National Institute Of Materials Physics - Romania

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5781 articles found

3511. Dynamics of Cyanophenyl Alkylbenzoate Molecules in a Surface Layer Adsorbed onto Aerosil. I. Cyanophenyl Hexylbenzoate

Authors: Frunza, S; Schonhals, A; Goering, H; Manaila-Maximean, D; Beica, T; Frunza, L; Moldovan, R

Published: 2008, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 495, 431, DOI: 10.1080/15421400802430448

A composite prepared front aerosil A380 and the liquid crystal (LC) 4-hexyl-4'-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel-Fulcher-Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration.

3512. Photocatalytic and structural properties of mixed titania and zirconia aerogels

Authors: Danciu, V; Baia, L; Cosoveanu, V; Baia, M; Vasiliu, F; Diamandescu, L; Teodorescu, CM; Feder, M; Popp, J

Published: 2008, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 2, 80, DOI:

A series of TiO2-ZrO2 aerogels with different ZrO2 contents (3-10 %) were prepared by an acid catalyzed sol-gel method followed by low-temperature supercritical drying and heat treatment. The structure and morphology of mixed aerogels were studied by means of Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), xenon sorption and elemental analysis. XRD spectra refinements show that the calcined TiO2-ZrO2 aerogels consist mainly in anatase phase accompanied by a small fraction of tetragonal ZrO2. The Raman spectra proof that the network of the calcined mixed aerogels mainly preserves an anatase structure similar to that of the calcined TiO2 aerogel. The TEM images reveal that the anatase particles present round shapes with a highly homogeneous size (about 3-8 nm). Areas containing chains of amorphous tetragonal zirconia were also identified by the TEM investigations. The addition of zirconia to titania significantly increased the surface area and stabilized the anatase phase of titania. The best catalytic efficiency for the salicylic acid photodecomposition was obtained for the aerogel with 9.39% zirconia that exhibited an apparent rate constant of about three times higher than that of the pure titania aerogel.

3513. Growth by MOCVD of (001) Bi-2223 superconducting thin films on (001) and (110) MgO substrates

Authors: Endo, K; Badica, P

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012202

Superconducting c-axis thin films of Bi2Sr2Ca2Cu3O10 (Bi-2223) were grown by MOCVD on (001) and (110) MgO substrates. Films on both substrates show, as revealed by AFM investigations, similar morphology composed of regular rectangular grains and relatively low roughness of about two or three half c-axis unit cell order. AFM images also suggest that films might have in-plane epitaxy. The films have zero resistance critical temperatures of 95.1K for the (001) MgO substrate and 75K for the (110) MgO substrate, respectively. Considering very large films-substrate lattice mismatch between (110) MgO and (ab)-plane of the superconducting phase the growth of indicated films is a surprise.

3514. Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: structure, chemistry, morphology, and magnetism

Authors: Teodorescu, CM; Socol, G; Negrila, C; Luca, D; Macovei, D

Published: 2008, NSTI NANOTECH 2008, VOL 1, TECHNICAL PROCEEDINGS: MATERIALS, FABRICATION, PARTICLES, AND CHARACTERIZATION, 438, DOI:

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated by (i) X-ray photoelectron spectroscopy, (ii) the local atomic order by X-ray absorption near-edge structure (XANES), (iii) the morphology of the films was investigated by atomic force microscopy (AFM), and (iv) magnetic properties were quantified by magneto-optical Kerr effect (MOKE). In most cases, the chemical composition of the host matrix was found to be the vanadium (5+) oxide V2O5 at the sample surface and lower ionization states (+2 and +3) in the bulk. Co ions are found either in high ionization state Co5+ (for samples synthesized in high vacuum condition, denoted by VO1), or with Co in lower ionization states Co4+ (for samples synthesized in a mixture of argon and oxygen atmosphere, denoted by VO2). Consistent information was obtained from chemical shifts from individual core level scans in XPS, compared with existent data in litterature, and the amplitude of the pre-edge peak in XANES, which is a sign of quadrupole 1s -> 3d dipole-forbidden transition and whose amplitude is proportional to the number of 3d vacancies per atom. AFM revealed big particles with sizes > 100 mu m for VO1 samples, whereas smaller nanoparticles with sizes ranging between 20 and 30 mu m were observed for VO2 samples. VO1 samples presented very high coercitive fields with a relatively low saturation magnetisation at room temperature, whereas VO2 samples presented double-loop hysteresis curves, indicating the coexistence of exchange bias between two kinds of magnetic moieties with strong anisotropy.

