The ability of large grain, REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ [(RE)BCO; RE =
rare earth] bulk superconductors to trap magnetic field is determined by their
critical current. With high trapped fields, however, bulk samples are subject
to a relatively large Lorentz force, and their performance is limited primarily
by their tensile strength. Consequently, sample reinforcement is the key to
performance improvement in these technologically important materials. In this
work, we report a trapped field of 17.6 T, the largest reported to date, in a
stack of two, silver-doped GdBCO superconducting bulk samples, each of diameter
25 mm, fabricated by top-seeded melt growth (TSMG) and reinforced with
shrink-fit stainless steel. This sample preparation technique has the advantage
of being relatively straightforward and inexpensive to implement and offers the
prospect of easy access to portable, high magnetic fields without any
requirement for a sustaining current source.The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic ﬁelds is determined by their critical current. With high trapped ﬁelds, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped ﬁeld of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-ﬁt stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic ﬁelds without any requirement for a sustaining current source.This is the final published version, distributed under a Creative Commons Attribution License. This can also be found on the publisher's website at: http://iopscience.iop.org/0953-2048/27/8/08200

Ainslie, MD

Campbell, AM

Cardwell, DA

Dennis, AR

Durrell, JH

Hellstrom, EE

Hull, J

Jaroszynski, J

Palmer, KGB

Shi, YH

Strasik, M

Superconductor Science and Technology

The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source

Apollo (Cambridge)

