National Institute Of Materials Physics - Romania

Publications

860 articles found

771. Micro-structure and magnetic properties of Fe-Cu nanocomposites for anisotropic permanent magnets

Authors: Kuncser, V; Valeanu, M; Lifei, F; Predoi, D; Jianu, A; Kappel, W; Codescu, M; Patroi, E; Pasuk, I; Bulinski, M; Filoti, G

Published: MAY 31 2005, JOURNAL OF ALLOYS AND COMPOUNDS, 395, 6, DOI: 10.1016/j.jallcom.2004.11.059

Melt spun ribbons of FexCu100-x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe-Cu ribbons has been obtained via Mossbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established. © 2004 Elsevier B.V. All rights reserved.

772. Reply to comment on 'magnetic relaxation phenomena and inter-particle interactions in nanosized gamma-Fe2O3 systems'

Authors: Predoi, D; Kuncser, V; Filoti, G

Published: APR 6 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 2254, DOI: 10.1088/0953-8984/17/13/N02

773. Heavy ion induced damage in NaCl and KCl crystals

Authors: Enculescu, M; Schwartz, K; Trautmann, C; Toulemonde, M

Published: APR 2005, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 229, 405, DOI: 10.1016/j.nimb.2004.12.131

NaCl and KCI single crystals were irradiated with swift heavy ions (An, Ph, Bi and U) of energy between 560 and 2640 MeV. The damage induced by the ions was studied using optical absorption spectroscopy, chemical etching and small-angle X-ray scattering. The damage in the tracks is dominated by the creation of simple color centers (F and V-3 centers). Their concentration increases as a function of fluence, saturating when single tracks overlap. The F-center containingly track zone extends up to several tens of nanometers. In addition to this large track halo, there exists also a much smaller region (1-2 nm) close to the ion trajectory in which defect aggregates are formed. Indications for this track core are given by the preferential chemical etchability of ion tracks and also by the anisotropic pattern produced by small-angle X-ray scattering. This halo-core track structure is very similar as in LiF, however given by the higher mobility of the primary hole centers in NaCl and KCl, recombination with electrons is facilitated resulting in a lower efficiency for defect creation. (c) 2005 Elsevier B.V. All rights reserved.

774. On the relaxation mechanisms of some radiation induced free radicals in polymers

Authors: Velter-Stefanescu, M; Duliu, OG; Preda, N

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 989, DOI:

Electronic spin-lattice relaxation measurements performed in X and Q bands on electrons irradiated polyethylene, polycarbonate and polymethylmethacrylate have shown that at liquid helium temperatures and higher, up to 12K, the spin-lattice relaxation time of radiation induced free radicals does not depend on the magnetic. field but varies inverse proportionally with the square of absolute temperature and free radicals concentration. These peculiarities have been well explained by admitting that below 77 K there is a considerable amount of stable free radicals coupled in pairs that mediate the spin-lattice relaxation process of single radicals. Consequently, we have proposed an analytical equation that, within this hypothesis, quantitatively describes all these particularities.

775. Ni nanowires electrodeposited in single ion track templates

Authors: Daub, M; Enculescu, I; Neumann, R; Spohr, R

Published: APR 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 870, DOI:

30-mu m thick polycarbonate foils were irradiated with single swift heavy ions (,e.g. Au 11.4 MeV/nucleon). After etching with solutions containing NaOH and methanol, templates containing single conical pores were obtained. The size and the shape of the nanopores depend on etching conditions such as time of etching, concentration and temperature of the etching solution. By electrochemical deposition of nickel, nanowires were grown in the single pores. The nanowires were contacted by sputtering a gold layer on top of the membrane. The magnetic measurements indicated that the nanowires possess around 1% anisotropic magnetoresistance. The current-voltage characteristic of a Ni nanowire shows a linear behavior for current densities smaller than 10(8) A/cm(2). The maximum current density that a single Ni wire can withstand was found to be 3x 10(8) A/cm(2).

776. Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots

Authors: Moldoveanu, V; Tolea, M; Aldea, A; Tanatar, B

Published: MAR 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.125338

A detailed description of the tunneling processes, within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Buttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot.

