National Institute Of Materials Physics - Romania

The problem of the thickness dependence of the dielectric constant, as well as the extrinsic contributions to its value, is analyzed for the case of epitaxial Pb(Zr,Ti)O-3 (PZT) thin films. It is shown that the frequency dependence of the measured capacitance is best simulated by an equivalent circuit incorporating the trap-containing capacitance of a Schottky contact. The thickness dependence of the dielectric constant, calculated using the formula of a plane capacitor, appears to be an extrinsic effect due to the interface phenomena in the metal-ferroelectric-metal structure. The intrinsic dielectric constant of the PZT material seems to be thickness independent and of low value of about 30-40. This is closer to the values estimated from Raman measurements or from quantum theories of ferroelectricity. The thickness independence is also proven by piezoresponse force microscopy measurements. The presence of traps is evidenced by the presence of a photovoltaic effect at subgap wavelengths.

Field Assisted Sintering Technique (FAST) is currently used to consolidate ceramic, metal and composite powders. This method combines a pulsed electrical current with simultaneous application of an external pressure. In this article we report fabrication by FAST of high density MgB2 ceramic with addition of 5% SiC and 5% B4C. The influence of sintering conditions such as temperature and time on bulk density, morphology and superconducting properties of the samples was investigated. All samples had bulk density of more than 90% of the theoretical one and the same critical temperature T-c = 38.5 K from magnetisation measurements. These preliminary results suggest that FAST is a promising method for processing of MgB2 superconductor.

Fine-grain BaTi0.87Sn0.13O3 (BTS13) powder was synthesized from an oxalate precursor and used to prepare sintered ferroelectric BTS13 ceramics. The evolution of the morphology and structure of a BTS13 precipitate precursor as a function of temperature was analyzed. We compare the sintering behavior as well as the dielectric properties of the BTS13 ceramics obtained using uniaxial and cold isostatic pressing.

Results regarding the growth of carbon nanostructures by using a low-pressure expanding radiofrequency plasma jet are reported. The growth was performed in an argon plasma injected with acetylene precursor, in the presence of ammonia or hydrogen, on a substrate covered with nickel or cobalt catalysts. Various forms of nanostructured carbon material were obtained, depending on the catalyst size, ammonia or hydrogen / acetylene ratio and substrate temperature. The results exemplified in the paper describe nickel nanoparticles coated in highly ordered graphite shells, granular carbon, two dimensional carbon nanoflakes, nanowires and nanorods.

High permittivity dielectric materials (epsilon(r) > 20) are used in mobile communications due to low dielectric loss in microwaves and millimeter waves, specific temperature dependence of the dielectric properties and high dielectric permittivity, which allows device miniaturization. Preparation of Ba(Zn1/3Ta2/3)O-3 ceramic materials (BZT) with optimal properties for applications requires special thermal treatments due to the difficult control of the cationic ordering. The samples were prepared by using solid-state reaction, doped with Eu2O3, ZrO2, Nb2O5, or Al2O3-Y2O3 and sintered at the following temperatures: 1400, 1500, 1550 degrees C for 2 or 3 hours. In order to improve the microwave properties, annealing treatments at 1410 degrees C for 10h or 30h were performed on ceramic samples. XRD was used for compositional and structural characterization. The XRD data allowed the study of the transition from the pseudo-cubic cell to the hexagonal cell. The dielectric properties were measured in the microwave range and were correlated with additives and structural properties. Sintering temperatures greater than 1500 degrees C are required for doped BZT compositions, in order to obtain high dielectric constant (epsilon(r) similar to 28) and high quality factor (Q = 10000 at 10 GHz).

Photoluminescence (PL) spectra recorded at room temperature (RT) in polycrystalline BaFCI:Eu2+ X-ray storage phosphor have shown the europium associated luminescences, a broad luminescence band at 385nm (3.22eV) with a 3.3 +/- 0.1 mu s PL lifetime accompanied by a luminescence line at about 368nm (3.37eV). The energy difference Delta EB-L =226cm(-1) between the two excited levels from which the europium associated band and line luminescences took place was computed. An additional broad luminescence band at about 425nm (2.92eV) with 0.7 +/- 0.1 mu s PL lifetime has been attributed to the europium luminescence perturbed by an impurity. Time resolved spectroscopy measurements have shown that no other impurity luminescences (like oxygen-vacancy centres) can be observed at RT except the europium associated ones.

