WELL-DEFINED GRAIN-BOUNDARY WEAK-LINKS IN SILVER-SHEATHED (TL0.78BI0.22)(SR0.8BA0.2)(2)CA2CU3OX TAPE CONDUCTORS

The key factors that make the thallium-based 1223 phase conductors att ractive candidates for high-field applications at 77 K are a high tran sition temperature (about 115 K) and an irreversibility line comparabl e to that of YBa2Cu3O7. However, a third important factor is the grain boundary behaviour. We have performed detailed current-voltage (I-V) studies of a silver-sheathed (Tl0.78Bi0.22)(Sr0.8Ba0.2)(2)Ca2Cu3Ox (Tl -1223/Ag) tape. Although the tape has a fairly high self-field critica l current density J(c) at 77 K (about 5 x 10(3) A cm(-2)), the I-V cha racteristics display the 'weak-link' signatures well known for YBa2Cu3 O7: linear dissipation above a sharply defined critical current, a pro nounced decrease of J(c) in a magnetic field H and hysteresis with res pect to field cycling. Broader implications for the superconductive be haviour of HTS grain boundaries are discussed.