Publications

An iterative method is presented for the characterization of lossy piezoelectric materials in the radial resonant mode, which provides a new formula to calculate the frequencies at which the electrical admittance is to be measured. This new method considerably reduces the measurement time by separating the programs for data acquisition and for calculus. The accuracies of the material constants for several soft and hard piezoceramic materials, covering a wide range of values of the planar coupling factor and mechanical quality factor, were tested and it was evidenced that they depended on the type of the piezoelectric material. The new method proved to be as accurate as, or in some cases even more accurate than, the other iterative methods. This new method can also be applied whenever the standard method does not allow the determination of the material constants. (C) 2002 Elsevier Science Ltd. All rights reserved.

Experimental results for the temperature dependence of density at normal pressure for two compositions of sodium lauryl sulphate/decanol/water solutions, exhibiting either a calamitic or a discotic lyotropic nematic phase at room temperature, are presented. Within the limits of experimental precision (+/-1 x 10(-5) g cm(-3)), the systems show no jump in density at the nematic to isotropic phase transition. Over the studied temperature range, the mean thermal expansion coefficients were also evaluated.

Broad-band dielectric spectroscopy (10(-2) to 10(9) Hz) is applied to investigate the molecular dynamics of NaY, a zeolite of faujasite structure. A do conductivity contribution and three relaxation processes were revealed in the dielectric spectra. The relaxation process in the high-frequency range ascribed to orientational fluctuations of water molecules inside the sodalite cages is studied in detail. As a main result it is found that its mean relaxation time has an unusual saddle-like temperature dependence, which obeys neither an Arrhenius nor a Vogel-Fulcher-Tammann law. A quantitative description was given on the basis of a model recently developed by Feldman et al. (J. Phys. Chem. B 2001, 105, 1845) to analyze the dynamics of water confined in nanoporous glasses. Reasonable values for the activation energies of orientational fluctuations and of defect formation as well as for the defect concentration were obtained.

In the field of pharmaceutical applications relying on encapsulation of drug by polymer coating, capsules based on biodegradable polymers with mean size of about 500 nm have been obtained by a patented emulsion-diffusion method. The morphology, size and structure of the nanocapsules (NC) control their pharmaceutical properties, especially release of the drug. Here is reported a decisive contribution of transmission electron microscopy (TEM) in revealing the controlled form and size of the NC: mean size values measured by TEM do agree with data obtained by laser granulometry. Moreover, the TEM magnification allows an estimation of the membrane thickness. (C) 2002 Elsevier Science B.V. All rights reserved.

Thin films of langasite (La3Ga5SiO14) have been prepared from langasite crystal targets by pulsed laser deposition. The films deposited at room temperature on silicon wafers are amorphous. After annealing at 700 degreesC, in normal atmosphere, the films crystallize as polycrystalline langasite oriented with the plane (001) parallel to the surface of the silicon substrate.

Broadband dielectric spectroscopy (10(-2)-10(9) Hz) was employed to study in a rather large temperature interval the molecular dynamics of the composite systems containing a nematic liquid crystal (LC), 4-n-pentyl-4'-cyanobiphenyl, confined to NaY, a zeolite with faujasite structure. Thermal analysis was also performed in order to get information concerning the loading of the zeolite and the possible phase transitions. No phase transitions and no relaxation processes of the LC molecules inside the pores were observed. But remarkably, in the presence of these molecules the dielectric behavior of the faujasite host changes due to modifications appeared in the mobility of Na+ ions. (C) 2002 Elsevier Science B.V. All rights reserved.

In-plane rho(ab) and out-of-plane p(c) resistivity measurements were performed on the same antiferromagnetic YBa2Cu3O6.25 single crystal over a wide range of temperatures T. rho(ab)(T) exhibits two crossovers with decreasing T: a crossover from metallic to weak localization behavior at 175 K and a second crossover to two-dimensional chiral variable-range hopping VRH behavior at 115 K. The latter reflects the topologic excitations induced in the spin system. rho(c)(T) displays an exp(-T-2/T-c,h(2)) dependence at high T and a VRH type dependence below 115 K. The T derivative of rho(c)(T) shows a kink at 32.65 K which we attribute to the antiferromagnetic ordering of the Cu( 1) spins.

The ferric form of the wild-type mouse neuroglobin (Ngb), a newly discovered heme protein which is primarily expressed in the brain of mammals, has been characterized by electron paramagnetic resonance (EPR) spectroscopy. The study reveals the simultaneous presence of two related structural forms in a wide range of pH values. The dominant low-spin form (>90%) with g-tensor principal values 3.15, 2.16 and 1.34 can be attributed to a His-Fe-III-His configuration. The high-spin form with g(perpendicular to) = 5.97 and g(parallel to) similar to 2, can be ascribed either to a hexacoordinated His-Fe-III-H2O form or to a pentacoordinated His-Fe-III. The high-spin to low-spin ratio is found to decrease with increasing pH values. (C) 2002 Elsevier Science B.V. All rights reserved.

Electrolytic colouration has been undertaken for not only a KCl crystal containing Pb2+ ions but also KCl crystals containing alkali impurity ions (Li+, Na+ and Rb+) besides Pb2+ ions. Absorption spectra of these electrolytically coloured crystals have been studied, together with the emission spectra. Similar spectra were observed among these crystals. Eight weak absorption bands are observed in the 290-700 nm region, while five intense bands are observed in the 200-290 nm region. It is suggested-that these bands arise from Pb- ions with tetragonal crystal field symmetry. The energy-level assignment for these absorption bands is made using the energy-level diagram of the isoelectronic Bi-0 atom with C-4v crystal field symmetry. It is suggested that an off-centre position effect of the Li+ ion is observed in the kCl crystal codoped with Li+ ions.. Broad emission bands at 1530 and 1580 nm have been. observed in electrolytically coloured KCl containing Pb2+ ions and KCl containing Li+ and Pb2+ ions, irrespectively.

We studied the YBa2Cu3O7-x bulk superconductor doped with BaZrO3 up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO3 doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO3 does not lead to degradation of the superconducting properties of bulk YBa2Cu3O7-x.