National Institute Of Materials Physics - Romania

Publications

5840 articles found

5011. Structural properties of amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons

Authors: Crisan, O; Le Breton, JM; Jianu, A; Teillet, J; Filoti, G

Published: NOV 14 1997, JOURNAL OF ALLOYS AND COMPOUNDS, 262, 389, DOI: 10.1016/S0925-8388(97)00417-9

The influence of Gd addition on the structural properties of Fe-Cu-Nb-Si-B nanocrystallized and amorphous alloys is studied. The crystallization temperature increases and the microstructure of the annealed samples changes. Gd addition induces the formation of Gd-Fe-B phases. In fully crystallized Fe-Gd-Cu-Nb-Si-B alloys the alpha-Fe(Si), Fe-Nb-B, Gd3Fe62B14 and Gd1.1Fe4B4 phases are observed. The evolution of the microstructure is followed as a function of the cumulative effects of annealing time and temperature. The results suggest the transformation of the metastable Gd3Fe62B14 phase into Gd1.1Fe4B4 and alpha-Fe. The hyperfine parameters of the Gd3Fe62B14 Mossbauer contribution are reported. (C) 1997 Elsevier Science S.A.

5012. Coulomb effects on the quantum transport of a two-dimensional electron system in periodic electric and magnetic fields

Authors: Manolescu, A; Gerhardts, RR

Published: OCT 15 1997, PHYSICAL REVIEW B, 56, 9718, DOI: 10.1103/PhysRevB.56.9707

The magnetoresistivity tensor of an interacting two-dimensional electron system with a lateral and unidirectional electric or magnetic modulation, in a perpendicular quantizing magnetic field, is calculated within the Kubo formalism. The influence of the spin splitting of the Landau bands and of the density of states (DOS) on the internal structure of the Shubnikov-de Haas oscillations is analyzed. The Coulomb electron-electron interaction is responsible for strong screening and exchange effects and is taken into account in a screened Hartree-Fock approximation, in which the exchange contribution is calculated self-consistently with the DOS at the Fermi level. This approximation describes both the exchange enhancement of the spin splitting and the formation of compressible edge strips, unlike the simpler Hartree and Hartree-Fock approximations, which yield either one or the other.

5013. Microhardness and elastic properties of bulk glasses and thin films of the GexSb40-xS60 family

Authors: Pamukchieva, V; Savova, E; Popescu, M

Published: OCT 1997, PHYSICS AND CHEMISTRY OF GLASSES, 38, 262, DOI:

The microhardness (H) of bulk glasses and thin films of the GexSb40-xS60 family has been measured. The volume of the micro-voids (Vh), their formation energy (Eh) and the modulus of elasticity (E) have been calculated. It is assumed that the bulk glasses and thin films have different elasticity levels due to the deformation of specific chemical bonds.

5014. Current-voltage characteristics of long Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes at different bending strains

Authors: AlMosawi, MK; Crisan, A; Beduz, C; Phillips, D; Haldar, P

Published: SEP 15 1997, PHYSICA C, 289, 69, DOI: 10.1016/S0921-4534(97)01587-6

The current-voltage (I-V) characteristics of long Bi2Sr2Ca2Cu3O10/Ag (Bi:2223) multifilamentary tapes with different bending strains were measured in a large range of dissipation (10(-12)-10(-3) V/cm). The experimental data were successfully interpreted in terms of current-induced unbinding of the thermally-created vortex-antivortex pairs by taking into account the size limitation of vortex fluctuations. It was found that the shape of the I-V curves can be explained by the contributions to the dissipation of superconducting intergrain contacts having different qualities (i.e. different critical currents) and by the appearance of micro-cracks during the bending process. (C) 1997 Elsevier Science B.V.

5015. Accurate equivalent-network modelling of GaAs/AlAs based resonant tunnelling diodes with thin barrier layers

Authors: Kwaspen, JJM; Lepsa, MI; van de Roer, TG; van der Vleuten, W

Published: SEP 11 1997, ELECTRONICS LETTERS, 33, 1658, DOI: 10.1049/el:19971107

The small-signal intrinsic impedance of GaAs/AlAs based resonant tunnelling diodes with thin barriers has been measured at room temperature over the full 0-2V bias-voltage and 0.05-40.05GHz frequency ranges, on stable, non-oscillating devices. The classical Esaki and the quantum-inductance equivalent circuits were used to model the impedance for CAD purposes. Information about the quasibound-state lifetime against bias-voltage was extracted.

