Ravi-Persad Sawh

Uranium chemistry and pinning centers in high temperature superconductor

Abstract A study has been made of compounds formed when up to 1% uranium oxide is added to YBa 2 Cu 3 O 7− δ (Y123), prior to texturing. Initially, the study was to create pinning centers by addition of UO 4 , followed by neutron fission of some of the 235 U component. It was found that, prior to fission, the chemical reaction of U provides sufficient pinning centers to approximately double J c . The U forms in deposits about 300 nm in diameter. The number of such deposits is proportional to the mass of U. TEM and XRD studies identify the compound as (U 0.6 Pt 0.4 )YBa 2 O 6 , a double perovskite. Neither T c nor creep vary with %U. The uranium compound has also been produced outside of Y123. The behavior of uranium in high temperature superconductor (HTS) is thus of interest not only for introducing pinning centers by neutron fission of U but also as an effective chemical method to increase J c .

Production run of 2 cm diameter YBCO trapped field magnets with surface field of 2 T at 77 K

A production run of 60 melt-textured YBCO trapped field magnets (TFMs), 2 cm in diameter, is reported. The TFMs have pinning centers of Y211 and (Pt0.4U0.6)YBa3O6 deposits and also damage tracks of ions from uranium fission. The resulting average trapped field at the center of the seed-side surface is 〈BT(r = 0)〉 = 2.04 T at 77 K. The rms spread in BT is 5.9%. TFM samples rejected due to problems with the SmBCO seeds totaled ~10%, and an additional 2% were lost due to cracking under magnetic pressure. Methods to significantly reduce these losses are discussed. The ratio of BT on the TFM bottom to that on the seed side is ~67%. Evidence of a method to raise this ratio to 1.0 is presented. The magnitude and time dependence of the radioactivity resulting from the uranium fission are reported.

Improved pinning regime by energetic ions using reduction of pinning potential

When ion damage is used to create pinning centers, full columnar pinning centers provide the largest pinning potential, Upin, but not the greatest Jc or pinned field, Bpin. Some of the characteristics of columnar defects which limit Jc and Bpin are discussed, including reduction of percolation path, and the need for a larger number of columns of damage, for pinning, than are usually estimated. It is concluded that columnar pinning centers are limited to Bpin<4 T, and also severely reduce Jc. Evidence is reviewed that aligned damage, or broken-columnar pinning centers, described herein, can provide orders of magnitude higher Jc, and higher pinned field, despite providing lower Upin. A pinning center morphology is discussed which utilizes multiple-in-line-damage (MILD). For, e.g., present day large grain HTS Jc, obtainable by MILD pinning, is estimated to be of the order of 106 A/cm2 at 77 K, even when crystal plane alignment and weak links are not improved. Pinned field is increased by over an order of magnitude. An experiment is proposed to confirm these observations, and to directly compare MILD to columnar pinning centers. It will also determine the optimum MILD structure. Other measurements of interest, made possible by the same data set, are described.

Tungsten and molybdenum double perovskites as pinning centers in melt-textured Y123

Abstract Y123+30 mol% Y211 powders were doped with tungsten and platinum, and textured. Microstructure studies show the presence of profuse spherical deposits, 200–300 nm in diameter. These deposits were identified as (W 0.5 Pt 0.5 )YBa 2 O 6 , a double perovskite. The size of the W-rich deposits is independent of the W doping level. There is no substitution of W into the Y123 matrix. For Pt doping >0.5 wt.%, trapped field is observed to increase monotonically up to 40% for W doping of up to 0.48 wt.%. We conservatively estimate that this corresponds to a 60% increase in J c at constant field. Thus (W 0.5 Pt 0.5 )YBa 2 O 6 double perovskites deposits act as pinning centers. Similar studies of molybdenum doping indicate deposits 200–300 nm, of (Mo 0.5 Pt 0.5 )YBa 2 O 6 , also a double perovskite. The (W 0.5 Pt 0.5 )YBa 2 O 6 and (Mo 0.5 Pt 0.5 )YBa 2 O 6 deposits are remarkably similar to the (U 0.6 Pt 0.4 )YBa 2 O 6 deposits found earlier in U-doped Y123. Therefore, W and Mo are suitable non-radioactive substitutes for U.

