National Institute Of Materials Physics - Romania

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892 articles found

821. Non-isothermal pyrolysation of the spray-frozen freeze dried complex nitrate in Bi(Pb)-Sr(Ba)-Ca-Cu system investigated by X-ray diffraction analysis

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

Published: 2000, EUROPEAN POWDER DIFFRACTION, PTS 1 AND 2, 321-3, 839, DOI: 10.4028/www.scientific.net/MSF.321-324.834

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu= 1.8:0,4:1.8:0.2:2.2:3.0, the precursor powder for synthesis of Bi-2223 superconducting phase, was obtained by spray-frozen, freeze drying technique. Samples of the nitrate precursor powder were placed in a heated furnace and extracted in air when temperature of the powder has attained values of 640, 660, 710, 750, 760, 778, 790, 815 and 850 degrees C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

822. Jahn-Teller configurations in natural spinels

Authors: Kuncser, V; Russo, U; Graziani, R; Della Giusta, A; Palade, P; Bulinski, M; Filoti, G

Published: DEC 1999, PHYSICS AND CHEMISTRY OF MINERALS, 27, 102, DOI: 10.1007/s002690050245

The local configurations and related hyperfine interactions of the tetrahedrally coordinated Fe2+ in two natural spinels were investigated. A special fitting procedure of the Mossbauer spectra is proposed in order to cover the involved mechanisms over the whole temperature range. The behavior of the quadrupole splitting vs temperature was successfully explained in terms of the split of the electronic 3d ground state doublet under a non-cooperative Jahn-Teller effect also providing estimates of the splitting energy for each configuration.

823. Phase formation during nonisothermal decomposition of the freeze-dried Bi : Pb : Sr : Ba : Ca : Cu=1.8 : 0.4 : 1.8 : 0.2 : 1.2 : 2.0 complex nitrate powder

Authors: Badica, P; Aldica, G

Published: OCT 1999, JOURNAL OF SUPERCONDUCTIVITY, 12, 615, DOI: 10.1023/A:1007739716481

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu = 1.8: 0.4:1.8:0.2:1.2:2.0 was obtained by spray-frozen, freeze-drying technique. Samples of the nitrate precursor powder were placed in a heated furnace (heating rate similar to 100 degrees C/min) and extracted in air when temperature of the powdered samples attained values of 439, 495, 550, 600, 640, 647, 717, 766, 814, and 850 degrees C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

824. Quasicrystals under pressure: a comparison between Ti-Zr-Ni and Al-Cu-Fe icosahedral phases

Authors: Ponkratz, U; Nicula, R; Jianu, A; Burkel, E

Published: AUG 1999, JOURNAL OF NON-CRYSTALLINE SOLIDS, 250, 848, DOI: 10.1016/S0022-3093(99)00190-8

Single-phase icosahedral phases (i-phases) of TiZrNi and AlCuFe alloys were obtained by single roller melt-spinning in protective argon atmosphere. The high-pressure (up to 25 GPa) compression properties of i-TiZrNi and i-AlCuFe were investigated by in situ energy-dispersive synchrotron radiation (SR) diffraction experiments using a gas-membrane diamond-anvil cell. In contrast to i-AlCuFe, reproducible deviations from the Birch-Murnaghan equation of state were noticed in the form of hysterezis loops during the first compression cycles of i-TiZrNi alloys. This response of quasicrystalline matter to hydrostatic pressure conditions is here reported for the first time. Plastic deformation processes and effects related to the small grain size of the rapidly-quenched TiZrNi alloys are tentatively considered as possible explanations. (C) 1999 Elsevier Science B.V. All rights reserved.

825. Nanostructure and phase distributions in (Cu0.80Fe0.20)(100-x)B-x melt spun ribbons studied by Mossbauer spectroscopy

Authors: Kuncser, V; Rosenberg, M; Yavari, AR; Filoti, G

Published: JUL 20 1999, JOURNAL OF ALLOYS AND COMPOUNDS, 289, 276, DOI: 10.1016/S0925-8388(99)00163-2

The analysis of the Mossbauer spectra of (Cu0.8Fe0.2)(100-x))B-x the melt spun ribbons taken at several temperatures between 4.2 K and room temperature allowed us to identify the occurring Fe-containing phases: alpha-Fe, gamma-Fe and <(Cu)under bar>Fe solid solutions. The presence of B inhibits the phase transformation from fee to bcc Fe. For x = 0 and 1, the amount of Fe in the <(Cu)under bar>Fe solid solution is close to 3 at.% solubility limit, while for x=3 besides crystalline Fe3B, a supersaturated phase of 9.7 at.% Fe in the <(Cu)under bar>Fe solid solution was found, showing a strong deviation from the binary equilibrium phase diagram. (C) 1999 Elsevier Science S.A. All rights reserved.

