National Institute Of Materials Physics - Romania

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5781 articles found

4031. Unconventional method of obtaining nanocrystallites from copper sulfides

Authors: Simonescu, CM; Teodorescu, VE; Patron, L; Giurginca, M; Capatina, C

Published: AUG 2005, REVISTA DE CHIMIE, 56, 812, DOI:

In this article the obtaining and characterization of a mixture of copper sulfide (CuS, Cu9S, CuS2) is presented. The characterization was made by IR Spectroscopy, X-ray Powder Diffraction (XRD) and Transmission Electron Microscopy (TEM). The sulfide mixture represents an aggregate of small particles with an average diameter of 20-30 nm and particles with the diameter of about 500 nm. The smaller particles have the cuboidal morphology and correspond to CuS2; the bigger ones have the hexagonal morphology and correspond to CuS, and Cu9S8.

4032. New exotic negative ions of In and Ga in KCI crystals

Authors: Topa, V; Apostol, E

Published: AUG 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2132, DOI:

In this paper we present the experimental proofs that in KCl:In+ and KCl:Ga+ crystals, after non-standard electrolytical colouring, all impurity ions are converted in negative species and occupy anionic positions. The optical absorption and emission, electron paramagnetic resonance, thermal treatment in a flux of N-2 or H-2 and the electrons migration from negative metal species of coloured part to uncoloured part drived by an electric field, have been used as experimental methods for the demonstration of the conversion of In+ and Ga+-ions in negative exotic ions. The last two methods are very important for proving the existence of negative metal ions in crystals. The first one was improved by our team in [1], and the second one reported here for the first time, was used as an indubitable test for the evidence of In and Ga negative metal ions in anionic sites, in electrolytical coloured samples.

4033. Fe-coverage-induced out-of-plane spin components of the antiferromagnetic spin structure in exchange-biased Fe/FeSn2 bilayers

Authors: Stromberg, F; Keune, W; Kuncser, VE; Westerholt, K

Published: AUG 2005, PHYSICAL REVIEW B, 72, DOI: 10.1103/PhysRevB.72.064440

Exchange-coupled Fe/FeSn2(001) bilayer systems consisting of a polycrystalline ferromagnetic Fe layer grown on an epitaxial antiferromagnetic FeSn2 layer have been prepared by molecular beam epitaxy and investigated by Fe-57 conversion electron Mossbauer spectroscopy and superconducting quantum interference device magnetometry. The systems show a significant exchange bias effect at low temperatures. Tracer layers of Fe-57 (in the Fe layer) and (FeSn2)-Fe-57 (in the FeSn2 layer) have been placed in the samples in order to probe the spontaneous spin orientation at different distances from the Fe/FeSn2 interface. The Fe spins in the ferromagnetic layer are preferentially oriented in the interfacial plane. In as-prepared samples the presence of the Fe top layer induces a striking out-of-plane component of the interfacial Fe spins in the antiferromagnetic FeSn2 film. This perpendicular component decreases in magnitude at a larger distance from the interface. A reorientation transition from out-of-plane toward in-plane spin orientation was observed in the interfacial FeSn2 layer with increasing age of the sample. This effect is correlated with an increased magnitude of the exchange bias field for the aged samples.

4034. Evidence for irradiation triggered nonuniform defects distribution in multiharmonic magnetic susceptibility of neutron irradiated YBa2Cu3O7-delta

Authors: Sandu, V; Popa, S; Di Gioacchino, D; Tripodi, P

Published: AUG 2005, JOURNAL OF SUPERCONDUCTIVITY, 18, 581, DOI: 10.1007/s10948-005-0040-z

Multiharmonic ac-magnetic susceptibility chi(1) chi(2), chi(3), of neutron irradiated Li-doped YBa2CU3O7-x, has revealed a nonmonotonic dependence of all harmonics on the neutron fluence. The irradiation has a strongly depressive influence on the intergrain connection suggesting an increase of the effective thickness of the intergranular Josephson junction at a neutron fluence of 0.98 x 10(17) cm(-2). Less damaged are the intragrain properties. A spectacular enhancement of the superconducting intragranular properties reflected in the characteristics of all harmonics was observed at highest fluence phi = 9.98 X 10(17) cm(-2). We assume that this effect results from the development of a space inhomogeneous distribution with alternating defectless and defect-rich regions.

4035. A(II)B(VI) photoresistances by sintering-sublimation technics

Authors: Nazarenco, I; Jaklovszky, J; Ghiordanescu, V; Sava, BA; Albeanu, I; Dumitrescu, M; Ulieru, D

Published: AUG 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2118, DOI:

The sintering-sublimation technics was conceived for increasing the reliability of the photoconductive devices, by decreasing the concentration of mobiles donors, which are utilised for photoconductive material sensibilisation. The technics combines three different technics: the vapor phase deposition; the vacuum vaporising; the ceramic sinterisation. This special technics gives stoichiometric layers. The technics avoids dissociation (the flash sublimation technic), sublimates the compounds from two different sources, on heated support, or sublimates the compound, with Cd excess. It was investigated, by mass spectrometry and optical absorption studies, the presence, in gaseous state, of the A(II)B(VI) molecules. The condensation on the substrate of the vapors is difficult because the Cd, Zn and Hg elements have difficulties in the condensation process. The Frenkel model, of the condensation, can be successfully used for describing the process. This model uses the observations concerning the difficulty of the condensation on supports of those elements, at higher temperatures than 90 - 195 K. It is supposed that the support surface is a potential surface, with holes and potential walls, whose dimensions can not overcome the interatomic distances, for polished surfaces.

