National Institute Of Materials Physics - Romania

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field H-c2 and irreversibility field H-irr when compared with pristine MgB2. The reason for better H-c2 and H-irr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of H-c2 and H-irr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T >= 24 K, there is mixed pinning (H <= 5 T).

Transport and thermodynamic properties were studied for the complex metallic compound beta-Al3Mg2, composed of 925 atoms per unit cell. beta-Al3Mg2 exhibits bulk superconductivity below T-c=0.87 K. An exponential temperature dependence of the specific heat well below T-c indicates BCS-like behavior with a nodeless gap of width Delta(BCS)(0)approximate to 1.5 K. The coherence length is derived as xi(0)=4.85x10(-8) m and the Ginzburg Landau parameter kappa(GL)approximate to 13, characterizing beta-Al3Mg2 as a type II superconductor. Superconductivity in beta-Al3Mg2 occurs in the absence of inversion symmetry of the crystal. Surprisingly, in spite of the rather complex crystal structure of beta-Al3Mg2, physical properties turn out to be quite simple.

FexOy-SiO2 nanocomposites were prepared by sol-gel method by using two SiO2 sources and Fe(SO4)(2)center dot 7H(2)O as raw materials. The amorphous gels were thermally treated up to 1000 degrees C. The initial gel and the thermally treated samples were characterized by thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) and infrared spectroscopy. The presence of hematite was confirmed by the obtained Mossbauer spectra which showed the characteristic sextet. The total amount and the size distributions of the hematite nanoparticles can be controlled via the initial precursors and subsequent by annealing conditions. (C) 2006 Elsevier B.V. All rights reserved.

High harmonics ac susceptibility measurements joint with neutron irradiations on quasi-bi-dimensional high temperature superconductor (quasi-2D-HTSC) are good tools to study the flux dynamics and its interaction with pinning processes in these superconductors. Flux neutron intensity of 5 * 10(17) n * cm(-2) shows a deep change in the flux pinning dynamics in Bi-Sr-Ca-Cu-O (BSCCO) system. Third harmonic susceptibility signal increases in amplitude after the neutron irradiation, followed by a rise of the pinning and critical current. Moreover, after the irradiation, the measurements underline the demise of the anomalous peak effect (PE) associated with a three-dime.nsional/two-dimensionaI (3D/2D) flux lattice transition.

Exchange-coupled Nd2Fe14B/alpha-Fe nanocomposite magnets receives considerable research interest, because these are regarded as promising candidate materials for permanent magnets, with an adequate performance while keeping the content of relatively expensive Nd low. The advantage of these new magnetic materials are the lower rare earth content (and consequently, cheaper magnets with better corrosion resistance). The Nd2Fe14B/alpha-Fe nanocomposites were obtained by recrystallisation treatment from an amorphous phase prepared by melt-spinning. For Fe contents, between 83 and 85 at. %, we have obtained the hardening of the alpha-Fe phase by exchange interactions between the hard Nd2Fe14B phase and the soft alpha-Fe phase. The evolution of the structure and main magnetic characteristics after different annealing conditions was studied. For all isotropic permanent magnets obtained from the prepared nanocomposites was measured a remanence ratio higher than 0.5, which confirm the existence of the mentioned exchange interactions.

