National Institute Of Materials Physics - Romania

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892 articles found

811. Sc addition to (Bi,Pb)(2)Sr2Ca2Cu3Ox freeze-dried ceramic

Authors: Badica, P; Aldica, G; Mandache, S

Published: AUG 2000, JOURNAL OF SUPERCONDUCTIVITY, 13, 533, DOI: 10.1023/A:1007812615073

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc2O3 was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis. X-ray diffraction (XRD), Scanning electron microscopy (SEM), and chi'(T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.

812. Field-induced phase transitions and giant magnetoresistance in Dy3Co single crystals

Authors: Baranov, NV; Bauer, E; Hauser, R; Galatanu, A; Aoki, Y; Sato, H

Published: JUL 2000, EUROPEAN PHYSICAL JOURNAL B, 16, 72, DOI: 10.1007/s100510070250

Electrical resistivity and calorimetric measurements on Dy3Co show that below the Neel temperature (T-N = 44 K) the non-collinear antiferromagnetic structure exhibits: field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K.

813. Synthesis of the Ca0.45Cu0.55O peritectic phase using a mixture of nitrate powders

Authors: Badica, P; Aldica, G; Alexe, G; Crisan, A

Published: APR 2000, MATERIALS LETTERS, 43, 184, DOI: 10.1016/S0167-577X(99)00256-6

The Ca0.45CU0.55O phase was successfully produced from a mixture of Ca and Cu nitrate powders with cation ratio Ca:Cu = 1:1. The heat treatment for decomposition and the synthesis of the indicated phase were pet-formed at 710 degrees C in air and flowing oxygen, respectively. The Ca0.45Cu0.55O phase concentration increases with synthesis time. After 209 h of heat treatment, the concentration of the Ca0.45Cu0.55O phase determined by X-ray diffraction (XRD) analysis has reached 87% and the kinetic data have indicated that the material enters the saturation stage of the reaction, The secondary phases are CaO and CuO. (C) 2000 Elsevier Science B.V. All rights reserved.

814. Physical properties of skutterudites YbxM4Sb12, M = Fe, Co, Rh, Ir

Authors: Bauer, E; Galatanu, A; Michor, H; Hilscher, G; Rogl, P; Boulet, P; Noel, H

Published: APR 2000, EUROPEAN PHYSICAL JOURNAL B, 14, 493, DOI: 10.1007/s100510051057

A series of compounds YbxM4Sb12, M = Fe, Co, FeCo, Rh,Ir, were synthesised by reaction sintering. From Rietveld refinements isotypism was determined in all cases with the LaFe4P12-(skutterudite)-type, space group Im (3) over bar - No. 204. These refinements also served to derive the Yb-content in the samples. There is a systematic trend for the Yb-occupancy in the parent lattice M4Sb12; revealing a gradual decrease of the Yb-content from x = 0.8 (M = Fe), x = 0.5 (FeCo), x = 0.2 (Co); x = 0.1 (Rh) to x approximate to 0.02 (Ir). This dependency seems to correlate with the thermal stability of the ternary compounds: a true ternary compound forms for M = Fe, whilst for M = Co, Rh, Ir stable binary skutterudite compounds MSb3 already exist. Measurements of various bulk properties revealed the absence of any long range magnetic order in this series of compounds. While the samples rich in Yb behave metallic like, the Rh and Ir based skutterudites show a semiconducting-like resistivity which at lower temperatures is characterised by variable range hopping in the presence of strong Coulomb interaction. Although Yb0.1Rh4Sb12 exhibits a Seebeck coefficient up to about 150 mu V/K, figures of merit ZT generally are below 0.1 near room temperature, primarily due to the large resistivities of the sintered material.

