KINETICS AND MECHANISM OF THE (BL,PB)2SR2CA2CU3O10 FORMATION REACTION IN SILVER-SHEATHED WIRES

A detailed kinetic and mechanistic analysis of the growth of the (Bi2-xPbx)Sr2Ca2Cu3O10 phase in silver-sheathed wires has been performed by an isothermal equilibration method. Silver tubes loaded with precursor powders were processed into wires using established metallurgical techniques. The wire specimens were immersed in a preheated equilibration apparatus, heat-treated at the desired temperature in 7.5% O2 for varying periods of time, then quenched in a room-temperature silicone oil bath. The results indicated that the kinetic data followed a nucleation growth model derived for a reaction at the interface between thin sheets and a fine powder or a fluid. Transmission electron microscopy confirmed the two-dimensional reaction geometry and revealed the presence of an amorphous phase at grain boundaries, where rapid transport diffusion appears to occur due to the absence of the stabilizing influence of the regular lattice. A reduction in activation energy was observed at temperatures greater-than-or-equal-to 819-degrees-C, which is tentatively attributed to the onset of a liquid-phase-controlled reaction (i.e. a phase boundary crossing). The effects of powder processing parameters and precursor particle size on the kinetic behavior and the growth rate of the (Bi2-xPbx)Sr2Ca2Cu3O10 phase are also discussed.