Y.T. Huang

Superconductivity with Tc(zero) above 105 K in Tl-containing septenary oxides with Y1Ba2Cu3Oy-like structure

Abstract A family of high- T c superconductors with T c(zero) above 105 K is described. The compounds are seven-element blends with the general formula Tl 0.5 Pb 0.5 Ca 0.8 A 0.2 Sr 2 Cu 2 O x ( A = Y ), and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). The results represent one of the first cases where chemical substitution has significantly raised T c in a known compound. The septenary compounds are tetragonal in structure with almost identical lattice parameters to those of now-famous Y 1 Ba 2 Cu 3 O y structure. The samples were highly reproducible, stable and remarkably homogeneous both in composition and structure. The preparative conditions were found to be much less stringent than those of other copper-based high- T c superconductors.

Improvement of resistive switching in NiO-based nanowires by inserting Pt layers

Nonpolar resistive switching is demonstrated in polycrystalline NiO-based nanowires. The lower switching voltages and narrower switching distributions are exhibited in multilayered NiO/Pt nanowires, compared to the monolithic NiO nanowires. The temperature dependence of resistance at low resistance state reveals the conduction is attributed to the hopping through percolation paths composed of oxygen-related defects. The inserted Pt layers behave as intermediate electrodes to reduce migration length of oxygen ions and to store the oxygen ions near the electrodes. Therefore, the localized formation/migration of oxygen ions confines the occurrence of percolation paths, leading to improvement of the switching parameters.

The fabrication and characterization of superconducting Tl-Pb-Ca-Pr-Sr-Cu-O compounds with Y1Ba2Cu3Oy-like structure and Tc (zero) up to 106 K

Bulk superconductivity with T c (zero) up to 106 K in a Tl 0.5 Pb 0.5 Ca 0.8 Pr 0.2 Sr 2 Cu 2 oxide with Y 1 Ba 2 Cu 3 O y -like structure was observed. A single phase, tetragonal structure with a = 0.382 ± 0.001 and c = 1.200 ± 0.001 nm and space group P4/mmm, was found in Tl 0.5 Pb 0.5 Ca 0.8 Pr 0.2 Sr 2 Cu 2 oxide samples by X-ray diffraction. T c (zero)s were found to be 78, 96, 106, 98, 45 and 25 K for Tl 0.5 Pb 0.5 Ca 1− x Pr x Sr 2 Cu 2 oxide samples with x = 0, 0.1, 0.2, 0.3, 0.5 and 0.7, respectively. For x = 1, the samples exhibited semiconducting behavior. All sintered samples were found to be remarkably homogeneous both in composition and structure. They were also exceptionally regular in microstructure and highly reproducible and stable. Although the six-metal blend makes the system appear to be rather complicated composition-wise, it is compensated by the relatively simple crystal structure. Furthermore, the preparation conditions are much less stringent than those for other copper-based high- T c superconductors.

Preparation of high purity 110 K phase in the (Bi, Pb)-Sr-Ca-Cu-O superconductor system using a solution method

Abstract A solution method has been demonstrated to purify the 110 K superconducting phase in the (Bi, Pb)-Sr-Ca-Cu-O system. Oxalic acid was used to chelate metal cations from the nitrates. The amount of chelating agent impacts the cation absorption in the precursor. The higher purity 110 K superconducting phase can be obtained in the shortest time and at the lowest sintering temperature. X-ray diffraction, resistivity and magnetization measurements all indicated positive evidence. The process can be expedited when the stoichiometry was enriched by calcium and copper. The preparation technique is considered valuable to purify multi-phase superconductors for mass production.

A new high-Tc superconducting Tl-Pb-Ca-Sr-Cu-O system

Abstract A new family of high-Tc superconducting material in the Tl-Pb-Ca-Sr-Cu-O system has been synthesized. Multiple phases with two-step superconducting transitions around 110 K and 80 K were observed resistively and magnetically. The 80 K phase has been tentatively identified to be (Tl, Pb)2Sr2CuOy by X-ray diffraction analysis. This material appears to have a relatively high flux pinning.

Synthesis and characterization for a new family of Tl-containing septenary oxides with Tc,zero above 105 K

Abstract Bulk superconductivity with Tc,zero above 105 K in Tl 0.5 Pb 0.5 (Ca 1−x A x )Sr 2 Cu 2 O y oxides (x=0.2, A=Y and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) with YBa 2 Cu 3 O y -like structure was observed. For A=Ce and mischmetal, Tc,zero was found to be 95 K and 101K with x=0.1. The results represent one of the few cases where chemical substitution has significantly raised Tc in a known compound. Single phase samples were prepared and were found to be highly reproducible and stable. Furthermore, the preparation conditions are much less stringent than those of other copper-based high-Tc superconductors.

Induced paramagnetism by low-field-cooled trapped flux in (Er0.2Ca0.8)Sr2(Tl0.5Pb0.5)Cu2Oy

Field-cooled magnetization measurements on the monophasic polycrystalline superconductor (Er0.2Ca0.8)Sr2(Tl0.5Pb0.5)Cu2Oy (Tc = 105 K) under various magnetic fields between 5.0 and 130 K are reported. For the cooling field Ha < 17 gauss, the strong paramagnetism induced by the trapped flux was observed. This novel effect is significantly different from the more familiar diamagnetic shielding of a superconductor.

Epitaxial growth of high-Tc superconducting Tl-Ca-Ba-Cu-O films by liquid phase epataxial process

Abstract Tl-Ca-Ba-Cu-O epitaxial films have been successfully grown on (001) MgO substrate by liquid phase epitaxial (LPE) process. The as-grown films showed an onset of superconductivity at about 140 K and zero resistance at 111 K. X-ray diffraction analysis suggests the films to be highly preferentially oriented with the c -axis perpendicular to the film surface. Observation from transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed the presence of both Tl-Ca 2 Ba 2 Cu 3 O y and TlCa 2 Ba 3 Cu 4 O y phases.

Rapid processing of Tl-Ba-Ca-Cu-O 2233 phase with Tc=127 K

Abstract As-sintered Tl-Ba-Ca-Cu-O 2223 samples were further annealed in an attempt to enhance the T c . Optimal conditions were determined according to the results of resistance and magnetic susceptibility characterizations. The highest T c , 127 K, was obtained when the sample was annealed at 820°C in flowing oxygen for 16 h. Weight change combined with a XRD and SEM/EDS study were used to explain the possible mechanism of enhancement.

A new series of (Tl1−yBiy)(Ca1−xYx)Sr2Cu2Oz superconductors with “1122” structure

Abstract Bulk superconductivity of Tl-based 1122-type structure was enhanced maximally up to 105 K in (Tl 1−y Bi y )(Ca 1−x Y x )Sr 2 Cu 2 O z system. By introducing substitution elements of Bi and Y for Tl and Ca respectively, the stability 1122 phase was improved as well as the bulk superconductivity was dominated.