Processing and properties of superconducting YBa2Cu3O7?x powder by single-step calcining in air

Abstract

The preparation of orthorhombic YBa2Cu3O7−x powder using only one calcination step is described. Yttrium and copper nitrates and barium carbonate were used as precursor materials. The use of an appropriate precipitant agent leading to a complete coprecipitation of the Y3+ and Cu2+ cations, promoted the rapid low temperature formation of YBa2Cu3O7−x (900‡C for 4 h) with no secondary phases. X-ray diffraction patterns of the obtained powder showed the presence of the only one phase with the orthorhombic structure, and lattice parameters ofa=0.3824 nm,b=0.3893 nm,c=1.1676 nm and σ=1.775 which correspond to a superconducting YBa2Cu3O6.95 phase. Sintering compacted samples at 940‡C several times led to highly dense (⩾ 96% theoretical density) superconductor bodies, and a value ofT c as high as 94 K was measured. Although AES depth profiles in fracture surfaces evidenced good grain boundaries of the superconducting YBa2Cu3O6.95 samples, however, theJ c measured was never greater than 15 A cm−2, which could be due to a weak-link region between highJ c grains. Slow degradation of the YBa2Cu3O6.95 ceramics seems to take place by the formation of Ba(OH)2 and YO(OH) at the superconductor surface in contact with the moisture of the air.