Advances in Science and Technology Vol. 47

Abstract: Nondestructive testing of superconducting blocks and thin films is a worth point in their development and their industrial production. The most intrinsic characteristic to be tested is the critical current, so is the maximum current can be carried in the superconducting state. The measurement of the current flowing through the samples could be done by direct transport experiments but this means using specific samples, and only the mean current may be found. Distribution of current density in the sample is more relevant because it yields the map of inhomogeneity of the samples, and its effect in the current density distribution. Magnetic measurements have been developed by Hall scanning and magneto-optical effect, allowing the mapping of the component of the magnetic field perpendicular to a surface of the sample created by the current distribution. By solving the inverse Biot-Savart problem a map of current densities can be obtained. We will present the status of the magnetic measurements obtained by exploring superconducting bulks and tapes magnetized by an external field and the magnetic map generated by the current carried through superconducting wires.

Abstract: The issues in the conventional sintering of the MgB2 superconductors have conducted to the discovery of a new way to densify this material. The new process is an “in situ” method that relies on the reactive liquid infiltration (RLI) of liquid Magnesium into Boron powders packed preform. The RLI process allows to obtain highly dense manufacts without the use of hot pressing apparatus and can be applied to the manufacture of large superconducting pieces. One of the peculiarities of the MgB2 superconductivity, that withstand up to 39 K, is represented by the relative insensitiveness of the supercurrent percolation to the orientation of the grain boundaries. This property allows to use polycrystalline material without loosing superconducting performance, granted that a good connectivity between the crystalline grains must be realized, as the RLI process allows to do. The microstructure of the bulk material obtained by RLI shows a variety of morphologies, according to the kind of the used Boron powders and to the process variables. A detailed analysis of the microstructure of the MgB2 obtained by RLI will be presented, as well as its analytical description and the correlation with the superconducting characteristics.

Abstract: We report on recent progress towards the continuous deposition of YBCO Coated Conductors (CC) by thermal co-evaporation. This is an attractive vacuum technique thanks to the simplicity, low cost and intrinsic uniformity over large areas. Recently, we published the in situ preparation route of 1 μm thick superconducting YBCO films deposited onto CeO2 buffered Ni biaxially textured tapes using a reel-to-reel system; end-to-end critical current densities Jc's at 77 K and self-field, measured by transport measurements are in the 1-2 MA/cm2 range for 1 m. long samples, with zero-resistance Tc= 87 K and transition widths DTc<3 K. In spite of the very good CC’s performances reported by a number of laboratories all over the world, several steps must be optimized in order to limit the CC production costs, in particular concerning the complexity of the CC architecture and the choice and optimization of the YBCO deposition technique. We specifically address the following critical points regarding: 1) the in situ oxidation of the YBCO layer using a novel supersonic nozzle, 2) the deposition by thermal or e-beam evaporation of thick crack-free CeO2 buffer layers and 3) the scaling-up of the deposition process using a new multichamber system.

Abstract: Addition of Ti and Zr to high-pressure (HP) synthesized MgB2 results in an increase of critical current density of the material due to the absorption of impurity hydrogen coming most likely from the materials of a high-pressure cell. The results of the studies of structure, critical current density, trapped field and mechanical characteristics are discussed. High-pressure synthesized MgB2 (with Ti additions) blocks were for the first time used in a SC electromotor at 20 K and demonstrated the efficiency similar to that of MT-YBCO bulk (at the same working temperature).

Abstract: The ruthenocuprates family is a very interesting class of materials, because of the coexistence of superconductivity and magnetic ordering. Ruthenocuprates include RuSr2RECu2O8 and RuSr2(RE,Ce)2Cu2O10- (RE = rare earth elements or Y). It is possible to synthesize samples of these phases with Gd, Eu or Sm with normal synthesis conditions. For the others high-pressure high-temperature (HPHT) synthesis is required. We had successfully synthesized the RuSr2Tb1.5Ce0.5Cu2O10 by HPHT technique, starting from RuO2, SrO2, Tb4O7, CeO2, CuO and Cu. Around 300 mg of the mixture was allowed to react in a flat-belt-typehigh- pressure apparatus at 6GPa and 1200 °C – 1550 °C. The optimised temperature of synthesis was found to be in the range between 1350 °C – 1450 °C. The as-synthesized compound crystallized with a structure belonging to the space group I4/mmm. DC magnetic susceptibility versus temperature plot for RuSr2Tb1.5Ce0.5Cu2O10 in an applied field of 10 Oe demonstrated magnetic transition at 150 K but the superconducting transition was not clearly observed. To our knowledge this is the first successful synthesis of the Tb based Ru-1222 phase.

Abstract: In this work we present the results about the preparation of the superconductor YBa2Cu3O7-δ (Y123) by sol-gel method using oxalate precursors. Samples were prepared by dissolving stoichiometric mixtures of Y(OOCCH3).4H2O, Ba(OOCCH3)2 and Cu(OOCCH3)2.H2O acetates in aqueous solution of oxalic acid, HCOO-COOH. The particulate sols obtained (2g.) were divided in two parts (1g. each one), from which the first one (sample A) was grinded, pellet, calcined (860°C) and sinterised at 860°C in oxidizing atmosphere, the second part (sample B) was grinded, pellet, calcined at 880°C and finally sinterized at 880°C in the same conditions, the XRD revels the superconductor Y123 is formed in the first sample, but coexisting with remarkable presence of the secondary phases: CuO, BaCu3O4, BaCO3 and BaCuO2, while in sample B the XRD reveals the obtention of high purity monophase Y123, the ac susceptibility measurement using a Quantum Design (SQUID) magnetometer indicates TC(onset)=92K for this sample, moreover the Rietveld refinement was done taking the orthorhombic Pmmm Y123 spatial group as a model, this suggest the cell´s parameters a=3.839Å, b=3.899 Å and c=11.702Å as reported in the literature. Therefore for the preparation of Y123-superconductor using oxalate precursors, the optimus sintering temperature is 880°C.

Abstract: Bi-Pb-Sr-Ca-Cu-O films were deposited at 270-420°C by organic chemical spray pyrolysis deposition with triphenyl bismuth (TPB), Pb(tmhd)2, Sr(tmhd)2, Ca(tmhd)2 and Cu(tmhd)2 (tmhd: 2,2,6,6-tetramethyl-3,5-heptanedione). A factorial experimental design was applied in order to study the interaction between composition, deposition temperature, annealing time and annealing temperature and the effect in the formation of crystalline phases. Results of their structural and chemical characterization are presented.

Abstract: The phase evolution and properties of strontium vanadate Sr3V2O8 were investigated using different method of preparation: solid state reaction and two wet-route techniques (colloidal route and sol-gel method - polymerized complex route). DTA/TG study showed decomposition temperatures of starting materials and helped to choose processing temperatures. XRD analysis was used to determine phase evolution. Solid state reaction sample contained a small amount of Sr3V2O8 as soon as after the first calcinations, but 100% of Sr3V2O8 did not appear until whole procedure was finished. Colloidal route turned out as an inconvenient method for preparation of pure phase. The final colloidal route sample contained 80% of Sr3V2O8, residual volume corresponded to Sr10V6O25. The most progressive phase evolution was detected in the sol-gel route. Raman an FTIR spectroscopy detected evolution of the first amount of Sr3V2O8 after gel heating at 250°C for 2 hours. The pure strontium vanadate was prepared after heating at 800°C for 2 hours.