National Institute Of Materials Physics - Romania

Publications

884 articles found

861. Structural modifications of the superconducting phases on Bi system by electron beam irradiation

Authors: Aldica, G; Geru, II; Puscasu, BM; Constantinescu, F; Badica, P

Published: JUN 1996, JOURNAL OF SUPERCONDUCTIVITY, 9, 276, DOI: 10.1007/BF00727547

Superconducting samples of the Bi(Pb)-Sr-Ca-Cu-O system have been irradiated in transmission electron microscopy (TEM) to investigate the effect of a relative high fluence of electrons with 75 and 100 keV energy on the microstructure of the material. The diffraction pattern images show a dramatic change from very uniform lattice spots at ab crystalline planes to a circular pattern corresponding to damage and breaking of the materials in very small crystallites.

862. Temperature-dependent collective pinning exponent in Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation

Authors: Crisan, A; Popa, S; Aldica, G; Jaklovszky, J

Published: JUN 1996, JOURNAL OF SUPERCONDUCTIVITY, 9, 291, DOI: 10.1007/BF00727550

The low-voltage-level current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation were measured in applied magnetic fields both parallel and perpendicular to the samples' surfaces. The data were interpreted within the framework of the collective pinning model. The collective pinning exponent alpha changes from approximate to 1 to approximate to 0.5 (close to T-c), while in the case of unoriented samples it is constant (and equal to the theoretical value 1/2) as shown in an earlier paper. We suggest that this temperature dependence of alpha is due to the change of the dimensionality of the intragranular vortices.

863. Annealing effects in amorphous Fe77Gd3B20 ribbons

Authors: Mihalca, I; Ercuta, A; Zaharie, I; Jianu, A; Kuncser, V; Filoti, G

Published: MAY 1996, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 157, 162, DOI: 10.1016/0304-8853(95)01143-9

The crystallization kinetics of the amorphous alloy Fe77Gd3B20 were examined under isothermal flash annealing conditions both in air and in a vacuum. Magnetic measurements were used to determine the reaction order (n) and activation energy (E(a)), and Mossbauer spectroscopy (MS) was used to identify some resulting crystalline phases.

864. A new approach for the growth of high-T-c superconductors

Authors: Jaklovszky, J; Aldica, G; Rusu, C; Crisan, A

Published: APR 1996, APPLIED SUPERCONDUCTIVITY, 4, 194, DOI: 10.1016/S0964-1807(96)00023-3

In this paper we describe a modified liquid phase separation (double crucible) method for growth of superconducting Bi-Sr-Ca-Cu-O single crystals. We used alumina crucibles. The morphology and composition of as-grown crystals were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The stacking thin-plate single crystals of maximum 5 x 6 x 0.1 mm(3) were obtained. (C) 1997 Elsevier Science Ltd. All rights reserved.

865. Structural modifications of superconducting phases in Bi(Pb)-Sr-Ca-Cu-O system

Authors: Badica, P; Aldica, G; Alexeev, AF; Gridasova, TY; Morozov, VV

Published: MAR 1 1996, PHYSICA C, 259, 96, DOI: 10.1016/0921-4534(96)00021-4

Bi-Pb-Sr-Ca-Cu-O samples were produced by the spray-frozen, vacuum-freeze drying method from nitrate solutions. Structural modifications of crystallites during sintering were investigated in agreement with bulk density, phase structure of the samples, critical temperature T-c and critical current density j(c). A model for morphology changes of (2223) crystallites during sintering was proposed. Maximum j(c) in a zero magnetic field is reached on samples sintered for 200-250 h.

866. Influence of pyrolization temperature on final characteristics of BSCCO ceramic produced by spray-frozen, freeze drying method

Authors: Badica, P; Aldica, G; Alexeev, AF; Gridasova, TY; Morozov, VV

Published: FEB 1 1996, JOURNAL OF MATERIALS SCIENCE LETTERS, 15, 188, DOI: 10.1007/BF00274446

867. SINTERING TIME AND PROPERTIES OF YBA2CU3O-SIMILAR-TO-7 SUPERCONDUCTING CERAMICS

Authors: ALDICA, G; POPESCU, M; POPA, S; BUNESCU, MC; BADICA, P

Published: JUN 1995, JOURNAL OF SUPERCONDUCTIVITY, 8, 372, DOI: 10.1007/BF00728173

The effect of annealing duration in oxygen flow on the superconducting characteristics of YBa2Cu3Osimilar to 7 bulk ceramics was investigated by structural, electrical, magnetic, and electronic microscopy investigations. The long-time annealing is deleterious for the superconductivity in YBa2Cu3Osimilar to 7 ceramics. The optimum time interval for annealing is from 20 to 70 h at 950 degrees C, and within this range the maximum value of T-c is obtained.

868. CURRENT-VOLTAGE CHARACTERISTICS OF HIGH-TEMPERATURE SUPERCONDUCTING CERAMICS WITH THE PEAK EFFECT IN THE MAGNETIC-FIELD DEPENDENCE OF THE TRANSPORT CRITICAL-CURRENT DENSITY

Authors: CRISAN, A

Published: APR 1995, JOURNAL OF SUPERCONDUCTIVITY, 8, 320, DOI: 10.1007/BF00732386

The experimentally determined current-voltage characteristics of high-T-c superconducting ceramics exhibiting the ''peak-effect'' in the magnetic field dependence of the transport critical-current density were described in terms of thermally activated flux creep at grain boundaries; taking into account the collective pinning of intergranular vortices. The peak effect was explained by considering the increase of the intergrain pinning potential at intermediate fields through the interaction of intergranular (Josephson type) vortices and intragranular (Abrikosov) vortices. The magnetic field dependences of the effective pinning potential and of the collective pinning exponent were experimentally determined, and the features of the I-V curves were explained through these dependences.

869. OPTICAL INDUCED MODIFICATIONS IN THIN-FILMS OF FE-PVA

Authors: FILOTI, G; KUNCSER, V; FRANKE, H; KARDINAL, T; MANIVANNAN, G

Published: MAR 1995, JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY-ARTICLES, 190, 320, DOI: 10.1007/BF02040006

The ferric chloride doped polymers change their refractive index under UV irradiation. The Fe:PVA is sensitive to ferric chloride concentration, molecular weight of PVA, and recording beam power. By Mossbauer investigations we tried to correlate the Fe2+ / Fe3+ ratio, as well as the temperature dependence of the energetical parameters with the assumed mechanisms which involve both the valence state of iron and a cross-linking reaction of PVA matrix.

870. ON A MODIFIED KIM ANDERSON MODEL OF THE CURRENT-VOLTAGE CHARACTERISTICS OF HIGH-TEMPERATURE SUPERCONDUCTING CERAMICS

Authors: CRISAN, A

Published: AUG 1994, JOURNAL OF SUPERCONDUCTIVITY, 7, 691, DOI: 10.1007/BF00721648

In comparison with the ''classical'' Kim-Anderson model for the current-voltage characteristics of type-II superconductors, another model with a less sharp pinning potential is presented. In addition to the approximate solution, for currents close to the critical one, a complete solution of this model is derived, which better fits the experimentally obtained current-voltage characteristics.