Publications

The experimentally determined current-voltage characteristics of high-T-c superconducting ceramics exhibiting the ''peak-effect'' in the magnetic field dependence of the transport critical-current density were described in terms of thermally activated flux creep at grain boundaries; taking into account the collective pinning of intergranular vortices. The peak effect was explained by considering the increase of the intergrain pinning potential at intermediate fields through the interaction of intergranular (Josephson type) vortices and intragranular (Abrikosov) vortices. The magnetic field dependences of the effective pinning potential and of the collective pinning exponent were experimentally determined, and the features of the I-V curves were explained through these dependences.

Within a one-dimensional model of off-centre diffusion it is shown that the diffusion coefficient depends on the number of off-centre sites and on their orientation relative to the host lattice.

Hemoglobin (Hb) interaction with Hg2+ ions was investigated by means of laser photolysis and spectroscopic methods. The binding of the gaseous axial ligands to the 3rd and 4th molecules of a Hb tetramer, 1(3)' and 1(4)', were found to show more relaxed features than in the absence of Hg2+ ions. A heme iron oxidation takes also place. The kinetic parameters of this interaction were compared with those of the systems Hb-Cu2+ and Hb-Sn(IV).

The new Fe(III) complexes with isosceles or equilateral configurations showed atypical magnetic behaviour with a magnetic susceptibility which does not obey the Curie-Weiss law. The experimental data were fitted by using the Heisenberg-Dirac-van Vleck model with a spin Hamiltonian containing different exchange parameters in the case of isosceles configuration. A more elaborate Hamiltonian, taking into account the perturbation arising from a biquadratic exchange interaction was used in the case of equilateral configurations.

The ferric chloride doped polymers change their refractive index under UV irradiation. The Fe:PVA is sensitive to ferric chloride concentration, molecular weight of PVA, and recording beam power. By Mossbauer investigations we tried to correlate the Fe2+ / Fe3+ ratio, as well as the temperature dependence of the energetical parameters with the assumed mechanisms which involve both the valence state of iron and a cross-linking reaction of PVA matrix.

Conductance transients of thick, sintered SnO2 films at step isothermal changes in oxygen partial pressure show exponential decays with time constants which cannot be explained in the frame of the usually accepted model of Schottky barriers at intergranular contacts dominating the conductance behavior. An alternative model, based on a linear relationship between conductance and the surface coverage by chemisorbed oxygen, accounts well for the experimental results. This model proposes the existence of a highly resistive sub-surface layer, isolating the surface from the bulk; it identifies the oxygen chemisorption sites with surface oxygen vacancies. The model is also supported by the temperature-variation of conductance in ''ramp'' experiments with increasing and decreasing temperature, and by the effects of prolonged heat treatments in oxygen upon the conductance and gas sensitivity.

The aqueous radioactive wastes are chemical systems as consequence of the variety of the contained cations. The chemical systems precipitated from FeSO4 and FeCl3 simulating the aqueous radioactive waste treatment have been investigated by Mossbauer spectroscopy. Mg2+, Ca2+ and Ba2+ were used as foreign cations, the calcium being among the main components of aqueous radioactive waste. The results showed the ion radius influence varying from Mg2+ and Ba2+ in the case of the precipitation from FeSO4. For the precipitation from ferric chloride the influences are not so evident.

The anion influence on the hydrated oxides formed by Fe3+ and Fe2+ precipitation simulating the aqueous radioactive waste treatment has been investigated by Mossbauer spectroscopy. Fe-3(SO4)(2), Fe(NO3)(3). FeCl3, FeSO4 and FeCl2 were used as iron sources and neutralized by NaOH. The obtained products contain several kinds of amorphous and/or poorly crystallized hydrated oxides, depending on the anion type which exists in solution and on the initial iron valence. The pH influence on the final precipitate is taken into account.

A method is proposed for estimating the energy of vanishing flow (also called balance energy, E(bal)) without reconstructing the reaction plane. This method is based on the shapes of experimental particle-particle azimuthal correlation functions. It is used for determining E(bal) for three systems: Ar + Al, Zn + Ti, Zn + Ni. The results are compared with estimations using flow-parameter analysis and with theoretical expectations.

The surface tension is calculated as the excess of the free energy per unit area, due to the presence of a surface layer, using Landau-de Gennes expansions, in the hypothesis of a first order transition in the bulk and taking into account the dependence of the surface free energy from the surface tilt angle. The surface order parameter is calculated and surface-ordered phase above the phase transition temperature has been found. A variety of calculated surface tension versus temperature curves with a jump at the phase transition, with positive or negative slope, well describing the experimental data from literature, have been attained.