Composite bulk superconducting magnets for high field applications

Lead Research Organisation: University of Cambridge
Department Name: Engineering


The RE-Ba-Cu-O (where RE= rare earth element such as Y, Nd, Sm, Gd, etc.) family of bulk, melt processed high temperature superconductors [(RE)BCO] is the subject of extensive world-wide research due primarily to their potential to trap large magnetic fields. This has been demonstrated spectacularly by the Cambridge Bulk Superconductivity Group, which recently set a new world record trapped field of 17.6 T at 26 K using these materials, breaking the previous world record that had stood for more than 10 years. The Cambridge Group has been at the forefront of research in this area for the past 20 years and, in addition to funding from EPSRC and other government sources, has attracted substantial and sustained industry funding.

Bulk (RE)BCO superconductors have reached an important and critical stage in their research and development. Their spectacular field generating properties have high potential for a range of sustainable engineering applications, including flywheel energy storage, motors and generators, magnetic separators, bio-medical applications and magnetic levitation devices. This proposal is for a unique and timely combination of fundamental materials research and the development of practical assemblies to generate practical magnetic fields using bulk superconductors that can be used routinely and commercially in engineering devices for the first time.

The main objective of the project will be to shape the magnetic field at low temperatures (50 K and 30 K), where critical current, and hence field generating capability, is significantly higher than at 77 K. Materials to improve the mechanical strength and the thermal conductivity for incorporation in the sample and assembly structures will be developed to obtain optimum performance in high field for samples and assemblies magnetised specifically by pulse magnetisation. It is becoming increasingly likely that rapidly developing cryo-cooler technology will enable practical applications at temperatures below 77 K, and this will drive the development of improved materials and new structures.

The single grain, (RE)BCO bulk superconductors developed with improved mechanical strength and thermal conductivity will be incorporated into assemblies of different composite shapes of different (RE)BCO materials to enable the control of magnetic field strength and distribution. The properties and performance of these assemblies will be compared with larger sized, individual samples of comparable surface areas at 77 K where the requirement for mechanical strength is relatively modest. Capability developed during our current EPSRC grant on multi-seeding will further enable the fabrication of multi-seeded, quasi-single grains, whose properties will be compared with an assembly of smaller, closely packed samples of similar sizes. The trapped field and levitation force of assemblies of different (RE)BCO superconductors arranged in different orders will be measured at 77 K and compared with the properties of conventional, single grains of the same size.

The project, which will continue to support outreach in UK school, colleges and universities, benefits from strong financial support of major international industrial collaborators, including the Boeing Company, Siemens and Bio-med (UK).

Planned Impact

The impact of this project will be particularly significant for the Cambridge Group our collaborators and our industrial partners, building on an outstanding research and collaboration track record. The project will also benefit both industry and academia worldwide: 1. Engineering industries involved in the manufacture of magnetic bearings, flywheel energy storage systems, high field permanent magnet devices and magnetic separators; 2. Companies manufacturing permanent magnet MRI/NMR systems; 3. High efficiency, compact motors and generators; 4. Devices being developed for healthcare applications, and non-invasive surgical devices, in particular. Researchers working on magnetic and transport properties, solidification and phase kinetics of (RE)BCO materials will also benefit directly from the results of the project. Samples will be supplied to the relevant UK academic institutions and schools free of charge, as required.

The main mechanism for impact and exploitation is via established collaborations with the Boeing Company, Siemens and Bio-med (UK), which, collectively, have a number of large, active (propriety) projects with potential users of bulk material in energy storage flywheel systems, non-destructive testing, non-invasive surgery and the all-electric aircraft. This is a ready-made market, and offers a unique opportunity for the UK to make a major impact on HTS technology on an international scale. Unfortunately, the details of these projects are proprietary, although they represent a combined potential market of many tens of millions of pounds in the medium to long term. State of the art bulk material and assemblies of bulk single and multi-seeded grains developed as part of this project will be incorporated into
these devices to yield a direct route to commercial exploitation. Other emerging applications relevant to developing sustainable engineering technologies include high efficiency, light and compact motors and generators, testing applications requiring a large magnetic field, stable magnetic bearings, fault current limiters and magnetic separators. Boeing and Siemens, in particular, have potential interest in these devices, and are well-placed to supply the global market. Cambridge has existing, formal IPR agreements in place with Boeing, Siemens and Bio-med (UK), including joint patent interests (Boeing and Bio-med), and the partnerships are well-positioned to fully exploit emerging bulk superconductor technology. The outputs with potential technical and commercial impact will be identified by comparing the properties, manufacturability and cost of state of the art bulk samples with the requirements of global industry, as defined by the partner industries. This will provide bench-mark data by which to evaluate the output of the project and enable comparison with competing technology (where it exists). Key properties are the magnitude of trapped field at 77 and 50 K, mechanical strength, thermal conductivity and production cost, which will be monitored by Boeing, Siemens and Bio-med (UK) as part of the project. In-situ performance of bulk samples in a wide range of devices will provide further technical output to quantify the capability of the bulk materials in an applications-specific environment.

