National Institute Of Materials Physics - Romania

Publications

892 articles found

601. Doped MgB2 prepared by field assisted sintering technique

Authors: Aldica, G; Sandu, V; Badica, P; Nishijima, G; Awaji, S; Watanabe, K

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012079

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field H-c2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T < 24 K (H <= 5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

602. Film-substrate lattice-engineering of HTS thin films

Authors: Endo, K; Badica, P

Published: 2008, INTERNATIONAL SYMPOSIUM ON LATTICE EFFECTS IN CUPRATE HIGH TEMPERATURE SUPERCONDUCTORS (LEHTSC2007), 108, DOI: 10.1088/1742-6596/108/1/012045

Materials may show anisotropic properties on different crystal directions and this is also the case of High Temperature Superconductors (HTS). To take advantage of the materials anisotropy one concept of interest is "orientation engineering" in thin films. This can be realized through the control of the film-substrate lattice relationship. Some examples in this regard are presented in this work and through comparative analysis we try to evaluate the viability of this approach and of he entcountered problems. It is expected that in the future this approach will generate new nano composite materials with new properties and effects leading to development of new devices with new or improved functionality.

603. Growth control of perovskite-related oxide thin films

Authors: Endo, K; Badica, P

Published: 2008, ACTA CRYSTALLOGRAPHICA A-FOUNDATION AND ADVANCES, 64, C559, DOI: 10.1107/S0108767308082056

604. Mechanical Treatments at Room Temperature of Nb3Sn Practical Wires: pre-torsion for wires with a different architecture

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

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012036

Room temperature mechanical treatment consisting of multiple torsion loadings (named pre-torsion) was applied to Nb3Sn composite wires with different architecture to reduce thermal residual compressive strain experienced by the superconducting Nb3Sn filaments. Due to this effect all investigated wires have shown enhancement of the critical current density, I-c up to 56% at 15T and 4.2K. Enhancement of I-c was larger for the reinforced wires than for the wires without reinforcement suggesting that reinforcement is useful in strain relaxation during pre-torsion. The best results were obtained when the position of the reinforcement was located in the outer region of the wire. Pre-torsion is similar or more efficient than cycles of bending loadings (named pre-bending). For pre-torsion important parameter is rotation angle per length of the wire and optimum conditions for short wires are valid for long wires. Uniformity of Ic along the length of a 99cm wire after pre-torsion was within +/-2.1A.

605. Flux-creep activation energy for pure and SiC doped MgB2 by ac-susceptibility measurements

Authors: Mihalache, V; Sandu, V; Aldica, G; Groza, JR

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012166

ac susceptibility measurements were performed on pure and SiC-doped MgB2 prepared by field assisted sintering technique in the temperature range between 5 and 50 K. The activation energy for flux-creep U for both samples was determined at dc-fields of 0, 0.5, and 5 T, respectively, from the frequency dependence of the imaginary part of the ac-susceptibility chi '' data measured in the range 17-9997 Hz. It was found that at 0 and 0.5 T, the activation energy of the SiC-doped sample is almost half the energy of the pristine sample whereas at 5 T the activation energies of the two samples become almost equal. A similar behaviour show the H-p(T) lines as obtained from susceptibility data in the range 0-9 T. For fields lower than 7.5 T, the H-p field of the doped sample is lower than for the undoped MgB2. At similar to 7.5 T, the H-p(T) for SiC doped MgB2 crosses over the H-p(T) of pure MgB2 and for higher fields the doped samples exhibits a higher irreversibility H-p. This field dependence was extracted from the evolution of the. chi(1)''(T) data. This behaviour is a consequence of the morphology of the sample with a large spread in grain size.

606. Growth by MOCVD of (001) Bi-2223 superconducting thin films on (001) and (110) MgO substrates

Authors: Endo, K; Badica, P

Published: 2008, 8TH EUROPEAN CONFERENCE ON APPLIED SUPERCONDUCTIVITY (EUCAS'07), 97, DOI: 10.1088/1742-6596/97/1/012202

Superconducting c-axis thin films of Bi2Sr2Ca2Cu3O10 (Bi-2223) were grown by MOCVD on (001) and (110) MgO substrates. Films on both substrates show, as revealed by AFM investigations, similar morphology composed of regular rectangular grains and relatively low roughness of about two or three half c-axis unit cell order. AFM images also suggest that films might have in-plane epitaxy. The films have zero resistance critical temperatures of 95.1K for the (001) MgO substrate and 75K for the (110) MgO substrate, respectively. Considering very large films-substrate lattice mismatch between (110) MgO and (ab)-plane of the superconducting phase the growth of indicated films is a surprise.