An experimental generator using high temperature superconducting quasi-permanent magnets

An experimental axial gap generator, with a 6 inch diameter rotor and quasi-permanent HTS magnets, was tested. A variable speed motor was used to rotate the rotor above a stator composed of copper wire wound in an 8 pole 3 phase configuration. The rotor and stator were both run in liquid nitrogen. A 3 phase variable resistor bank was the generator load. The HTS magnets were YBCO, each 2 cm in diameter and 1 cm thick, melt textured using a SmBCO seed, containing 60% excess Y, and about 1%Pt. They were not irradiated, and could trap maximum fields of 3500-4000 G. First, the HTS magnets were field cooled in a field of approximately 2200 G, provided by the stator while carrying 25 Amperes. The HTS magnets trapped about 1800 G. In this configuration, the generator was run for about one hour at speeds up to 2000 RPM, and developed a maximum power output of 33 Watts. Next the HTS magnets were warmed, and then zero field cooled. In this case, activation was accomplished by a /spl sim/15 ms pulse on the stator, providing 5000 G of pulsed field. The HTS magnets trapped about 2600 G. The generator was then run for about 1 hour at speeds up to 2265 RPM, and developed a maximum power output of just under 100 Watts.<<ETX>>

The effects of uranium doping and thermal neutron irradiation on the pinning properties of Ag/Bi-2223 tapes

Abstract Ag/Bi-2223 multifilamentary tapes doped with small quantities of UO 4 powder were prepared by the powder-in-tube process. Thermal-neutron irradiation of these tapes induces fission of the 235 U atoms, with the resulting fission fragments creating randomly splayed defects. An observed increase in J c after irradiation as well as a shift in the irreversibility line to higher fields, in comparison to nondoped samples, implies an improvement in the flux pinning of the tapes as a result of this process. Dynamic magnetization relaxation measurements were conducted to elucidate the behaviour of the effective activation energy (U eff ) as a function of current density. Changes in the activation energy after irradiation are examined and related to changes in the pinning potential of the tapes.

On the current transport limitations in Bi-based high temperature superconducting tapes

The transport critical current densities, Jc, of superconducting (Bi, Pb)2Sr2Ca2Cu3Ox/Ag tapes were measured before and after employing a special radiation technique, which leaves the grain boundary properties largely unaffected. We identify two regions separated by a temperature dependent crossover field Hgb−p. In the low field region, Jc is limited by the transport currents across the grain boundaries, which remain unchanged after irradiation. Above Hgb−p, Jc is limited by flux pinning. In this field region, the artificial defects optimize flux pinning and enhance Jc.

Improved pinning by multiple in-line damage

Columnar pinning centres provide the largest pinning potential, Upin ,b utnot the greatest Jc or pinnable field, Bpin .C haracteristics of ion-generated columnar defects which limit Jc and Bpin are discussed, including reduction of the percolation path, and the need for a larger number of columns of damage, for pinning, than are usually estimated. It is concluded that columnar pinning centres limit Bpin to less than 4 T, and also severely reduce Jc .T hegoal of maximizing Upin ,v ia columnar centres, appears to have obscured a more rewarding approach and resulted in neglect of a large regime of ion interactions. Evidence is reviewed that multiple in-line damage (MILD), described herein, can provide orders of magnitude higher Jc and Bpin ,d espite providing lower Upin .T he MILDpinning centre morphology is discussed, and it is estimated that for present-day large grain high Tc superconductors, a Jc value of ∼10 6 Ac m −2 is obtainable at 77 K, even when crystal plane alignment and weak links are not improved. In addition, the pinned field is increased by over an order of magnitude. An experiment is proposed to confirm these calculations, directly compare MILD pinning to continuous columnar pinning, and determine the optimum MILD structure. Applications of MILD pinning are discussed.

A Study of Pulsed Activation of Trapped Field Magnets—Part 1: Effects of Pulse Height and Creep

We report experimental results of a study of pulsed field activation of YBCO trapped field magnets (TFMs), at 77 K, in which the applied field covers an area smaller than the TFM. Data on a subset of experiments, in which pulse amplitude is varied, show the effects on trapped field distribution and creep. At low pulse height, trapped field is observed at the sample periphery, even at applied field well below BC1. At increased fields, the measured J is not limited to JC or zero, as in the Bean model, but varies continuously from 0 to JC, possibly as a result of averaging over the spatially varying applied field. For partial activation, creep is a function of r. As the critical state is approached, the spread in creep rates collapses to a single value. Full activation is obtained by a single pulse of applied field approximately 3.3 times maximum trapped field on the TFM surface.

Characteristics of Trapped Field Magnet Activation by Flux Pumping

Currents are set up in high temperature superconducting (HTS) material when it is cooled in a magnetic field (called field-cooling: FC), or exposed to a magnetic field after cooling (called zero-field-cooling: ZFC). The HTS material can be treated to make these currents persistent, thereby forming “trapped field magnets” (TFMs). TFMs have been improved over the past two decades by the efforts of a large number of worldwide research groups. However, applications using TFMs have lagged, in part due to the problem of high fields needed for activation. We describe herein experiments to characterize the behavior of TFM activation using repeated applications of permanent ferromagnets (called “pumping”). Significant partial activation is obtained using a non-uniform pumping field which is higher in the center of the HTS than at the periphery. ZFC, followed by pumping with such a field, results in trapping the full applied field, instead of half the applied field, as is trapped by zero-field-cooling followed by application of a uniform field. We also find that for FC partial activation, and subsequent additional activation by pumping, the resulting fields are additive. Additionally, we conclude that one contributing, positive component of activation by fluxoid pumping is the phenomenon of creep.