Processing and Properties of BarShaped SingleSee

Authors: Abdulmajeed Almalki Devendra K Namburi Mazen BaAbbad Anthony R Dennis K Y Huang AbdulAziz Almutairi J H Durrell David A Cardwell

Publish Date: 2016/12/30

Volume: 30, Issue: 6, Pages: 1397-1403

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Abstract

The fabrication of REBaCuO bulk superconductors where RE is a rareearth element such as Y Gd and Sm is both time consuming and expensive due to the complexity of the melt process and the slow growth rate of large single grains In this study different approaches to the fabrication of barshaped bulk YBCO superconductors are investigated and compared using single and multipleseeding techniques via topseeded melt growth TSMG Both the microstructural and superconducting properties of the bulk samples are investigated including trapped field critical current density critical temperature and levitation force The results of this study indicate that in general the superconducting properties of YBCO fabricated by a singleseeded process are significantly better than those of samples fabricated by a fourseeded process for nonbridge seeds The differences between the samples are less pronounced in the levitation force measurements however In this paper we attempt to explain the reasons for the similarities and differences observed between bulk samples fabricated by the different single and multiseeded processesYBaCuO YBCO and the related cuprates containing different rareearth RE elements is one of the socalled type II hightemperature superconducting oxide ceramic materials with a critical temperature of 90–92 K 1 2 YBCO in bulk form can be machined relatively easily at room temperature and is therefore a good candidate for practical applications such as trapped field magnets flywheel energy storage and noncontact bearings for frictionfree motors 1 2 3 The critical current density J c and the trapped magnetic flux density B t are the two most important properties of YBCO bulk superconductors from an application perspective 4 5 In addition the trapped magnetic flux density is proportional to the critical current density J c and the diameter of the supercurrent loop d ie B mathrm tpropto J mathrm c× d which is an important figure of merit for bulk superconductors 5Various growth techniques including melttextured growth MTG melt powder melt growth MPMG powder melting process PMP and topseeded melt growth TSMG 6 7 8 9 have been developed over the past 25 years in order to fabricate YBCO bulk superconductors in large single grain form Of these the TSMG technique has been studied the most extensively and has emerged as a reliable process for fabricating large REBCO bulk single grainsSample microstructure plays a key role in determining the flux pinning strength in type II superconductors The presence of nonsuperconducting phases in the form of defects ideally of the order of magnitude in size of the coherence length 1–3 nm at around 20 K impedes the motion of flux vortices and as a result increases the trapped field B t It has been found that a reduction in the size of RE 2BaCuO 5 ‘RE211’ secondaryphase particles embedded within the superconducting REBa 2Cu 3 O 7−y ‘RE123’phase matrix that occur naturally as a result of the melt growth process improves the critical current density of REBCO 10 11 12 The concentration of Y211 particles in YBCO is known to vary considerably from the region close to the seed crystal to the edge/bottom of the sample 13 This is an unavoidable effect which occurs due to particle pushing effects associated with the YBa 2Cu 3 O 7−x Y123 growth front Recently composition grading techniques were employed to minimize inhomogeneities in the Y211 concentration throughout the volume of the sample thereby achieving greater uniformity in its superconducting properties 9 10 14The TSMG technique involves the incongruent melting of the Y123 phase above its peritectic temperature T p ∼ 1005 ∘C in air which decomposes to Y211 solid and a liquid phase comprising BaCuO 2 and CuO A seed crystal whose melting temperature is above the T p of the seeded bulk but with similar crystal lattice parameters and having phase stability with the melt is employed for seeding and subsequent growth of YBCO into a single grain The growth rate of the Y123 phase is of the order of 02–04 mm/h 12 13 which limits practically the ultimate size of the final single grain Additionally it has been reported previously that large samples exhibit increasing crystallographic misorientation of the Y123 phase with increasing distance from the seed 14 As a result it is not easy to fabricate REBCO single grains reliably in sizes of diameter larger than ∼ 40 mm The increasing drive towards applications requires the development of methods to produce larger REBCO bulk samples both reliably and economicallyIn this context multiseeding is one approach that can be used to fabricate largesized REBCO in quasisingle grain form and reduce simultaneously the processing time In particular magnetic levitation forces and stiffness ratios for both multi and singleseeded bulk superconductors have been observed to increase with increasing cooling times 15 Seeding techniques have been employed to grow YBCO single grains 16 with both 0 ∘–0 ∘ and 45 ∘–45 ∘ orientations of the seed Here for instance 0 ∘–0 ∘ represents the angles between the direction of the long axis of a bridge seed and the a direction corresponding to the 100 direction for the tetragonal YBCO phase of the two seeds respectively 16 17 The bridge technique has enabled significant improvement in the alignment of the seeds and has eliminated the sharp decrease in trapped field observed at the centre of YBCO bulk superconductors fabricated using independent ie nonbridge seeds A detailed comparison between melt processes using 0 ∘–0 ∘ and 45 ∘–45 ∘ bridge seeds has been reported by Shi et al 17 This clarified that the 45 ∘–45 ∘ bridge seeding results in cleaner grain boundaries in comparison with 0 ∘–0 ∘ bridge seedsGdBCO and YBCO bulk superconductors have also been fabricated via a multiseeding technique employing welloriented NdBCO thin film seeds 18 19 The use of multiseeds arranged asymmetrically with 110/110 orientation resulted in a promising approach for enlarging the domain size of GdBCO bulk samples However the NdBCO thin film seeds used to grow YBCO were damaged partly during processing due to contact with the liquidphase components at elevated temperature 20 21 which resulted in the undesirable growth of Y123 grains with a 001/103 orientation This partial melting of seed crystals can be avoided to a certain extent by invoking either twostep cooling during the melt process 22 or by employing the buffer technique 23 24 25 Also it has been reported that the YBCO single grains grow much larger if the cooling time at the growth temperature is increased 12 13The levitation force generated between the bulk sample and the load is the most critical parameter for the successful development of systems such as flywheels and contactless bearings In this paper we compare the key applied properties of singleseeded and multiseeded YBCO barshaped samples in order to determine whether multiseeding represents an effective route for the production of larger samples as required in several applications

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