This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 131.111.184.102This content was downloaded on 11/07/2014 at 13:29Please note that terms and conditions apply.A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steelView the table of contents for this issue, or go to the journal homepage for more2014 Supercond. Sci. Technol. 27 082001(http://iopscience.iop.org/0953-2048/27/8/082001)Home Search Collections Journals About Contact us My IOPscienceFast Track CommunicationA trapped field of 17.6T in melt-processed,bulk Gd-Ba-Cu-O reinforced with shrink-fitsteelJ H Durrell1, A R Dennis1, J Jaroszynski2, M D Ainslie1, K G B Palmer1,Y-H Shi1, A M Campbell1, J Hull3, M Strasik3, E E Hellstrom2 andD A Cardwell11Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK2Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University,2031 East Paul Dirac Drive, Tallahassee, FL 32310, USA3Boeing Company, Seattle, WA, USAE-mail: jhd25@cam.ac.ukReceived 16 May 2014, revised 30 May 2014Accepted for publication 11 June 2014Published 25 June 2014AbstractThe ability of large-grain (RE)Ba2Cu3O7− δ ((RE)BCO; RE= rare earth) bulk superconductors totrap magnetic fields is determined by their critical current. With high trapped fields, however,bulk samples are subject to a relatively large Lorentz force, and their performance is limitedprimarily by their tensile strength. Consequently, sample reinforcement is the key to performanceimprovement in these technologically important materials. In this work, we report a trapped fieldof 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconductingbulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced withshrink-fit stainless steel. This sample preparation technique has the advantage of being relativelystraightforward and inexpensive to implement, and offers the prospect of easy access to portable,high magnetic fields without any requirement for a sustaining current source.Keywords: bulk superconductor, high magnetic field, critical current, top-seeded melt growth(Some figures may appear in colour only in the online journal)IntroductionIt has long been known that, in addition to fabricating solenoidsfrom wire or tape, type-II superconducting materials can be usedto trap magnetic fields when fabricated in the form of well-connected bulks [1, 2]. Top-seeded melt growth (TSMG) hasemerged over the past 25 years as a practical route for fabri-cating large, single grains of the rare earth (RE) cuprate familyof high-temperature superconductors (HTS) of composition(RE)Ba2Cu3O7− δ ((RE)BCO). As a result, these materials havesignificant potential for application, effectively, as high-fieldpermanent magnets [3]. The performance of these magnets at77 K is limited by the critical current carrying capacity of thebulk superconductor. Nevertheless, fields of up to 2 T have beenachieved in 20mm diameter superconducting bulk samples [4]and up to 3 T in samples of 65mm diameter [5] at 77 K.The critical current density (Jc) of HTS is enhanced attemperatures lower than 77 K, and significantly larger mag-netic fields can be trapped. Notably, Tomita and Murakamireported a trapped field of 17.24 T at 29 K in an arrangementof two YBa2Cu3O7− δ (YBCO) samples of 26 mm diameterimpregnated with Wood’s metal and resin and reinforced withcarbon fibre [6]. Fuchs et al also reported a trapped field of16 T at 24 K in a Zr-doped and Ag-impregnated YBCOsample of 25 mm diameter placed inside a reinforcing stain-less-steel tube [7]. The prospect of generating portable highfields that are available outside the bore of a superconductingsolenoid is now a distinct possibility, given that considerable Superconductor Science and TechnologySupercond. Sci. Technol. 27 (2014) 082001 (5pp) doi:10.1088/0953-2048/27/8/0820010953-2048/14/082001+05$33.00 © 2014 IOP Publishing Ltd Printed in the UK1Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Made open access 7 July 2014 progress is being made in developing practical systems formagnetizing and refrigerating such samples to operatingtemperatures well below 77 K [8]. Additionally, thesesuperconducting bulk permanent magnets operate in what iseffectively ‘persistent mode’, which cannot yet be achieved insolenoids fabricated from YBCO-coated conductors due tothe difficulty in making superconducting joints.There are significant challenges, beyond merely produ-cing a material that exhibits a sufficiently large critical currentdensity, in trapping very high fields in (RE)BCO bulksuperconductors. First, the motion of flux during the ramp-down of any applied, external magnetic field generates asignificant amount of heat, which can lead to the formation ofthermal instabilities and the consequent catastrophicquenching of the bulk superconductor [6]. Second, the forcesexerted on the sample during magnetization due to the Lor-entz force are large and can lead to mechanical failure, withunreinforced samples typically failing at ∼7–8 T [9, 10]. Theforces in a bulk sample during the ramp-down of the mag-netizing field are tensile in nature [11], and the resultingtensile stresses are not resisted well by the brittle ceramic-likenature of the material. Moreover, as it is the resistance tofracture that determines the tensile performance and, giventhat the TSMG process generates many voids and cracks inthe sample microstructure, a wide variation in field-trappingperformance is to be expected from sample to sample [12].The aim of the work presented here was to develop asuperconducting bulk system that could exhibit peak trappedfields as large, or larger, than those currently reported in theliterature using conventional sample processing and reinfor-cement techniques that are relatively simple to implement.MethodsReinforcement analysisWe have previously developed Ag-containing GdBCO sam-ples that exhibit good field-trapping performance at 77 K [13]and