National Institute Of Materials Physics - Romania

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.

ZSM-5 samples with a silicon-to-aluminium atomic ratio of 25 were impregnated with gallium, indium or thallium salts. The acidic and textural properties of the samples were determined either before or after reduction. The acidity was investigated by infrared spectroscopy using pyridine (Py) as test molecule, titration with KOH or Py as well as ammonia adsorption. Differences were revealed concerning the dispersion of impregnated species and the distribution of the acidic sites. The sites containing ionic thallium were spectroscopically identified; the other sites were indirectly evidenced. Under our impregnation conditions, gallium species seem to enter inside the zeolite to a higher extent than was reported for similar samples. The contribution of indium to the sample acidity was reduced because it is deposited as a distinct phase onto the external surface. An adsorption mechanism operating during impregnation was proposed on the basis of electrochemical measurements; it was used to account for the acidity data. The textural properties were interpreted in terms of the deposition of the impregnation species near the pore mouth, hindering the entrance of the test molecules.

We compare a set of results concerning our measurement of the optical absorption (OA), photoconductivity (PC), thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) on the same crystal. We also investigate the effects of various preillumination treatments at room or liquid nitrogen temperature. We conclude that illumination with infrared light (rather than solely red) plays an important role which has been underestimated in the past.

The effects of pulsed excimer laser irradiation (lambda = 308 nm, tau = 10 ns) on the magnetic and structural properties of amorphous and nanocrystalline Fe73.5Cu1Nb3Si22.5-xBx (x = 6 and 9) were investigated by transmission Mossbauer spectroscopy, scanning electron microscopy and energy-dispersive X-ray analysis. In the amorphous specimens, two types of compositionally-dependent kinetics have been deduced for the laser-induced changes to the easy magnetization axis: controlled magnetic anisotropy (x = 9) and random distribution of magnetic moment directions (x = 6). Micrometer-size crystalline precipitates of alpha-Fe have been observed in the Fe73.5Cu1Nb3Si16.5B6 sample irradiated at the highest repetition rate. Pulsed excimer laser treatment of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 has been found to enhance the fraction of crystalline Fe-Si alloy (18 at. %) with DO3 structure, and to determine a random distribution of magnetic moment directions in the remaining amorphous grain boundary phase. Excimer laser processing is suggested as a powerful tool for controlling the magnetic anisotropy and phase equilibrium in amorphous and nanocrystalline materials