We have successfully demonstrated trapping of a magnetic field of 17.6 T in a bulk superconductor at 26 K. This is about 0.4 T in excess of the previous record field obtained by Tomita et al.. Bulk superconductors offer the prospect of convenient, permanent magnet like, fields but of much larger intensity with the very best rare earth magnets achieving little more than 1 T.
The question that arises is as to why it has taken 10 years to achieve a fairly moderate increase on Tomita et al.’s result. To understand this it is important to consider that the limiting factor to the high field performance of such bulks in superconductor applications is not the critical current density of the superconductor itself. In fact, it is the tensile strength of the superconductor. The strains inside a bulk superconductor during charging to 17 T can reach ~100 MPa with the strain scaling as B2. As the superconductors in question are brittle ceramics it is clear that this is a challenging problem. Moreover in materials where fracture toughness is important there is often a wide spread of performance since generally it is the one “worst crack” that determines performance.
To achieve high trapped fields, some sort of mechanical reinforcement of the sample is required. Two classes of techniques have been exploited in the literature, steel banding where due to differential thermal contraction a pre-stress is applied to the bulk, and reinforcement with carbon fibre epoxy . We were interested in a quick and easy method which led to our selecting steel banding. We realised, however, that the pre-stress introduced by differential thermal expansion is relatively modest and looked to improve it. We hit upon the simple, and indeed frequently employed in engineering, solution of “shrink fitting”. We carefully machined our bulks to be perfectly cylindrical and prepared slightly under size stainless steel (304L) rings. When heated, these rings expanded and could be slipped over the bulks, as the rings cooled they contracted applying pre-stress to the bulk superconductor. This pre-stress then further increased as the samples were cooled to measurement temperature. In this way we sought to avoid tensile stress sufficient to break them and were able to achieve a field of 17.6 T.

We acknowledge funding from EPSRC (EP/K02910X/1) and The Boeing Company.

Studies in nanocrystalline low temperature superconductors have shown decreases in critical temperature (T_c) along with increases in the upper critical field (B_c2(0)) [1, 2]. However, few studies have been done on nanocrystalline high-temperature superconductors. In this work, we present our methodology and results from fabricating nanocrystalline high-temperature superconducting materials. Mechanical ball milling, hot isostatic pressing and oxygen annealing have been used to fabricate nanocrystalline YBa₂Cu₃O_7-x and a nanocrystalline material that is a mixture of YBa₂Cu₃O_7-x, Y₂BaCuO₅ and CeO₂, in which the secondary phases act as pinning centres [3]. The microstructure of the material was characterised using XRD and SEM. The milled material have grain sizes of ~ 5 nm, which increased to ~ 50 nm after hot isostatic pressing and oxygen annealing. The superconducting properties were characterised using a.c. susceptibility, magnetisation and resistivity to study the effects of grain size on T_c, B_c2(0) and critical current density (J_c) in fields up to 8.5 T. For the single phase nanocrystalline material, T_c and B_c2(0) were reduced to 81 K and 66 T. Magnetisation measurements showed paramagnetic hysteresis behaviour even at T < T_c. We also present computational simulations on the properties and effects of grain boundaries on the resistivity of polycrystalline materials, in support of experimental data. We will present our most recent findings at the conference.
References:
1. Niu, H.J. and D.P. Hampshire, Disordered Nanocrystalline Superconducting PbMo₆S₈ with Very Large Upper Critical Field. Physical Review Letters, 2003. 91(2): p. 027002.
2. Taylor, D.M.J., M. Al-Jawad, and D.P. Hampshire, A new paradigm for fabricating bulk high-field superconductors. Superconductor Science & Technology, 2008. 21: p. 125006.
3. Shi, Y., et al., Synthesis of YBa₂Cu₃O_7-d and Y₂BaCuO₅ Nanocrystalline Powders for YBCO Superconductors Using Carbon Nanotube Templates. ACS Nano, 2012. 6(6): p. 5395-5403.

We would like to thank: Dr Douglas Halliday, Dr Budhika Mendis and Mr Leon Bowen from Durham University Physics Department. We would also like to thank Dr Yunhua Shi and Prof. David Cardwell from Cambridge University Bulk Superconductivity Group. This work was funded by the EPSRC.

The goal of reinforcing bulk superconductors is strengthening the base material to improve the capacity to maintain high magnetic fields during application. The current highest magnetic fields are limited by the materials strength of the bulk superconductors which fail during magnetization. By adding additional material strength we aim to boost the charging capabilities of the bulk superconductors produced and improve their overall lifespan in use. Creating high strength, high field bulks provides opportunities for their use in applications.

Our current world record field bulk which trapped 17.6 T is accompanied by multiple other bulks which are capable of trapping 12 T without failure. The capability to regularly produce high field bulk superconductors promotes their use in applications which benefit greatly from increased field strength.

This work was supported by the Boeing Company. Part of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490, the State of Florida, and the U.S. Department of Energy.

