Publications

The molecular dynamics of 4-octyl-4'-cyanobiphenyl and 4-pentyl-4'-cyanobiphenyl, confined to the mesopores with diameters less than 25 Angstrom of the molecular sieve AlMCN-41, was investigated over a wide temperature range using broadband dielectric spectroscopy. As expected, the phase transitions of the bulk liquid crystals cannot be detected for the confined molecules. A new, slow relaxational process occurs over the whole temperature range due to the motions of the molecules confined in the pores; the temperature dependence of the characteristic frequency obeys a Vogel-Fulcher-Tamman (VFT) law associated with a glassy state. The orientational order within these small pores is mostly dominated by the local environment. (C) 1999 Elsevier Science B.V. All rights reserved.

Thermally stimulated current (TSC) measurements performed in the 100 K-400 K temperature range on Bi4Ti3O12 (BiT) thin films annealed at 550 degrees C and 700 degrees C had revealed two trapping levels having activation energies of 0.55 eV and 0.6 eV. The total trap concentration was estimated at 10(15) cm(-3) for the samples annealed at 550 degrees C and 3 x 10(15) cm(-3) for a 700 degrees C annealing and the trap capture cross-section was estimated about 10(-18) cm(2). From the temperature dependence of the dark current in the temperature range 20 degrees C-120 degrees C the conduction mechanism activation energy was found to be about 0.956-0.978 eV. The electrical conductivity depends not only on the sample annealing temperature but also whether the measurement is performed in vacuum or air. The results on the dark conductivity are discussed considering the influence of oxygen atoms and oxygen vacancies.

The paper describes the structural properties of some Fe80-xCrxP20 (1 less than or equal to x less than or equal to 7) mesoscopic (similar to 1 mu m) particles produced by mechanical alloying. The influence of the atomic concentration of Cr and that of the milling time are examined. The hyperfine held distribution obtained from the room temperature Mossbauer spectra of the as-milled samples exhibits three well separated peaks, associated with different chemical short range order, and depending on the atomic concentration of Cr atoms. (C) 1999 Elsevier Science B.V. All rights reserved.

Amorphous C/Ni superlattice films designed as normal-incidence reflector for 5 nm have been grown on float-glass substrates by magnetron sputter deposition in Ar discharge. A comprehensive set of characterization techniques has been applied: grazing X-ray reflection (0.154 nm), atomic force microscopy and transmission electron microscopy in order to determine the quality of the structure. By comparing the results, it could be concluded that RF-magnetron sputtering technique is a good choice for growing such layered synthetic microstructures. (C) 1999 Elsevier Science B.V. All rights reserved.

The contrast variation method, applied in a neutron small angle scattering experiment, was used to simultaneously obtain information about molecular and magnetic structure of ferrofluids. Limitations caused by size polydispersity of particles were considered. (C) 1999 Elsevier Science B.V. All rights reserved.

We have considered the stationary state of a one-dimensional Gaussian polymer chain of length Nl (N is number of segments, l is the Kuhn length) subjected to a one-parameter class of exactly solvable, symmetric, repulsive potentials with multiple (two or three) minima. We have calculated analytically and numerically the exact Green's function G(R\R-';L) and the mean-square end-to-end distance [(R-R-')(2)], respectively, of the polymer chain. The instanton approximation is translated in polymer physics language and used to analyze the conformation of the polymer chain in its ground state, by evaluating the average number of folds that connect the potential minima. All quantities are functions of the barrier height and of the separation between wells. Our results show that for a given length N, the polymer expands with increasing barrier width until a maximum value of [(R-R-')(2)] is reached. Afterwards, the polymer apparently collapses in one of the wells. This behavior defines a critical length N* and may offer the possibility of applications in separation and pattern recognition processes. (C) 1999 American Institute of Physics. [S0021-9606(99)50122-0].

Soft type piezoceramics with stoichiometric composition Pb(Nb0.02Li0.007Zr0.51Ti0.463)O-3 were prepared by the usual ceramic technique. Several samples with lead oxide in excess of up to 4% wt, were prepared. Careful measurements of the density of the sintered disc-shaped materials and the piezoelectric properties (piezoelectric voltage constant, g(31), piezoelectric strain constant, d(31), dielectric permittivity,epsilon(r), electromechanical planar coupling coefficient, k(p), mechanical quality factor, Q(m)) were carried out. All piezoelectric properties were spectacularly enhanced by addition of small amount of lead oxide. The maximum values were reached for samples containing 2-3 wt.% PbO, regardless the sintering temperature. The experimental results were discussed in terms of the two competitive processes: structural rearrangement and dissolution-reprecipitation, which are active in the sintering process in the presence on the excess PbO liquid phase. The last process seems to be dominant and explains the improvement on the piezoelectric properties of PZT materials.

Thin films prepared by pulsed laser deposition and single crystal implanted LiNbO3 wafers used in optoelectronic technology were studied by transmission electron microscopy. The use of high electron beam density for high resolution imaging induced a decomposition effect in the implanted LiNbO3 lattice. It is shown that the most probable decomposition leads to the formation of NbO2 nuclei by a topotactic solid state reaction along the pseudo-cubic directions of the lithium niobate matrix. It is also shown that powdered LiNbO3 specimens or samples obtained by preparation of cross section of LiNbO3 thin films are more stable in the electron beam, compared with the samples obtained from implanted LiNbO3 wafers.

The effect of low amounts of dysprosium, samarium and manganese (0.1 - 0.5 at.%) on the properties of the chalcogenide glassy semiconductors As2S3 and As2Se3 is investigated The metals have significant influence on the structural, optical, electrical and photoelectric characteristics. The fundamental absorption edge shifts towards longer wavelengths with doping. The largest shift was observed for As2S3 glasses doped with Dy and Mn. The conductivity is significantly increased by addition of Mn and Dy. All the dopants hamper the photoconductivity with the exception of 0.5 at. % Dy in As2Se3 which generates a broad secondary band in the photoconductivity spectrum near 1.05 eV, ascribed to small amounts of crystalline phase. Close correlation was revealed between the medium range order in the network, the metal electronegativity and the activation energy of conductivity.

Structural and magnetic properties of melt-spun Cu-Co-(Fe-Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred. (C) 1999 Elsevier Science B.V. All rights reserved.