B Zeimetz

Large low-field magnetoresistance over a wide temperature range induced by weak-link grain boundaries in La0.7Ca0.3MnO3

Polycrystalline bulk porous samples with a large number of weak-link grain boundaries and high density polycrystalline bulk samples with strong-link boundaries were synthesized by conventional solid-state reaction and a partial melting technique. The weak-link samples showed large magnetoresistance (MR,Δρ/ρH=[ρ0−ρH]/ρ0), 20–30%, at a low magnetic field of 300 mT and over a wide temperature range from the magnetic transition at 235 to 77 K. The partially melted samples exhibited the same magnetoresistance behavior as that of single crystals with a maximum peak MR of 15% at a narrow temperature range around the ferromagnetic transition. It is suggested that grain boundaries are necessary but not sufficient to account for the MR at low field over a large temperature range. Weak-link grain boundaries rather than strong-link boundaries are responsible for the MR at low field over a large temperature range.

Preparation of Ag - Bi-2223 tape by controlling the phase evolution prior to sintering

The critical current densities and microstructures of tapes prepared using various pre-sintering thermomechanical treatments were compared. The phase composition prior to sintering was found to have a strong effect on the rate of 2223 phase growth and texture and, consequently, final transport properties. The difference in optimum requisite tape sintering processes when using powder with the same stoichiometry from different powder processes or even using identical powder obtained from the same source is due to variations in the content and composition of the secondary phases in the calcinated powder and/or the different phase transformations that occur due to different thermomechanical treatments prior to the sintering procedure. The optimum phase composition before sintering is a mixture of the main phase (textured mature Pb-2212 and Pb free 2212) and a small amount of secondary phases consisting of SrCaCuO and with small particle size. BiSrCuO, CuO and CaCuO are all detrimental to final tape performance characteristics and should be minimized in precursor powder. The optimum phase composition can be prepared by appropriate pre-sintering preparation and thermomechanical procedures. The particle size of the oxide in the Ag sheath can be decreased to a large extent by the deformation process. A critical current density of and a high (1 T, 77 K)/ of 23.4% has been achieved reproducibly using short sintering times and one intermediate pressing procedure. The reproducibility is a direct consequence of controlling the phase composition of the oxide core prior to sintering.

Improvement of grain connectivity in Bi2223/Ag tapes by reducing Bi2201 phase at grain boundaries

Abstract The critical current of Bi2223/Ag tapes with different secondary phase content was measured in an applied magnetic field at 77 K. The phase content was altered using different cooling rates after the final sintering, or by using an additional sintering stage at a lower temperature. The difference in strong link grain connectivity in the tapes was obtained by extracting strong link critical current ( I c0 s ) from the I c vs. H curves. Flux pinning was monitored via the field of the peak of pinning force density. Secondary phase content was assessed using X-ray diffraction. SEM analysis was used to study the secondary phase distribution and microstructure in the tapes. Normally prepared tapes showed an initial increase of normalized I c0 s with J c0 . For J c0 >20 kA/cm 2 , however, normalized I c0 s saturated at about 35% [J. Horvat, W.G. Wang, R. Bhasale, Y.C. Guo, H.K. Liu, S.X. Dou, Physica C 275 (1997) 327], while tapes with low Bi2201 content did not show this saturation of normalized I c0 s . This resulted in a substantial improvement of field dependence of I c over conventional PIT tapes. For the field direction parallel to the tape plane, 28% of I c0 was retained at 1 T. SEM analysis revealed that the Bi2201 was located between the grains. It is suggested that by removing Bi2201 from low angle tilt boundaries, a higher number of strong links was formed in the tapes.

Microhardness anisotropy of Bi-2212 crystals

Abstract The microhardness of a high quality Bi-2212 crystal, grown by using the temperature gradient method, was measured with the direction of the applied force parallel, and perpendicular to the ( a , b ) crystallographic plane. The measurement was carried out using a technique which is capable to monitor both the applied force, and the displacement of the indenter relative to the sample surface. A maximum force of 10 mN was used, applied in 20 steps in a square root sequence on a diamond pyramid indenter, having a tip radius of 1 μm. From the load–unload cycle, a number of physical parameters of the Bi-2212 phase were deduced: Youngs modulus, Poissons ratio, the yield strength in compression, the elastic recovery rate, and the energy loss during the indentation process. The results show a strong hardness anisotropy between the two directions of the applied force: 3.78 GPa for the applied force perpendicular, and 0.78 GPa for the applied force parallel to the ( a , b ) crystallographic plane.

