Superconducting bulk magnets: Very high trapped fields and cracking

Improved trapped fields are reported for bulk melt-textured YBa2Cu3O7-delta (YBCO) material in the temperature range between 20 and 50 K. Trapped fiel ds up to 12.2 T were obtained at 22 K on the surface of single YBCO disks ( with Ag and Zn additions). In YBCO minimagnets, maximum trapped fields of 1 6 T (at 24 K) and of 11,2 T (at 47 K) were achieved using (Zn+Ag) and Zn ad ditions, respectively. In all cases, the YBCO disks were encapsulated in st eel tubes in order to reinforce the material against the large tensile stre ss acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequenc e of flux jumps due to thermomagnetic instabilities in the temperature rang e betweeen 20 and 30 K. (C) 2001 American Institute of Physics.