Izumi Sankawa

Preparation of Y-Ba-Cu-O Superconducting Tape by Atmospheric Plasma Spraying

Y-Ba-Cu-O superconducting tapes over 10 m long were prepared by an atmospheric plasma spraying method. The tapes were annealed at a maximum temperature of 900°C in an oxygen flow. The tapes consisted of a Y-Ba-Cu-O layer and a Ni alloy substrate. After annealing at 900°C, Tc onset and Tc offset of the tape were 94 K and 88 K, respectively. The critical current density at liquid nitrogen temperature was about 6 A/cm2 in the length range of 7.8 m. The superconductivity of the tapes was observed in the liquid nitrogen until it was bent to a curvature radius of 60 mm.

Superconductivity in YBa2Cu3O7-δ Thick Films by the Atmospheric Plasma Powder Spraying

Thick films composed of Y-Ba-Cu-O (e.g. 70 µm thick) were prepared on Ni alloy substrates at various temperatures by the atmospheric plasma powder spraying method. By increasing the substrate temperature during the spraying, the amount of unknown structure in the film sprayed was lowered and the structure of the film sprayed on 600°C substrate was tetragonal in phase. After the postannealing at 600°C, this film revealed superconducting temperatures for onset and zero resistance of 92 K and 86 K, respectively.

Orthorhombic-to-Tetragonal Phase Transition in the Y-Ba-(Cu, Co)-O System

X-ray powder diffraction, thermal analysis and resistivity measurements have been carried out for the YBa2(Cu1-x, Cox)3O7-δ system with 0<x<0.333. With increasing Co content, weight loss in the thermogravimetric (TG) curve steadily decreases and the structure transforms from orthorhombic to tetragonal. Around this phase transition at x=0.02, Tc starts to decrease. This phenomenon is considered to be ascribed to the disordering of oxygen sandwiched between Ba planes.

Microwave Surface Resistance Studies of YBa2Cu3O7-δ Single Crystal

The microwave surface resistance of a superconducting YBa2Cu3O7-δ single crystal at 24 GHz was measured as a function of temperature and magnetic field. Below the superconducting transition temperature of 91 K, the microwave surface resistance rapidly decreased and was saturated below 75 K at zero magnetic field. Above 60 K, the microwave surface resistance increased linearly with the magnetic field. No hysteresis and no residual resistance were observed in the magnetic field cycling.

YBa2Cu3Oy Annealed under High-Pressure Oxidizing Atmosphere

Heat treatment under high-pressure oxidizing atmosphere (oxygen partial pressure of 400 atm) is found to lower the Tc of the superconductor YBa2Cu3Oy. The powder X-ray diffraction pattern and the oxygen content measurement suggest that the excess oxygen occupies the oxygen defect site between the Ba-Ba layers.

MICROWAVE SURFACE RESISTANCE OF YBa2Cu3O7-δ

The magnetic field dependence of microwave surface resistance of both single crystals and sintered polycrystals of YBa2Cu3O7-δ has been measured at 24 GHz. The resistance decreases below superconducting transition temperature and reaches constant residual resistance. The value is much lower for single crystals than for polycrystals. Furthermore polycrystals showed low field magnetoresistance and high field one with hysteresis which resembles the magnetization behavior of typical type II superconductor. The single crystal, however, showed nearly no magnetoresistance. The magnetoresistance observed is proved to be a inherent phenomena to sintered polycrystal which is composed of grains and contains many voids.