National Institute Of Materials Physics - Romania

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892 articles found

331. The influence of heating rate on superconducting characteristics of MgB2 obtained by spark plasma sintering technique

Authors: Aldica, G; Burdusel, M; Popa, S; Enculescu, M; Pasuk, I; Badica, P

Published: DEC 15 2015, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 519, 189, DOI: 10.1016/j.physc.2015.10.004

Superconducting bulks of MgB2 were obtained by the Spark Plasma Sintering (SPS) technique. Different heating rates of 20, 100, 235, 355, and 475 degrees C/min were used. Samples have high density, above 95%. The onset critical temperature T-c, is about 38.8 K. There is an optimum heating rate of similar to 100 degrees C/min to maximize the critical current density J(c0), the irreversibility field H-irr, the product (J(c0) x mu H-0(irr)), and to partially avoid formation of undesirable flux jumps at low temperatures. Significant microstructure differences were revealed for samples processed with low and high heating rates in respect to grain boundaries. (C) 2015 Elsevier B.V. All rights reserved.

332. Behaviour of the second magnetization peak in La2-xSrxCuO4 single crystals upon entering the doping domain of static stripe order

Authors: Miu, L; Ionescu, AM; Ivan, I; Miu, D; Adachi, T; Omori, K; Koike, Y

Published: DEC 15 2015, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 519, 84, DOI: 10.1016/j.physc.2015.08.013

The occurrence of a second magnetization peak (SMP) on the dc magnetization curves of superconducting single crystals with random quenched disorder in increasing external magnetic field H is quite common. Many models for this intriguing phenomenon have been proposed so far, but its origin is still under strong debate. Optimally doped and overdoped La2-xSrxCuO4 single crystals (x >= 0.15) exhibit an SMP over a large temperature interval, up to very close to the critical temperature. By decreasing doping, it was found that with H oriented along the c axis the SMP completely disappears in the doping domain of well developed static charge and spin stripes (x similar to 1/8), reappearing for x <= 0.10. This behaviour follows the instability of the Bragg vortex glass generated by the static stripe order (as revealed by small-angle neutron scattering experiments), which is confirmed by our magnetization relaxation measurements. If H is along the (a, b) planes, the SMP is observed for all investigated specimens. The presented analysis supports the scenario in which the SMP is generated by the pinning-induced disordering of the low-field Bragg vortex glass in the dynamic conditions of dc magnetic measurements. (C) 2015 Elsevier B.V. All rights reserved.

333. Magnetization reversal via a Stoner-Wohlfarth model with bi-dimensional angular distribution of easy axis

Authors: Kuncser, A; Kuncser, V

Published: DEC 1 2015, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 395, 40, DOI: 10.1016/j.jmmm.2015.07.035

A numerical extension of the simple Stoner-Wohlfarth model to the case of bi-dimensional angular distributions of easy axis is provided. The results are particularized in case of step-like, Gaussian-like and user defined distributions. In spite of its simplicity, the model can be applied to magnetically textured thin films and multilayers with in-plane magnetic anisotropy, independently on the texture source. Exemplifications are provided for a simple ferromagnetic textured FeCo film as well as for a FeMn/FeCoi CufFeCo spin valve structure. (C) 2015 Elsevier B.V. All rights reserved.

334. Ruthenium oxide-hematite magnetic ceramic nanostructures

Authors: Stroh, C; Tolea, F; Valeanu, M; Diamandescu, L; Xu, TH; Sorescu, M

Published: DEC 2015, CERAMICS INTERNATIONAL, 41, 14375, DOI: 10.1016/j.ceramint.2015.07.070

xRuO(2)-(1-x)alpha-Fe2O3 (x=0.1, 0.3,0.5 and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of RuO2 and alpha-Fe2O3 mixtures for 0-12 h of ball milling. Mossbauer spectroscopy, X-ray diffraction (XRD), magnetic measurements, simultaneous differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were employed to investigate the phase evolution of xRuO(2)- (1-x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for alpha-Fe2O3 (hematite) to four sextets and a doublet upon duration of the milling process with ruthenium oxide. Rietveld refinement of the XRD patterns yielded the values of lattice parameters as function of composition and milling times. The presence of Ru-substituted hematite and Fe-doped ruthenium oxide was evidenced and correlated with differences between ionic radii of Fe3+ and Ru4+. Magnetic measurements recorded at 5 and 300 K in applied magnetic fields of 50,000 and 100,000 Oe showed that the estimated saturation magnetization of the milled samples increased with ball milling time while preserving a multidomain magnetic structure. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and 1 T, for samples milled for 0 and 8 h of mechanochemical activation. These results correlate well with the DSC-TGA measurements, which support the formation of mixedoxide solid solutions in the system under investigation. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

