K. Salama

Influences of Y2BaCuO5 particle size and content on the transport critical current density of YBa2Cu3Ox superconductor

Abstract Transport critical current density anisotropy measurements have been conducted on liquid-phase processed YBa 2 Cu 3 O x superconductors (123) with differing Y 2 BaCuO 5 (211) content and particle size. It is found that for specimens with micron-size 211 inclusions and low 211 content, the critical current density, J c , increases with 211. In contrast, J c is determined to decrease with 211 content when the specimens contain large 211 particles and high volume fraction of 211. Moreover, the ability of 211 to act as pinning sites appears to diminish at moderate magnetic fields, whereas the crystal defects that are associated with the 123–211 interfaces appear to be effective pinning centers at high fields. It is proposed that the variation in J c with 211 content is not unique, but depends on the 211 content and the particle size, as well as the strength of the applied magnetic field.

Flux pinning by dislocations in deformed melt-textured YBa2Cu3Ox superconductors

Abstract The potential of flux pinning by dislocations to increase the critical current density (Jc) of melt-textured YBa2Cu3Ox superconductors in a variety of orientations of magnetic field (H) with respect to the a-b plane was studied. The melt-textured samples were subjected to high temperature uniaxial and isostatic deformation to increase the dislocation density. A broad peak in Jc is observed in these samples at H‖c-axis and is attributed to point pinning by dislocations generated in the a-b plane by the deformation process. A sharp peak in Jc is seen at 45° to the a-b plane in some isostatically deformed samples. Edge type 1 6 [031] partial dislocations located at the boundary of the stacking faults observed along the [ 1 10] zone axis of these samples are proposed to contribute to this peak. The results indicate that the Jc of melt-textured samples can be enhanced in magnetic field orientations other than H‖a-b by introduction of dislocations through high-temperature deformation processing.

TEM and critical current density studies of melt-textured YBa2Cu3Ox with silver and Y2BaCuO5 additions

Abstract TEM studies have been conducted on oriented domains of melt-textured YBa2Cu3Ox/15 wt.% silver (123/Ag) composites with various amounts of Y2BaCuO5 (211) additions. The observed microstructural defects are compared with the transport critical current densities, Jc, measured on the same domains in an effort to ascertain the likely flux pinning centers responsible for Jc and its increase with 211 addition in melt-textured 123. In the TEM studies, defects such as dislocations, pile-ups and loops, Ag and 211 inclusions are observed. The characteristics and densities of dislocations, pile-ups and loops within the bulk HTS do not appear to vary with Ag and 211 additions, and dislocation densities remain on the order of 109 cm−2 for all specimens. In addition to the typically observed large 20 to 60 μm Ag inclusions, we found that much smaller Ag particles of 2 to 200 nm are dispersed throughout the bulk in all specimens. Even though the small dimensions of these particles may allow them to be potential pinning sites, they do not appear to be responsible for the increase in Jc of the specimens examined. With 211 additions, the amount of 211 particles ranging in size from 0.2 to 2 μm is increased. It is observed that while large 211 inclusions possess defect-free interfaces, the interfaces of micron-size 211 particles with their high surface curvature are associated with numerous dislocations, where the localized dislocation density is on the order of 1010 cm−2. These interface-associated defects are believed to be responsible for the enhancement in Jc with an increasing amount of 211 addition.

High rate melt texturing of Nd1−xBa2+xCu3O7−δ type superconductors

Melt texturing is one of the most prominent techniques available to fabricate bulk high‐temperature superconductors with superior current transport and magnetic properties. In the case of Y1Ba2Cu3Ox, melt textured samples can have a Jc on the order of 105 A/cm2 at 77 K and zero applied magnetic field, and in excess of 103 A/cm2 at 77 K and 30 T. However, a serious drawback of the melt textured Y1Ba2Cu3Ox compound, which severely restricts its application, is the very slow rate (1 mm/h) needed to develop the highly textured planar front necessary for optimum superconducting properties. RE‐123 compounds containing the rare earth element neodymium have been recently found to recrystallize much more rapidly, indicating the possibility of their high speed melt texturing. By processing bars (50 mm×5 mm×5 mm) of these compounds through a high‐temperature gradient, fully textured microstructures were obtained at rates up to 50 mm/h, which are more than 50 times faster than those employed in Y1Ba2Cu3Ox. After oxyg...