Two practical Nb3Sn composite wires (short samples) were investigated from the points of view of critical current, I-c, and cracks evolution versus number of bending load cycles. Bending was applied manually at room temperature using a block-form with curved surface corresponding to applied bending strains of 0.5% and 0.8%. It was applied in plane, alternatively in one direction and in the opposite one up to 130 times. We shall use for this mechanical treatment term pre-bending. The wires had approximately the same diameter but a different architecture: one was of conventional type (standard US-Japan wire, Hitachi Cable) and the second one was CuNb reinforced wire (Furukawa Electric Co. Ltd.) with reinforcement located near-the-edge. It was found that critical current maximizes gradually with the number of bendings and the maximum value of I-c, is attained for a number of bending loadings, N-pb, of about 15. Beyond this optimum N-pb, critical current is constant. Optimum N-pb is likely not dependent on the wire, applied field or pre-bending strains. Results suggest that Cu is important in the work hardening process. Work hardening during pre-bending is imposing limitations in application of this technically convenient mechanical treatment for release of the residual strain and further enhancement of I-c. Semi-quantitative analysis of the electron microscopy observations shows that for our experimental conditions the density of cracks is approximately constant being in agreement with described I-c, behavior. Other possible consequences of our results are discussed. Among the most interesting issues is possibility of the 3D independent control of yielding and hence of residual strain release. Several new practical ideas that will need future confirmation are proposed: e.g. application of loading-unloading cycles of torsion treatment in combination with tensile or pre-bending treatments might be useful.

In order to be used for medical applications, a material must exhibit many specific characteristics, of which the most fundamental is related to biocompatibility. Lately, the iron oxide nanoparticles are preferred for biomedical applications because they present a superparamagnetic behaviour at room temperature. The preparation method and conditions of iron oxide nanoparticles represents some of the most important challenges that will determine the particle size and shape, the size distribution, the surface chemistry of the particles and consequently their magnetic properties. The samples were characterized by X-ray diffractions (XRD), transmission electron microscopy (TEM) and infrared spectroscopy.

The influence of the Mn, Se and Sb impurities on the structure and morphology of US thin films grown on p(+) Si wafers was studied. The starting powders were mixed in the same molar ratios (0.3%) and deposited in the same conditions by vacuum thermal evaporation. X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and reflectance studies made on thermal treated thin films (573 K, 2 h in air) evidenced that thin films have a hexagonal oriented structure, and that dopants enter into the US lattice merely by substitution. The dopant nature influences the thin film thickness and chemical composition. The doped US thin films have roughness in nanometer region and a reflectivity lower than 40%. Silicon substrate acts as a template and favors the retention of Mn and scatters the Sb dopants. The CdS:Se thin film is thicker than CdS:Mn and CdS:Sb ones and is a mixture of doped and undoped nanocrystals. (C) 2007 Elsevier B.V All rights reserved.

The synthesis by pulsed laser deposition technique of zinc oxide thin films suitable for gas sensing applications is herein reported. The ZnO targets were irradiated by an UV KrF* (lambda = 248 nm, tau(FWHM) similar to 7 ns) excimer laser source, operated at 2.8 J/cm(2) incident fluence value, whilst the substrates consisted of SiO2(001) wafers heated at 150 degrees C during the thin films growth process. The experiments were performed in an oxygen dynamic pressure of 10 Pa. Structural and optical properties of the thin films were investigated. The obtained results have demonstrated that the films are c-axis oriented. Their average transmission in the visible-infrared spectral region was found to be about 85%. The equivalent refractive indexes and extinction coefficients were very close to those of the tabulated reference values. Doping with 0.5% An and coating with 100 pulses of Au clusters caused but a very slight decrease (with a few percent) of both transmission and refractive index values. The coatings with the most appropriate optical properties as waveguides have been selected and their behavior was tested for butane sensing. (C) 2007 Elsevier B.V. All rights reserved.