contain a more homogenous Jc distribution than YBCO[14]. Unlike the Ag-GdBCO samples fabricated by othergroups, we do not observe a significant deleterious effect onJc for an Ag content of up to 15 wt% AgO2. This is a con-sequence of the large number of voids and cracks generatedduring the TSMG process, which can accommodate theexcess Ag without affecting the properties of the continuoussuperconducting matrix. As a result, it was not necessary toenhance the pinning performance of the materials studied hereby introducing an extra pinning phase to the Gd-123microstructure.Previous investigators have used stainless-steel rings toreinforce bulk superconductors, relying on the pressure thatoccurs at the superconductor/stainless steel interface due todifferential thermal contraction on cooling from room tem-perature to the desired measurement temperature. Here, we firstdetermined the magnitude of the interface pressure that wouldresult on the samples using a standard method [15] for ana-lysing the shrink-fitting process and the published values for themechanical properties of (RE)BCO [16]. It was found that thiswould equate to a relatively modest 55MPa for a reinforcingring of 24 mm in diameter and 3mm thickness cooled to 28 K,which is the same order of magnitude as the tensile strengthanticipated in a GdBCO superconducting bulk containing Ag[10]. The required tensile strength of a material for a particularmaximum field can be estimated [6] as 0.282 B2 MPa. Therequired tensile strength at a field of 18 T is therefore ∼91MPa,which would result in the mechanical failure of most samples,since the internal compressive stress will be less than theinterface pressure. The samples were machined precisely to adiameter of 24.15mm and were reinforced by a stainless-steelring of internal diameter 24mm to increase the pre-stress. Therings were heated to >300 °C to enable them to fit onto thesuperconductor. This configuration was calculated to provide apre-stress of ∼250MPa, which is a significant improvement onthe stress achieved from simple steel banding. It should benoted that, while this compressive stress is added uniformly, thesample interior may not experience a homogeneous stress dis-tribution due to the presence of voids in the sample, which maylead to specific regions of much lower compressive stress.Sample preparation and measurementsCylindrical bulk samples ∼25 mm in diameter and ∼13 mm inheight were grown using the top-seeded melt growth processdescribed elsewhere [13], and then machined to a diameter of24.15 mm. The samples exhibited trapped fields of ∼0.9 T at77 K. A 304 Stainless Steel ring was then heated to a tem-perature >300 °C and shrink-fitted to each sample.The samples were combined into stacks of two, andjoined by a thermally conductive epoxy resin, as shown infigure 1, for measurement purposes. A linear array of fiveFigure 1. The assembled stack of two GdBCO bulk samples, each24.15 mm in diameter and 15 mm high. The samples were reinforcedwith a 3 mm thick ring fabricated from 304 Stainless Steel. Therecess visible in the ring served to accommodate the measurementwiring. The two bulk samples were then assembled into a stack withan array of five Hall probes at their interface using STYCASTthermally conductive epoxy resin. The Hall probes were arrangedevenly in a line across the sample at −8, −4, 0, 4 and 8 mm from thecentre of the sample.2Supercond. Sci. Technol. 27 (2014) 082001Lakeshore HGT-2101 Hall sensors was placed at the centre ofeach stack at positions −8, −4, 0, 4 and 8 mm from the centreof the sample (i.e. between the two halves). These sensorswere supplied un-calibrated and exhibited some non-linearity,which was characterized at 100 K. The output voltage of theHall sensor was measured at the temperature at which themagnetizing field was applied. The temperature of the two-sample stack was measured using a Cernox sensor, and thesample temperature was stabilized using a wire-wound heaterwrapped around the stack.The Hall sensors were driven with a 100 Hz, 10 μA peaksine wave using a Keithley 6221 current source, and the Hallvoltage was measured using a lock-in amplifier for each Hallprobe. The samples were magnetized in the bore of the SCM-218 T superconducting magnet at the National High MagneticField Laboratory, Florida State University. The desired mag-netizing field was applied with the temperature of the stacksmaintained at 100 K, after which the samples were cooled tothe measurement temperature. Finally, the external field wasramped down at a rate of 0.015 T min−1.Results and discussionTwo similar stacks were magnetized initially by field coolingfrom 16 T at 28 K. One stack suffered some cracking andtrapped a maximum field of 10 T, whereas the second two-sample stack trapped a field of 15.4 T.A third sample was then magnetized with an applied fieldof 17.8 T at 26 K. The fields recorded during the magnetizingprocess of this stack are shown in figure 2. A trapped peakfield of 17.6 T was achieved at the end of the magnetizingprocess, which represents the largest field trapped in a bulksuperconductor at any temperature reported in the literatureto date.To monitor any reduction in trapped field due to fluxcreep, the sample was maintained at 26 K for a period of160 min, as shown in figure 3. Flux creep, the rate of whichdecreases with time, was observed as expected [17]. The fluxcreep behaviour over the relatively short time period mea-sured was initially logarithmic but then dropped morequickly.The sample was then warmed slowly at a rate of0.5 K min−1 and the variation of the trapped field with tem-perature recorded. Figure 4(a) shows the field at the centre ofthe sample and figure 4(b) the evolution of the trapped fieldprofile with increasing temperature. This data set suggests thatthe critical current performance of the samples employedwould potentially