3515. Analytical approximations of the NIEL in semiconductor detectors for HEP

Authors: Lazanu, I; Lazanu, S

Published: 2008, ROMANIAN REPORTS IN PHYSICS, 60, 78, DOI:

An important tool for the development of devices (detectors, cell solar, electronic circuit components) for high energy accelerator facilities or for space utilization, where new missions and experiments will be operated, is to find new materials with harder radiation properties. The radiation fields in these environments are extremely complex and the tests of the behaviour of different materials and devices for concrete situations are difficult to realise and very expensive. Thus, scaling of degradation effects would represent a useful tool and it is the main aim of the present contribution. Some analytical expressions for NIEL that suggest possible scaling formula are given.

3516. MODELING OF TRAP DISCHARGING PROCESSES IN MULTIPLE QUANTUM WELL STRUCTURES

Authors: Ciurea, ML; Iancu, V; Stavarache, I; Lepadatu, AM; Rusnac, E

Published: 2008, CAS: 2008 INTERNATIONAL SEMICONDUCTOR CONFERENCE, PROCEEDINGS, +, DOI: 10.1109/SMICND.2008.4703332

The paper presents the modeling of trap discharging processes in Multiple Quantum Well nanostructures. The coupling between trapping and detrapping phenomena, due to the CaF2 buffer layers is discussed. The relative role of tunneling and displacement currents is also analyzed. The model allows the determination of trap parameters that are not directly measurable. The results are in good agreement with the experimental data.

3517. Structure and magnetic properties of Fe/Fe oxide clusters

Authors: Crisan, O; von Haeften, K; Ellis, AM; Binns, C

Published: 2008, JOURNAL OF NANOPARTICLE RESEARCH, 10, 199, DOI: 10.1007/s11051-008-9463-3

Fe clusters have been synthesised in ultrahigh-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.

3518. The synthesis and characterization of poly vinyl chloride chemically modified with C-60

Authors: Rusen, E; Marculescu, B; Butac, L; Preda, N; Mihut, L

Published: MAY-JUN 2008, FULLERENES NANOTUBES AND CARBON NANOSTRUCTURES, 16, 185, DOI: 10.1080/15363830802042563

The fullerenated poly vinyl chloride was synthesized by the addition reaction of azido-substituted poly vinyl chloride with C-60. Structural characterization carried out using Raman and FTIR spectroscopy confirm the presence Of C-60 chemically bound in the new compound. Also, the photoluminescence properties of fullerenated poly vinyl chloride are considerably changed from those Of C-60.

3519. Spectroscopic ellipsometry study of amorphous SrxBa1-xNb2O6 thin films obtained by pulsed laser deposition

Authors: Ion, V; Galca, AC; Scarisoreanu, ND; Filipescu, M; Dinescu, M

Published: 2008, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 5, 5, +, DOI: 10.1002/pssc.200777818

Optical properties of amorphous Strontium Barium Niobate (a-SBN) thin film are investigated using spectroscopic ellipsometry. Since the SBN can be applied to optoelectronic devices, the dispersion of refractive index is desirable. The films are obtained by Pulsed Laser Deposition (PLD) onto MgO and Si substrates by targeting a SBN:60 monocrystal: The dielectric function of a-SBN is approximated using a single Tauc-Lorentz oscillator model. Thicknesses of the films and of their rough layer are in agreement with SEM and AFM results. The dispersion of the refractive index is presented in the 1 - 5 eV range.

3520. Fractal antennas for wireless communications

Authors: Nicolaescu, I; Goga, G; Banciu, MG; Ioachim, A

Published: 2008, 2008 IEEE INTERNATIONAL WORKSHOP ON ANTENNA TECHNOLOGY: SMALL ANTENNAS AND NOVEL METAMATERIALS, CONFERENCE PROCEEDINGS, +, DOI:

Design, simulation fabrication and measurement of fractal antennas for wireless communications are presented in this paper Two types of geometries, Le. Sierpinski gasket and Koch curve, were investigated The proposed antennas are compact, can operate for multiple frequency bands and exhibit small mutual coupling.