National Institute Of Materials Physics - Romania

We introduce a theoretical analysis of the temporal and spatial evolution of the surface topography of solids following interference between incident and scattered pulsed laser beams. The essential role played by the nonlinear delayed feedback in the laser-radiation-surface system is considered. We show that it finally determines the surface topography evolution from pulse to pulse. In order to complete the analysis, numerical calculations have been conducted under the hypothesis of strong attenuation of laser radiation into the sample and of a limited heat diffusion during the action of a laser pulse. We predict an evolution from very simple to complex (chaotic) structures under multiple-pulse-laser irradiation of solid surfaces. This evolution is determined by some key irradiation parameters; initial surface microrelief, incident laser intensity, and the number of applied laser pulses. Experiments were performed in order to check the main predictions of the theoretical analysis. The system of transversal excited atmospheric pressure-CO2 laser radiation (lambda = 10.6 mum)-interacting with fused silica was chosen as appropriate for performing test experiments. Optical microscopy studies of laser-treated zones evidenced special modifications of the surface topography in good accordance with the conclusions following from the theoretical analysis. The theoretical analysis is also in good agreement with some available data from the literature, at the same time providing a coherent interpretation of previously unexplained behaviors.

The peculiarities of the photocapacitance effect in dry processed Cu2S-CdS heterojunction, have been investigated. The light flux dependence of capacitance is analysed. The main effect of low-level illumination is the narrowing of the 'intrinsic' layer that is formed as a consequence of Cu diffusion from the Cu2S layer. It was found that, in photocapacitance effects, a major role is played by the intermediate recombination centres present in the near-interface region.

The time-dependence of the conductance G(t) of SnO2:Pt:Sb gas sensors after introducing CO in atmospheres containing oxygen and water vapour was investigated. Depending on experimental conditions, different G(t) behaviours were observed over different temperature ranges: either monotonic increases towards steady-state values, or self-sustained oscillations. Part I describes and analyzes the non-oscillatory behaviour, Part II the oscillatory behaviour. A unitary model is proposed, which offers a qualitative explanation for both, by taking into account an assembly of interdependent surface phenomena, including adsorption, desorption, spill-over and chemical reactions of oxygen, carbon monoxide and water. The role of water vapour is emphasized. Though speculative, the model is consistent with a broad range of experimental results and it also allows for some general considerations about utilizing the SnO2:Pt:Sb material as CO sensor and as oxidation catalyst.

Plastic deformation of glassy ribbons of Fe80B20 leads to the partial destruction of the short-range compositional order of the material. The electrochemical potential measured between two electrodes made from glass ribbons of composition Fe40Ni40B20 shows a non-linear dependence of potential on the uniaxial deformation of one electrode. These effects explain the reduction of corrosion resistance of the glassy metallic alloys subjected to deformation.

The Gd2Fe14-xMxC compounds form solid solutions in the composition ranges x less-than-or-equal-to 1.0 for M = Cu or V and x less-than-or-equal-to 1.5 for M = Si or Ni The Curie temperatures increase when Fe is replaced by Ni, Cu and Si and decrease when this is substituted by V. The above behaviour is correlated with site occupancy of substitutional elements in P4(2)/mnmm-type lattice. The decrease of magnetizations, at 4.2 K as result of substitutions, is greater than that expected from simple dilution behaviour. The magnetic data are analysed in correlation with those obtained in corresponding borides.

The temperature dependence of the resistivity and magnetoresistance-elastoresistance of r.f.-sputtered Ni-Si thin films with variable silicon content (0-24 at.%) (determined by Rutherford backscattering spectroscopy) annealed to 300 degrees C was measured. The films, investigated by X-ray diffraction and transmission electron microscopy, exhibit a polycrystalline structure of the Ni f.c.c. type with a disorder degree which increases with the silicon content and a metallic-type conduction mechanism. The ferromagnetic properties of Ni are still preserved in the Ni-Si thin films up to 8.8 at.% silicon content.

The competitive effects of iron substitution by silicon and of the interstitial nitrogen atoms on the structural and magnetic properties of R2Fe17-xSix compounds, with R=Y or Gd, are analysed. The values of Curie temperatures and of mean iron moments are correlated to the exchange interactions (between iron atoms) in the parent and nitrogenated compounds. The magnetic behaviour of iron is analysed on the basis of the mean field model and it is shown to present mainly a localized character in the interstitial compounds.

A model for conduction that fits SnO2 polycrystalline layers with a grain size below 50 nm is presented. Quantitative evaluations using bulk parameters, found in the literature, and the assumption that surface oxygen vacancies are sites for atmospheric oxygen chemisorption permit a simple formulation of the problem: conduction by homogeneously distributed electrons along cylindrical filaments. An expression for the mobility is proposed, in which diffuse surface scattering is taken into account. On its basis, a relationship between the electrical mobility and the concentration of the reducing gas is theoretically derived for the first time. The predictions of the model for the dependence of conductance, concentration of conduction electrons and their mobility on the reducing-gas concentration are compared with experimental data and found to be in good agreement.

The introduction of more reactive precursors for Pb and Sr (oxalates), as well as Ca (citrate) and the use of a Bi nitrate decomposition route, has increased the percentage of the high-T-c (2223) phase in the Bi-Sr-Ca-Cu-O (BSCCO) system. Partial substitution of Bi(Pb) with Sb gives an almost single (2223) phase sample. In addition, a single (2212) phase sample is obtained when high purity Bi2O3 is used as a precursor, whereas Bi acetate leads to semiconducting behavior. The morphology of the samples is studied with a scanning electron microscope (SEM), the stoichiometry with energy-dispersive x-ray analysis (EDAX), and the structure with x-ray diffraction (XRD), while the superconducting properties are investigated by de-resistivity, ac-susceptibility, and SQUID magnetometry techniques.