M. Däumling

Reversible suppression of the so-called fishtail effect in ultra pure single crystals of YBa2Cu3O7−δ achieved by proper oxygenation

We report on the absence of the so-called fishtail or peak effect in the magnetisation curves of properly oxygenated high purity (5N) single crystals of YBa2Cu3O7−δ. As we recently reported [1] high pressure oxygenation procedures are suitable to produce a monotonous dependence of the critical current density with the magnetic field. In this article we report furthermore how to reversibly produce or remove this effect on the same crystal by proper oxygenation methods. Along with the high purity of the samples, — due to crystal growth in non reactive BaZrO3 crucibles —, this proves that at least in pure samples the fishtail effect is due to different oxygenation states and/or oxygen distributions. In less pure samples, however, other sources for this anomaly like metallic impurities can act as an additional source for the fishtail anomaly.

Hot rolling of silver-sheated Bi (2223) tapes

Abstract Silver-sheated Bi(2223) tapes have been prepared using a newly developed hot-roll apparatus where the rolls can be heated. up to 850°C. At 77 K and 0 T, a critical current density of 18 000 A/cm 2 has been measured on tapes after hot rolling at 800°C. Microstructural analyses and X-ray diffraction have been performed. The field dependence and the anisotropy of the critical current were investigated at 77 K. The hot-rolled tapes show a higher density and better grain alignment compared to cold-rolled tapes. The critical current density at 77 K and 1 T is 3500 A/cm 2 , i.e., it decreases by a factor of 5 will respect to the value in zero field. This is a considerable improvement compared to the published data on cold-rolled tapes where this factor is almost 7 for our reference tape and between 8 and 12 in the published data.

Mechanically induced flux pinning in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Abstract It is shown that mechanical deformation enhances intragrain flux pinning in silver-sheathed tapes of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x (2223) material. Tapes with zero transport critical current density ( J ct ) - but nonzero magnetic hysteresis - were produced with prereacted 2223 powders. Comparisons of the hysteresis in tapes directly after a heat treatment and after a subsequent pressing step show a clear increase of the hysteresis, almost a factor of three at 20 K and 3 T, corresponding to a five-fold increase in J c . The J c values can be lowered again by an additional heat treatment. Bent platelets can be observed in the microstructure after, but not before, pressing, suggesting that dislocations that could have been introduced in the pressing step may act as pinning centers.

Factors determining the magnetic field generated by a solenoid made with a superconductor having critical current anisotropy

The magnetic field generation in a simple solenoid is reconsidered for the case where the magnetic field is generated by a superconductor with anisotropy in its critical current density. In this case the influence of the radial magnetic field at the solenoid ends on the weak direction of the conductor has to be taken into account. Instead of the usual load line which stems from the maximum axial field at the inner turns, two load lines must be considered: one as usual, and the second one representing the radial field at the coil end. The maximum field generated by the solenoid is determined by which load line meets its respective jc-H curve first. For tapes of (Bi,Pb)2Sr2Ca2Cu3Ox it is the radial field at the solenoid ends which determines the central field which can be generated by the solenoid. This is also the case for most other anisotropic superconductors even with a moderate jc anisotropy of, for example, two. Insert coils in a background field can significantly raise the maximum central field as the ratio between axial and radial fields is different. This gain for a magnet made from Bi(2223) tapes is of the order of 30% (at T= 77 K). Some alternatives for maximum field generation using anisotropic tapes are discussed.

Monotonic dependence of jc on magnetic field is twinned crystals of YBa2Cu3O7−δ and ErBa2Cu3O7−δ

Abstract We have investigated the magnetic hysteresis and in particular the “fishtail” anomaly in crystals of YBa 2 Cu 3 O 7−δ and ErBa 2 Cu 3 O 7−δ , oxygenated in various fashions. For a high-pressure (100 bar) oxygenation this anomaly disappears, but only at high annealing temperature (700°C). It is concluded that for proper oxygenation the diffusion distance for oxygen must be larger than the size of the crystals, leading to a more homogeneous oxygen distribution throughout the crystal.