777. Photoluminescence of manganese- and copper-doped CdS nanowires

Authors: Ghiordanescu, V; Sima, M; Enculescu, I; Grecu, MN; Mihut, L; Secu, M; Neumann, R

Published: FEB 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 454, DOI: 10.1002/pssa.200406927

Arrays of US: Mn2+ : Cu+ micro- and nanowires grown in polycarbonate ion-track templates exhibit photoluminescence in the spectral domain ranging from 500 to 800 mn at room temperature. A comparison with similar US and CdS:Mn2+ wire arrays is presented. The individual contributions to the emission spectra of Cu+ and Mn2+ ions in the US matrix are explained using their energy level schemes. Also SEM, EDX and EPR data are given or these wires. (c) 2005 WILEY-VCH Verlag GmbH A Co. KGaA, Weinheim.

778. Effect of UV radiation on the semi-interpenetrating polymer networks based on polyurethane and epoxy maleate of bisphenol A

Authors: Rosu, L; Cascaval, CN; Ciobanu, C; Rosu, D; Ion, ED; Morosanu, C; Enachescu, M

Published: JAN 15 2005, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 169, 185, DOI: 10.1016/j.jphotochem.2004.06.008

The behavior of two semi-interpenetrating polymer networks (S-IPNs) based on polyurethane (PU) and epoxy maleate of bisphenol A (EMBA) under exposure to UV radiation for long period of times (up to 200 h) was investigated. The networks obtained by the specific mixture under certain thermal treatment included in their structures crosslinked EMBA and linear PU. Advanced photooxidative degradation took place for long time of UV irradiation (over 160 h, especially), with almost a total destruction of the raw structures. The important modification of both the surface and of the morphology of the tested samples with the time of irradiation was also analyzed. Simultaneously, as compared to results obtained for short time of UV radiations (up to 10 h), where the mechanical properties of the tested samples improved, long time of UV radiations led to deterioration of many of the mechanical properties. (C) 2004 Elsevier B.V. All rights reserved.

779. Vibration modes analysis by x-ray topography in quartz and langasite resonators

Authors: Mateescu, I; Capelle, B; Detaint, J; Johnson, G; Dumitrache, L; Bran, C

Published: 2005, PROCEEDINGS OF THE 2004 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM AND EXPOSITION, 584, DOI:

This paper presents the results of the vibration modes measurements by X-ray topography in SC-cut quartz and Y-cut unpolished langasite resonators. A comparison of these results with X-ray diffraction topography images on AT-cut quartz resonators and Y-cut polished langasite resonators pointed out the behavior of mass-loading effect with plate orientation angle and with the surface state of the piezoelectric substrate. The results of the X-ray topography investigations are compared with electrical measurements performed on the same resonators. 5MHz Sawyer, plan parallel SC-cut quartz and Y-cut unpolished langasite resonators, with 14mm plate diameter and various electrode thickness and diameters have been investigated by Xray topography on fundamental, third and fifth overtones. The measurements were performed by conventional transmission Laue setting using white beam synchrotron radiation at LURE/DCI, Orsay, France. The study by X-ray topography on SC-cut quartz resonators and unpolished Y-cut langasite resonators pointed out a good agreement with the results obtained on the same resonators by electrical measurements. The conclusion is that the SC-cut quartz resonator characteristics present a similarly harmonic dependence with those of the Y-cut langasite resonators thus revealing the stress-compensated feature of the Y-cut in langasite crystal.

780. Surface-enhanced Raman scattering studies on C-60 fullerene self-assemblies

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

Published: 2005, CARBON, 43, 9, DOI: 10.1016/j.carbon.2004.08.020

Surface-enhanced Raman scattering (SERS) was used to investigate C-60 self-assembling in solvents like pyrrolidine (Py) and Nmethyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C-60 aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C-60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at similar to342cm(-1). In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I-h C-60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800cm(-1) is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C-60 in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C-60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original H-g(l) band at similar to269cm(-1) with two others at similar to255 and similar to246cm(-1). In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118cm(-1), which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C60 clusters of different size. (C) 2004 Published by Elsevier Ltd.