G.W. Qiao

Effects of nano-ZrO2 particles on the superconductivity of Pb-doped BSCCO

The major limitations of Bi-system superconductor application are the intergrain weak links and weak flux pinning capability. In order to promote the practical application of Bi-system superconductor, effective flux pinning centers must be artificially introduced into HTS to enhance the flux pinning capability. With those goals, we have investigated the effects of nanometer-sized ZrO2 addition on flux pinning capability of the bulk Pb-doped Bi-2223 by using of X-ray diffraction (XRD), DC resistivity, rho, AC susceptibility, c, and magnetic hysteresis loops measurements. We have prepared samples by Bi,Bi1.8Pb0.4Sr2Ca2.2Cu3O10+y (nano-ZrO2)(x) with x range from 0 to 0.5. The results show that, with the increasing of x-, the T-c (onset) drops down gradually. However, the measurements of the critical current density, J(c), show that appropriate amount-doped nano-ZrO2 greatly enhance the values of J(c) under magnetic fields. According to the results of XRD patterns, there are slight reflection peaks of ZrO2-related nonsuperconducting compounds. It is believed that these nonsuperconducting ZrO2-related compounds act as effective flux pinning centers

ELECTRICAL-RESISTIVITY OF NANOCRYSTALLINE FE-CU-SI-B ALLOYS OBTAINED BY CRYSTALLIZATION OF THE AMORPHOUS ALLOY

Abstract Nanocrystalline Fe-Cu-Si-B alloys, with grain sizes of 30–90 nm, were prepared by crystallization of the parent amorphous alloy. Electrical resistivity of 30 nm-grained Fe-Cu-Si-B samples measured at room temperature was found to be higher than that of the amorphous material. With increasing grain size, the resistivity declines rapidly, which is in good agreement with the theoretical analysis based on the electron scattering of interfaces.

Magnetic properties and structure of R2Co17−xTix intermetallic compounds (R=Pr or Nd)

The effect of substitution of Ti for Co in compounds R2Co17−xTix (R=Pr or Nd, and x=0.0–2.0) on the structure and the magnetic properties has been investigated. It is found that the solubility of Ti in the compounds R2Co17−xTix (R=Pr or Nd) is around 1.0. The R2Co16Ti (R=Pr or Nd) are isostructures to their parent compounds (rhombohedral), and its unit cell is expanded. The spontaneous magnetization μs and Curie temperature TC decrease with increasing Ti content, owing to the magnetic dilution and the reduction of the moment of Co sublattice caused by Ti substitution. Moreover, an anomaly in the M-T curves of Pr2Co16Ti and Nd2Co17−xTix compounds with x=0.5 and 1.0 may be attributed to a spin reorientation. This transition may originate from the change of crystal electron field due to the preferential occupation of Ti atoms on specific crystallographic sites.

STUDY OF AG/OXIDE INTERFACE OF BI-2223 SILVER-SHEATHED SUPERCONDUCTING COMPOSITE

The composition and microstructure of the Ag/Bi-2223 oxide interface in the silver-sheathed superconducting composite have been analyzed by Auger electron spectroscopy, electron probe microanalysis, x-ray-diffraction analysis, scanning electron microscopy, and energy-dispersive analysis x-ray spectroscopy. The results reveal that a diffusion layer exists in the Ag/oxide interface, and the width of the diffusion layer is greater in high-J(c) samples than in low ones. It is also found that bismuth oxide may vaporize through the silver cladding at high temperature, and the deficiency of bismuth brings about the formation of nonsuperconducting phase

High-field magnetization of doped Bi-series oxide superconductors

The high-field magnetization measurements of Zn- and Ga-doped sintered Bi-based compounds were carried out in a 40 T high-field facility. The critical current density and the flux pinning force of the Bi-series oxide superconductors were calculated using Bean’s critical state model. It is found that both the critical current density and the flux pinning force of the Zn-doped oxide compounds are decreased slightly with increasing Zn content. However, the critical current density and the flux pinning force of the Ga-doped oxide superconductors diminish significantly with enhancement of dopant content. It seems that the superconductivity of Bi-series superconductors is much more severely destroyed by a trivalent dopant than by a divalent one.

