National Institute Of Materials Physics - Romania

The temperature dependence of the conductivity (between 15 and 300 K) and optical transmission spectra (between 0.8-3.5 mu m) have been measured on r.f.-sputtered Si100-xNix with 0 less than or equal to x less than or equal to 15. The films were characterized by Rutherford backscattering spectroscopy and X-ray diffraction. Changes in the optical sub-bandgap structure, with corresponding changes in the conduction mechanism take place by varying the Ni content. The films are amorphous, showing a main broad diffraction peak. Its position deviates, for this concentrationrange, from 2(kF) (k(F) = Fermi radius), the alloy losing the Hume-Rothery character present for 50 less than or equal to x less than or equal to 60.

The effect of LiOH addition to YBa2Cu3O7-x is presented. The crystal structure remains orthorhombic, but the orthorhombic splitting decreases from 0.0171 at 2 at. % Li to 0.0158 at 50 at. % Li. The critical temperature exhibits a decrease from 91-.18 to 83.79 K in the same range of lithium concentrations. From the magnetization curves were obtained the lower critical fields and the intragranular critical current density. The latter exhibits a negative power law, j(c) proportional to B-alpha, with alpha approximate to 1/2. V-I characteristics are typical for grained superconducting materials: V proportional to(I-I-c)(n(B,T)). The transport critical current dependence on temperature and magnetic field is also presented.

The authors obtained the Fe-57 Mossbauer spectra of two pumpellyite samples obtained from two different mineral sources in the Romanian Carpathians. Using a specialized computation program of the quadrupole splitting spectral parameter, QSCOMP, the observed quadrupole doublets were attributed to the ferric and ferrous ions in octahedral sites. The refined analysis of spectral parameters permitted us to characterize both the oxidation state, the electronic configuration and the coordination symmetry with the distortions and occupancies of pumpellyite constituting iron ions. The differences of the spectral parameters giving the data of the iron abundance, the chemical iron-oxygen bonds, the deformation and occupancies of iron octahedra, observed at the two studied samples, are discussed and correlated by the different genesis conditions of two pumpellyite-bearing occurrences.

An extended Williams-Landel-Ferry equation is derived within the free volume approximation. This equation relates the jump frequency of an arbitrary molecule (around the glass transition temperature) to the segmental jump frequency at the glass transition temperature. The connection with the usual WLF equation is discussed.

The current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.

An attempt has been made to put in evidence the modified V-K centres in low temperature UV irradiated Kl:Cl crystals. The interpretation of the results seems to favour a new type of V-K centre localised near an impurity halogen instead of a heteronuclear V-K centre.

Electron spin resonance investigations on polyethylene-polyaniline blends are reported. The temperature dependence of resonance line parameters, in the temperature range 270-400 K is investigated in detail.

We measured the temperature dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals. The penetration field H* was determined, at different temperatures, from the field dependence of chi '' in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near T-c, which are consistent with the usually reported data for this material.

The spectrum and the eigenstates of a finite two-dimensional tight-binding electronic system, with Dirichlet boundary conditions, in a magnetic field and with an external linear potential are studied. The eigenstates show an equipotential character, and may cross the plaquette in the direction perpendicular to the electric field. When leads are added to the plaquette, the channels carrying the current may be shortcut by equipotentials, resulting in additional plateaus situated between the usual integer-quantum-Hall-effect plateaus. This ideals confirmed by a numerical calculation within the four-terminal Landauer-Buttiker approach.

Pd-Tm/gamma-Al2O3 catalysts containing 0.1% Pd-1% Tm, 0.3% Pd-1% Tm and 0.5% Pd-1.% Tm, respectively have been obtained by successive impregnation of gamma-Al2O3, first with palladium and then with thulium. Introduction of Tm leads to an increase in the conversion of vinylbenzenes compared to that with monometallic Pd/gamma l-Al2O3 catalysts. For monocomponent Pd/gamma-Al2O3 catalysts, turnover frequency (TOF) is unchanged. For bicomponent Pd-Tm/gamma-Al2O3 catalysts, if one considers that the reaction occurs only on the Pd particles, then introduction of Tm seems to increase the TOF. However, by taking Tm into consideration when calculating the TOF, then the bicomponent catalysts are seen to behave as the monocomponent catalysts. This behaviour is highlighted also by using the Koros-Nowak criterion. If Tm is not considered to be an active species, vinylbenzene hydrogenation appears to be controlled by mass transfer processes. However, the texture characteristics of the bicomponent catalysts are not sufficiently modified by Tm introduction to support such a hypothesis. On the contrary, by considering the Tm participation in the reaction, the slope of the Koros-Nowak dependence is included in a range typical for kinetically controlled reactions.