Publications

Two series of PT-type materials doped with Eu+3 with the formula PbxEuyTi0.98Mn0.02O3 (x=0.98, 1.00 and y=0.00, 0.02, 0.04, 0.08, 0.12) were prepared by conventional ceramic technique. The compounds with compensated and uncompensated valences were sintered between 1150 and 1220degreesC. The two types of compounds and the content of the donor ion, Eu+3 , influence the mechanical and dielectric properties of sintered samples. The spectra and data obtained on Eu-151 Mossbauer measurements are discussed.

It is shown that self-organization can be revealed in non-crystalline materials. Experimental and modelling studies are discussed. The importance of the Boolchand intermediary phase for the chalcogenide network glasses with self-organization is pointed out. The basical principles of self-organization are the key for the production of nano-materials with new properties.

Hybrid polymer based on selenium-sulphur doped by paraffins has been deposited by pulsed laser deposition. The properties of the thin films were investigated. Sample stabilization was carried out by annealing. A nano-crystalline fraction was obtained in the films annealed at 100degreesC. The structure of the nanocrystals corresponds to the Se3S5 crystallographic form. The films exhibit optical memory properties and are suggested for use in optical phase change memories.

The thermal stability of amorphous Al2O3 films (similar to8 and 80 nut thick) deposited by atomic layer deposition on HF-last and thin SiO2 covered (001) Si substrates is studied by transmission electron microscopy. The layers are in- and ex-situ annealed in the same temperature range.

(Ba1-xSrx)TiO3 solid solution (BST), with different molar compositions (x = 0.25, 0.5, 0.75, 0.9) Were prepared by conventional solid-state reaction from raw materials. Structural parameters, crystallite sizes and bulk densities were determined. Perovskite type polycrystalline structure of the BST ceramics was revealed by X-ray diffraction (XRD) data. The dependence of permittivity and losses at low frequency (1 kHz) was analyzed. The microwave investigations at room temperature revealed dielectric constant around 1000 and loss smaller than 1% at 1.1 GHz. The results indicate that some BST dielectric ceramics are suitable in paraelectric phase for microwave devices. The dielectric constant can be adjusted by using a DC-bias field and such materials are appropriate for manufacturing electric field controlled components such as resonators, phase shifters etc.

Starting from the idea that energies of adsorption, dissociation and surface reactions, involved in gas interaction with SnO2 chemoresistive sensors, are gas-specifics, we present a new method for improving selectivity, which consist in combining the variations of two physical parameters: thermal effect Delta T and electrical resistance R.

As-grown 4H-SiC epitaxial layers were investigated by deep-level transient spectroscopy to study the formation of the well-known Z(1,2) defect with energy levels normally detected at about E-C-0.7 eV. Chemical vapor deposition, applying various nitrogen-doping concentrations and C/Si ratios (1.2-3) in the gas phase, was used to prepare the samples. The Z(1,2) defect concentration was observed to increase with the incorporated nitrogen concentration. The dependence was linear for medium C/Si ratios (1.5-2.5). The highest and lowest applied C/Si ratios (3 and 1.2) enhanced and suppressed the Z(1,2) defect formation, respectively. This behavior tentatively suggests a complex of nitrogen with interstitial carbon atoms or, less probably, silicon vacancies. In particular, the correlation between the Z(1,2) defect formation and the nitrogen incorporation was clearly shown in the present investigation, in contradiction to conclusions of other authors. Previously reported negative-U properties of the Z(1,2) deep-level defects could be confirmed. A 1:1 relation between the concentrations of Z(1) and Z(2) was obtained for the present as-grown epitaxial layers. (C) 2002 American Institute of Physics.

The DyNi5-xAlx compounds crystallize in CaCu5-type structure for an aluminum content xless than or equal to1.5 and in HoNi2.6Ga2.4 type for xgreater than or equal to2. The saturation moments per formula unit extrapolated at 0 K, in composition range x<2 are similar to9.2 mu(B) in agreement with x-ray photoelectron spectroscopy studies which show negligible crystal field effects. For xgreater than or equal to2 a mictomagnetic type contribution to magnetic ordering develops which becomes dominant only for the DyNi2Al3 compound. Band structure calculations on DyNi5-xAlx show that the Dy 5d band is polarized parallel to the 4f moment. For DyNi5 the nickel moments at 2c and 3g sites are antiparallel oriented to that of Dy. The nickel contributions to magnetization decrease when increasing aluminum content and are near nil for xgreater than or equal to2. Above the Curie points the reciprocal susceptibilities follow linear dependencies. The effective nickel moments decrease from 2.11 mu(B)/atom (x=0) to 0.87 mu(B)/atom (x=2.5). The magnetic behavior of nickel is analyzed in models which take into account the electron correlation effects in d bands. (C) 2002 American Institute of Physics.

The crystallization of amorphous SmxFe80-xB20 melt spun ribbons is studied over an extended range of composition (0 < x < 8). Differential scanning calorimetry scans reveal different endo- and exothermic effects depending upon the composition. X-ray diffraction and Mossbauer studies for samples annealed at temperatures close to the onset of the first exothermic effect prove that, during the primary crystallization of the amorphous ribbons, both alpha-Fe and Fe3B phases appear-with Sm ions randomly accommodating the Fe sites in the tetragonal Fe3B lattice-while the subsequent exothermic reactions correspond to the decomposition of the metastable Fe3B phase into alpha-Fe and Fe2B and to the formation of the ternary Sm1.1Fe4B4 phase. Fully crystallized ribbons with Sm contents lower than about 7 at.% show the co-existence between alpha-Fe and Fe2B soft magnetic phases and the Sm2Fe14B magnetic one. The enhancement of magnetic properties with increasing relative proportion of magnetic phase is discussed and correlated with exchange coupling and spin wave stiffness data obtained from the magnetic measurements.

The effect of foreign impurity atoms (Dy and Pr) in as-evaporated amorphous As2Se3 thin films on the photodarkening relaxation was investigated. The photodarkening amplitude depends on the kind and concentration of rare-earth dopant, as well as on the thickness of the sample. The observed variation of photodarkening amplitude with the sample thickness is due on the amount of absorbed photons generating the photodarkening effect. The photodarkening kinetics is described by a stretched exponential function with the dispersion parameter alpha less than or equal to 0.5, which indicates high dispersion of the relaxation process in untreated as-deposited chalcogenide films.