An investigation of stoichiometry and thermo-mechanical processing parameters of (Pb,Bi)2Sr2Ca2Cu3Ox superconducting tapes

This thesis details an investigation into some of the processing parameters associated with the production of (Pb,Bi)-2223 tapes. Both monocore and multifilamentary (Pb,Bi)-2223 silver-sheathed superconducting tapes have been processed and characterised. The relative conversion of (Pb,Bi)-2212 to (Pb,Bi)-2223 has been measured using X-Ray diffraction. The critical current density and the associated magnetic field dependence have also been measured. The parameters investigated include the stoichiometry of the precursor powder, the mechanical deformation of multifilamentary tapes, the effect of the cooling rate, and the effect of twisting a multifilamentary tape. Two precursor powders were compared. The Endo composition powder ((Pb0.34,Bi1.84)Sr1.91Ca2.03Cu3.06Ox) and a second powder, that had an additional 10 % excess of (Pb,Bi)-2212 added to the Endo composition ((Pb0.374,Bi2.024)Sr2.101Ca2.133Cu3.266Oy). Both the Endo and 10 % excess precursor powders were packed into thin, and thick silver sheaths, and a silver alloy (Ag(99.5%), Ni(0.2%), Mg(0.28%)). Of the three separate studies the thick silver sheath samples produced the best results for both precursor powders. The Endo composition tapes had better characteristics than the 10 % excess tapes, with an lc of 17 A and 14 A, respectively. The mechanical deformation experienced by 37 filament multifilamentary tapes before thermal processing was studied. The final wire diameter, the rolling steps used to convert the wire to a tape were investigated. It was found that tapes with thin, high aspect ration filaments produced a higher critical current density. High aspect ration filaments were produced within the conductor using small rolling steps of 20 μm followed by one or two 100 μm steps to the as-rolled thickness. Through attempting to optimise both the diameter and the rolling regime a 20 % improvement in the Jc was obtained from 12 KAcm-2 to 14.5 KAcm-2. Using the improved deformation scheme from the above tapes were subjected to various cooling rates. The magnetic field dependence of the tapes was improved using a slow cooling rate (1 deg C/hr 818- 730 deg C). The critical current density of the tapes could be improved from a maximum of 13 KAcm-2 to 15.5 KAcm-2 when a secondary dwell was applied. The maximum Jc obtained for twisted tapes was 6.5 KAcm-2 this is far lower than those obtained for untwisted tapes. The increased deformation and elongation of the filaments is believed to be responsible for this effect. The filaments of a twisted tape were observed to remain uncoupled to a field of 10 mT at 43 Hz, producing lower losses than the theoretical plot for an untwisted tape. (author)