Samples of (Bi,Pb)(2)Sr2Ca2Cu3O10 (Bi(Pb)-2223) phase have been fabricated from two different oxide precursors with the same stoichiometry, synthesized by solid state reaction in the air at 815 degrees C for 20 h. The first precursor was prepared by mixing Bi2O3, PbO, SrCO3, CaCO3 and CuO powders, while the second one through mixing Bi2O3 and PbO with Sr2Ca2Cu3Ox produced from SrCO3, CaCO3 and CuO powders. Pellets from the two precursors were prepared in the same conditions; heat treatment time was varied and intermediate grindings were applied. Although the maximum attained Bi(Pb)2223 phase amount was about 50% and above 90% in the samples from the two precursors, respectively, the samples from the first precursor have generally shown better superconducting properties such as the critical temperature from resistivity measurements and intragrain critical current J(cg) from magnetic susceptibility measurements. For our particular case the influence of the final heat treatment is relatively low and at the same time the precursor plays a major role in controlling growth processes and final superconducting properties. Lower room-temperature Seebeck coefficient of the thermoelectric power for the samples fabricated from the first precursor suggests that one mechanism through which this control is realized might be the modification of the oxygenation level in the samples produced from different precursors. It was also found that for the samples prepared from the two precursors, intermediate grinding after 110h or more of heat treatment enhances density, almost does not influence J(cg) (or slightly improves it), decreases T-c, and does not influence significantly the amount of Bi(Pb)-2223 phase.

Field Assisted Sintering Technique (FAST) is currently used to consolidate ceramic, metal and composite powders. This method combines a pulsed electrical current with simultaneous application of an external pressure. In this article we report fabrication by FAST of high density MgB2 ceramic with addition of 5% SiC and 5% B4C. The influence of sintering conditions such as temperature and time on bulk density, morphology and superconducting properties of the samples was investigated. All samples had bulk density of more than 90% of the theoretical one and the same critical temperature T-c = 38.5 K from magnetisation measurements. These preliminary results suggest that FAST is a promising method for processing of MgB2 superconductor.

Two practical Nb3Sn composite wires (short samples) were investigated from the points of view of critical current, I-c, and cracks evolution versus number of bending load cycles. Bending was applied manually at room temperature using a block-form with curved surface corresponding to applied bending strains of 0.5% and 0.8%. It was applied in plane, alternatively in one direction and in the opposite one up to 130 times. We shall use for this mechanical treatment term pre-bending. The wires had approximately the same diameter but a different architecture: one was of conventional type (standard US-Japan wire, Hitachi Cable) and the second one was CuNb reinforced wire (Furukawa Electric Co. Ltd.) with reinforcement located near-the-edge. It was found that critical current maximizes gradually with the number of bendings and the maximum value of I-c, is attained for a number of bending loadings, N-pb, of about 15. Beyond this optimum N-pb, critical current is constant. Optimum N-pb is likely not dependent on the wire, applied field or pre-bending strains. Results suggest that Cu is important in the work hardening process. Work hardening during pre-bending is imposing limitations in application of this technically convenient mechanical treatment for release of the residual strain and further enhancement of I-c. Semi-quantitative analysis of the electron microscopy observations shows that for our experimental conditions the density of cracks is approximately constant being in agreement with described I-c, behavior. Other possible consequences of our results are discussed. Among the most interesting issues is possibility of the 3D independent control of yielding and hence of residual strain release. Several new practical ideas that will need future confirmation are proposed: e.g. application of loading-unloading cycles of torsion treatment in combination with tensile or pre-bending treatments might be useful.

The iron oxide-based nanomaterials have a great importance because of their impact on a wide number of industries. They have many properties and applications in domains as: environmental protection, biomedical, catalysis, information displays and electronics. In order to avoid the tendency of nanoparticles to aggregate, they are often included in sol-gel derived silica matrices, thus being also ensured a homogeneous dispersion of the ultra-fine metal oxide particles in the host matrix. The reasons of the attempt are both an economic and a non-pollutant one, tacking into account the fact that it is well known that aqueous silica sol is cheaper and less toxic than TEOS. The Fe3O4 was introduced in the reaction mixture as aqueous suspension. A final iron content related to SiO2 of 3% wt. was chosen for the prepared nanocomposite. Thermal analysis, IR spectroscopy, XRD and TEM methods have been used for the structural characterisation,. Some experiments of water depollution (from As) have been proceeded using the prepared Fe3O4-SiO2 nanocomposite.

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its position's field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature superconductors.