Effect of deformation of parameters on interface morphology of silver-sheathed high-temperature superconductor tapes

Abstract We have produced a series of Ag sheathed BiSrCaCuO tapes with a systematical variation of some of the deformation parameters to investigate effects on the distortion of the sheath-core interface, which often shows a wavy morphology (“sausaging”). The increase of the powder density has been measured directly by monitoring the volume of the tapes during the deformation. Using silver particles as a “tracer”, the non-uniform, wavy powder flow can be observed also inside the bulk oxide core. We develop a simple model of the deformation process, based on a relaxation concept, and compare its quantitative predictions with our experimental results and with those of other groups.

Critical current enhancement in (Bi, Pb)2Sr2Ca2Cu3O10 tapes via isotropic quasi-columnar defects, induced by uranium fission products

Abstract UO4 are added to precursor powders of (Bi, Pb)2Sr2Ca2Cu3O10 and then textured by PIT technique into AgBiSCCO tapes. Tapes are then irradiated with thermal neutrons to fission235U. Fission products create short quasi-columnar defects that act as pinning centers. Transport properties, before and after irradiation, are investigated. Several features emerge from these studies. Birr increases by a factor of ∼2 at fluence, Fn∼2×1016 cm−2. Ic increases by a factor ∼ 4, for B//ab= 2T, and by factor ∼ 100, for B//c= 0.8T. The highest Ic enhancements shifts toward higher fluence as the applied field increases. The intrinsic BiSCCO anisotrophy is reduced up to a factor of 100.

Grain connectivity and flux pinning in Bi-2223/Ag PIT tapes

Abstract A hot-pressing technique was used to prepare Ag-sheathed Bi-2223 multifilamentary tapes in order to study its effect on grain connectivity and flux pinning. The self field critical current density (Jc) after hot-pressing was significantly enhanced, the maximum increase reached more than double than before. Hot pressing raised the Jc (B=0) from 22 000 A/cm2 (Ic=18 A) to 56 800 A/cm2 (Ic=36.5 A) for 81-filament tape and from 26 000 A/cm2 (Ic=36.2 A) to 56 000 A/cm2 (Ic=56.6 A) for 19-filament tape. But, the Jc in applied magnetic field was not improved or degraded by hot-pressing. The Jc retention in magnetic field of 1 T decreased from 33% for the original tape to 21% for the hot-pressed 81-filamentary tapes and remained the same (24%) for 19-filamentary tapes before and after hot-pressing. The effect of cooling rate on connectivity and flux pinning in hot-pressed Bi-2223 PIT tape has been investigated. The fraction of strong links (IC0s/IC0) was improved from 64% to 73% for 81-filament tape due to hot pressing and slow cooling. However it was reduced to 42% for hot-pressed and normally cooled tape. Microstructural analysis showed that hot-pressing chiefly improved grain connectivity, increased core density, improved crystal alignment, recovered micro-cracks and reduced secondary phase impurities. Hot-pressing technique reduced some defects, but at the same time, it created different type of defects, so the whole amount of defects was not much changed. Therefore, hot pressing caused different influence on flux pinning for different original tapes. These results suggest that limitation of Jc in Bi-2223 tapes is multiple and potential for further improvement of Jc is great.

Improved critical current of superconducting Bi2223-Ag tapes for current lead application by addition of 'large' silver particles

The effect of silver additions on the microstructure and superconducting properties of (Bi2223) - Ag tapes has been investigated. We find a strong dependence on both the silver content and especially the silver particle size. The addition of small () particles results in a slight reduction of the critical current. Large () particles are squeezed into long flat layers, leading to better grain alignment, grain density and phase purity of the 2223 phase along these layers. As a consequence, the critical current is increased by up to 60% in self-magnetic field, and even more in an external magnetic field.

Recrystallization effects and grain size in Bi-2223 tapes

Bi-2223 precursor powders with different stoichiometries and synthesized using different heat treatments were compared with powders obtained from external sources in powder-in-tube tapes. In the final microstructure, it was found that the Bi-2223 grain size varied considerably between tapes made from different powders. The critical current density was found to increase with increasing grain size. Among the factors found to determine the Bi-2223 grain size were powder stoichiometry and Bi-2212 recrystallization rate.

Critical current density significantly enhanced by hot pressing in Bi-2223/Ag multifilamentary tapes

Ag-sheathed Bi-2223 multifilamentary tapes were prepared with a hot-pressing technique to study its effect on grain connectivity. The critical current density after hot-pressing was significantly enhanced with the maximum increase reached more than double that before. Hot pressing raised the from to for 81-filament tape. However, the critical current density in an applied magnetic field was not improved by hot-pressing. The in a magnetic field of 1 T decreased from 33% of for the original tape to 21% of for the hot-pressed tapes before and after hot pressing. The fraction of strong links was improved from 64% to 73% for the 81-filament tape as a result of hot pressing. Microstructural analysis showed that hot pressing chiefly improved grain connectivity, increased core density, recovered microcracks and reduced secondary phase impurities.