Hot rolling of Bi1.6Pb0.4Sr2Ca2Cu3Oxsuperconducting pellets

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Abstract
Alignment of grains has been studied in sintered pellets of Bi1.6Pb0.4Sr2Ca2Cu3Ox 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 (Tc) and the transport critical current density (Jc) 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, Tc 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.