High field magnetization of YBa2(Cu,Zn)3O7-δ superconductors

Abstract The high field magnetization of sintered YBa 2 Cu 3- x Zn x O 7- δ compounds was investigated. By means of Beans critical state model it is derived from magnetization measurements that the slope of critical current density vs. external magnetic field curve becomes less negative and the flux-pinning force diminishes significantly with increasing zinc content. The magnetic field corresponding to the maximum of the pinning force decreases sharply with increasing zinc content. It is thought that the destruction of superconductivity in YBa 2 Cu 3 O 7- δ compounds by zinc doping is due to the change in electronic structure in the CuO 2 planes as well as the reduction in orthorhombic distortion.

STUDY OF THE CRYSTALLIZATION PROCESS OF BI1.8PB0.2SR2CA2CU3OZ GLASS USING TRANSMISSION ELECTRON-MICROSCOPY

Crystallization of the Bi1.8Pb0.2Sr2Ca2Cu3Oz (BPSCCO) glass has been investigated by transmission electron microscopy in the temperature range from 500-degrees-C to 860-degrees-C. Experimental results show that the microstructure of the annealed BPSCCO glass is closely determined by the annealing temperatures. The superconducting phases are block-shaped when the BPSCCO glass is annealed at a temperature lower than 600-degrees-C, while the superconducting phases adopt a lamella like shape, with length/width ratio of about 4-5, above 600-degrees-C. The 2212 phase exists from 600-degrees-C to 860-degrees-C, but the 2223 is stable only in the narrow range between 800-degrees-C and 850-degrees-C. High-resolution lattice images revealed that the superconducting phases formed via the glass process are composed of several narrow lamellae with a common c-axis, and there are no secondary phases at the interfaces between the lamellae. It has also been found that the crystal defects can be produced through intergrowth of the 2212 and 2201 phases.

Anomalies in the M(T) curves of the ceramic superconductor Y0.8Pr0.2Ba2Cu2.90Li0.10O7−δ

Abstract In the present study, some anomalies, the paramagnetic Meissner effect (PME), the diamagnetic dip effect (DDE) in the ZFC and FC M(T) curves and the glass transition, were observed for the ceramic superconductor Y 0.8 Pr 0.2 Ba 2 Cu 2.90 Li 0.10 O 7−δ . The PME vanishes at fields higher than 5 Oe. The DDE and the glass transition peak shift to lower temperature with increasing field, accompanied by a slight broadening of DDE. All of these anomalies are discussed in relation to the Josephson coupling loops in this superconductor sample.

SOLIDIFICATION CHARACTERISTICS OF TEXTURED 123-PHASE IN YBCO BY LASER FLOATING-ZONE LEVELING (LFZL) METHOD

Because of incongruent melting in YBa2Cu3O7-x, it is difficult to grow the textured 123 phase directly from the laser-heated molten zone in the laser floating zone (LFZ) method. To overcome this problem, the laser floating zone leveling (LFZL) method was utilized following a pseudo-binary diagram along the Y2BaCuO5-YBa2Cu3O7-x tie line and a crystal rod of textured 123 phases was directly grown from the molten zone. Experimental results indicate that the orientation of the textured tetragonal 123 phases is perpendicular to the c-axis and that the T(c) of the grown crystal rod is 91.2 K after annealing at 550-degrees-C for 40 h in flowing oxygen. Also, the solidified morphologies of some crystal rods grown at different growth rates were observed.

AN INVESTIGATION OF MELT-TEXTURED HIGH-TC SUPERCONDUCTOR WIRES MADE BY LASER HEATED PEDESTAL GROWTH TECHNIQUE

Abstract The growth of high Tc oxide superconductors both in Y and Bi systems by the laser floating zone melt technique has been studied. The seeding and feeding rods for the process were prepared by common powder metallurgy. The as-grown crystals are 0.5 –1.5 mm in diameters and show high densities and textured structures with c axis normal to the growth axis which would be greatly helpful to reduce the weak links between grain boundaries and enhance the transport critical current density. High transport Jc of 1880 A/cm2 at 77K and OT was found in Bi2Sr2CaCu2Ox textured fibers and Pb was also added in the specimens in order to raise the Tc and Jc. The samples were also characterized by X-ray diffraction, metallography, TEM, SEM and EPMA.