support the maximum trapped field of17.6 T at temperatures of up to 32 K. It is immediatelyapparent from the variation of Jc with distance that the fieldtrapped in this sample is not limited by the current carryingcapability of the superconductor, as the profile is flatter thanwould be expected for full penetration. Furthermore, thesample was still capable of trapping almost 10 T at 50 K, atemperature which, significantly, is achievable using a single-stage cryocooler. Figure 4(b) shows that the trapped fieldprofile changes to one corresponding to a sample limited bycritical current as the temperature of the two-sample stackincreases. It is apparent in figure 4(b) that the centre Hallprobe was not aligned fully with the centre of the field dis-tribution. This may have been due to slight asymmetry of thesuperconducting properties of the sample, or slight mis-alignment of the two components of the stack.The sample was then re-magnetized in a separate, butotherwise identical process, using an 18 T field, but failed dueto cracking as the external field was reduced. Similarly, afurther, fourth sample stack failed under a magnetizing fieldof 18 T, suggesting that the maximum trapped field observedin this study corresponds approximately to the limit ofmechanical performance for the shrink-fit reinforcementmethod employed to reinforce the samples.A spread in performance of four sample stacks wasobserved in this study for similar magnetizing processes. ThisFigure 2. The field measured at the interface between the twosamples in the stack by an array of Hall probes (the distancesindicated are from the centre of the sample) as the magnetizing fieldwas ramped down.Figure 3. The reduction in trapped field due to flux creep at the endof the magnetizing sweep with the sample temperature maintainedat 26 K.3Supercond. Sci. Technol. 27 (2014) 082001is not surprising, and indeed is characteristic of a brittlematerial that fails typically by fast fracture. Although theWeibull modulus for GdBCO fabricated by TSMG is notavailable in the literature, the TSMG process is one that isknown to generate a large number of cracks of differentlengths in large, single grain samples. It is expected, there-fore, that wide variation in sample performance will beobserved between apparently similar samples from the samegrowth batch. Statistically, this also means that samples witha larger volume will exhibit poorer performance [18].The question remains why, given the large predictedinterface pressure, samples still failed at stresses lower thanthe applied pre-stress. The answer to this is likely to be two-fold: the difficulty in providing a well-defined and smooth(RE)BCO surface to mate with the reinforcing ring, and thefact that the internal pre-stress provided is somewhat smallerthan the interface pressure. This could be addressed by usinga larger shrink-fit temperature, although this would riskdamaging the samples. The choice of an alternative reinfor-cement material to 304 Stainless Steel is limited, given thatthis material exhibits an unrivalled combination of stiffnessand yield strength while still possessing a significant coeffi-cient of thermal expansion. Another route would be tofabricate GdBCO with enhanced fracture toughness, perhapsvia the use of techniques such as fibre reinforcement to bridgecracks as they form during melt processing.ConclusionThe best-performing two-sample stack of bulk GdBCO con-taining Ag trapped a peak magnetic field in excess of 17.6 Tfrom a 17.8 T magnetizing field at 26 K. This is the highestfield trapped in a bulk superconductor reported to date at anytemperature.The applicability of the relatively simple technique ofapplying a shrink-fit steel reinforcement to the samples hasbeen demonstrated clearly. This is a well understood and easyto implement technique that is applied regularly to ceramicmaterials [19]. Apart from the addition of Ag and the con-sequent minor adjustment of the thermal process duringgrowth, the GdBCO samples were prepared in air by a con-ventional cold-seeding top-seeded melt process. The techni-que described here, therefore, provides a practical route togenerating very high fields to be trapped in single grain, (RE)BCO superconducting bulk samples. The ceramic nature ofthese samples leads to a wide variation in fracture toughness,and the use of compressive pre-stress is an effective route toenhanced field trapping performance.AcknowledgementsThis work was supported by the Boeing Company and by theEngineering and Physical Sciences Research Council (grantnumber EP/K02910X/1). Part of this work was performed atthe National High Magnetic Field Laboratory, which is sup-ported by National Science Foundation Cooperative Agree-ment no. DMR-1157490, the State of Florida, and the USDepartment of Energy.References[1] Echarri A 1966 Remanent induction in a hard superconductorPhys. Lett. 20 619–21[2] Frankel D J 1979 Critical‐state model for the determination ofcritical currents in disk‐shaped superconductors J. Appl.Phys. 50 5402–7[3] Hull J R and Murakami M 2004 Applications of bulk high-temperature superconductors Proc. IEEE 92 1705–18[4] Ravi-Persad S et al 2013 Production run of 2 cm diameterYBCO trapped field magnets with surface field of 2 T at77 K Supercond. Sci. Technol. 26 105014[5] Nariki S, Sakai N and Murakami M 2005 Melt-processedGd–Ba–Cu–O superconductor with trapped field of 3 T at77 K Supercond. Sci. Technol. 18 S126[6] Tomita M and Murakami M 2003 High-temperaturesuperconductor bulk magnets that can trap magnetic fields ofover 17 tesla at 29 K Nature 421 517–20[7] Gruss S et al 2001 Superconducting bulk magnets: veryhigh trapped fields and cracking Appl. Phys. Lett. 793131–3Figure 4. 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A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

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