J. A. Parrell

Magnetic granularity, percolation and preferential current flow in a silver-sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape

Abstract Magneto-optical imaging of the flux penetration into a Ag sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape has been used to extract the current flow paths in a magnetic field applied perpendicular to the c-axis. Using the large aspect ratio of the plate-like grain structure and the slab geometry of the sample to simplify the current-flow geometry, we converted the magneto-optical signal into two-dimensional (2D) field and current distributions. We found that the current patterns were very non-uniform and sensitive to weak magnetic fields of ∼ 400–800 Oe, even at 10 K. Current streamlines show that the effective current-carrying cross-section of the tape strongly depends on the field. Magnetization currents flow preferentially near the silver sheath, while the tape center supports mainly percolative and granular current patterns consisting of an array of macroscopic current loops whose long dimensions are of the order of the tape thickness. By comparing contour maps of the local Jc values with the microstructural images, we found that the high-Jc regions correlate with colonies of well-aligned long grains which are preferentially located near the silver interface, while the less aligned structure of smaller grains in the central part of the tape is associated with the granular behavior and much lower current-carrying capability. The wide distribution of the local Jc(x, y) revealed by magneto-optical imaging indicates that the performance of BSCCO-2223 tapes can be significantly improved if a larger fraction of well aligned grains can be produced more uniformly throughout the tape cross-section.

Direct evidence for residual, preferentially-oriented cracks in rolled and pressed Ag-clad BSCCO-2223 tapes and their effect on the critical current density

We have studied the connectivity of rolled and pressed Ag-sheathed tapes through three deformation and heat treatment cycles using magneto-optical imaging. In pressed samples, a transport critical current density of (77 K, 0 T) was reached after two heat treatment steps, and increased to after a third heat treatment. Critical current density values in rolled samples also reached after two heat treatments, but decreased to after the subsequent deformation and third heat treatment. Magneto-optical imaging using fields applied perpendicular to the rolling plane revealed that flux penetrated the superconducting core mainly through defects oriented perpendicular to the direction of current flow in rolled samples, and parallel to the direction of current flow in pressed samples. In those samples which had received more than one heat treatment, the flux is believed to penetrate through cracks produced during the deformation steps which do not heal during the subsequent heat treatments due to lack of sufficient residual liquid phase.

Enhancement of the 77 K irreversibility field and critical current density of (Bi,Pb)2Sr2Ca 2Cu3Ox tapes by manipulation of the final cooling rate

By manipulating the cooling rate from the final heat treatment, we have raised the 77 K, self‐field critical current density (Jc) of multifilament (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes by a factor of 3, and the irreversibility field (H*) by more than 50%. The Jc of samples cooled in 7.5% O2 from their reaction temperature of 825 °C increased from ∼8 to ∼24 kA/cm2 and H*(77 K) increased from ∼120 to ∼200 mT as the cooling rate was decreased from 5 to 0.016 °C/min. The results unambiguously show that the flux pinning properties of 2223 tapes can be improved by simple changes in wire processing.

Latest Improvements of Current Carrying Capability of Niobium Tin and Its Magnet Applications

Practical high field superconducting magnets are exclusively built with Nb3Sn multi-filamentary composites. Over the last few years there have been significant improvements in the current carrying capability of Nb3Sn strand, and these improvements offer the possibility to build more efficient and higher field strength magnets. The Nb3Sn composite requirements are somewhat different depending on the application, such as magnetically confined fusion, high energy physics accelerators, and solenoids for NMR or laboratory magnets, and thus require different designs. We will present the current status of Nb3Sn strand development at Oxford Instruments, Superconducting Technology (OST) for these applications, along with magnet results

On the role of Vickers and Knoop microhardness as a guide to developing high critical current density Ag-clad BSCCO-2223 tapes

Abstract Because there are many parameters influencing the critical current density (Jc) of Ag sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes, good and easily measured correlation variables to Jc are highly desirable. Yamada et al. have recently shown that very high Jc values can be obtained when the BSCCO core is rolled to a high Vickers microhardness (Hv) prior to heat treatment. The present experiment extends the correlation by showing that there is a linear relation between the Hv of reacted tapes and Jc. Measurement of the Knoop microhardness (HK) shows that the hardness is highly anisotropic. HK measured with the long indentor axis parallel to the a-b planes shows no dependence on reaction time or Jc, while HK measured parallel to the c-axis is strongly correlated. A high microhardness appears to be directly correlated to a high-density BSCCO core with a small crack density. Such a core has good connectivity which directly translates to a high Jc value.

