free molecules. This mechanism is as a rule accompanied by a metal-molecule or molecule-metal charge transfer, which partly accounts for the success of the SERS studies on conducting polymers. Unfortunately, these studies have revealed that the chemical effects at the polymer-metal interface varied substantially depending on the various types of polymer and metallic support. In this context, polyaniline containing two different entities (a reduced and an oxidized state) in its repeating units exhibits specific alterations in its SERS spectra depending on the type of metallic support. This paper presents new results concerning the structure of emeraldine-base and emeraldine-salt polyaniline thin films deposited on rough Ag and Au supports. The effect on the SERS spectra of the polymer-metal and polymer-ambient interface chemical reactions is also described. The presence of an interface compound depending on the oxidizing properties of the metallic support has a strong influence on the SERS spectra, no matter how the PAN films were deposited on the support (whether by solvent evaporation or by an electrochemical process, i.e. cyclic voltammetry). When an emeraldine base is doped with HSO4- ions, it turns into an emeraldine salt, which exhibits a disordered state in its macromolecular chain, leading to a modified profile of the similar to 1162 cm(-1) Raman line which is associated with the C-H bond of the quinoid ring. The Lorentzian profile is altered by the addition of a Gaussian profile component. For a rough Ag support, the SERS spectra show that the transformation of emeraldine base films into emeraldine salt films is a reversible process, Copyright (C) 1999 John Wiley & Sons, Ltd.

We solve numerically the time-dependent self-consistent random phase approximation Dyson equations for a system of electrons and holes of a direct gap semiconductor excited by an ultrashort laser pulse interacting with LO phonons in equilibrium. Special care is devoted to the proper treatment of the prehistory of the system by including the polaron effects. We show that for weak Frohlich coupling (alpha=0.1), the generalized Kadanoff-Baym relationship, which enables a one-time formulation, is excellently verified while for intermediate coupling(alpha=1) the deviations become important. [S0163-1829(99)03943-0].