607. Structure and magnetic properties of Fe/Fe oxide clusters

Authors: Crisan, O; von Haeften, K; Ellis, AM; Binns, C

Published: 2008, JOURNAL OF NANOPARTICLE RESEARCH, 10, 199, DOI: 10.1007/s11051-008-9463-3

Fe clusters have been synthesised in ultrahigh-vacuum chamber using a gas-stabilized cluster aggregation method that ensures good control of the cluster size and naturally oxidized in order to obtain Fe/Fe oxide core-shell nanoparticles. The morphology of an individual nanoparticle, as revealed by transmission electron microscopy, consists of a Fe core of an average diameter of 4.4 nm surrounded by an oxide shell of uniform thickness of about 1.2 nm in average. The nanoparticles may be assimilated with a ferro-/antiferromagnetic (FM/AF) system. The morpho-structural features have been correlated with magnetic measurements on the core-shell nanoparticles. A significant exchange bias effect has been measured, when the sample was field-cooled under an applied field of 3 T. As the morphology of core-shell nanoclusters is much more complicated than in FM/AF bilayers of regular thickness due to the particular geometry of the coronal AF layer, the shape and surface anisotropy have to be taken into account for a correct interpretation of the magnetic data.

608. Photochemistry aspects of the laser pyrolysis addressing the preparation of oxide semiconductor photocatalysts

Authors: Alexandrescu, R; Morjan, I; Dumitrache, F; Scarisoreanu, M; Soare, I; Fleaca, C; Birjega, R; Popovici, E; Gavrila, L; Prodan, G; Ciupina, V; Filoti, G; Kuncser, V; Vekas, L

Published: 2008, INTERNATIONAL JOURNAL OF PHOTOENERGY, 2008, DOI: 10.1155/2008/604181

The laser pyrolysis is a powerful and a versatile tool for the gas-phase synthesis of nanoparticles. In this paper, some fundamental and applicative characteristics of this technique are outlined and recent results obtained in the preparation of gamma iron oxide (gamma-Fe2O3) and titania (TiO2) semiconductor nanostructures are illustrated. Nanosized iron oxide particles (4 to 9 nm diameter values) have been directly synthesized by the laser-induced pyrolysis of a mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene (as sensitizer). Temperature-dependent Mossbauer spectroscopy shows that mainly maghemite is present in the sample obtained at higher laser power. The use of selected Fe2O3 samples for the preparation of water-dispersed magnetic nanofluids is also discussed. TiO2 nanoparticles comprising a mixture of anatase and rutile phases were synthesized via the laser pyrolysis of TiCl4- (vapors) based gas-phase mixtures. High precursor concentration of the oxidizer was found to favor the prevalent anatase phase (about 90%) in the titania nanopowders. Copyright (C) 2008 R. Alexandrescu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

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

Published: 2008, PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIEN, 100, 082026, DOI: 10.1088/1742-6596/100/8/082026

Co-MgO granular films presenting TMR effects were prepared by thermo-ionic vacuum arc (TVA) method with the simultaneous ignition of plasma in Co and MgO vapors, respectively. The processing method is suitable for the simultaneous preparation of films of different relative content of Co in the MgO insulating matrix. 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. This mechanism was suggested by the comportment of as prepared and thermally annealed samples.

610. A SOLID STATE NANO-GENERATOR: CONCEPT, DESIGN AND THEORETICAL ESTIMATIONS

Authors: Vopsaroiu, M; Cain, MG; Kuncser, V; Blackburn, J

Published: 2008, FUNCTIONALIZED NANOSCALE MATERIALS, DEVICES AND SYSTEMS, +, DOI: 10.1007/978-1-4020-8903-9_44

Nano-technology is a very attractive area of research and innovation because it allows the current trends in miniaturization to continue. The transition from micro scale to nano scale devices has already taken place in many applications such as electronics, magnetic recording and nano-biophysics. However, as we scale down the size of the structures and devices, it becomes obvious that the classical behavior will break down at the nano-scale and an interesting superposition of classical and quantum effects will emerge. Therefore, the validity of classical physics is questioned and many aspects of physics are now being revisited from the point of view of nano-technologies. In line with the new developments in miniaturization and nano-technologies, we propose in this letter a simple mechanism that applies the Faraday effect at the nano-scale in order to create a possible solid-state energy nano-generator device. The proposed nano-generator functionality is based oil what we shall call the Super-Paramagnetic Electromotive Force (SPEF) effect. This has the potential to produce a very small voltage on short time scales by converting directly thermal energy at room temperature to electromotive energy Without the need for external work or mechanical motion.