Fabrication and characterization of mercurocuprate superconductors on silver substrates.

摘要

The HgBa2CaCu2O6+delta (Hg-1212) and HgBa2Ca2Cu3O8+delta (Hg-1223) superconductors posses excellent superconducting properties, especially above 77 K. To be commercially viable, however, manufacturing wire/tapes of these superconductors has to be addressed. This research studies the fabrication and characterization of doped Hg-1212 and Hg-1223 superconductors grown on silver substrates using dip-coating and pulsed laser deposition (PLD) techniques. The main goal of this thesis is to optimize the synthesis parameters, understand the growth mechanism, and investigate the influences of the silver interface on the microstructure and superconducting properties of doped Hg-1212/Hg-1223 films.; A process methodology is firstly developed to fabricate Pb and Re doped Hg-1223 thick films on silver substrates using the dip-coating method. The phase content, microstructure, irreversibility behavior, and critical current density of the dip-coated films are investigated by x-ray diffraction (XRD), scanning electron microscopic (SEM), transmission electron microscopy (TEM), Superconductor QUantum Interference Device (SQUID), magneto-optical imaging (MOI). The Re-doped films show the dominant phase of (Hg,Re)-1223, characterized by an onset critical temperature (Tc) of 131 K. The transport critical current (Ic) is measured at 4.2 K, self-field using the standard four-point probe method with a 1 muV/cm criterion. The transport critical current density, Jtranc , at 4.2 K and self-field is ∼3 x 103 A/cm 2. The limitations to the critical currents are discussed in detail.; The comparison of Pb and Re doping effects on microstructure and irreversibility are studied. The Pb-doped films have much larger colony size, compared to that of the Re-doped films, whereas the Re-doping significantly decreases the structural anisotropy gamma, which consequently increases the vortex line stiffness and enhances flux pinning.; (Hg,Re)-1212 thin films are also synthesized on silver substrates using the PLD method. The optimization studies of the synthesis parameters show that quality films are those deposited at 200 mJ for 30 min followed by an annealing of 670°C for 10 h. The as-prepared films exhibit c-axis orientation but without in-plane texture, which mainly results from the polycrystalline structure of silver. Although the average Jtranc (4.2 K, self-field) of the PLD thin films, ∼3 x 10 4 A/cm2, is about one order-of-magnitude higher than that of the dip-coated thick films, it is about two orders-of-magnitude lower than the corresponding magnetization critical current density, Jmagc . The lower Jtranc value is attributed to the poor grain connectivity and film texture, and cracks in the grains formed because of the differences in the coefficient of thermal expansion between the film and the substrate.; The laser deposited (Hg,Re)-1212 film/silver interface microstructure is studied by SEM and TEM, showing that the substrate surface morphology significantly affect the film nucleation and growth. The (Hg,Re)-1212 film growth mechanism is investigated by Atomic Force Microscopy (AFM). A two-stage growth mode is also proposed.