Study on enhancement of Tc (⩾130 K) inTlBa2Ca2Cu3Oy superconductors

Abstract

A TlBa2Ca2Cu3Oy (Tl-1223) superconductor with anideal composition has a much higher Tc than the established value(~120 K). The intrinsic Tc arises by suppression of Tlsubstitution for Ba and Ca sites, which inherently occurs in samples preparedby conventional methods. The substitution of Tl was suppressed by preservingthe samples in a heavily over-doped state (Tc~100 K) which wasrealized by a high-pressure (solid and oxygen) synthesis method. In addition,we found several synthesis conditions for which we could obtain high-Tcsamples with good reproducibility. Residual carbon in the precursor was foundto be harmful for superconductivity, so that it had to be reduced as much aspossible before the high-pressure synthesis. The synthesis temperature andstarting concentration of Tl, which also affect Tc, had to be controlledproperly to enhance Tc. Samples show that Tc⩾130 K bypost-annealing in a reducing atmosphere at 650-700 °C for 0.5 h to 12 h.The sample prepared with the starting composition ofTl0.75Ba2Ca2Cu3.25Oy had an almost ideal grain compositionand showed a Tc of 133.5 K. This Tc is comparable withthat of HgBa2Ca2Cu3Oy (Hg-1223) which is known to have the highestTc in superconductors.