Xiao Ling

Microstructures of an MTG YBa2Cu3Oy superconductor

Microstructures of a YBa2Cu3Oy superconductor having a Jc value of 23800 A cm-2 at 77 K in a 2 T magnetic field, prepared by the MTG technique, were studied by TEM. The grains of the 123 phase in the superconductor preferentially align with mutually parallel a-b conductive planes. The granular Y2BaCuO5 and CuO are dispersed in the 123 phase and many dislocations, dislocation loops and stacking faults exist in the 123 phase. These crystal defects may be the main flux pinning centres and may play an important role in the increase of the Jc value in a magnetic field.

Surface pinning and influence of field reduction on magnetic relaxation in YBa2Cu3O7 bulk sample

Abstract Differences between the magnetic relaxation due to surface barriers and bulk pinning is studied. It is shown that a small field reduction can affect the two relaxation processes quite differently. Experimental results on a melt-gown YBa 2 Cu 3 O 7 bulk sample and theoretical calculations based on surface pinning support this conclusion. We suggest that field-reduction relaxation experiments can be used to probe surface pinning.

Peak Effects in EuBa2Cu3Oy Crystal Prepared by Melt-Textured Growth Method

The magnetization curves of EuBa2Cu3Oy crystal prepared by melt-textured growth method were measured from 1.5 to 87 K up to field 7.5 T for H || C and H ⊥ C. Peak effects were observed at 60, 70 and 73 K for H || C, and 70, 73 and 77 K for H ⊥ C. It is shown that the magnetic field Hp at which the magnetization or critical current reach their extremum is approximately independent of temperature and the peak effects observed can be explained by matching effects.

FABRICATION AND PROPERTIES OF HIGH Jc Y—Ba—Cu—O SUPERCONDUCTOR BY (MTG) PROCESS

Superconducting Y—Ba—Cu—O bulk materials were fabricated by melt—textured growth (MTG) method, where cooling slowly under temperature gradient produces an aligned structure exhibiting high Jc value. The critical current density was measured by continuous dc four—probe method with criterion of 1μV/cm. Jc value has reached 23, 800 A/cm 2 at 77K under 2T magnetic field perpendicular to the current. The magnetic field dependences of Jc from 1 to 7 Tesla show the anisotropy for two orientations (H┴I, H┴a—b plane and H┴I, H//a—b plane). The X—ray diffraction patterns show the crystals preferentially aligned. The SEM and TEM observations of the MTG sample reveal that the sample has dense layered—structure with regular alignment of plate—shaped crystals. Twinning planes of fairly high density exist. A great quantity of dislocations, dislocation loops and stacking faults exist around 211 phase. These crystal defects may be main flux pinning centers. We have measured the zero—field cooled (ZFC) and the field cooled (FC) magnetizations as functions of temperature at various external dc magnetic field. The dependence of irreversibility point T * on 2/3 power of field H for MTG sample was demonstrated. The flux jumping was found in the hysteresis loop below 10K.