National Institute Of Materials Physics - Romania

Publications

892 articles found

611. Electrical conduction in initial field assisted sintering stages

Authors: Aldica, G; Khodash, V; Badica, P; Groza, JR

Published: DEC 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3870, DOI:

Field Activated Sintering Techniques (FAST) are increasingly used to consolidate metal and ceramic powders. They combine application of a pulsed electrical current with external pressure to activate the densification process. The nature of electrical field/current interaction with powder systems is not clear yet. In this regard, in-situ transport measurements and microstructural analyses were carried out during pulsed current application in the FAST processing of Ni powders. Our experimental results were compared with existing electrical conduction theories and observed effects were relatively well explained qualitatively within these theories. Under electrical field it was confirmed that no voltage-current phase-shift occurs at elevated operating interfacial temperatures. For the field situation a higher applied pressure is decreasing the contact instability among particles contacts and of the sintering onset temperature. Microstructural investigations of the FAST-sintered samples indicate rather distribution of the higher density regions and a considerable increase of neck formation above 340 degrees C.

612. Tunneling magneto-resistance granular thin films deposited by thermo-ionic vacuum arc technique

Authors: Mustata, I; Anghel, A; Lungu, CP; Pontilian, O; Kuncser, V; Schinteie, G

Published: DEC 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 3820, DOI:

Co-MgO granular films presenting TMR effects were prepared by thermo-ionic vacuum arc method with the simultaneous ignition of plasma in Co and MgO vapors. Morphologic, structural and magnetic behaviors were analyzed in as prepared and annealed samples. The influence of the Co content on the magnetic properties of the prepared films was analyzed, in correlation with tunneling magneto-resistance effects. The tunneling magneto-resistance effect is maximal for certain Co content. This behavior was interpreted by the contrary effects of decreasing the average size of the magnetic grains, and hence the average inter-grains distance at higher Co relative content, and the enhanced magnetic disorder in very fine grains dispersed in the insulating matrix.

613. Coexistence of superconductivity and antiferromagnetism in HgBa2Ca4Cu5Oy: Multiharmonic susceptibility and vortex dynamics study

Authors: Crisan, A; Tanaka, Y; Iyo, A; Shivagan, DD; Shirage, PM; Tokiwa, K; Watanabe, T; Cosereanu, L; Button, TW; Abell, JS

Published: DEC 2007, PHYSICAL REVIEW B, 76, DOI: 10.1103/PhysRevB.76.212508

We have studied the multiharmonic ac susceptibility response of HgBa2Ca4Cu5Oy (Hg:1245), a multilayered high-temperature superconductor (HTS) having two crystallographically inequivalent CuO2 planes in a unit cell with very imbalanced carrier concentrations. Vortex melting lines are well described by the commonly accepted model of melting with moderate anisotropy factors of 40-50, depending on the doping level. The diamagnetic response with applied fields parallel to superconducting (a,b) planes also shows a quite robust supercurrent along the c axis. Our results are also discussed in connection with contradictory models and experiments regarding the coexistence of superconductivity and antiferromagnetism in HTSs.

614. Growth of stacked heterostructures of SIS-type with S=YBCOor BSCCO and I = (Ba, Ca) CuO2 or (Sr, Ca) CuO2

Authors: Endo, K; Badica, P

Published: NOV 2007, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 20, S436, DOI: 10.1088/0953-2048/20/11/S25

