Publications

A combined sintering-sublimation method has been used to grow CdS or CdSeS photo-conducting polycrystalline thin films. By a fast firing technique, the films are grown on Cr2O3-x ceramic pellet sintered substrate. The film growing and the fast firing sintering processes take place simultaneously. The sintering-sublimation technique has been designed to increase the reliability of photoconducting devices, mainly by decreasing the donors' concentration which determines a higher sensitivity. This process becomes more effective when pellets containing raw CdS, respectively CdSeS, and a less reactive component - such as nonstoichiometric chromium oxide, are used. This oxide provides surfaces where thin CdS or CdSeS layers are crystallizing. At the same time, the nonstoichiometric Cr2O3-x can react with the oxygen allowing a more efficient control of the properties of CdS and CdSeS thin layer which is generated as a result of the above-mentioned sublimation-condensation process.

EPR and TEM investigations on sublimated solid Alq(3), [tris(8-hydroxyquinoline)aluminum(Ill)] are reported. Different paramagnetic centers in microcrystalline Alq3 fractions as synthesized and annealed are observed. Their nature, thermal treatment dependence, as well as correlation with the crystal structure are analyzed. The existence of randomly oriented triplets and quartets in annealed Alq3 fractions is evidenced from the temperature dependence of X- and Q-band EPR spectra. The spin Hamiltonian parameters are given and proposals for the model structures are made.

High quality superconducting thin films of HTS have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, design and performance.

Unusual conductivity behavior, is observed in the semiconducting phase of granular manganites. It is characterized by, at a certain temperature, of a switching process between two activation energies. A core-shell model is developed to explain the presence of the conductivity of the two activation energies. The model suggests that inside the disordered thin outer layer the energy bands are split into isolated energy levels. Additional correlations between paramagnetic behavior and the existence of the two activation energies within the core-shell model are furnished by the ESR spectroscopic technique. (c) 2006 Published by Elsevier Ltd.

The influence of uranium doping up to 2 at. % on grain size, electrical resistivity and dielectric constant of barium titanate ceramics has been investigated. The samples were prepared by the conventional ceramic technique using pure raw materials. The mixed oxides were calcined at 1100 degrees C. The pressed pellets were sintered at temperatures between 1300 divided by 1500 degrees C in controlled atmosphere of argon and oxygen. The electrical resistivity drastically decreased, from 10(9) to 10(3) Omega(.)cm, with increasing uranium content up to 1.5 at. % and increased thereafter to about 10(7)Omega center dot cm, for 2 at. % U. The dielectric constant slightly increased with increasing U-content, up to about I at. % U and then shows a sudden increase with about two orders of magnitude, having a sharp maximum at 1.5 at. % U, when sintered in argon atmosphere compared to only half an order of magnitude when sintered in oxygen atmosphere. The results are discussed in terms of the possible A and B sites occupied by uranium as well as the boundary layer enriched with vacancies.

The behaviour of the dielectric and piezoelectric properties of doped PZT ceramics, with variable lead content, has been investigated from -50 degrees C to room temperature. The PZT materials were prepared by the oxide technique and samples were sintered as pellets, at various temperatures. Niobium and lithium ions have been substituted for Ti4+, Zr4+ in the PZT matrix, and a small amount of Bi2O3 was added to cause A-position vacancies. The lead content in the four materials prepared varies in a narrow range, from 0.97 to 1.029 lead atoms, while the concentration of all other components are maintained constant. The microstructure of samples, evaluated by SEM technique and XRD analysis evidenced the presence of the tetragonal phase characteristic for the PZT 52/48. The dielectric and piezoelectric constants measured on poled samples depends on the sintering temperature, defects and domain walls, lead content.

The SANS facility existing at the Pitesti 14MW TRIGA reactor is presented. The main characteristics and the preliminary evaluation of the installation performances are given. A monochromatic neutron beam with 1.5 angstrom <= lambda <= 5 angstrom is produced by a mechanical velocity selector with helical slots. A fruitful partnership was established between INR Pitesti (Romania) and JINR Dubna (Russia). The first step in this cooperation consists in the manufacturing in Dubna of a battery of gas-filled positional detectors devoted to the SANS instrument.

We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Buttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano, lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano, dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.

Nanocomposites of iron /iron oxide/polyoxocarbosilane were prepared by the IR co-pyrolysis of Fe(CO)(5) and HMDSO. Their morphologies, chemical content and thermal behavior were studied by different analytical techniques. For examining the sensing capabilities of the low-polymer content nanocomposite thick films, the variation of the electrical resistance to CO and CH4 gases was tested. A promising behavior as concerning the sensitivity and the selectivity was found.