Eishi Tanaka

Microstructures and dissociated dislocations in YBa2Cu3O7−x prepared by a “grain-coating” technique

Abstract Microstructures of YBCO prepared by a “grain-coating” technique were observed through transmission electron microscopy. The coating material of Ag or Bi 2 O 3 is detected by EDS analysis only at grain boundaries and not inside grains in which abundant dislocations are found. Most of the dislocations are dissociated into partial dislocations with stacking faults. Two types of stacking fault are observed: One has a large width and is terminated by straight or smooth partial dislocations, the other is terminated by curved dislocations of which the separation is alternately large and small. The stacking fault vector is found to be 1 6 [0 3 1] , being consistent with an extra CuO layer in the 123 matrix. The coating material improves the weak-link at grain boundaries and increases the J c , while plenty of dislocations associated with stacking faults act as effective pinning centers and contribute to increasing the J c in a high magnetic field.

Introduction of dislocation into YBa2Cu3O7-δ and effect on Jc characteristics

Dislocations accompanied by stacking faults are found in intragrain regions of polycrystalline YBa2Cu3O7-δ which is prepared by a solid-state reaction process. The process is characterized by the inclusion of high-temperature sintering. Transport critical current density Jc is measured in a liquid nitrogen bath at magnetic fields H up to 1.5 T. Jc at H>0.3 T hardly deteriorates, producing a nearly flat Jc-H characteristic at high fields. The result shows that the dislocations probably act as pinning centers for magnetic flux.

Charge–spin–orbital coupling in CuO

Abstract The electron pairing mechanism for superconductivity in cuprates is generally attributed to magnetic interactions because superconductivity occurs on the verge of frustrated low-dimensional antiferromagnetic order in Cu–O planes or chains. CuO is the simplest copper oxide that may be the fundamental one not just because till now only compounds containing the Cu–O–Cu bond have produced the high- T c superconductivity but also because recently we found that charge stripes exist in this simplest compound. Here we report strong charge–spin–orbital coupling in CuO, which shows that CuO is a reference system for studying the electron correlation in cuprates.

Microstructures Near The Y2BaCuO5 Particle in Melt-Processed YBa2Cu3Oy

Transmission electron microscopy (TEM) was used to investigate the microstructures near the Y2BaCuO5 (211)/YBa2Cu3Oy (123) interface. Lattices of the 123 phase are elastically strained around the particles of 211 phase due to thermal and elastic mismatch between the two phases. The strained region is far larger than that of a dislocation. Thorn-like twin platelets are formed from the surfaces of 211 particles and dislocations are generated from most of the 211 particles irrespective of whether a TEM specimen is prepared by dimple-grinding or not. Stacking faults are freguently observed in the vicinity of the 211 particle, although they are restricted on (001) planes which intersect with the particle. The stacking fault consists of excess single layer of CuO, or double layer of CuO and Ba or Y planes, giving rise to local deviation from the 123 stoichiometry. These lattice strain, dislocations and stacking faults may play an important role in flux-pinning and account for the observed superconductor characteristics: improvement in Jc under magnetic fields and an increase of Jc with the volume fraction of 211 particles.

Reply to 'Comments on "evidence of the hydrogen release mechanism in bulk MgH2"'

In a comment on our Article “Evidence of the hydrogen release mechanism in bulk MgH2”, Surrey et al. assert that the MgH2 sample we studied was not MgH2 at any time but rather MgO; and that the transformation we observed was the formation of Kirkendall voids due to the outward diffusion of Mg. We address these issues in this reply.

Creation of Dislocation in YBa2Cu3O7-δ Ceramics and Effect on Jc

High-density dislocations were introduced into YBa2Cu3O7-δ ceramics by a solid-state reaction method. The ordinarily calcined ceramics were heated at 1000 °C, then crushed, pelletized and sintered at 900 °C. Investigations using TEM suggested that high-density dislocations were generated during crushing the large crystallites grown by the high-temperature heating. A high intragranular critical current density of ~1010 A m-2 at 77 K was obtained for the superconductor, showing that this kind of dislocations probably serve as a pinning force for the magnetic flux.

An experimental approach to the improvement of critical current density in high-TC YBa2Cu3O x ceramics

Abstract A “grain coating” technique was developed to improve the intergranular links in YBa2Cu3O x ceramics. Thin layers of additives such as Ag, SnO2, Bi2O3 were introduced into the grain boundaries by vacuum evaporation. Notable Jc increments due to grain coating were observed. TEM investigation revealed that dislocations accompanied by stacking faults existed in these samples.

High Resolution Electron Microscopy of Sintered YBa2Cu3O7-x

Sintered specimens have been observed through high resolution electron microscopy. Lattices of orthorhombic structure are slightly distorted in the vicinity of a twin boundary. Three types of grain boundaries (GBs) are present: (1)Some grain boundaries are parallel to a C-plane of one grain where a lot of stacking faults is concentrated along the GB. (2) The others have amorphous layers of 1∼4 nm in thickness at the boundaries. Lattice distortion and stacking faults were not found near the GBs. (3)A coincidence tilt boundary accompanies neither stacking fault nor amorphous layer, but an axial length varys near the GB. Cracks parallel to a C-plane are frequently presnt. A width of crack is in the range of 1 nm ∼ several tens of nm. A stacking fault present in an interior of grain consists of extra planes of Ba layer and Cu-O chain. The lattice distortion at a twin boundary, a nano-crack and an isolated stacking fault in the internal region of a grain may act as an effective pinning center but most of the structures mentioned above seem to be responsible for low Jc value.