EFFECTS OF COMPOSITION AND PROCESSING ON THE SUPERCONDUCTIVITY OF LA1+ZBA2-ZCU3OY

The effects of composition, processing time (t), temperature (T) and O-2 partial pressure (p(O2)) on the superconducting transition temperature, T-c, were investigated for La1+zBa2-zCu3Oy. Tallon and Flower's prediction of T-c approximate to 100 K [Physica C 204 (1993) 237] at z = 0 could not be attained. Instead, the T-c maximum of 93-94 K is similar to that of the yttria analog. As z was increased, the T-c values increased gradually by a few kelvin until z = 0.2, which is at the orthorhombic to tetragonal transition, and then decreased more abruptly, with no superconductivity for z greater than or equal to 0.5. Crucial steps in the process at all z values include the highest temperature anneal ( 1065 degrees C) in 0.1 MPa O-2, reduction of the oxygen content at 950 degrees C and cooling to 300-400 degrees C, both at 100-1000 Pa O-2, and reoxygenation at 0.1 MPa O-2 during slow cooling (5-20 degrees C/min) to < 150 degrees C. Additional experiments demonstrated that the intragranular critical current was less than that for the yttria analog. Values of T-c decreased fairly monotonically with decreasing y at z = 0.08, and a La excess of 0.2 and Cu nonstoichiometry on both sides of Cu/(La + Ba) = 1 generally depressed T-c.