We investigated MOCVD growth of c-axis two-layer and three-layer stacked heterostructures on (100) SrTiO3 of SI, IS or SIS type. S denotes high temperature superconductor thin films of YBa2Cu3O7 (Y-123), Bi2Sr2Ca2Cu3O10 (Bi-2223) or Bi2Sr2CaCu2O8 (Bi-2212), while I stands for insulating thin films of (Ba, Ca) CuO2 or (Sr, Ca) CuO2. X-ray diffraction, atomic force microscopy and measurements of resistivity versus temperature were performed after the deposition of each layer. The best results, suitable for further device fabrication, were obtained for the SIS structure (100) SrTiO3/Bi-2223/(Sr, Ca) CuO2/Bi-2212. Similar structures with I = (Ba, Ca) CuO2 have shown higher impurity phases and lower superconducting quality of the top superconducting layer. For two-layer or three-layer structures, Bi-2223 as a top layer could not be obtained and, instead, a phase with Bi-2212 formed. Structures with (100) SrTiO3/Y-123/(Sr, Ca) CuO2/Y-123 or (100) SrTiO3/Y-123/(Ba, Ca) CuO2/Y-123 were also promising, but a current leak often occurred due to greater roughness. Structures with Y-123 are less sensitive to growth temperature than those with BSCCO.

615. Discovery of Li-2(Pd,Pt)(3)B superconductors

Authors: Badica, P; Togano, K; Takeya, H; Hirata, K; Awaji, S; Watanabe, K

Published: SEP 1 2007, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 460, 94, DOI: 10.1016/j.physc.2007.03.163

Critical temperature T-c of the Li-2(Pd1-xPtx)(3) B was reported to be 7-8 K for x = 0 and 2.2-2.8 K for x = 1. In this article we present our preliminary results on behavior of magnetization-temperature curves with starting composition of Pd-B precursor, y-Li concentration in LiyPd3B and post-annealing of the Pd-end compound. Results suggest that to maximize T-c ratio Pd: B should be close to 3: 1, while y-Li has to be optimum. The lowest T-c for LiyPd3B was 4.4-4.6 K, while post-annealings at 560 degrees C allowed enhancement of T-c up to 8.2-8.4 K. Compositions Li(2)Z(3)B with Z = Ni, Ru, Rh, Re, Ag are not superconducting down to 1.8 K. Exception is composition with Re showing superconductivity due to Re3B compound. All samples were prepared by arc melting. (c) 2007 Elsevier B.V. All rights reserved.

616. Thermally and pressure activated phase evolution in Fe-Pt-Nb-B melt spun ribbons

Authors: Crisan, AD; Nicula, R; Crisan, O; Burkel, E

Published: SEP 2007, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 27, 1282, DOI: 10.1016/j.msec.2006.06.033

The Fe-Pt-rich alloys has recently attracted a lot of interest for their potential development of the hard magnetic L1(0) (also denoted gamma(1)) FePt phase with high magnetocrystalline anisotropy. An alloy with the nominal composition Fe65Pt25Nb2B8 has been synthesised by rapid quenching of the melt. The phase evolution in the as-cast ribbons was monitored by energy dispersive X-ray diffraction of the synchrotron radiation. It is shown that the Fe65Pt25Nb2B8 alloy is completely crystallized in the as-cast state. With increasing temperature a partial disorder-order phase transformation with occurrence of hard magnetic L1(0) phase is observed. This transformation is directly related to thermal and pressure effects in the sample. With applying pressure, the phase transformation between cubic and tetragonal FePt symmetry is more pronounced and is accompanied by a strong uniaxial lattice expansion. The results are extremely promising for the direct formation from the melt of an exchange coupled hard-soft magnetic alloy with initial formation of the tetragonal hard magnetic L1(0) FePt phase. (C) 2006 Elsevier B.V. All rights reserved.

617. Hybrid approach to the synthesis of FePt/Fe3B nanocomposite magnets

Authors: Crisan, O; Angelakeris, M; Kehagias, T; Filoti, G

Published: SEP 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2737, DOI:

Exchange coupling between alternatively dispersed hard and soft magnetic phases, gives rise to exchange spring magnets with enhanced energy product (BH)(max) that are believed to represent a novel class of permanent magnets with improved properties. The synthesis of a hybrid nanocomposite system made of soft magnetic Fe3B melt spun ribbon and hard magnetic Fe-Pt thin film, obtained by e-beam co-deposition in ultrahigh vacuum onto the ribbons, is reported. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction are used for structural characterization of these hybrid samples. Magnetic measurements provide observation of a drastic decrease of the saturation magnetization, compatible with the exchange coupling between hard and soft magnetic phases in the melt spun/thin film samples.

