Hot rolling of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x superconducting pellets

Alignment of grains has been studied in sintered pellets of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x upon rolling in hot condition. The pellets of diameter 12.7 mm and thickness 2.5-4.5 mm were kept in a pre-heated furnace for 15 min. Two furnace temperatures, namely 865 and 880 degrees C, were used in two different sets of experiments. The hot pellets were wrapped in ZrO₂ cloth and rolled in a roller at room temperature. During transfer from the furnace to the roller the temperature at the surface of the pellet dropped by 10-15 degrees C. About 50% reduction in thickness was imparted to the pellets in a single pass. Considerable alignment of platelet-like grains was observed in hot-rolled pellets with the help of a scanning electron microscope (SEM), but superconductivity was destroyed due to the formation of a glassy phase upon rapid quenching (during rolling) of the liquid phase, which was formed by partial melting. By annealing the rolled pellets at 840 degrees C for 24 h in air the superconductivity was recovered. The superconducting transition temperature at zero resistance (T_c) and the transport critical current density (J_c) at 77 K of the pellets, rolled after pre-heating at 865 degrees C and subsequently annealed, were 105.5+or-1 K and 124+or-7 A cm^-2 respectively, compared with 100.1+or-0.6 K and 41+or-4 A cm^-2 for the sintered pellets. However, T_c of the pellets rolled after heating at 880 degrees C reached only 60+or-2 K upon subsequent annealing. Rolling increased the density and hardness of the pellets.