TRANSMISSION ELECTRON-MICROSCOPY STUDY OF TWINS IN MELT-TEXTURED YBA2CU3OX SUPERCONDUCTORS SUBJECTED TO DEFORMATION

Melt-textured YBa2Cu3Ox(123) superconductors have been shown to posses s large critical current densities J(c) when the current is flowing al ong the strongly superconducting a-b planes. In order to further enhan ce J(c), the number of effective flux pinning centres has to be increa sed. Whether or not twin boundaries are effective pinning sites is of particular interest since twins are inherent defects found in 123. In this investigation, the twin structure of 123 have been modified by me chanical deformation. It is found that the twin density has been incre ased by deformation processing, and secondary as well as tertiary twin s have been generated to relieve the higher level of stresses. In addi tion, complex twin features and extensive twin interactions are seen i n the deformed superconductors. Transport J(c) measurements, however, revealed that J(c) does not increase monotonically with decreasing twi n spacing, thereby indicating that twin boundaries are not dominant pi nning centres at moderate magnetic field strengths.