INSTITUTE OF PHYSICS PUBLISHING SUPERCONDUCTOR SCIENCE AND TECHNOLOGY Supercond.Sci.Technol. 17 (2004)263–268 PII:S0953-2048(04)68836-1 Effect of hot uniaxial pressing on the microstructure and critical current density of (Bi, Pb)-2223 tapes H Fujii 1 , V Garnier 2 , E Giannini 2 and R Fl ¨ ukiger 2 1 SuperconductingMaterialsCenter,NationalInstituteforMaterialsScience,1-2-1,Sengen, Tsukuba305-0047,Japan 2 DepartmentofCondensedMatterPhysics,UniversityofGeneva,24 QuaiErnest-Ansermet,CH-1211,Geneva4,Switzerland Received23August2003 Published10December2003 Onlineat stacks.iop.org/SUST/17/263 (DOI:10.1088/0953-2048/17/2/005) Abstract Pressuresupto10MPawereapplieduniaxiallyto(Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O y ((Bi,Pb)-2223)tapesduringeitherthefirstorsecondstageoftheheat treatmentprocessinair. Afterbothannealingstages,anincreaseofthe oxidefilamentdensityofthetapeswasobservedwhenapplying pressure. Forthefirststage,thecriticalcurrent(I c )ofthetapeswasvery sensitivetotheappliedpressure,andthereproducibilitywaspoor. This behaviourisrelatedtothelargeamountofliquidinvolvedinthe reaction.Thisliquidwaseasilypushedoutfromthecentretotheedgesof thetapesbypressure,whichcausedlocaldeviationsofthecompositionand hence,adecreaseofvolumefractionof(Bi,Pb)-2223. Incontrast,applying pressureonlyduringthesecondstagewhenamoderateamountofliquidis producedbroughtaboutthehighdensificationandthereproducible enhancementof I c andcriticalcurrentdensity(J c )valuesbyabout40and 100%,respectively. Thisenhancementisattributedtothehigh densificationandimprovedcouplingofgrainsduetotheuniaxial pressure. 1. Introduction Among the Bi-based high critical temperature (T c ) superconductors, described as Bi 2 Sr 2 Ca n−1 Cu n O y (Bi-22 (n − 1)n; n = 1, 2, 3), the Pb-substituted (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O y ((Bi, Pb)-2223) phase shows the highest T c of about 110 K. At present, the (Bi, Pb)-2223 phase is the most promising material for application to wires and tapes. Although Ag-sheathed (Bi, Pb)-2223 tapes show critical currentdensity(J c )valuesofabout75and30kAcm −2 forshort and industrial lengths, respectively [1, 2], locally J c reaches valuesashighas250kAcm −2 over50–100 μmlongregions [3]. The reason for the degradation of J c inlongtapesisthe presence of many voids and impurity phases, which act as obstacles for grain connectivity and hence, supercurrent path [4,5]. Inordertoreduceporosity,intermediaterollingand/or pressing are carried out during the fabrication process. By rolling and/or pressing, many cracks are introduced, which cause a deterioration of J c . These cracks are healed by subsequentheattreatment,butthisprocessrequireslongtime, andhence,increasesthefabricationcost. Recently high-pressure (HP) processing was performed with a hot isostatic pressing (HIP) furnace (∼200 MPa) to improvethedensityofoxidefilamentsof(Bi,Pb)-2223tapes and to suppress the lead evaporation from the tapes [6–10]. Another possible benefit from adopting this technique is the shorteningoftheheattreatmenttime,i.e.toachievesingle-step heat treatment without intermediate rolling or pressing. The densityofthefilamentswasindeedimproved,andthe J c was enhancedbyHPprocessing. Alternatively,highdensification oftheoxidefilamentscanalsobeachievedbytheapplication of uniaxial pressure. This is thought to increase J c , also by an improvement of grain alignment. This can be achieved probably more effectively by applying pressure uniaxially rather than isostatically. Hot uniaxial pressing was recently improvedforprocessing(Bi,Pb)-2223bulk[11],and J c values up to 20 kA cm −2 at 77 K and 0 T were obtained under pressures up to 30 MPa [12]. As an alternative technique 0953-2048/04/020263+06$30.00 ©2004IOPPublishingLtd PrintedintheUK 263