335. Bipolar Conductance Switching of Single Anthradithiophene Molecules

Authors: Borca, B; Schendel, V; Petuya, R; Pentegov, I; Michnowicz, T; Kraft, U; Klauk, H; Arnau, A; Wahl, P; Schlickum, U; Kern, K

Published: DEC 2015, ACS NANO, 9, 12512, DOI: 10.1021/acsnano.5b06000

Single molecular switches are basic device elements in organic electronics. The pentacene analogue anthradithiophene (ADT) shows a fully reversible binary switching between different adsorption conformations on a metallic surface accompanied by a charge transfer. These transitions are activated locally in single molecules in a low-temperature scanning tunneling microscope. The switching induces changes between bistable orbital structures and energy level alignment at the interface. The most stable geometry, the "off" state, which all molecules adopt upon evaporation, corresponds to a short adsorption distance at which the electronic interactions of the acene rings bend the central part of the molecule toward the surface accompanied by a significant charge transfer from the metallic surface to the ADT molecules. This leads to a shift of the lowest unoccupied molecular orbital down to the Fermi level (E-F). In the "on" state the molecule has a flat geometry at a larger distance from the surface; consequently the interaction is weaker, resulting in a negligible charge transfer with an orbital structure resembling the highest occupied molecular orbital when imaged close to E-F. The potential barrier between these two states can be overcome reversibly by injecting charge carriers locally into individual molecules. Voltage-controlled current traces show a hysteresis characteristic of a bipolar switching behavior. The interpretation is supported by first-principles calculations.

336. Effect of thermal treatments on the structural and magnetic transitions in melt-spun Ni-Fe-Ga-(Co) ribbons

Authors: Tolea, F; Sofronie, M; Crisan, AD; Enculescu, M; Kuncser, V; Valeanu, M

Published: NOV 25 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 650, 670, DOI: 10.1016/j.jallcom.2015.07.296

The effect of thermal treatments on the martensitic transformation in three representative Ni-Fe-Ga alloys with or without Co substitutions has been studied by calorimetry, X-ray diffractometry, scanning electron microscopy and magnetometry. The alloys were prepared as ribbons, by the melt spinning technique. The thermal treatments promote a reduction of the martensitic transformation temperature in all investigated samples, with the most pronounced decrease for the alloys with lower Ga content. Three different mechanisms induced by specific thermal treatments and responsible for the characteristic behaviour of the martensitic transformation, with respect to temperature and heat of transition, were observed and discussed in details. (C) 2015 Elsevier B.V. All rights reserved.

337. Low-activation W-Si-C composites for fusion application

Authors: Ivekovic, A; Galatanu, A; Novak, S

Published: NOV 2015, FUSION ENGINEERING AND DESIGN, 100, 645, DOI: 10.1016/j.fusengdes.2015.08.013

W-Si-C composites were fabricated by active filler controlled pyrolysis of W powder (high tungsten content) and W-SiC powder mixtures (low tungsten content), infiltrated by a preceramic polymer and heat treated at temperatures from 1600 to 2000 degrees C. Material with high volume fraction of W in initial powder-polymer mixture, formed a composite material composed of W, W2C and W5Si3 with closed porosity in a single polymer infiltration and pyrolysis (PIP) cycle. After heat treatment at 1700 degrees C the material exhibited flexural strength above 350 MPa, hardness of 7.8 GPa and indentation modulus of 250 GPa. Room temperature thermal conductivity of the composite was rather low, 23 Wm(-1) K-1, however, thermal conductivity increased with increasing temperature achieving 35 Wm(-1) K-1 at 1000 degrees C. The effect of W as active filler in W-SiC powder mixtures with low volume fraction of tungsten was negligible. Therefore, six polymer infiltration and pyrolysis cycles were used to achieve significant densification with 15% porosity. The material fabricated at 1800 degrees C was composed of SiC, WC and WSi2 and exhibited flexural strength of 400 MPa and room temperature thermal conductivity of 100 W m(-1) K-1, which decreased to 32 W m(-1) K-1 at 1000 degrees C. (C) 2015 Elsevier B.V. All rights reserved.