618. Crystallization processes in Fe-Pt-Nb-B melt spun ribbons

Authors: Crisan, AD; Crisan, O; Randrianantoandro, N; Valeanu, M; Morariu, M; Burkel, E

Published: SEP 2007, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 27, 1285, DOI: 10.1016/j.msec.2006.08.031

Intermetallic Fe-Pt-Nb-B alloys with 3 different compositions have been synthesized by rapid solidification technique and their phase structure was characterized by means of X-ray diffraction and Mossbauer spectrometry. It is shown that Fe68Pt21Nb2B9 and Fe65Pt25Nb2B8 as-east samples consist mainly of A1 soft magnetic f.c.c. Fe-Pt phase, while the Fe68Pt13Nb2B17 as-cast sample exhibits topological short-range order, typical for amorphous ribbons. Crystallization processes in the amorphous sample and phase evolution with the temperature have been studied using differential scanning calorimetry (DSC). The occurrence of exothermic peaks is related to structural transformations in the alloys and the crystallization process is shown to be highly dependent upon the heating rate in the DSC process. (C) 2006 Elsevier B.V. All rights reserved.

619. Crystallization processes and phase evolution in amorphous Fe-Pt-Nb-B alloys

Authors: Crisan, O; Crisan, AD; Randrianantoandro, N; Nicula, R; Burkel, E

Published: AUG 16 2007, JOURNAL OF ALLOYS AND COMPOUNDS, 440, L7, DOI: 10.1016/j.jallcom.2006.08.331

Fe-Pt system is nowadays widely studied due to its potential applications as magnetic recording media. The hard magnetic FePt L1(0) phase has extremely promising potential as permanent magnet with high magnetocrystalline anisotropy. Of recent interest is also the developing of the hard magnetic phase from an amorphous precursor by appropriate crystallization processes. The melt-spun amorphous Fe68Pt13Nb2B17 alloy has been submitted to dynamical annealing and its phase transformation during the process has been monitored by differential scanning calorimetry and in situ energy-dispersive X-ray diffraction of the synchrotron radiation. In the first stage of crystallization, alpha-Fe and cubic FePt phases are formed from the amorphous precursor. At around 600 degrees C superlattice Bragg reflections corresponding to tetragonal FePt are indexed in the XRD spectra and a-Fe phase diminishes drastically. Finally, between 900 degrees C and 975 degrees C the tetragonal superlattice peaks disappear and cubic FePt phase is formed again. This reversible order-disorder transformation is accompanied by a strong uniaxial lattice expansion of the cubic FePt unit cell. The system show promising features for the co-existence of hard and soft exchange coupled magnetic phases crystallized from FePt-based amorphous precursors. (c) 2006 Elsevier B.V. All rights reserved.

620. Maximization of the critical current of practical Nb3Sn wires through complex mechanical treatments at room temperature

Authors: Badica, P; Oguro, H; Awaji, S; Nishijima, G; Watanabe, K

Published: AUG 2007, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 20, 813, DOI: 10.1088/0953-2048/20/8/015

Relaxation of the thermal residual strain in Nb3Sn wires is realized through cycles of multiple torsion loadings ('pre-torsion') at room temperature. As a consequence, the critical current, I-c, is enhanced. This effect is stronger than previously reported mechanical treatments of multiple bending ('pre-bending'). The maximum I-c is attained for complex mechanical treatments of pre-torsion and pre-bending. Complex treatments allow efficient and relatively independent control of residual strain relaxation over all three directions of the wire, resulting in enhancement of I-c towards the theoretical limit of the material. Our findings have an immediate positive impact on the performance of Nb3Sn wires and, hence, on their applications (e.g. react-and-wind coils).