Publications

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.

The supercurrent-transport properties of YBa(2)Cu(3)O(similar to)7 (Y: 123) polycrystalline hulk sintered samples with random grain orientation were thoroughly investigated. The dissipation process in these materials occurs due to intergranular flux motion, rather than by weak-link quenching. It was found that the model which takes into consideration collective creep of intergranular vortices is self-consistent in describing the low-voltage-level current-voltage characteristics and the intergranular irreversibility line of high-T-c superconducting ceramics.

Gd(2)Fe(14-x)M(x)C compounds, with M = Ni, Si, Cu or V and x less than or equal to 1.5 were investigated by Fe-57 Mossbauer spectroscopy. A preferential substitution of iron sites by the M elements is evidenced. The hyperfine parameters are discussed in correlation with the data obtained by magnetic measurements.

In Part I a general equation was derived for the intensity diffracted by a matrix phase containing intergrowths. It was applied to the case of Bi2Sr2Can-1CunOy phases (n = 2, 3) with stacking defects (''wrong'' number n of Cu-O planes in a half-cell). A least-squares program based on this equation yields the nature, concentration, and short-range order of the defect intergrowths. The program uses the peak shifts which are induced by the presence of defects. The analysis is exemplified on several BSCCO samples, whose defect state informs about the formation steps of the superconducting phases.

The presence of intergrowth with a different spacing inside a layered phase causes significant alterations in the X-ray diffraction pattern, concerning both line positions and widths. An analysis of these changes, based on a Hendricks-Teller-type formalism yields estimates about the defect nature, concentration and short-range order. The procedure was exemplified by simulation of XRD patterns for Bi-based oxidic superconductors, in which intergrowths of related phases are often present inside the matrix phase.

Temperature induced surface alignment transitions in nematic liquid crystals are analysed by means of an elastic model in which the effective splay-bend surface-like elastic constant is taken into account. Experimental data in hybrid cells are analysed and the value of the effective splay-bend elastic constant is found to be in good agreement with that obtained in homogeneous samples for the same liquid crystal. The validity of the model is successfully checked on data from the literature, for such different temperature dependences as those of MBBA at the free surface and on two types of SiO(x) deposited surfaces, and for E7 on a fluoropolymeric surface, exhibiting a temperature induced surface alignment transition from planar to homeotropic.