BISMUTH ACTIVITIES IN A LEAD-DOPED BISMUTH-2223 SUPERCONDUCTOR

Observations are reported on the thermal decomposition of a Pb doped B i2Sr2Ca2Cu3Oy (Bi-2223) sample. Experimental methods include different ial thermal analysis (DTA), thermogravimetric analysis (TGA) using a v acuum balance, and vapor-pressure measurements by Knudsen effusion usi ng (1) mass spectrometry, (2) a vacuum balance, and (3) an external ba lance for mass transport detection, DTA performed under N-2 showed end othermic peaks at 722 degrees C and 864 degrees C. TGA under vacuum sh owed mass loss beginning at 700 degrees C with a peak at 750 degrees C . Mass spectrometry showed that O-2(g) was lost from the sample at tem peratures between ca. 650 degrees C and 675 degrees C. It was conclude d that the DTA peak at 722 degrees C, the mass loss in TGA, and the O- 2 loss by mass spectrometry were all associated with a change in oxida tion state of Cu from +2 to +1. Vapor pressures of Bi(g) from the Bi-2 223 superconductor samples yielded the activity of Bi in the Bi-2223 p hase as 0.073 +/- 0.016 and the relative chemical potential of Bi in t he Bi-2223 phase of - 17.4 +/- 1.4 kT/mol at 800 K.