338. B4C in ex-situ spark plasma sintered MgB2

Authors: Burdusel, M; Aldica, G; Popa, S; Enculescu, M; Mihalache, V; Kuncser, A; Pasuk, I; Badica, P

Published: OCT 2015, CURRENT APPLIED PHYSICS, 15, 1270, DOI: 10.1016/j.cap.2015.07.017

Powder mixtures of MgB2 and B4C with composition ((MgB2) + (B4C) x, x = 0.005, 0.01, 0.03) were consolidated by Spark Plasma Sintering at 1150 degrees C for 3 min. The average particle size of B4C raw powder was relatively high of 4 mm. Despite this, it is shown that processing processes are fast and, as in the case of the in-situ routes, for our ex-situ method carbon substitutes for the boron in the crystal lattice of MgB2. Specifics of microstructure are discussed based on electron microscopy observations. Carbon substitution and microstructure contribute to enhancement of the critical current density J(c) at high magnetic fields and of the irreversibility field H-irr. Samples are shown to be in the point pinning limit with some tendency toward the grain boundary pinning depending on B4C doping amount and temperature. An optimum composition is found for x = 0.01: for this sample, at 20 K, a J(c) of 100 A/cm(2) is obtained at 5.35 T. This value is higher than for the pristine MgB2 sample and for an optimum ex-situ nano-SiC-doped sample obtained for the same SPS processing conditions. (C) 2015 Elsevier B.V. All rights reserved.

339. Consolidation of W-Ta composites: Hot isostatic pressing and spark and pulse plasma sintering

Authors: Dias, M; Guerreiro, F; Correia, JB; Galatanu, A; Rosinski, M; Monge, MA; Munoz, A; Alves, E; Carvalho, PA

Published: OCT 2015, FUSION ENGINEERING AND DESIGN, 98-99, 1955, DOI: 10.1016/j.fusengdes.2015.06.178

Composites consisting of tantalum fiber/powder dispersed in a nanostructured W matrix have been consolidated by spark and pulse plasma sintering as well as by hot isostatic pressing. The microstructural observations revealed that the tungsten-tantalum fiber composites consolidated by hot isostatic pressing and pulse plasma sintering presented a continuous layer of Ta2O5 phase at the W/Ta interfaces, while the samples consolidated by spark plasma sintering evidenced a Ta+Ta2O5 eutectic mixture due to the higher temperature of this consolidation process. Similar results have been obtained for the tungsten-tantalum powder composites. A (W, Ta) solid solution was detected around the prior nanostructured W particles in tungsten-tantalum powder composites consolidated by spark and pulse plasma sintering. Higher densifications were obtained for composites consolidated by hot isostatic pressing and pulse plasma sintering. (C) 2015 Elsevier B.V. All rights reserved.

340. Effect of Mn addition on the thermal stability and magnetic properties of rapidly-quenched L1(0) FePt alloys

Authors: Crisan, O; Crisan, AD; Mercioniu, I; Pantelica, D; Pantelica, A; Vaucher, S; Nicula, R; Stir, M; Vasiliu, F

Published: OCT 2015, INTERMETALLICS, 65, 87, DOI: 10.1016/j.intermet.2015.06.008

Nano-composite magnets with L1(0) structure derived from binary FePt alloys and prepared as melt-spun ribbons are of current interest due to their higher operating temperature and the ability to be cast as a two-phase magnet with exchange spring magnetic properties, as both soft and hard magnetic phase may emerge from the same metastable precursor, i.e. the disordered cubic A1 phase. The present paper studies the effect of Mn addition on the thermal stability and phase structure, on the abundance of the hard magnetic phase and relative proportion of the soft ones, on the microstructure of the alloy as a function of temperature and on the overall magnetic properties. The interplay of the various magnetic sublattices in the ordering of the L1(0) phases as a consequence of introducing antiferromagnetically coupled Mn atoms in the alloy composition is discussed and interpreted in terms of microstructural changes induced by this addition as revealed by high resolution transmission electron microscopy and Xray diffraction. The temperature evolution of the phase composition and structural parameters is monitored using synchrotron radiation powder diffraction, while the compositional aspects are investigated using proton-induced X-ray emission and energy dispersive X-ray spectroscopy. Magnetic measurements reveal the magnetic parameters of interest (coercivity, remanence, Curie temperature, saturation magnetization), as well as the exchange-coupled two-phase nature of these magnets and provide information that hints at possible spin reorientation transitions in the Mn-containing planes of the L1(0) superlattice. (C) 2015 Elsevier Ltd. All rights reserved.