Influence of low temperature–low oxygen pressure post-annealing on critical current density of Bi(Pb)2223/Ag superconductors

Abstract

A Bi(Pb)2223/Ag multifilamentary tape was manufactured using the traditional PIT method. A series of the short samples cut from the tape were used to investigate the effect of low temperature–low oxygen pressure post-annealing on the critical current density of the samples. Two series of the air-sintered superconducting samples with the different intermediate pressing times and total sintering time were in situ post-annealed, respectively, under the reduced atmosphere taking the post-annealing temperature, time and oxygen partial pressure as variations. The results indicate that there are different effects of the post-annealing on the critical current density for the samples in the two series. Post-annealing at the temperatures from 760°C to 810°C for a short time can improve the J c values up to 1.5–2.5 times for the samples in series 1, and the maximum value can be achieved at 780°C, which makes the J c A/ J c B– T (A: after the post-annealing; B: before the post-annealing) curve present a single-peak form, whereas the J c values can be obviously improved up to 1.3–2.0 times at 820°C and slightly improved at 780°C for the samples in series 2, which makes the J c A/ J c B– T curve present a double-peak form. XRD, SEM and ac susceptibility measurements indicate that the two different effects of the post-annealing, which closely depends on the sintering time of the samples before post-annealing, should be attributed to the two different phase transformations relating to the formation of (Bi,Pb)2212 and Bi2223, respectively, during the post-annealing. Formation of (Bi,Pb)2212 from Pb-poor Bi2212 and (Pb,Bi) 4− α (Sr,Ca) 5− β CuO x (Pb451) at 780°C for the samples with a shorter sintering time has an effect of improving the transition temperature of 2212 from 68–74 to 76–80 K and, therefore, overcomes the obstruction of the phase on the critical current at liquid nitrogen temperature and obviously improves the critical current density of the samples. The improvement of the J c values for the samples with a longer sintering time after being treated at 820°C can be attributed to the formation of Bi2223 from Bi2212, Pb451 and (Sr,Ca) 14Cu 24O x . It seems that the effectiveness of post-annealing strongly depends on the sintering time of the samples before post-annealing: the formation of BiPb2212 at 780°C needs a shorter time (relatively larger amount of Bi2212) in advance, whereas the formation of Bi2223 at 820°C needs a longer time (decreased amount of Bi2212 and increased amount of (Sr,Ca) 14Cu 24O x ). Among the two phase transformations, the formation of Pb-enhanced Bi2212 at 780°C seems to be of utmost importance and has a universal meaning for the manufacture of the Bi2223 tapes, which solves the intrinsic problem of the partial decomposition of Bi2223 during the furnace cooling and may present a new idea of improving the J c values through adjusting the transition temperature of the superconducting secondary phases besides the exhausted effort of making the Bi2223 single phase. The J c values after post-annealed at 780°C for a short time for the samples with a shorter sintering time before the treatment can be comparable with that of the longer time sintered samples and the samples after post-annealed at 820°C, which also reflects the importance of the transformation of Bi2212 into BiPb2212 on the critical current density of the tapes and makes the mechanism deserve to be further investigated.