Study of growth kinetics in melt-textured YBa 2 Cu 3 O 7− x

Directional solidification has been shown to be a successful way of achieving high current densities in bulk YBCO. The lack of understanding of the growth kinetics, however, makes it difficult to fabricate longer samples and reduce the processing times. To study the growth kinetics, quenching experiments of undoped YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} (Y-123) and Y-123 doped with Pt and Nd from above the peritectic temperature with different holding times, {ital t}, were conducted. The results of these experiments indicate that the average 211 particle size varies as {ital t}{sup 1/3}. Growth rate experiments were also conducted on these samples to determine the maximum growth rate for plane front solidification, {ital R}{sub max}. This quantity was measured for undoped and doped Y-123 and its was found that the addition of Pt did not increase {ital R}{sub max} while the addition of Nd doubled the growth rate. Using the coarsening results together with the growth rate experiments, the diffusivity of Y in liquid and the 211-liquid interfacial energy for undoped and doped Y-123 were calculated. {copyright} {ital 1996 Materials Research Society.}

Effects of ionic doping on superconducting properties of melt textured YBa2(Cu1-xMx)3O7-δ (M = Co, Ni, Zn or Ga) large grains

The work reported in this paper aims at a better understanding of doping effects on high temperature superconducting YBa2Cu3O7?? (YBCO or M-doped YBCO), in the form of large grains, fabricated by the top seeded melt-growth process. The study focused on the performance of YBCO large grains doped with Co, Ga, Ni and Zn substituting for the Cu site. The rate of Tc suppression with dopant concentration varies significantly among different dopants. Zn?doping shows the highest depression rate, followed by Ni, Ga and Co?doping. This we attribute to the fact that Zn and Ni substitute at Cu?(2) sites on CuO2 planes in the lattice, whereas Co and Ga occupy Cu (1) sites along CuO chains and they have different magnetic moments. Trapped field, levitation force and Jc are enhanced due to pinning caused by local suppressed superconducting regions, and the enhancement effects are the highest in the case of Zn doping.

Critical current properties and the nature of the electromagnetic coupling in melt-textured YBa2Cu3O6+x bicrystals of general misorientation

Abstract Detailed characterization of five (5°, 7°, 11°, 14°, 38°) general misorientation YBa 2 Cu 3 O 6+ x bicrystals produced by the melt-texture, liquid-phase-removal method shows that their field-dependent critical current density characteristic ( J c ( B )) undergoes a progressive, rather than a sharp transition with increasing misorientation. The 5° bicrystal had strongly coupled characteristics, while the 7°, 11° and 14° bicrystals had mixed characteristics, indicative of parallel weakly and strongly coupled paths crossing the grain boundary. This was true even for the 38° bicrystal. We conclude that some strongly coupled channels can remain open in even very high angle, generally misoriented bicrystals. As a whole these bicrystals exhibit a slower fall of J c ( B ) with misorientation angle than do thin film [001] tilt bicrystals.

Transport current density in bulk oriented-grained YBa2Cu3Ox/ silver composites

Abstract A liquid-phase processing method has been developed to manufacture bulk oriented-grained YBa 2 Cu 3 O x /silver (1-2-3/Ag) superconducting composites with large current carrying capacity and enhanced mechanical properties. It is found that addition of up to 25 wt.% Ag does not significantly affect the transport critical current density of these composites. Furthermore, J ct of 1-2-3/Ag composites is weakly dependent on applied magnetic field indicating that weak-links are largely eliminated through texturing. In a nominal 15 wt.% Ag, the values of J ct at 77 K are 86 000 A/cm 2 and 18 500 A/cm 2 at zero and 1.5 T applied field, respectively.

Critical current anisotropy in liquid phase processed YBa2Cu3Ox superconductors

Abstract The critical current anisotropy of liquid phase processed oriented-grained superconductors at 77 K has been studied with magnetic field aligned at different angles to the a-b plane. Two peaks, one at H ‖ a - b plane and another at H perpendicular to a-b plane are observed. The width of the a-b plane peak appears to be strongly sensitive to the degree of alignment of the a-b plane and in turn to intrinsic pinning. The peak at H perpendicular to the a-b plane is not affected by the volume fraction of 2-1-1 precipitates but appears to be related to twin spacing.

Enhancement of critical current density in YBa 2 Cu 3 O x superconductor by mechanical deformation

The critical current density of melt-textured YBa 2 Cu 3 O x superconductor has been enhanced by mechanical deformation at a high temperature. Hot deformation at 45° to both the slip plane (001) and the slip directions [100]/[010] has resulted in a high density of dislocation loops and stacking faults. The deformed samples are found to exhibit a critical current density (J c ) at H  c -axis as high as that at H  a-b plane at 1.5 T and 77 K. A J c of 35300 A/cm 2 has been achieved at H  c (1.5 T and 77 K) which is twice as high as that observed in undeformed samples. The enhanced J c in this magnetic field orientation is attributed to pinning by the defects created by mechanical deformation. This pinning mechanism is found to be effective over a wide angle between the magnetic field and the a-b plane and thus results in a marked reduction in the critical current anisotropy.