Variations of superconducting transition temperature in YbBa_2Cu_3O_(7-δ) ceramics by Gd substitution

摘要

So-called RE-123 superconductors are said to generally exhibit superconductivity at the transition temperature T_c around 90K with RE (rare-earth) ions from Nd to Lu. However there is some tendency that T_c decreases with decrease in RE ionic radius. Especially for Yb-123, T_c often becomes below 90K. We find that the smaller the ionic radius is, the lower the melting temperature becomes for RE-123 ceramics. In fact, Yb-123 ceramics often melt above 910°C, while Gd-123 ceramics can be sintered above 950°C without melting. From this tendency we suppose that such T_c lowering is due to insufficient sintering temperature, resulting in insufficient solid state reaction and poor crystal growth. In this work, we try to raise T_c in Yb-123 ceramics by increasing the sintering temperature through Gd substitution. We find that the optimum sintering temperature is 930°C in Yb_(0.7)Gd_(0.3)Ba_2Cu_3O_(7-δ) ceramics with the highest T_c of 91.8K (zero resistivity).