826. Structural refinement effect on the magnetoresistive properties of annealed melt spun Cu-Co-(Fe-Si) ribbons

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

Published: MAY 1999, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 195, 436, DOI: 10.1016/S0304-8853(99)00160-2

Structural and magnetic properties of melt-spun Cu-Co-(Fe-Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred. (C) 1999 Elsevier Science B.V. All rights reserved.

827. Intermediate frustration in [Fe3O(CH3COO)(6)(H2O)(3)] NO3 center dot 4(H2O) trinuclear cluster

Authors: Filoti, G; Bartolome, J; Dickson, DPE; Rillo, C; Prisecaru, I; Jovmir, T; Kuncser, V; Turta, C

Published: MAY 1999, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 196, 563, DOI: 10.1016/S0304-8853(98)00825-7

The frustration of the trinuclear clusters with antiferromagnetic intracluster interaction was investigated at low temperature and in applied magnetic field, by Mossbauer, susceptibility and magnetisation measurements. The spin configuration and the electronic state are described as a function of the temperature and the intensity of the applied field. The values of the exchange integrals are provided. (C) 1999 Elsevier Science B.V. All rights reserved.

828. Structural and magnetic properties of magnetoresistive Cu-Co-Fe ribbons

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

Published: MAY 1999, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 196, 469, DOI: 10.1016/S0304-8853(98)00822-1

Melt-spun Cu80Co10Fe10 ribbons are shown to exhibit a nanogranular structure with magnetic alpha-Fe(Co) grains segregated from the Cu-rich matrix after appropriate annealing. The Cu matrix purification as well as the magnetic properties of the alpha-Fe(Co) grains existing in a range of sizes are studied using X-ray diffraction, Mossbauer spectrometry and magnetic measurements. There is an optimum annealing for obtaining a high magnetoresistive effect. (C) 1999 Elsevier Science B.V. All rights reserved.

829. Fe-57 Mossbauer study of Pr-m(Fe, MO)(n) compounds with m : n=2 : 17 and 1 : 12

Authors: Morariu, M; Lazar, DP; Galatanu, A; Plugaru, N; Kuncser, V; Filoti, G; Hilscher, G; Kottar, A

Published: MAR 30 1999, JOURNAL OF ALLOYS AND COMPOUNDS, 285, 47, DOI: 10.1016/S0925-8388(98)01055-X

The effects of Mo substitution in Pr2Fe17-xMox and PrFe12-xMox compounds were investigated by X-ray diffraction, magnetic measurements and Mossbauer spectroscopy. Different fitting procedures, using magnetic sextets at low temperature, magnetic sextets and quadrupole doublets at room temperature and paramagnetic doublets well above the Curie points, had to be considered in order to analyze the Mossbauer spectra. The Mossbauer data obtained on the Mo-containing compounds stand for distributions of the hyperfine fields due to the random distributions of Mo over the 6c sites in the 2:17 and 8i sites in the 1:12 compounds. The variations in the isomer shifts and hyperfine fields with composition are discussed in relationship to the local environment details at the different iron sites. Site-specific information on the electronic effects due to Fe(Sd)-Mo(4d) states hybridization are derived. (C) 1999 Elsevier Science S.A. All rights reserved.

830. One step synthesis of Bi(Pb)-2223 phase in Bi(Pb)-Sr(Ba)-Ca-Cu nitrate freeze dried powder

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

Published: MAR 1999, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 12, 167, DOI: 10.1088/0953-2048/12/3/010

One step synthesis of the superconducting Bi(Pb)-2223 phase during heat treatment for non-equilibrium decomposition of the freeze dried Bi:Pb:Sr:Ba:Ca:Cu = 1.8.0.4: 1.8:0.2:2.2:3.0 nitrate powder has been studied. The heating rate was similar to 100 degrees C min(-1) and samples were maintained in air at 848 degrees C for 0, 5, 10, 20, 30, 60, 90, 240 and 1080 min. Each batch was investigated by x-ray diffraction analysis and ac magnetic susceptibility chi'(T). Evolution of phases and kinetics of phase formarion-decomposition processes were discussed. similar to 80% of Bi(Pb)-2223 has formed indicating the possibility of lowering the processing time of the freeze dried precursor powders by one order of magnitude comparative to the multistep synthesis route (from hundreds to tens of hours). A 30 min heat treatment step at 840 degrees C performed on the way up to the synthesis temperature is a forthcoming operation in order to block the formation of secondary undesired phases (e.g. (Ca1-xSrx)(14)Cu24O41).