Publications

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.

The orientation properties of some complexes of chromium (III) or cobalt (II) with oxygen-containing ligands are presented. The orientation obtained is mostly homeotropic. The possibility of anchoring by coordination of liquid crystal (LC) molecules to the transition metal ion within the alignment layer is discussed on the basis of spectroscopic arguments.

The problem of computing the neutron optics of curved-crystal spectrometers is reviewed. The conditions of focusing in scattering and the limitations to achievable resolutions are discussed. Experience with programs for resolution-intensity optimization is presented. Computational examples are given of high resolution configurations with bent crystals and open beams, showing significant intensity gain over conventional techniques. Quasi-elastic scattering measurements at energy transfer resolutions below 10 mu eV are shown to be feasible at high luminosity. Curved monochromators for high resolution neutron powder diffraction, combining the beam focusing onto sample with focusing in scattering, are also discussed.

CdO whisker crystals with micrometre thickness and length of up to 4.5 mm were grown by a simple technology described in previous publications of the authors. The whiskers grow on CdS crystal substrates, previously covered by a thin metal or silicon layer, on annealing in air at atmospheric pressure. The growth initiator used was Ag, Au, Al, Si, Mn, Ni, Pd or Pt. A study was carried out of the effect of impurities on the conditions and efficiency of growth, as well as on the morphology and composition of the whiskers obtained. The small crystals were examined using an optical microscope, a scanning electron microscope, X-ray structural analysis and X-ray microanalysis. The maximum speed of growth obtained with Au was about 1 mm h-1. The whiskers grow with a face-centred cubic lattice. The largest crystals are in the form of a parallelipiped with facets of {100}. Data are obtained which support the vapour-liquid-solid mechanism of crystal growth. The possibility is also shown of growing whiskers on zinc or cadmium plates covered with a thin layer of Cu, Ag, Au or Al by applying the same technology.

The coherent one-phonon scattering cross section measured in time-of-flight (TOF) neutron diffractometry is derived for any ratio between the sample-to-detector flight path and the total flight path. For the particular case of scattering by acoustic phonons in monocrystals, the differential cross section is described in terms of the scattering surface in four-dimensional space (Q(e,omega)), where HBARQ(e) is the momentum transfer for elastic scattering and HBARQ(e) is the energy transfer. This cross section is required in calculating the thermal diffuse scattering (TDS) correction for TOF neutron diffractometry.

Pd-Dy/gamma-Al2O8 and Dy/gamma-Al2O3 catalysts were prepared using the wet impregnation method. As supports was used Pd/gamma-Al2O3 and gamma-Al2O3. BFETM, TED and SAED analysis of these catallysts revelead the presence of the rare earth element as aluminate, oxide or unidentified forms. The rare earth compounds synthesis is controlled by the palladium percents on gamma-Al2O3.

An evaluation of the tilt angle variation observed recently when unrubbed polyvinylic alcohol is deposited on obliquely evaporated SiO layers is presented. The method of calculation supposes that the variation of the tilt angle is due to the modification of the initial topography of SiO during the coverage process with the polymeric film. The Laplace equation was obtained by minimizing the elastic energy of the system in the uniconstant approximation and was analytically solved. The found variation of the tilt angle is in good agreement with the experimentally observed one.

A high pressure technique for the direct synthesis of polycrystalline HgTe in quartz ampoules is presented. An excess of mercury as well as a quasi-static regime of the inert gas pressure al melt temperatures up to 750 degrees C have been used. X-ray diffraction and SEM confirmed the structural homogeneity of the synthesized material.

Mossbauer measurements performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both as thin films and ribbons show a dependence of the spins orientation and [H-hyp] versus temperature, Gd content and preparation conditions. Increasing the Gd content, the initial low anisotropy disappears and [H-hyp] decreases. A sharp increase of the anisotopy with temperature in ribbons with low Gd concentration is evidenced.