National Institute Of Materials Physics - Romania

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5781 articles found

3831. A distinctive signature in the Raman and photoluminescence spectra of intercalated PbI2

Authors: Preda, N; Mihut, L; Baibarac, M; Baltog, I; Lefrant, S

Published: OCT 4 2006, JOURNAL OF PHYSICS-CONDENSED MATTER, 18, 8912, DOI: 10.1088/0953-8984/18/39/020

Through correlated studies using scanning electron microscopy (SEM), Raman spectroscopy and low-temperature photoluminescence (PL) we have demonstrated that the intercalated PbI2 with ammonia, poly (vinyl alcohol) and polyacrylamide are characterized by a distinctive signature in the Raman and photoluminescence spectra. After intercalation, the Raman spectrum of PbI2 reveals an orthorhombic structure, identical with that observed on the micrometric scale, of KPbI3 rod-like particles resulting from the reaction between Pb(NO3)(2) and KI, carried out in liquid media such as ethanol and acetonitrile. The rods and the intercalated PbI2 are characterized by a new and strong emission band at 2.23 eV (about 550 nm) that appears at 77 K under an excitation wavelength of about 340 nm. Modification of the Raman and PL spectra results from a compressing effect produced by the penetration between the PbI2 layers of different molecular species. The compression acting primarily on the iodine layers has the result of lowering the contribution of the 5p states of I- ions to the constitution of the electronic level situated at the top of valence band of PbI2, so that the photoluminescence of intercalated PbI2 acquires the characteristics of Pb2+ emission when it is dissolved in an alkali halide crystal.

3832. Electrosynthesis of the poly(N-vinyl carbazole)/carbon nanotubes composite for applications in the supercapacitors field

Authors: Baibarac, M; Gomez-Romero, P; Lira-Cantu, M; Casan-Pastor, N; Mestres, N; Lefrant, S

Published: OCT 2006, EUROPEAN POLYMER JOURNAL, 42, 2312, DOI: 10.1016/j.eurpolymj.2006.05.019

Electrochemical polymerization of N-vinyl carbazole (VK) on carbon nanotube (CN) films was studied by cyclic voltammetry in LiClO4/acetonitrile solutions. Cyclic voltammograms recorded on a blank Pt electrode were compared with those obtained when single or multi-walled CN films were deposited on the Pt electrode; in the latter case, a down-shift of the VK reduction peak potential was observed. Functionalization of CNs with poly(N-vinyl carbazole) (PVK) was invoked by Raman scattering and UV-VIS-NIR spectroscopic studies. The influence of sweep rate on the electrochemical properties of the PVK/CN nanocomposite and the performance of supercapacitors constructed using PVK-functionalized single-walled carbon nanotube electrodes were also evaluated. (c) 2006 Elsevier Ltd. All rights reserved.

3833. Magnetic interactions in NdFeB bulk permanent magnets with additions

Authors: Chiriac, H; Lupu, N; Chiriac, C; Valeanu, M; Kuncser, V

Published: OCT 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1769, DOI:

NdFeB-based bulk nanocomposite permanent magnets (with Dy and Co partially substituting Nd and Fe, respectively) with additions of Early Transition Metals (ETM = Mo, Ti, Zr, Cu, Nb, V, W), in form of rods with diameters ranging from 0.5 to 0.8 mm, have been prepared by devitrification annealing of amorphous and partly-amorphous precursors produced by injection die casting. A fully amorphous structure was obtained for rods with the diameter as large as 0.6 mm. The as-cast rod samples are magnetically soft and the magnetic hardness develops as the samples are devitrified to the optimum nanostructure. The best-achieved hard magnetic properties have been obtained for the optimum devitrified (15 min at 630 degrees C) Nd(3)Dy(1)F(3)e(66)Co(10)B(20) samples of 0.6 mm in diameter: H-i(c) = 296 kA/m, mu M-0(r) = 0.86 T, Mr/M-max = 0.65 and (BH)(max), = 74 kJ/m. The Mossbauer spectra indicate the coexistence of 4 magnetic phases: 2:14:1, alpha-Fe, Fe3B and a short-range ordered intermetallic Fe-B-type phase.

3834. Mossbauer study of the hyperfine interactions and spin dynamics in alpha-iron(II) phthalocyanine

Authors: Filoti, G; Kuz'min, MD; Bartolome, J

Published: OCT 2006, PHYSICAL REVIEW B, 74, DOI: 10.1103/PhysRevB.74.134420

The Fe-57 Mossbauer spectroscopy on alpha-iron(II) phthalocyanine (FePc) as a function of temperature (1.3 < T < 295 K) and applied field (0 < B < 10 T) has been used to study the peculiar magnetic properties of this ferromagnetic quasilinear chain type compound. One sextet with an internal hyperfine field B-int=66.2 T was observed at 1.3 K, a very large value for a bivalent iron with S=1 pointing to the existence of large positive orbital and dipolar contributions in the investigated FePc. Under an applied field, the experimental spectra exhibited two nonequivalent Fe positions, due to spin canting, with the values for the hyperfine fields of the split sextets increasing with increasing field, an indication that unlike most cases, B-int in alpha-FePc is positive, i.e., parallel to the magnetic moment of iron. Therefore, the origin of the large hyperfine field is the orbital moment rather than the Fermi's contact interaction. This fact is ascribed to the orbital degeneracy of the ground state of Fe(II) in the present configuration, where an unpaired hole occupies the orbital doublet (d(xz), d(yz)). This feature supports and explains the magnetization and susceptibility data as well as the anomalously high hyperfine field observed at Fe-57 nucleus. The relaxational behavior in the ac susceptibility and Mossbauer spectra found in the region 5-20 K was ascribed to solitonlike motion of domain walls within the magnetic chains, with a single-kink activation energy of 72 K.

