National Institute Of Materials Physics - Romania

(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.

Homogeneous thin films have been obtained by UV-excimer pulsed laser deposition from a PMMA bulk material. The structure of films is at variance with the structure of the bulk polymer. The size of the ordered domains, as well as the structural features, evidenced by X-ray diffraction in the deposited films, depend on the fluence of the ultraviolet laser pulse used in the ablation process and on the temperature of the substrate. The radial distribution functions of the bulk PMMA material and thin films show differences in the short range order ascribed to the breaking of the polymer chains and monomer dissociation during deposition process.

Calcium orthophosphates (CaP) and hydroxyapatite (HA) were intensively studied in order to design and develop a new generation of bioactive and osteoconductive bone prostheses. The main drawback now in the CaP and HA thin films processing persists in their poor mechanical characteristics, namely hardness, tensile and cohesive strength, and adherence to the metallic substrate. We report here a critical comparison between the microstructure and mechanical properties of HA and CaP thin films grown by two methods. The films were grown by KrF* pulsed laser deposition (PLD) or KrF* pulsed laser deposition assisted by in situ ultraviolet radiation emitted by a low pressure Hg lamp (UV-assisted PLD). The PLD films were deposited at room temperature, in vacuum on Ti-5Al-2.5Fe alloy substrate previously coated with a TiN buffer layer. After deposition the films were annealed in ambient air at 500-600 C. The UV-assisted PLD films were grown in (10(-2)-10(-1) Pa) oxygen directly on Ti-5Al-2.5Fe substrates heated at 500-600 degreesC. The films grown by classical PLD are crystalline and stoichiometric. The films grown by UV-assisted PLD were crystalline and exhibit the best mechanical characteristics with values of hardness and Young modulus of 6-7 and 150-170 GPa, respectively, which are unusually high for the calcium phosphate ceramics. To the difference of PLD films, in the case of UV-assisted PLD, the GIXRD spectra show the decomposition of HA in Ca2P2O7, Ca2P2O9 and CaO. The UV lamp radiation enhanced the gas reactivity and atoms mobility during processing, increasing the tensile strength of the film, while the HA structure was destroyed. (C) 2002 Kluwer Academic Publishers.

A survey of the main NDT methods used to evaluate the quality and properties of wood and wooden structures is given. Among the most important ones there arc visual inspection, sonic and ultrasonic stress wave. Some other methods include: deflection, electrical, isotope and Xray methods. The ultrasonic methods seem to be the most suitable ones in many cases especially for quality control purposes, but these methods depend on the quality of the piezoelectric transducers. Consequently, the construction and characteristics of a new type of piezoelectric transducer are reported. The new transducer is a composite type transducer, consisting of a sandwich type piezoactive element and a steel cylinder glued together. The sandwich piezoactive element was made of five thinner discs of 20 mm diameter and 2 mm thickness. The whole construction has the resonant frequency at 60 kHz, proper to wood investigation. The transducer can be successfully used for NDT of wood, either in the trough transmission or pulse-echo arrangements. The transducers were tested for sound velocity measurements in different wood samples.

The structural configurations that govern the behavior of the amorphous chalcogenides under excitation are the nano-scale, disordered, layer-like clusters. The competition between breaking, formation and switching of bonds at the cluster boundaries, accompanied by precipitation-dissolution phenomena, gives rise to fractal morphology of these clusters and to the reversible scalar photo-structural effects. The photoinduced anisotropy phenomena are explained in the frame of a model with stimulation of chiral elements induced by polarized light in the disordered layers of the clusters.