Phase transformation and critical current density of (Bi, Pb)-2223/Ag superconducting tapes by a low temperature–low oxygen pressure post-annealing method

A low temperature-low oxygen pressure post-annealing method was introduced for a series of Bi(Pb)-2223/Ag multifilamentary meter-length rolled tapes manufactured using the different precursor powders. The universal effectiveness of the transformation of Bi2212 into BiPb2212 by the post-annealing on the improvement of the critical current density as reported earlier for the short pressed samples is examined. The results indicate that for most of the air-sintered tapes the critical current densities are improved up to 1.5-2.5 times after the short time post-annealing at 780 degrees C under 90% N-2-10% O-2 atmosphere and only a little exception with J(c) slightly decreased or increased after the treatment was observed. XRD and SEM measurements reveal that there are two different phase assemblages in the fully reacted tapes with J(c) increased (Bi2223 + 2212 + Bi3Pb0.6Sr2CaCu3Ox + (Pb, Bi)(4-alpha)(Sr, Ca)(5-beta)CuOx(Pb451) + (Sr, Ca)(14)Cu24Ox and Bi2223 + 2212 + Pb451 + (Sr, Ca)(14)Cu24Ox, respectively) due to the difference of Pb content in the precursor powders. Therefore, there have been different phase assemblage changes of Bi2223 + 2212*(increased) + Pb451(decreased) + (Sr, Ca)(14)Cu24Ox and Bi2223 + 2212* (increased) + (Sr, Ca)(24)Cu24Ox, respectively, after the post-annealing. An improved grain alignment and colony boundary can also be observed besides the decrease of the non-superconducting impurities and the increase of 2212. XRD measurements also indicate that some undesired reactions may happen for the slightly decreased or increased tapes during the post-annealing, which should be responsible for the ineffectiveness of the treatment in these tapes. Ac susceptibility measurements for the J(c) improved samples show an obvious increase of the transition temperature of 2212 from 68 to 76 K after the post-annealing. It can be concluded that the low temperature-low oxygen pressure post-annealing method is an effective means considering the eliminating of the weak links. The mechanism of the treatment can be determined as the overcoming of the main weak links of Pb-poor Bi2212 by the transformation of this lower T-c phase (68 K) to the higher T-c phase (76 K) cooperated simultaneously with the elimination of the weak links of the non-superconducting impurities during the transformation. This mechanism, which concentrates on the increase of the transition temperature of the low T-c phase instead of the exhausted efforts of making the single Bi2223 phase, may imply a shorter sintering time and a decreased number of the intermediate deformation. Therefore, it may present some new ideas on the manufacture of the Bi2223 tapes operated at the liquid nitrogen temperature, which makes the method noteworthy and further investigated. (C) 2000 Elsevier Science B.V. All rights reserved.