National Institute Of Materials Physics - Romania

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5781 articles found

4241. Mossbauer study of manganese-doped magnetite below the Verwey transition

Authors: Sorescu, M; Diamandescu, L; Brand, RA; Tarabasanu-Mihaila, D

Published: FEB 2004, MATERIALS LETTERS, 58, 888, DOI: 10.1016/j.matlet.2003.07.028

Samples of FC3-xMnxO4 (x=0-0.6) were studied below the Verwey transition (T=122, 77, and 4.2 K) by Fe-57 Mossbauer spectroscopy. Considering that Mn2+ substitutes for Fe2+ on octahedral sites, we inferred the existence of five subspectra at all temperatures, one for Fe3+ ions on tetrahedral sites, two for Fe3+ ions on octahedral sites, and two for Fe2+ ions on octahedral sites. The hyperfine magnetic fields of one Fe3+ sublattice and two Fe2+ sublattices decrease slightly with increasing the amount x of Mn substitutions. The results are consistent with the existence of localized octahedral Fe3+ and Fe2+ spin states below the Verwey temperature. (C) 2003 Elsevier B.V. All rights reserved.

4242. Spin magnetization of a strongly correlated electron gas confined in a two-dimensional finite lattice

Authors: Nita, M; Dinu, V; Aldea, A; Tanatar, B

Published: FEB 2004, PHYSICAL REVIEW B, 69, DOI: 10.1103/PhysRevB.69.073106

The influence of disorder and interaction effects on the ground state polarization of the two-dimensional correlated electron gas is studied by numerical investigations of the unrestricted Hartree-Fock approach. With the model of Anderson disorder a continuous increase of the spin magnetization until the fully polarized regime is obtained. The ferromagnetic ground state is found to be favorable when the electron number is lowered and the interaction and disorder parameters are suitably chosen.

4243. Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Authors: Beica, T; Moldovan, R; Tintaru, M; Enache, I; Frunza, S

Published: FEB 2004, CRYSTAL RESEARCH AND TECHNOLOGY, 39, 156, DOI: 10.1002/crat.200310163

Optical bireffingence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10(-3) was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (C) 2004 WILEY-VCR Verlag GmbH & Co. KGaA, Weinheim.

4244. Soft chemical synthesis and characterisation of some substituted ferrites

Authors: Olar, R; Badea, M; Diamandescu, L; Cristurean, E; Marinescu, D; Mihaila-Tarabasanu, D; Stanica, N; Brezeanu, M

Published: JAN 28 2004, JOURNAL OF ALLOYS AND COMPOUNDS, 363, 262, DOI: 10.1016/S0925-8388(03)00452-3

Substituted ferrites with the chemical composition MxFe1-xFe2O4-nH(2)O (M: Mn(II), x: 0.35, n: 0.4; M: Ni(II), x: 0.38, n: 1; M: Zn(II), x: 0.45, n: 0.35) have been prepared by a soft chemical method using as starting materials magnetite and suitable acetates. These compounds have been characterised by elemental chemical analysis, thermal analysis, IR spectroscopy, X-ray diffraction on powder, transmission electron microscopy studies as well as Mossbauer spectroscopy and magnetic measurements at room temperature. The X-ray diffraction study indicates that the substituted ferrites adopt a cubic structure and the size of particles determined from the diffraction peak is fairly close to that indicated for agglomerates by transmission electron microscopy. The magnetic methods agree to the substitution of Fe(II) in the octahedral positions preserving the inversed spinel structure. (C) 2003 Elsevier B.V. All rights reserved.

4245. Critical current density and magnetic-field irreversibility of Na0.35CoO2 center dot 1.3H(2)O superconductor

Authors: Badica, P; Kondo, T; Togano, K

Published: JAN 26 2004, APPLIED PHYSICS LETTERS, 84, 561, DOI: 10.1063/1.1641512

Critical current density J(c) of Na0.35CoO2.1.3H(2)O superconductor (T-c=4.4 K) has been estimated from magnetization measurements: J(c)(1.9 K,0 T)=5x10(4) A/cm(2), J(c) (1.9 K,0.4 T)=4x10(2) A/cm(2) and J(c) (3.4 K,0 T)=10(3) A/cm(2). J(c) versus applied magnetic field H cannot be fitted, as for high-T-c superconductors (HTS), by a power law for any temperature. The temperature dependence for a given H is not of the linear type as for MgB2, but is close to quasiexponential (associated with the thermally activated flux creep) as for HTS. Irreversibility field data points H-irr versus (1-T/T-c) obey a power law with power exponent n=3.4. The irreversibility line is located below, but in vicinity of, the line for Bi-2223. Details of morphology are also presented. (C) 2004 American Institute of Physics.

