The phase equilibria near (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+␦ ((Bi,Pb)-2223) was studied in the temperature range of 750°-820°C in air and in reducing atmospheres. In air, (Bi,Pb)-2223 was in equilibrium with one or more of the following phases: (Bi,Pb) 2 Sr 2 CaCu 2 O 8+␦ ((Bi,Pb)-2212), (Sr,Ca) 14 Cu 24 O 41 (14:24), (Ca,Sr) 2 CuO 3 , (Bi,Pb) 4 (Sr,Ca) 5 -CuO x (451), Ca 2 PbO 4 , and CuO. Ca 2 PbO 4 and the 451 phase were not in equilibrium with (Bi,Pb)-2223 at an oxygen partial pressure ( p O 2 ) of 10 −3 atm, which is indicative of higher lead solubility in (Bi,Pb)-2223 under more-reducing conditions. In a second set of experiments, the lead content in both (Bi,Pb)-2212 single crystals and (Bi,Pb)-2223 powders was investigated as a function of p O 2 at a constant temperature of 750°C. Lead solubility in (Bi,Pb)-2212 and (Bi,Pb)-2223 increased as p O 2 decreased.

No abstract

Reaction couples of either ((Bi,Pb)-2212) single crystals with `' powder or (Bi-2212) single crystals with `' powder were used to study the formation reaction of (Bi,Pb)-2223. The samples were annealed between and , in air, for 65 h. Microstructural observations were made of the different geometrical interfaces. No (Bi,Pb)-2223 platelets were formed below . Above , the (Bi,Pb)-2212 and (Bi-2212) crystals were found to break up near the interfaces with the powders, and (Bi,Pb)-2223 platelets were formed adjoining the (Bi,Pb)-2212 and (Bi-2212) crystallites. No evidence for large quantitites of liquids was found, although small, rounded particles having a liquid droplet appearance of a Bi/Pb rich phase of stoichiometry near the phase were observed on many of the (Bi,Pb)-2223 platelet surfaces. The experiments suggest that the formation of (Bi,Pb)-2223 occurs simultaneously with decomposition of (Bi,Pb)-2212 and the presence of liquid droplets, providing further support for the dissolution - reprecipitation model for phase formation.

No abstract

No abstract

No abstract

Transport currents of up to 140 A have been injected into melt-grown YBCO to measure AC and DC critical currents. Values of critical current densities of 16000 A cm-2 (zero field), 2000 A cm-2 (1 T) and 550 A cm-2 (5 T) were obtained at 77 K. The 50 Hz AC critical current densities are also reported, with values of 23000 A cm-2 and 12000 A cm-2 (RMS) in applied fields of 1 and 5 T respectively at 77 K. Silver contact pads enabled steady-state currents of up to 7000 A per cm2 of contact area to be injected into the sample without any observable heating effects in applied fields up to 5 T. It should be noted that the 50 Hz results are believed to correspond to the onset of flux depinning in contrast with the DC measurements where a 3 mu V cm-1 detection level for flux creep was applied.

The upper stability line for Bi2Sr2CaCu2O8+α (Bi‐2212) was determined over the temperature range 540°–800°C and the oxygen partial pressure range 1.5 ×101‐7.4× 10‐3 atm using coulometric titration and high oxygen pressure annealing. The decomposition products resulting from Bi‐2212 oxidation are CuO, two alkaline‐earth bismuthates, and the Raveau phase, Bi11Sr9Cu5Oy. The main alkaline‐earth bismuthate is a known phase of cation composition Bi:Sr:Ca = 2.00:2.48:1.19 (Bi:Sr:Ca 9:11:5). The other alkaline‐earth bismuthate is a minor phase of cation composition Bi:Sr:Ca = 2.00:0.53:0.39. The kinetics for Bi‐2212 decomposition are very slow; e.g., at 700°C in air, the reaction is incomplete after 7 days.