A comparison of lead-doped Bi-Sr-Ca-Cu-O superconductors at the '2223' and '2234' compositions

Superconducting Bi-Pb-Sr-Ca-Cu-O ceramics have been prepared at the Bi 1.2 Pb 0.8 Sr 2 Ca 2 Cu 3 O x (2223) and Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu 4 O y (2234) compositions. The properties and microstructures after sintering in air at 850 degrees C for times from 2 to 156 hours are described and compared. For the 2223 composition all heat treatment times result in multiphase samples. Bulk superconductivity giving zero resistance at 106 K and a maximum J c of 300 A cm -2 at 77 K is achieved after 50 h heat treatment. When sintered in excess of 40 h samples of the 2234 composition consist predominately of the high-T c , n=3, phase. J c is again maximised after 50 h heat treatment but at lower value of 150 A cm -2 and T c (R=0)=103 K. For longer sintering times J c of the 2234 composition falls to zero after approximately 100 h at 850 degrees C. Samples of both compositions densify rapidly during the first two hours of sintering. Further heat treatment results in a large de-densification, the reasons for which are described in detail. A decrease in lead content with increasing sintering time is observed. The implications of these observations for the production of high-quality, bulk Bi-Pb-Sr-Ca-Cu-O superconducting ceramics are discussed.