4036. Atomic scale structure of Ge30As4S66

Authors: Sava, F; Anghel, A; Kaban, I; Hoyer, W; Popescu, M

Published: AUG 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 1975, DOI:

The structure of a glassy sample of Ge30As4S66 has been measured by means of synchrotron radiation. X - ray diffraction data were processed and the atomo-electronic radial function (AERDF) was calculated. The high resolution AERDF curve allowed for an accurate determination of the first order distances between the atoms and the coordination numbers. A model of spatial structure, based on the concept of raft configuration was tested. An other model of the structure based on the continuous random network concept (CRN) was tried. The general conclusion drawn from this study supports the idea that raft and disordered configurations interlinked so that the dangling bonds in the structure be minimized, is the best model for such type of glass.

4037. High-temperature stiffness and damping measurements to monitor the glassy intergranular phase in liquid-phase-sintered silicon carbides

Authors: Roebben, G; Van der Biest, O; Sciti, D; Bellosi, A; Sarbu, C; Lauwagie, T

Published: AUG 2005, JOURNAL OF THE AMERICAN CERAMIC SOCIETY, 88, 2158, DOI: 10.1111/j.1551-2916.2005.00373.x

Detailed stiffness and internal friction (Q(-1)) versus temperature curves were obtained for liquid-phase-sintered silicon carbides using advanced resonant beam analysis up to 1400 degrees C. As-sintered materials display a stable Q(-1)-peak near 1100 degrees C, superimposed on an increasing background. The change of stiffness associated with the damping peak is quantitatively related to the amount of matter in pockets of the amorphous intergranular phase in which the refractory SiC matrix grains are embedded. The successful removal of the amorphous pockets by annealing at 1900 degrees C is deduced from the disappearance of the damping peak and confirmed with transmission electron microscopy.

4038. The influence of neutron irradiation on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz superconducting phase: The role of the grain edge

Authors: Mihalache, V; Totovana, A; Sandu, V; Popa, S; Aldica, G; Iyo, A

Published: AUG 2005, JOURNAL OF SUPERCONDUCTIVITY, 18, 467, DOI: 10.1007/s10948-005-0029-7

Using the transport and magnetization measurements, the influence of neutron irradiation at a fluence of 5 x 10(17) n cm(-2) on (B0.65C0.35)Ba1.4Sr0.6Ca2Cu3Oz has been investigated. The neutron irradiation was found to decrease critical temperature and transport critical current density, increase the residual and normal state resistivity, and improve the intragranular critical current density with 1.6 x 10(5) A/cm(2) (at 77.3 K and in the applied field up to 160 kA m) and Delta M-irr/Delta M-nonirr ratio (up to factor of 3) at highest field used for investigation. The field dependence of this ratio, which is below the unity at very low field but higher than I at high fields, correlated with the shape of the hysteretic loops as well as with the change of the transport parameters after irradiation suggests the role of the irradiation-induced effects on the grain edges. We discuss these effects in the framework of the Bean-Livingstone surface barriers and geometrical barriers.

4039. Surface-enhanced Raman scattering, photoluminescence and viscosity studies on C-60 aggregates in N-metryl-2-pyrrolidinone

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

Published: AUG 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2172, DOI:

Correlated studies of Surface Enhanced Raman Scattering (SERS) spectroscopy, photoluminescence (PL) and viscosity on aggregation of C-60 fullerene in N-methyl-2-pyrrolidinone (NMP) solutions were performed. The aggregation is a result of storing of the solutions at the room temperature for different periods of time. The modified phonon spectrum in the aggregate phase is a consequence of the intermolecular interactions that lead to a large reduction in parent I-h C-60 symmetry. The Raman range below 800 cm(-1) is the most diagnostic for aggregate assignment. Substitution of the original of H, (1) band from similar to 269 cm(-1) with a new one at similar to 255 cm(-1) relates the formation of (C-60, NMP) complexes. In the second stage of aggregation, this band shifts at similar to 246 cm(-1) and a new band, associate to the interball interaction, is formed at similar to 94 cm(-1). PL emission of the C-60 solutes in NMP presents two components, one in red (1.92-2.00 eV) and another in green (2.18-2.3 eV), which originate in two successive processes. Their weights depend on C-60 concentration and on storage time. In the beginning are formed quickly molecular complexes (C-60, NMP) by charge transfer, which on associate slowly in stable aggregates. A dependence of specific viscosity on C-60 solution concentration is observed too.

4040. Peculiar orientation of nematic liquid crystals on rubbed polyvinylimidazole

Authors: Frunza, S; Beica, T; Moldovan, R; Zgura, I; Frunza, L

Published: AUG 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 2157, DOI:

Alignment properties of rubbed films of polyvinyl imidazole were studied by observing with the polarizing microscope the features of the nematic liquid crystal (LC) droplets onto these layers. It was found that the observing of the droplets can reveal the direction of easy axis on the alignment substrate not only in the case of LC having the director normal to the LC/air interface but also in the case of the director tangent to this interface, when no disclination line could be seen. We observed that some of the LCs are oriented parallel to the rubbing direction, but the other LCs are perpendicularly oriented. This peculiar behavior is at variance to that found on rubbed layers of polyvinyl alcohol. The role of the end-groups in the structure of the LCs on the alignment properties is discussed.