National Institute Of Materials Physics - Romania

Publications

892 articles found

871. 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.

872. 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.

873. Icosahedral order in tetrahedrally-close-packed (TCP) clusters

Authors: Nicula, R; Manaila, R; Devenyi, A; Jianu, A

Published: 1995, PROCEEDINGS OF THE 5TH INTERNATIONAL CONFERENCE ON QUASICRYSTALS, 272, DOI:

A sequential algorithm, leading to three-dimensional polytetrahedral dense random packing structures is employed to simulate the atom-by-atom growth of relatively small clusters, under a large variety of physical assumptions. The computer model developed can provide information about the influence of experimental growth conditions on the topological and energetical parameters of the cluster : average coordination number, atomic pair distribution function, the degree of local tetrahedral perfection and specific bonding energy as a function of cluster size. The degree of icosahedral order built-in during the simulated growth process can be estimated from the pair distribution function and is of particular interest, due to its relevance to quasicrystalline thin films investigation.

874. THIN-FILMS OF ICOSAHEDRAL AL-MN PHASES - ASSESSMENT OF STRUCTURAL PERFECTION

Authors: MANAILA, R; POPESCU, R; KORONY, G; JIANU, A; DEVENYI, A; BARNA, PB; CSANADY, A; DEVENYI, AM

Published: OCT 15 1994, THIN SOLID FILMS, 251, 18, DOI: 10.1016/0040-6090(94)90833-8

The icosahedral metastable Al-Mn phase was obtained in thin films prepared by the high temperature sequential vapor deposition method. Analysis of X-ray diffraction line profiles allows an estimate of the random phason strain, revealing the presence of a highly disordered icosahedral fraction. Its concentration is correlated with the degree of crystallographic perfection of the Al substrate. Thereby, the crucial role of Al diffusion in the formation of the icosahedral phase is evidenced.

875. 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.

876. ICOSAHEDRAL ALLOYS GROWN IN AL EXCESS - PROBLEMS OF DISORDER AND STABILITY

Authors: MANAILA, R; JIANU, A; POPESCU, R; DEVENYI, A

Published: MAR 21 1994, JOURNAL OF PHYSICS-CONDENSED MATTER, 6, 2320, DOI: 10.1088/0953-8984/6/12/004

Phonon- and random-phason-type structural disorder was measured in Al-transition metal (Tm) ternary alloys, prepared with Al excess by rapid quenching. The disorder parameters correlate with the calculations on the energy stability of Al12TM icosahedra. Thus, substitution of 'unstable' elements (Fe and Co) for 'stable' elements (Cr and Mn) causes a strong increase in both types of disorder, leading finally to crystallization. This chemical trend is modulated around special compositions of the TM sublattice. The degree of structural order correlates with the thermal stability of icosahedral phases.

877. HYPERFINE FIELDS AND MAGNETIC-ANISOTROPY IN FE100-XGDX THIN-FILMS AND FE80-XGDXB20 (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-24) THIN-FILMS AND RIBBONS

Authors: KUNCSER, V; FILOTI, G; SERBANESCU, MD; JIANU, A; CRISTEA, L; DORMANN, JL; NOGUES, M; MAKNANI, J

Published: 1994, HYPERFINE INTERACTIONS, 94, 2326, DOI: 10.1007/BF02063782

Mossbauer measurements performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both as thin films and ribbons show a dependence of the spins orientation and [H-hyp] versus temperature, Gd content and preparation conditions. Increasing the Gd content, the initial low anisotropy disappears and [H-hyp] decreases. A sharp increase of the anisotopy with temperature in ribbons with low Gd concentration is evidenced.

878. MAGNETIC-PROPERTIES OF FE100-XGDX THIN-FILMS AND FE80-XGDXB20 THIN-FILMS AND RIBBONS (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-20)

Authors: FILOTI, G; KUNCSER, V; SERBANESCU, MD; JIANU, A; CRISTEA, L; NOGUES, M; DORMANN, JL; MAKNANI, J

Published: NOV 1993, IEEE TRANSACTIONS ON MAGNETICS, 29, 3125, DOI: 10.1109/20.280879

Magnetic and Mossbauer experiments were performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both in thin films and ribbons for x less-than-or-equal-to 20. The ferrimagnetic order is perturbed by the presence of B and Fe-interactions are.different. For small Gd content, the low anisotropy is in the film plane, while for greater Gd the anisotropy goes out of the plane giving typical features of hysteresis loops and strong decrease of Mossbauer hyperfine fields.

879. DISSIPATION IN CURRENT-CARRYING HIGH-T(C) SUPERCONDUCTING CERAMICS IN APPLIED MAGNETIC-FIELD

Authors: MIU, L; POPA, S; CRISAN, A; ALDICA, G; JAKLOVSZKY, J

Published: AUG 1993, JOURNAL OF SUPERCONDUCTIVITY, 6, 284, DOI: 10.1007/BF00626270

The dissipation mechanism in high-T(c) superconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.

880. THEORETICAL AND EXPERIMENTAL-STUDY OF CYLINDRICAL GAS-FLOW CEMS DETECTORS

Authors: KUNCSER, V; FILOTI, G

Published: JUL 1993, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 82, 191, DOI: 10.1016/0168-583X(93)95100-J

The operational range of the gas counters used in conversion electron Mossbauer spectroscopy (CEMS) is at the limit between the proportional counter and Geiger Muller domains. Starting from the theory of phenomena in gas detectors under space charge conditions, a general expression for the multiplication factor was obtained. The theoretical results, presented graphically, are discussed in connection with experimental data.