5016. The Au-substituted Al-Cu-Fe icosahedral phase: Evidence for bond hybridization

Authors: Popescu, R; Macovei, D; Manciu, M; Zavaliche, F; Fratiloiu, D; Jianu, A; Devenyi, A; Manaila, R; Xie, Y; Hu, T; Orton, BR; Cernik, RJ; Tang, CC

Published: SEP 8 1997, JOURNAL OF PHYSICS-CONDENSED MATTER, 9, 7540, DOI: 10.1088/0953-8984/9/36/003

Au-substituted Al-Cu-Fe icosahedral phases, Al62Cu25.5-xAuxFe12.5 (x less than or equal to 7) were prepared by melt spinning. EDS data showed that Cu as well as Al atoms are effectively substituted for with Au. The hypercubic lattice parameter of as-quenched samples slowly increases with x, while phonon-and phason-type disorders are not affected by Au substitution. Experimental structure factors for 'sum' lines were used to derive an average radius of the atomic hypersurfaces. Comparison of experimental versus calculated ratios of structure factors for different models yielded information about the Au distribution over the icosahedral sites. It was inferred that a fraction of the gold shapes its Al surrounding in a AuAl2-type configuration. Evidence was found for d-sp hybridization of Au-Al bonds, on the basis of the 'white-line' area at the Au L-3 absorption edge.

5017. Correction for thermal diffuse scattering in time-of-flight neutron diffraction

Authors: Popa, NC; Willis, BTM

Published: SEP 1 1997, ACTA CRYSTALLOGRAPHICA SECTION A, 53, 545, DOI: 10.1107/S0108767397006004

A fast numerical algorithm is presented for calculating the TDS correction to Bragg peaks recorded in time-of-flight neutron diffraction studies on single crystals. The algorithm allows two average sound velocities, together with the structural parameters, to be treated as free parameters in a refinement program. The correction does not require, therefore, a prior knowledge of the elastic constants of the sample. The model is tested on a simulated set of reflection data.

5018. Diode-like SnO2 gas detection devices

Authors: Ionescu, R; Vancu, A; Buta, F

Published: SEP 1997, SENSORS AND ACTUATORS B-CHEMICAL, 43, 131, DOI: 10.1016/S0925-4005(97)00142-1

In usual chemoresistive SnO2 gas sensors, space charge accumulation at the electrodes and at intergranular contacts can contribute in the hystereses, irreproducibilities and time variations in electrical conductance which hinder the operation of these devices as transducers. For a thorough investigation of the effects of electrical polarisation, samples with highly asymmetric electrode contacts, in which polarisation is strong and which behave like diodes were used. On the other hand, we tentatively found the operation principle of a gas sensitive SnO2 device which avoids polarisation phenomena. (C) 1997 Elsevier Science S.A.

5019. A model of the metallic surface-emitting second harmonic generator

Authors: Popa, A; Lazarescu, MF; Dabu, R; Stratan, A

Published: SEP 1997, IEEE JOURNAL OF QUANTUM ELECTRONICS, 33, 1480, DOI: 10.1109/3.622625

We present an accurate calculation of the intensity of the second harmonic radiation generated by the reflection of the incident radiation from a metal surface, This radiation is due to two currents, The first current results from the rapid variation of the normal component of the oscillating electric field in the vicinity of the surface and depends on both conduction and bound electrons, The second one results from the action of the Lorentz force on conduction electrons, We show that if the adiabatic and the Thomas-Fermi assumptions are considered, then an accurate calculation of these currents is possible and leads to a simple relation of the intensity of the second harmonic radiation.

5020. Host/guest interactions in nanoporous materials .1. The embedding of chiral salen manganese(III) complex into mesoporous silicates

Authors: Frunza, L; Kosslick, H; Landmesser, H; Hoft, E; Fricke, R

Published: AUG 29 1997, JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL, 123, 187, DOI: 10.1016/S1381-1169(97)00046-0

The embedding of a enantioselective homogeneous catalyst - the chiral manganese(III) cationic complex of salen type (Jacobsen complex I) - into the pores of mesoporous substituted silicates is investigated. The host/guest interaction is studied by different methods: TG-DTA, UV-VIS, and FTIR. The results are consistent with a location of the large Mn complex inside the mesopores. Catalytic tests in olefin epoxidation prove the fully maintenance of the catalytic activity and stereoselectivity of the complex after embedding. This promising result indicates that MCM-like materials might have perspectives as supports for the heterogenization of homogeneous catalysts. (C) 1997 Published by Elsevier Science B.V.