3835. Magneto-sensitive nickel nanowires fabricated by electrodeposition into multi- and single-ion track templates

Authors: Ohgai, T; Enculescu, I; Zet, C; Westerberg, L; Hjort, K; Spohr, R; Neumann, R

Published: OCT 2006, JOURNAL OF APPLIED ELECTROCHEMISTRY, 36, 1162, DOI: 10.1007/s10800-006-9200-5

Polycarbonate templates of ( 30 +/- 1) mu m thickness containing cylindrical etched-track nanochannels of ( 500 +/- 50) nm diameter were used for electrodeposition of Ni nanowires. Using 10(4) channels per cm(2), the most favourable deposition potential of -1.0 V was determined in a potentiostatic mode by varying the deposition potential with respect to an Ag/AgCl reference electrode over a range between -0.1 V and -1.5 V. The deposition efficiency at -1.0 V was estimated around 10%. The resulting single wires had a resistance around 200 W and showed an anisotropic magnetoresistance (AMR) effect of 1%, applicable to directionally sensitive magnetic field sensors.

3836. Review on Bi-Sr-Ca-Cu-O whiskers

Authors: Badica, P; Togano, K; Awaji, S; Watanabe, K; Kumakura, H

Published: OCT 2006, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 19, R99, DOI: 10.1088/0953-2048/19/10/R01

We review the current status of the growth, characterization and applications of Bi-Sr-Ca-Cu-O (BSCCO) whiskers from the materials science point of view. Our analysis leads to the conclusion that reconsideration of the importance of the whisker studies is necessary; whiskers can play a major role in studies of the growth mechanism and defects, properties control and their understanding, and new devices and applications. Arguments favourable to the idea that BSCCO whiskers can possibly have a significant impact on future synthesis of nano-objects in this system or other high-T-c superconducting systems, as well as in investigation and understanding of the growth processes under elevated magnetic fields, are discussed. Emphasis is made on superconducting whiskers, but non-superconducting ones are also briefly addressed.

3837. Structure of bulk glassy As2Se3 and As2S3

Authors: Georgescu, G; Sava, F; Rares-Medianu, M

Published: OCT 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1805, DOI:

The structure of bulk glassy As2Se3 has been investigated by X-ray diffraction. The structure was compared with that of bulk glass As2S3 measured in the same conditions. Similarities and differences have been pointed out.

3838. Characterization of BaTi4O9 ceramics by Raman spectroscopy and XPS after ion etching

Authors: Catalin, N; Cernea, M

Published: OCT 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1883, DOI:

Barium tetratitanate (BaTi4O9) ceramic prepared by coprecipitation method was analyzed by Raman spectroscopy and by X-ray photoelectron spectroscopy (XPS). The orthorhombic crystalline structure of BaTi4O9 ceramic sintered at 1300 degrees C for 2h in air was identified on the Raman spectrum. The sample was etched in argon plasma and then analysed by XPS. The etching process did not influence significantly the XPS spectra of barium and titan suggesting a good compositional homogeneity of the BaTi4O9 ceramic prepared by oxalate method.

3839. Paracrystallinity or randomness: a challenge for noncrystalline structure

Authors: Popescu, M; Leonovici, M

Published: OCT 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 1837, DOI:

The structure of low coordinated disordered materials can be understood in the frame of basically different theories: the formation of a continuous random network with all the valence bonds satisfied, the formation of small clusters (amorphites) having all the bond at the margins satisfied (closed clusters) or open clusters with many dangling bonds that are partially compensated due to the formation of valence alternation pairs, and last but not least the formation of so-called nanoparacrystalline structures, i.e. deformed nano-crystalline structures. We show that an intermediate model based on a mixture of closed and open clusters as well as layer like configurations can explain the structure and properties of the binary arsenic chalcogenide glasses.

3840. In situ structural changes during toluene complete oxidation on supported EuCoO3 monitored with Eu-151 Mossbauer spectroscopy

Authors: Alifanti, M; Florea, M; Filotti, G; Kuncser, V; Cortes-Corberan, V; Parvulescu, VI

Published: SEP 30 2006, CATALYSIS TODAY, 117, 336, DOI: 10.1016/j.cattod.2006.05.036

Ceria-zirconia supported (10 and 20 wt.% EuCoO3) versus bulk EuCoO3 perovskite were prepared via citrate decomposition at 700 degrees C and tested for toluene complete oxidation. S-BET, XRD, XPS, H-2-TPD and in situ Eu-151 Mossbauer investigations were performed. Electron delocalization processes around the Eu cations are induced by the reaction, as proved via the evolution of the Mossbauer parameters. All characterization data indicate the formation of a well-dispersed EuCoO3 at the surface of Ce0.9Zr0.1O2 for the 10 wt.% loading and larger crystallite formation for 20 wt.% loading. In terms of intrinsic activity for toluene combustion, supported catalysts were more active than bulk EuCoO3. All structural changes during operation time are reversible. (C) 2006 Elsevier B.V. All rights reserved.