Improved-Fabrication-Process-for-Nb3Sn

IEEETRANSACTIONSONAPPLIEDSUPERCONDUCTIVITY,VOL.14,NO.2,JUNE2004987ImprovedFabricationProcessforNb3SnSuperconductorswithTa-SnPowderCoreHiroyukiKato,TakayoshiMiyazaki,TakashiHase,MamoruHamada,andKyojiTachikawaAbstract—Nb3Snsuperconductorsmadebythepowder-in-tube(PIT)processhavebeendevelopedforapplicationtoparticleacceleratormagnets,highfieldmagnetsandmanyotherfields.Recently,newlydevelopedPITprocesswireshavingtheTa-Sncore/Nb(Nb-Ta)sheathcompositehavearousedinterestsincethesewiresshowexcellenthighfieldperformanceupto23Tat4.2K.WefoundthatthesuperconductingcharacteristicsofthePITprocesswireswereimprovedbylocatingNb-TarodsintheTa-Sncore.ItisknownastheRIC(rod-in-core)process.MultifilamentarywiresfabricatedbytheRICprocessusingaCustabilizerwerestudied.Toimprovecriticalcurrentdensity(Jc)inRICwires,differentsheathmaterialswerestudied.ThewiresusingTasheathandCustabilizerhaveshownthehighestJcperformance,thoughnoreactedsuperconductinglayerexistedattheinterfacebetweenthecoreandthesheath.WeinvestigatedthecompositionandthemicrostructureofNb3SnlayerinthewireswithNbandTasheath.InthisreportwediscusstheoriginofhighJintheTasheathedRICwiresthroughthecompositionandmicrostructureanalysis.IndexTerms—Criticalcurrentdensity,highfield,multifilamen-tarywire,Nb3Sn,Ta-Snpowder.I.INTRODUCTIONTODAYbronzeprocessedpracticalwireisthemostcommonlyusedforhighfieldNMRmagnetsandmanyotherfacilities.Highfieldsuperconductingmagnetsarerequiredtogeneratehigherandhigherfields.Therefore,variousattemptstoenhancethecriticalcurrentofbronze-pro-cessedwiresathighfieldhavebeendone[1]–[3].Thoughdevelopmentstoenhanceofthebronze-processedwirehavebeencontinued,isstilllowerthanthatofotherprocessedconductorssuchasinternal-tinprocessandpowder-in-tube(PIT)process,becauseofthelimitationinthetincontentofthebronzematrix.Recently,newlydevelopedPITprocessconductorshavingtheTa-Sncore/Nb(Nb-Ta)sheathcompositehavearousedin-terestsincetheseconductorsshowexcellenthighfieldperfor-manceupto23Tat4.2K[4].ItwasstudiedthatasmallamountofadditionalCutothepowderenhancesnon-Cuinfieldslowerthan22T.SinglecorePITwireswiththeabove-men-tionedconfigurationwerefabricated[5].WehavebeentryingtoManuscriptreceivedOctober20,2003.H.Kato,T.Miyazaki,andM.HamadaarewithKobeSteel,Ltd.,Kobe,Hyogo651-2271,Japan(e-mail:[e-mail]).T.HaseiswithJapanSuperconductorTechnologyInc.(JASTEC),Ki-takyushu,Fukuoka,Japan.K.TachikawaiswithTokaiUniversity,Hiratuka,Kanagawa259-1292,Japan.DigitalObjectIdentifier10.1109/TASC.2004.830363Fig.1.ComparisonoffabricationprocessbetweenPITwireandRICwire.fabricatemultifilamentaryconductorsbythePITprocess.How-everitisdifficulttoobtainthesetypesofwireofhighcharac-teristics,becauseofthefabricationdifficultyduetothebreakingofsheathsinextrusionordrawing.InthisstudythePITprocessformultifilamentaryconductorhasbeenimproved.ItisknownastheRICprocess(rod-in-core).TwoexperimentalstepsfordevelopmenthavebeendoneonRICprocessedwire.AtfirsttheperformanceofRICwireswascomparedwiththatofconventionalPITprocessones.ThentheRICwiresusingadifferentsheathmaterialwerestudiedtoim-prove.II.EXPERIMENTA.WireFabricationTheatomicratioofTapowdertoSnpowderwas6to5,and2wt%CuwasaddedtotheTa-Snmixedpowderinordertodecreasetheoptimumreactiontemperatureandtoenhance.Themixedpowderwasreactedat95for10hoursinvacuum.Fig.1showsthecomparisonofthefabricationprocessbetweenthePITandtheRICwires.IntheRICprocess,Nb-TarodsarelocatedintheTa-Snpowdercore.AftertheheattreatmentthelayerisformedbothontheNbsheathorNb-Tasheath1051-8223/04$20.00©2004IEEE988IEEETRANSACTIONSONAPPLIEDSUPERCONDUCTIVITY,VOL.14,NO.2,JUNE2004TABLEISPECIFICATIONSOFSAMPLEWIREATFIRSTSTEPTABLEIISPECIFICATIONSOFSAMPLEWIREATSECONDSTEPandtheNb-Tarods.Atfirst,theRICwireswithoutCustabilizerwerecomparedwiththesinglecorePITwire.Thewirespecifi-cationsareshowninTableI.ThentoimproveofRICwiresthesheathmaterialwasinvestigated.TheRICwireswerefab-ricatedinthesamewayasthefirststep,exceptforchangingthesheathmaterialfromNb-TatoNborTa.Thewirespecifica-tionsareshowninTableII.AllthesespecimensinthesecondstephavetheCuouterstabilizer.B.EvaluationAllspecimenswerereactedat800for80h.Aftertheheattreatment,thestructuresoflayerswereobservedbyscanningelectronmicroscopy(SEM).Inaddition,theconcen-trationofTa,Nb,SnandCuwasresearchedbyenergydispersiveX-rayanalysis(EDX).Thenon-Cupropertiesat4.2Kofallspecimensweremeasuredbetween14Tand25TattheNationalInstituteforMaterialsScienceandKOBESTEEL.Electricfieldcriteriontodefinewas1.Wedefinethelayerasthecriticalcurrentdividedbythesumofareas.III.RESULTSANDDISCUSSIONSA.MicrostructureFigs.2and3showthecrosssectionalviewsofthesampleEandthesampleF.InthesampleE,thethinnerreactedlayerisobservedattheinsideofNbsheaththanthatobservedattheout-sideofNbTafilaments.Ontheotherhands,noreactedlayercanbeobservedattheboundarybetweentheTasheathandTa-SnpowdercoreinthesampleF.Fig.4showsSEMstructuresoflayers.Notsomuchdifferencecanbeseeninthegrainsizeinthelayers,betweenthePITwireandtheRICones.B.EDXAnalysisTableIIIistheresultoftheEDXanalysisonthelayerformedinthesamplesfromAtoF