A novel method for the direct synthesis of the Y2Ba4Cu7O15−x superconductor

Abstract

We have developed a novel and simple method for the formation of the Y2Ba4Cu7O15−x (Y-247) superconductor at atmospheric oxygen pressure, using cuprous cyanide as the starting copper compound. Cuprous cyanide was found to act as a reaction accelerator and getter of oxygen during the formation of Y-247. Y-247 is prepared by solid-state reaction of Y2O3, BaCO3 and Cu2(CN)2 at the thermal stability temperature of Y-247 (860 °C) for 12 h. However, we found that the formation of the Y-247 phase could be accomplished in just a few minutes due to the high reactivity of cuprous cyanide. We carry out a longer time reaction to reduce the amount of simultaneous growth of a small amount of impurity phases. A predominantly single-phase Y-247 compound (confirmed by x-ray diffraction) with Tc(R = 0) = 93 K and Tc(onset) = 95 K could be achieved very easily by this method. It is worth emphasizing that, even in the quenched Y-247 from 860 °C, Tc(R = 0) = 93 K is confirmed by resistivity and magnetic measurements. The observations have shown that oxygen-deficient YBa2Cu3O6+x blocks in the Y-247 unit cell had enough oxygen required for 95 K superconductivity even at 860 °C. However, the amount of superconductivity was found to increase with post-annealing in an oxygen atmosphere at 350 °C.