Suppression of magnetic granularity by transport current in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Using the magneto‐optical technique, transport current flow in (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) Ag‐sheathed tapes has been imaged. For zero transport current, I, the magnetization currents display a pronounced macroscopic magnetic granularity (on a scale larger than the actual grain size) characterized by a percolative distribution of current loops. By applying I≊0.8Ic this granular behavior was largely replaced by a more uniform current flow. Due to the significant dependence of Ic on E in BSCCO, the extent to which the magnetic granularity is observed strongly depends on the magnetic prehistory and on the electric field E, whose characteristic values are very different for magnetization and transport measurements.

Study of powder density, Ag:superconductor ratio, and microhardness of BSCCO-2212 Ag-sheathed wires and tapes during wire drawing and rolling

Densification of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ (BSCCO-2212) powder cores in Ag-clad wires and tapes has been studied as a function of strain and initial packing density. Density was measured directly by etching away the silver and weighing the remaining core. A universal feature in both wires and tapes was the attainment of maximum densities (/spl sim/80%) at intermediate strains of /spl sim/4. The linear correlation between microhardness and density found for round wires breaks down for rolled tapes, apparently because the core separates into dense locks separated by cracks.<<ETX>>

Current transfer lengths and the origin of linear components in the voltage - current curves of Ag-sheathed BSCCO components

The commonly observed linear sections in the voltage - current characteristics of Ag-sheathed BSCCO-2212 and BSCCO-2223 composites were studied. One source of such characteristics is due to ohmic losses in the silver at the current feed points. Characteristic current transfer lengths are of the order of millimetres at 4.2 K and tenths of millimetres at 77 K. However, linear components can also be observed well away from current feed points. These were found to be associated with regions of locally reduced critical current, which provoke local transfer of the excess current into the Ag, thus producing local, ohmic V - I characteristics. Because of the finite current transfer length, some ohmic voltage can be detected even outside regions of reduced critical current. When the silver was removed from lower critical current regions, ohmic voltages were no longer observed, even when the critical current was severely reduced by local damage with a laser. Within the range of electric field covered by this experiment , we conclude that all ohmic voltages are produced by current flow in Ag.

On the role of pre-existing, unhealed cracks on the bending strain response of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Studies of the transport critical current (Ic), magnetization, magnetic flux penetration, and microstructure of pressed and rolled Ag-clad (Bi,Pb)2Sr2Ca2Cu3Ox tapes (2223) have been made as a function of bending strain. Pressed tapes exhibited markedly less degradation of Ic from strain than did rolled tapes, while the magnetization of pressed tapes declined much more rapidly with bending strain than did either of the transport currents. Magneto-optical imaging of nonbent pressed samples revealed a network of flux-penetrated defect channels that were primarily oriented parallel to the tape axis. Bending such samples to a small strain increased the visibility of these defects, believed to be cracks. This network correlates well to the cracks produced in intermediate thermomechanical processing deformation steps. The greater sensitivity of the transport current of rolled samples to bending is further direct proof of the fact that the tape “remembers” the cracks induced in the core during intermediate deform...

Connectivity and flux pinning improvements in Ag-clad BSCCO-2223 tapes produced by changes in the cooling rate

The rate at which Ag-clad (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes are cooled from their final reaction heat treatment influences both the intergranular connectivity and intragranular flux pinning strength of the polycrystalline filaments. As the cooling rate from 825 °C to 730 °C in 7.5% O 2 was decreased over a range of 5 °C/min to 0.005 °C/min, J c (77 K, 0 T) increased from ∼8 to ∼24 kA/cm 2 , and the irreversibility field increased from, ∼120 to, ∼200 mT. The J c (4.2 K, 0 T) increased in a similar fashion. Cooling slowly also sharpened the critical temperature transition and increased the critical onset temperature from 107 K to 109 K. These improvements in the superconducting properties occurred despite partial decomposition of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x phase into non-superconducting impurity phases during the slow cooling. A microstructural basis for these multiple effects is described.