Search for a silver-sheathed 1212-type superconductor with a grain colony microstructure: M1Sr2Y1-xCaxCu2Oz with M=(Bi,Pb), (Cd,Pb), and (Fe,Pb)

We have investigated the possibility of forming a silver-sheathed 1212-type high-critical-temperature superconductor (HTS) with the same type of grain colony microstructure (GCM) that develops in the silver-sheathed Bi-2223 (Ag/Bi-2223) composite conductor. The basis for this study is that a silver-sheathed 1212-type superconductor having a GCM should exhibit 1) good long-length transport critical current properties, 2) better in-field performance than Ag/Bi-2223 due to the significantly shorter blocking layer distance in 1212-type as opposed to 2223-type HTS phases, and 3) long-length transport current capacity equaling or exceeding that of Ag/Bi-2223 in present-day applications. Three series of M₁Sr₂Y_1-xCa_XCu₂O_z (M-1212) compositions with M = (Bi,Pb), (Cd,Pb), and (Fe,Pb) were prepared in silver-sheathed/flat wire form by the powder-in-tube method. For M = (Bi,Pb), we were able to obtain over 80% conversion to a 1212-phase in a relatively short time (6 h) for oxygen partial pressures in the range from 0.08 to 0.21 atm and temperatures from 900°C to 940°C. The (Bi,Pb)-1212 phase grew with a GCM that had the same type of fiber texture found in Ag/Bi-2223 composites. However, within the range of processing conditions tolerated by a silver sheath, no transport critical current was achieved in any of the Ag/(Bi,Pb)-1212 specimens, including ones that showed the presence of an HTS phase. Various oxygen doping treatments performed at very high (ca. 88 atm O₂), intermediate (1 atm O₂), and low oxygen partial pressures (0.01 atm O₂) failed to induce a global HTS phase that carried a transport current. The preponderance of evidence from these studies indicated that any superconductivity detected in Ag/(Bi,Pb)-1212 specimens was most likely due to trace amounts of either Bi-2201 or Bi-2212. For M = (Cd,Pb) and (Fe,Pb), M-1212 phases in appreciable quantity did form in the silver-sheathed composites after heat treatment, but those M-1212 phases had granular microstructures. In the case of Ag/(Cd,Pb)-1212, the superconductivity that was detected in several specimens appeared to originate from a 2212-like phase. In the case of Ag/(Fe,Pb)-1212, there was clear evidence for the formation of a superconduc- ting 1212 phase, but neither the Ag/(Cd,Pb)-1212 nor the Ag/(Fe,Pb)-1212 specimens exhibited a measurable transport critical current, presumably (in the latter case at least) because of the absence of a GCM.