The goal of this study is to investigate the mechanisms that are responsible for the dissipation in various polycrystalline high-temperature superconductors. The importance of grain connectivity for the conductors to sustain a large supercurrent is demonstrated. The question in how far poor grain connectivity reduces the critical current density and influences its behavior in magnetic field is addressed. The location of the dissipation mechanisms is revealed before the mechanisms are studied independently.
Several techniques are applied to study the effect of the grain structure on the overall critical current of the superconducting samples. The magnetic knife technique is introduced to obtain the spatial distribution in overall critical current density of YBCO coated conductors. The critical current density shows large variations across the width of the conductor, especially in samples that are produced with pulsed-laser-deposition.
The variations in macroscopic critical current density of YBCO coated conductors are related to their grain structure by combining the magnetic knife technique and magneto-optical imaging. Flux penetration at high-angle grain boundaries and defects is visualized with a resolution of several micrometers and correlated to variations in the microscopic critical current density. A qualitative image of the current carrying capabilities of the conductor is obtained by integrating the intensity of flux that is trapped in superconducting colonies over a length scale of several millimeters along the conductor length.
Clusters of high-angle grain boundaries and large defects reduce the critical current density on a length scale of several millimeters. Current meanders around high-angle grain boundaries and defects, resulting in a component transverse to the direction of the transport current. For this reason current is also severely limited by defects that run along the sample length.
The combination of magneto-optical imaging and the magnetic knife shows a better grain connectivity in YBCO coated conductors that are produced with metal-organic-deposition, compared to those produced with pulsed-laser-deposition. A three-dimensional YBCO layer provides a supercurrent with additional current paths to avoid clusters of high-angle grain boundaries and defects. This results in a smaller variation in critical current density across the width and along the length of the conductor.
The location of the weak links in the current path of Bi-2223 tapes is studied by visualizing the damage to the grain structure, caused by applied strain. Experiments where the decrease in critical current is measured as function of longitudinal strain show that the critical current in low field decreases faster than the critical current in high field. This indicates that weak links where dissipation at low field occurs are more affected by strain than well-connected grains and are therefore mechanically weak. The location of the weak links in the current path is obtained by visualizing the damage to the grain structure caused by longitudinal strain with magneto-optical imaging.
Studying the effects of strain by visualizing cracks with magneto-optical imaging relates the production process to the grain structure of polycrystalline high-temperature superconductors. The formation of colony boundaries that run over the entire width of filaments in Bi-2223 tapes is a direct result of intermediate rolling steps during the production process. Partly-healed microcracks form mechanically weak connections over the entire width of the conductor and act as weak links in the current path.
A difference in phase formation and the absence of intermediate deformation steps in the production of Bi-2212 tapes results in the formation of colony boundaries that do not run filament-wide. This is demonstrated by much slower crack propagation where filaments in Bi-2212 tapes do not break instantly when strain is applied. Cracks propagate rather slowly over the width of the filament when the applied strain is increased.
The grain structure of YBCO coated conductors is a direct result of the grain structure of the metallic substrate. The formation of cracks is not influenced by the superconducting layer, but entirely by the substrate. Longitudinal strain affects colonies and colony boundaries in a similar way, which results in a slower degradation in critical current compared to Bi‑2223 tapes when the critical strain is exceeded.
Current flow in polycrystalline high-temperature superconductors is analyzed in the viewpoint of the parallel path model, which describes two separate dissipation mechanisms that limit the critical current density. According to the model, current at low magnetic field is limited by weak links, while the current at high magnetic field is carried by a backbone of strongly-linked grains where the current is limited by intra-granular flux motion. Although multiple current paths in parallel are present in polycrystalline high-temperature superconductors, current flow can be accurately described as two paths in parallel. This is demonstrated for the case of Bi-2223 tapes, where the model is carefully tested on several experiments and many samples.
As a first test, the critical current density of Bi-2223 tapes is scaled as function of magnetic field for different field angles. The results show that the critical current density can only be scaled as function of magnetic field parallel to the c-axis of the grains when the contributions of both current paths are taken into account separately. The differences between both current paths become evident. The mechanism that limits the critical current density in the strongly-linked backbone changes when the temperature changed, while that of the weakly-linked network does not. As a result the mechanisms are located at different parts in the grain structure.
The contribution of both current paths can be physically separated by preparing a weak-link-free powder from a Bi-2223 tape. An independent analysis of the mechanisms that limit the critical current in both current paths is possible. Dissipation in the strongly-linked backbone is the result of flux motion within the grains and can be described in good agreement with classical flux creep theory. Dissipation in the weakly-linked network does not occur in Josephson weak links located at high-angle grain boundaries. The power-law dependence that describes the dependence of the critical current density on magnetic field according to Josephson behavior does not apply. The good agreement between experiment and classical flux motion theory shows that dissipation occurs at low-angle grain boundaries and is the result of inter-granular flux motion.
It is demonstrated that although both dissipation mechanisms in the parallel path model occur at different locations in the grain structure, they are not completely independent. An interaction between inter-granular Abrikosov-Josephson vortices and intra-granular Abrikosov vortices is demonstrated by studying the change in pinning due to a variation in oxygen concentration of the grains. Both mechanisms show a similar dependence on oxygen concentration, although different characteristic time scales apply. Inter-granular pinning is reduced faster than intra-granular pinning when oxygen is removed. It is concluded that oxygen is first removed from the grains close to the grain boundaries. Inter-granular pinning is reduced faster due to the interaction between inter-granular flux lines with intra-granular flux lines close to the grain boundaries. For the same reason, pinning at grain boundaries is restored before pinning within the grains, after oxygen diffused back into the tape.
Inter-granular flux pinning at low-angle grain boundaries and intra-granular flux pinning within the grains determine the behavior of the critical current density of polycrystalline high-temperature superconductors in general and not only in Bi-2223 tapes. This conclusion is obtained when various high-temperature superconductors are studied, ranging from studies that reveal the influence of the grain structure of YBCO coated conductors on the overall critical current density to the observed double step in the magnetic field dependence of the critical current of Bi-2223 tapes, Bi-2212 tapes and YBCO coated conductors. Although a comparable grain structure in a large variety of polycrystalline high-temperature superconductors result in a comparable behavior of the critical current density, differences in pinning strength between conductors occur due to different intrinsic properties such as anisotropy.
The main conclusion of the research presented here is that poor grain connectivity in granular high-temperature superconductors not only reduces the effective cross-section of the current path, but is also responsible for the degradation of the critical current at low magnetic field. Inter-granular flux pinning is reduced when the grain boundary angle is increased. The overall critical current density can be largely raised by increasing grain connectivity, but also by shifting the boundary between strongly- and weakly-coupled grains to higher angles, by using proper doping.