Hot deformation effects on critical currents in (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes

We have subjected fully reacted high-quality pressed silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes to a hot deformation treatment at various temperatures and times. The critical current density jc of these tapes before deformation is about 34 kA/cm2 in self-field at 77 K. Some dependence exists upon the length of the deformation treatment but the deformation-induced effects can be summarized as follows: The self-field jc at 77 K is enhanced to about 42 kA/cm2, about a 25% increase. The field dependence of jc is weakened significantly and thus we still measure about 1000 A/cm2 at 77 K, 0.5 T (H⊥ tape), which is the highest values measured in tapes up to date. Thus even though these tapes do not have the highest self-field jc values they mark new standards for jc values in high magnetic fields for non-irradiated tapes. At lower measuring temperatures strongly enhanced in field jc values are also observed. The irreversibility line appears to be shifted to higher field for both orientations of the tape with respect to the magnetic field. Using transmission electron microscopy we find an enhanced density of basal plane dislocations generated by the deformation at high temperature.

TEM study of hot deformation effects on (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes

Abstract The microstructural effects of deformation at high temperatures on silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes have been studied by means of transmission electron microscopy. Hot deformation is found to cause bending of the grains by introducing dislocations apparently without generating cracks. Short-time deformation raises the dislocation density by one order of magnitude and yields higher dislocation densities compared to long-time deformation. The observed dislocations mainly have 1 2 〈110〉 Burgers vectors and form irregular networks, both before and after the hot deformation. The significantly increased critical current density of hot deformed tapes can be explained by the introduction of the observed defect structures, which are potential flux pinning sites.

Critical currents in (Bi,Pb)2Sr2Ca2Cu3Ox powder

Abstract We have performed magnetic measurements of the critical current density ( J c ) in powders of the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x material. Considering flux relaxation, and applying the same criteria used for resistive J c measurements, we arrive at J c ≈2×10 4 A / cm 2 (77 K, 0 T). This value is considerably less than the highest values obtained in silver sheathed tapes of the same material, indicating that deformation induced flux pinning and possibly even grain boundary pinning may play some role in determining the J c in tapes.

Measurement of the current density alinf the c-axis in silver sheated (Bi, Pb)2SR2Ca2Cu3Ox tapes

We measured the magnetic properties of silver sheathed Bi2223 tapes with emphasis on the case where H ⊥ c. All measurements were carried out at 77K. Three regimes were dfound in which the mechanism determining the magnetic hysteresis is different. Below the irreversibility line for H | c the magnetic hysteresis for H ⊥ c is enlarged due to slight misalignments of the tape with respect to the magnetic field. No solution for the anisotropic critical state is found. Just above this irreversibility line the current along a,b-direction appears to be the limiting factor for the critical state. Resistively and inductively measured Jc are equal at this point. In still fields (> 0.4T) the critical state is determined by current transport along the c-direction. This current is two to three orders of magnitude lowe than the critical current in a,b direction.

Electromagnetic behavior and ac loss in a multi-layer superconducting power cable

Abstract The electromagnetic behavior of a superconducting power cable consisting of a layered structure is described. As input for the model the voltage–current characteristics of the tapes used to make the cable, and the self and mutualinductance of the layers are used. Current and voltage wave shapes can then be calculated for each layer as a function of externally applied ac by numerical integration of the equations describing the coupled layers. Due to the non-linear V–I characteristics of the layers layer currents become non-sinusoidal in the vicinity of the layer-critical current. Losses are computed by multiplying voltage and current in each layer separately and integrating over one period. Transient conditions have also been modeled – for example a quick shutdown of the cable current can result in circulating currents between layers even though no external current is flowing. Decay constants for this current depend on the resistance between layers and in the case of insulated layers on the joint resistance.