Influence of composition and long term annealing on the formation of the 110K phase in the Bi-Pb-Sr-Ca-Cu-O system

Abstract

We report the preparation of polycrystalline samples with nominal composition (Bi1-xPbx)2Sr2Ca2Cu3+yO10-δ with 0 ≤ × ≤ 1 and y = 0 and 0.5 starting from the metal-oxides and carbonates. The prereacted material was pressed into pellets and sintered in air at 830°C in several steps up to 785 hours. Resistively measured critical temperatures Tc (midpoint) between 108K and 114K were achieved for 0.2 ≤ × ≤ 0.5. Tc,o and the onset of the transition observed by ac-susceptibility is usually 3K lower. X-ray diffraction studies show that long term annealing is essential for the formation of the 110K high-Tc-phase. DC-magnetization measurements exhibit after 143h annealing steps in the Meissner as well as in the shielding signal at 112K and 80K. The 80K features disappear after annealing for 785 hours. The extended levitation of the samples in inhomogeneous magnetic fields after cooling in liquid nitrogen shows that the volume fraction of the 110K phase is very large