815. Studies concerning secondary synthesis processes in [Bi(Pb)](2)[Sr(Ba)](2)Ca(2)Cu(3)Oy freeze dried superconducting ceramic

Authors: Badica, P; Aldica, G; Bunescu, MC; Nemyrovsky, AV

Published: APR 2000, JOURNAL OF MATERIALS SCIENCE LETTERS, 19, 563, DOI: 10.1023/A:1006765825483

816. Structural and magnetic investigation of nonstoichiometric YFe10V2 and its interstitial carbide prepared by arc-melting

Authors: Plugaru, N; Lazar, DP; Galatanu, A; Morariu, M; Wiesinger, G; Kottar, A; Vasile, E

Published: MAR 14 2000, JOURNAL OF ALLOYS AND COMPOUNDS, 299, 54, DOI: 10.1016/S0925-8388(99)00791-4

A far-from-equilibrium preparation route and using a particular combination of starting materials led us to obtain nonstoichiometric Y(Fe,V)(12) and its interstitial carbide. Structural analysis and EDAX results reveal the formation of the main 1:12 phase with the ThMn12-type structure, with 13-15% vacancies at the 3d metal sites. Insertion of interstitial carbon increases the T-C and magnetization of the 1:12 phase, whereas the unit cell volume remains unchanged. The effects of vacancies and interstitial carbon on the Mossbauer hyperfine fields and isomer shifts are discussed with reference to the calculated quantities. (C) 2000 Elsevier Science S.A. All rights reserved.

817. Influence of the Jahn-Teller effect on the linewidth of the Mossbauer spectra of natural spinels

Authors: Kuncser, V; Russo, U; Graziani, R; Della Giusta, A; Filoti, G

Published: FEB 21 2000, JOURNAL OF PHYSICS-CONDENSED MATTER, 12, 1460, DOI: 10.1088/0953-8984/12/7/326

The temperature dependence of the Mossbauer doublet linewidth of tetrahedrally coordinated Fe2+ ions, distorted by the Jahn-Teller effect, was analysed by means of a new mathematical approach. The idea was that the linewidth depends on a distribution of quadrupole splittings that are in turn dependent on temperature. The variation of the quadrupole splitting was supposed to be connected with the temperature dependent population of the ground state E-g orbitals, split by the local Jahn-Teller effect. The model was applied to three sets of experimental data. The barrier splitting Delta between the two E-g electronic states as well as their widths, delta Delta, were obtained.

818. Experimental investigation of the nonlinear optical response in Fe : PVA

Authors: Bulinski, M; Iova, I; Belea, A; Kuncser, V; Filoti, G

Published: JAN 2000, JOURNAL OF MATERIALS SCIENCE LETTERS, 19, 28, DOI: 10.1023/A:1006739412409

819. Grain-size refinement in Al-Mn-Ce rapidly-quenched alloys

Authors: Nicula, R; Jianu, A; Holzhuter, G; Barfels, T; Burkel, E

Published: 2000, METASTABLE, MECHANICALLY ALLOYED AND NANOCRYSTALLINE MATERIALS, PTS 1 AND 2, 343-3, 32, DOI: 10.4028/www.scientific.net/MSF.343-346.27

Alloys with nominal composition Al94-xMn6Cex (x = 1-4 at.%) and Al99-yMnyCe1 (y = 6, 8 and 10 at.%) were prepared by melt-spinning in protective Ar atmosphere and examined using transmission and scanning electron microscopy (TEM/SEM), selected-area electron diffraction (SAED) and high energy-resolution synchrotron radiation powder diffraction. Depending on the alloy composition, the average size of the icosahedral phase grains could be varied between 2-5 nm up to several hundred nm. The observed grain-size refinement effects open a way for the microstructure control of these novel quasicrystal-based nanocomposite materials and for the fine-tuning of their macroscopic behaviour.

820. Electron microscopy and synchrotron radiation powder diffraction study of icosahedral Ti-Zr-Ni alloys

Authors: Vasiliu, F; Jianu, A; Nicula, R; Burkel, E

Published: 2000, METASTABLE, MECHANICALLY ALLOYED AND NANOCRYSTALLINE MATERIALS, PTS 1 AND 2, 343-3, 36, DOI: 10.4028/www.scientific.net/MSF.343-346.33

The formation of quasicrystalline (QC) single-phases at compositions close to Ti53Zr27Ni20 was investigated using transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and high resolution powder diffraction using synchrotron radiation. The primary Ti-Zr-Ni alloys were melt-spun in protective Ar atmosphere using peripheral wheel speeds omega = 20 m/s and omega = 25 m/s. TEM and SAED investigations revealed that significant microstructural differences occur even for rather small changes of the rapid quenching rate. Grain refinement and improved material homogeneity were found to be responsible for the observed gain in the ductility of the as-quenched ribbons.