Publications

This paper presents some of the more relevant results concerning the mass-loading influence on quartz resonators and monolithic filter characteristics, and recent experimental results about the mass-loading effect on electrical parameters of langasite resonators. Based on the Ballato's transmission-line analogs of the trapped-energy resonators vibrating in thickness-shear mode, the mass-loading effect on resonator characteristics was studied. The effective mass-loading, motional inductance and quality factor of quartz and langasite resonators, as a function of harmonic order for various electrode parameters, were computed. In the frame of the Tiersten's analysis of monolithic structure, the theoretical calculations of the coupling coefficient of two-pole quartz and langasite monolithic filters as a function of mass-loading were performed. The analysis of these results shows that the mass-loading influence on characteristics of langasite resonators and monolithic filters is lower than on those of the quartz devices.

The W-band (95 GHz) ODMR spectra of AgCl nanocrystals (NCs) embedded in a KCl single crystal lattice have been measured for various orientations of the magnetic field in a 100} crystal plane, Only self-trapped exciton (STE) signals are observed while no ODMR is found from distant electron-hole pairs. This confirms the conclusion of a previous ODMR study in the Q-band (at 35 GHz) concerning the spatial confinement effect and impurity exclusion on exciton recombination in NCs of AgCl. Through increased resolution, more accurate g- and zero-field splitting values of the triplet state were obtained in good agreement with those of the STE in bulk AgCl.

In this paper the results of electrical measurements of the mass-loading influence on AT-cut quartz resonator parameters are compared with those obtained by X-ray topography analysis of the same resonators. Based on the Ballato's transmission-line analogs of the trapped-energy resonators vibrating in thickness-shear mode, the mass-loading effect on resonator characteristics was studied and the effective mass-loading, motional inductance and quality factor of quartz resonators were computed. X-ray topography measurements were performed by transmission diffraction using the synchrotron radiation. Sawyer plan-parallel polished AT quartz resonators with 14mm diameter, 5MHz frequency, Au electrodes with various structures, diameters and thickness were used in experiments. The results of the X-ray topography investigations are in agreement with the previous ones obtained by electrical measurements [3,5,6,9].

This work describes the study of PbSe1-xTex film electrodeposition on glassy carbon and ITO electrodes in acid solutions (0.1 M HNO3), using cyclic voltammetry and rotating-disk techniques. Electrochemical impedance spectroscopy study of PbSe1-xTex film in 0.1M HNO3 were also presented.

Thin films of YBa2Cu3O7-x have been obtained on NdGaO3 substrate by pulsed laser deposition with an excimer KrF* (lambda =248 nm, tau (FWHM)greater than or equal to 20 ns) laser source from a stoichiometric target. In the present paper we report details of the technology of pulsed laser deposition of high-temperature superconductor thin films, substrates' requirements and high-temperature superconductor thin film characterisation. The study of some parameters' intercorelations for optimisation of the technology of the pulsed laser deposition of high-temperature superconductor thin films is also analysed Moreover, the contacting technology and techniques of characterisation are described.

Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values B-4(0) = 0.0009(3) cm(-1), B2 = 0.0006(2) cm(-1) and 4 4 B-4(4) = 0.0017(3) cm(-1), to be considered together with the second-order parameters D = -1.1677(7) cm(-1) and E = -0.0135(6) cm(-1).

In this work, the thermal degradation of PP blends with previously treated sisal fiber (10mm), and gamma irradiated in air at different doses (10, 25, 50, 60, 70 kGy), is studied. The treatment to which sisal fibers were previously submitted were: (a) the addition of a coupling agent (Silane A-172), 1% by weight, (b) a chemical treatment at different times (1/2 and 1 h) with NaOH, and (c) the addition of a functionalized polypropylene known as Polybond 3150 and 3200, 5% by weight. After the treatment, mixtures of PP with 20% of sisal fibers were prepared. Their thermograms were analyzed using different integral methods in order to evaluate the activation energy. For the blend of PP with sisal fibers, previously treated for 1 h with NaOH, the largest value amounted to 60 kJ/mol (approximately) for irradiation doses under 40 kGy. At higher irradiation doses a decay on the activation energy was observed. When the fibers were treated with NaOH for 30 min, the activation energy decreased from 55 kJ/mol to values under 45 kJ/mol, when subjected to high irradiation doses (> 50 kGy). This can be explained by the fact that, when sisal fibers are treated with NaOH solutions over a longer time, impurities and lignin are better removed resulting in a higher surface roughness; as a consequence a higher polymer to fiber interaction is possible, which, in turn, makes the blend more stable at low irradiation doses. Instead, when the sisal fibers were treated with silane, the activation energy for thermal degradation decreased at higher irradiation doses until an inversion of the tendency was observed for the 50 kGy dose; this may imply that this irradiation induces the production of PP-filler bridges. With composites containing compatibilizers, a similar behavior as with silane was observed but with the inversion on the tendency at 70 kGy. From these results, it can be concluded that a l-h treatment with NaOH produces a higher thermal stability than the others. Moreover, this treatment has the best profit/cost ratio, which makes it commercially profitable when compared with others. The mathematical analysis shows that the blend of PP with NaOH-treated sisal fiber at the two treatment times (1 h and 1/2 h), showed competing degradation and single decomposition processes for the irradiation intervals between 0-50 kGy and 50-70 kGy, respectively. Regarding the blends of PP with silane-treated sisal fiber or Polybond addition showed bimolecular decomposition behavior for both irradiation intervals. (C) 2001 Elsevier Science Ltd. All rights reserved.

The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI <5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-EN ponders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp(3), as in the case of c-BN, (C) 2001 Elsevier Science B.V. All rights reserved.

The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S = 1/2, local symmetry axis along one of the crystal [111] axes and principal g values g(parallel to) = 2.0032 +/- 0.0009 and g(perpendicular to) = 2.0094 +/- 0.0005 at T = 10 K. (C) 2001 Elsevier Science B.V. All rights reserved.

A method to produce composition modulated Bi2-xSbxTe3 alloys by electrodeposition has been developed. The electrolyte, which consists in a HCl 6M (pH = 0) aqueous solution, allows the codeposition of bismuth, antimony and tellurium to be accomplished at room temperature on glassy carbon, platinum or nickel electrode. The composition of the films. their crystal structures and morphology were studied as function of electrochemical parameters and bath composition. It is shown that the electrodeposits are monophasic and exhibit a polycrystalline state. The electrical resistivity was of the order of 7-10 mOmegacm, whereas the electrodeposition rate value was 30-50 mum/h for the films with good thermoelectric properties.