High-quality single domain SmBCO bulks have been successfully produced in the air by top seeded infiltration and growth (TSIG) method, with a new solid phase of (Sm2O3 +xBaCuO2) and the liquid phase of (Y2O3+6CuO+10BaCuO2), x =0.9, 1.0, 1.1, 1.2, 1.3 and1.4. The results show that the onset critical temperature (Tco) of the SmBCO bulks increases with the increasing of x, because of the BaCuO2 addition reduced the substitution of Sm on Ba site in Sm1+xBa2-xCu3O7 crystal; when x =1.4, the Tco have reached 96K. The amount and the size of Sm211 particles of the single domain SmBCO bulks decreases with the increasing of BaCuO2 addition x. Both of the levitation force and trapped field of the samples indicate that the levitation force (trapped field) increases from 38.47 N(0.2330T) to 44.34N(0.2680T) with the increasing of x from 0.9 to 1.2, and then decreases from 44.34N(0.2680T) to 37.94N(0.2085T) with the increasing of x from 1.2 to 1.4, the largest levitation force of 44.34N and trapped field of 0.2680T are obtained with the optimal value of x=1.2. This result provides a very important way to fabricate high-quality single domain SmBCO bulks in air.

This work was supported by the National Natural Science Foundation in China(No.51342001, 50872079), the Keygrant Project of Chinese Ministry of Education (No.311033), Research Fund for the Doctoral Program of Higher Education of China(No.20120202110003), the Key Program of Science and Technology innovation team of Shaanxi Province (2014KTC-18)

Synthesis under reducing atmosphere is a well-known method to suppress partial substitution of RE for Ba in RE123 with RE=La~Gd. In our previous study, post-annealing under moderately low oxygen partial pressure 0.1~1 kPa at high temperature ~800°C prior to the oxygen annealing at low temperatures largely improves critical current properties of melt-solidified bulks and sintered bulks of RE123 even for compounds containing heavy RE elements, such as Y and Er. By combining carrier overdoping, intergrain J_c of RE123 sintered bulks was largely enhanced. For example, intergrain J_c of Y123 sintered bulk at 20 K in low field was improved from ~300 A cm^-2 to ~6000 A cm^-2 by addition of reductive post annealing process and Ca-doping. Such reductive post-annealing also enhances irreversibility fields of RE123 sintered bulk. These means that characteristics both at grain boundaries and in RE123 crystals are improved by reductive post-annealing, though this process does not largely change chemical composition and microstructure. In addition, we recently developed a new and easy method to synthesize RE247 phase and, therefore, various physical properties including critical current characteristics of RE247 can be systematically studied. Partial substitution of RE for Ba is more serious for RE247, because moderately high oxygen pressure is indispensable for phase formation. We have found that reductive post-annealing at ~600°C for a long time is effective for enhancing T_c and intergrain J_c of sintered bulk specimen of Y247 and Nd247. Since crystal size and shape of RE247 sintered bulks are largely different from those of RE123 sintered bulks, RE247 might be a new candidate of superconducting materials. Intrinsic effects of the reductive post-annealing process for sintered bulks of RE123 and RE247 as well as RE123 melt-solidified bulks and derivable potentials of these RE-Ba-Cu-O materials will be discussed.

We present recent progress in QMG materials in Nippon Steel & Sumitomo Metal Corporation. QMG is a high Jc bulk material, which consists of single crystalline RE123 phase with finely dispersed RE211 particles. HTS bulk materials fabricated by advanced QMG method have significant potential for high-field engineering applications, since they can trap high magnetic fields depending on their Jc and size. Gd-based QMG is very attractive for various bulk applications such as flywheel energy storage systems, ship motors, wind power generators etc. In generally, when LRE-Ba-Cu-O (LRE: light rare elements = Nd, Sm, Eu and Gd) bulk superconductors are melt-grown in air, RE/Ba substitution in 123 phase causes a lowering of Tc. However, Tc of Gd-based QMG is higher than that of Y-system, even when they are melt-grown in air. The trapped magnetic field of Gd-based QMG 60 mm in diameter at 77K is twice as large as that of Y-based QMG with a similar size due to their excellent Jc properties. An enlargement in diameter of bulk leads to enhance the trapped magnetic field. The large Gd-basd QMG up to 150mm in diameter was fabricated by incorporating the RE compositional gradient method. Compact NMR/MRI spectrometers are one of promising applications of bulk superconductors. Eu-based QMG is suitable for NMR magnets. NMR applications require extremely homogenous magnetic fields. In the case of Gd-system, very large paramagnetic moment of Gd ion reduces homogeneity in magnetic fields. In Eu-system, smaller paramagnetic moment of Eu ion improves the field homogeneity in the bulk. We will also report the field trapping properties of QMG at low temperatures.

We thank the Communauté Française de Belgique for cryofluid, and equipment grants, under reference ARC 11/16-03.