4246. Nanocrystalline rhombohedral In2O3 synthesized by hydrothermal and postannealing pathways

Authors: Sorescu, M; Diamandescu, L; Tarabasanu-Mihaila, D; Teodorescu, VS

Published: JAN 15 2004, JOURNAL OF MATERIALS SCIENCE, 39, 677, DOI: 10.1023/B:JMSC.0000011529.01603.fc

4247. Ba/Zr=1 : 1 freeze-dried and conventional chloride powders: synthesis of BaZrO3 and phase formation

Authors: Badica, P; Aldica, G; Iyo, A; Bradea, I; Jaklovszky, J

Published: JAN 2004, MATERIALS LETTERS, 58, 256, DOI: 10.1016/S0167-577X(03)00455-5

Precursor powders of Ba/Zr = 1:1 chloride have been prepared by freeze drying (FD) and conventional mixing. A simple, laboratory-made apparatus for sublimation of the frozen water from the cryo-particles is presented. Both powders have the same phase formation behavior. Up to 500-600 degreesC, Zr chloride transforms into ZrO2. At high temperatures, BaCl2 starts melting and decomposing and two subsequent processes take place: (a) chlorination of the Zr atoms from the oxide and (b) formation of BaZrO3 phase, These processes depend on treatment atmosphere and reactivity of the starting powders. Freeze-dried powders with higher reactivity and/or O-2 atmosphere are suitable to obtain high content of BaZrO3 phase. In the freeze-dried powder, decomposed in O-2 atmosphere at 1190 degreesC for 20 h, 90% of BaZrO3 phase content was attained. No new phases were observed during decomposition of the freeze-dried powder. In the as-synthesized freeze-dried powder, some XRD lines remained unidentified. (C) 2003 Elsevier B.V. All rights reserved.

4248. Synthesis of spinelic oxides using polynuclear malates coordination compounds. Nickel ferrite

Authors: Mindru, I; Patron, L; Carp, O; Marinescu, G; Pascu, M; Pascu, G; Diamandescu, L

Published: MAR-APR 2004, REVUE ROUMAINE DE CHIMIE, 49, 245, DOI:

Nickel ferrite was obtained by the thermal decomposition of three malates coordination compounds: [Fe2Ni(C4H4O5)(2.5)(OH)(2)]NO3.5H(2)O, [Fe2Ni(C4H8N2O3)(4)](NO3)(8).24H(2)O and (NH4)[Fe2Ni(C4H4O5)(3)(OH)(3)].3H(2)O. The solid state transformations were investigated by means of TG/DTA analysis, IR, X-ray diffraction, Mossbauer, magnetic and BET investigations. The solid state mechanisms, as well as the characteristics of the final products are strongly influenced by the nature of the precursors, especially by their homogeneity at atomic scale and composition.

4249. Microanalysis of Pb(Zr, Ti, W, Li)O-3 piezoeceramic

Authors: Ramer, R; Dimitriu, E; Boerasu, L; Luga, A

Published: 2004, EURO CERAMICS VIII, PTS 1-3, 264-268, 1176, DOI: 10.4028/www.scientific.net/KEM.264-268.1173

A PZT-type material with the composition near the morphotropic phase boundary and the chemical formula of PbZr0.51Ti0.465W0.015Li0.01O3 is studied. in this material, both tetragonal and rhombohedral phase are present. The proportion of the two phases in material depends on the sintering temperature. The nondestructive microanalysis is performed with an electron probe microanalyser (EPMA), equipped with wavelength dispersive spectrometers and energy dispersive X-ray analyzer. The X-ray elemental line scans are carried out on the following chemical elements: Pb, Zr, Ti and W. The electron and X-ray images provide maps with the elemental distribution in the lead matrix, in phases, inclusions and grain boundaries. Based on microanalysis results, dielectric and piezoelectric properties are discussed.

4250. Facet oxidation and degradation of AlGaAs/GaAs pulsed laser diodes

Authors: Ghita, RV; Lazarescu, MF; Manea, AS; Logofatu, C; Vasile, E; Ciupina, V

Published: 2004, ROMOPTO 2003: SEVENTH CONFERENCE ON OPTICS, 5581, 273, DOI: 10.1117/12.582822

Oxide films grow in atmosphere as a result of oxidation reaction and mass transport of the element through the oxide film. In most III-V compounds (as GaAs) semiconductor oxides of III-element and V-element are formed individually. The AlGaAs/GaAs laser diodes were exposed to facet oxidation at mirror surfaces in normal environmental conditions for a storage process. It was observed a decrease of optical output power in the early stage of experiment together with a constancy of the ratio P/P(0) for the long time term (month). We suggest the development at the mirror laser facet of a natural oxide film as Ga2O3 and GaAsO.. We present a SEM (Scanning Electron Microscopy) image of a natural oxide film grown in atmosphere at the surface of AlGaAs as laser facet, as well as a picture of a facet oxidation of a laser device. The EDS (Electron Dispersion Spectrum) of a natural oxide put into evidence the O (Kalpha) signal. Dielectric coating using Al2O3 and a-Si layers protected a part of laser devices, and at the mirror surface a reflectivity of 70% in the emission range of the laser (890nm) was measured. The emitted power for dielectric coated devices was constant, so the rapid degradation process was slowed down.