Effect of Lead Loss and Sheath Structure on Phase Formation and Alignment in (Bi,Pb)2Sr2Ca2Cu3O10+δ/Ag Composite Conductors

The effects of metallic constituent evaporation and sheath structure on grain growth and alignment in silver-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi-2223)/Ag composites have been investigated by inductively coupled plasma/atomic emission spectroscopy (ICP/AES), X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Specimens of Bi-2223/Ag composites fabricated by the oxide-powder-in-tube technique were peeled (opened) lengthwise to expose the ceramic powder core, and then heat-treated in 0.075 atm of oxygen for selected temperatures and times. The results were compared with those for as-processed samples with closed silver sheaths treated under identical conditions. ICP/AES analysis indicated that lead was the only metallic element to undergo substantial evaporation during annealing of opened samples. The lead-release process in parallel with the Bi-2223 formation reaction had an activation energy of ∼25 kJ/mol. Lead loss from the opened samples resulted in incomplete conversion to Bi-2223. The combined results show that the silver sheath effectively prevents evaporative lead loss, preserves and promotes densification, and induces texturing of the layered phases.

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