The SRA appointed to the project will undertake the impact activities in close collaboration with the PI and Co-I. Prof. Cardwell and Dr Durrell have extensive experience of the generation and protection of IPR, and are well placed to oversee and manage this project. Cambridge Enterprise (CE) (a University body established to advise and support the commercial exploitation of emerging technology) will be involved closely with any transfer of manufacturing technology outside the University. CE employs specialists in IPR, market research and contract law at no direct cost to the project, and will play a key role in the exploitation of the commercial potential of the material developed during this research.


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Congreve J (2018) The successful incorporation of Ag into single grain, Y-Ba-Cu-O bulk superconductors in Superconductor Science and Technology

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Goodfellow A (2016) Microstructural evolution in multiseeded YBCO bulk samples grown by the TSMG process in Superconductor Science and Technology

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Huang K (2018) Spatial Distribution of Flexural Strength in Y-Ba-Cu-O Bulk Superconductors in IEEE Transactions on Applied Superconductivity

Description 1. Shaping the magnetic field to achieve the highest levitation force at 77K
Three bar-shaped 60 x 20 x 12 mm3 YBCO samples have been prepared; one single-seeded (which is very difficult to produce), one multi-seeded and one consisting of three square samples packed together. The levitation forces of these three samples have been analysed using various permanent magnet arrays. The observed observations of the maximum levitation force can be explained using the theory of static levitation combined with an analysis of the induced magnetic moment of each sample. We find that, while the single grain sample exhibits the highest trapped field, the largest levitation force arises from the multi-seeded sample due to its higher magnetic moment originating from an induced Eddy current. This implies that there is potential for the use of arrays of smaller samples in place of single large grain samples in certain systems and that the magnetic flux can be shaped by assembling the samples to obtaining the maximum levitation force.

2. Understanding the effect of the size and aspect ratio of the single grain (RE)BCO to their superconducting properties trapped field and total flux
Limitation in size is one of the issues that hinders the applications of high temperature superconducting bulk materials. The use multiple samples is inevitable to achieve the required superconducting properties. The dimensions and the arrangements of the samples all play an important role in shaping the resulting superconducting properties. In order to gain a better understanding of bulk single grain superconductors for applications, three YBCO single grains of different diameters were fabricated and their trapped fields and total flux measured at 77 K after cutting the samples into sub-specimens of varying successive height. The effects of the sizes and aspect ratios of the single grain YBCO on their superconducting properties, trapped field and total flux are being investigated.

3. The successful incorporation of Ag into single grain, RE-Ba-Cu-O bulk superconductors (RE=Sm, Gd and Y) for improving the mechanical properties of the bulk ceramic superconductors. The YBCO-Ag system has proved difficult to grow into single grains after incorporating Ag within individual single grains. Growth rates have been measured for the first time and YBCO-Ag single grains of up to 25 mm in diameter have been grown reliably. The challenge for the SmBCO system is control of its superconducting properties during melt processing. The severe substitution of Sm on the Ba when melt processed in air makes the superconducting transition temperature Tc vary within a given sample. A new Y-123 substrate has been incorporated within he process that yields a consistent Tc throughout the whole sample, which is a significant breakthrough. A SmBCO-Ag single grain of 30 mm in diameter fabricated by this modified technique traps magnetic field of up to 1.0 T, which is the highest worldwide.

4. (RE)BCO single grains with various (RE)Gd-Y, Gd-Eu system combinations have been processed successfully and their properties investigated. Detailed studies of the sample microstructures, superconducting critical transition temperature, superconducting current density and the process techniques themselves have been carried out. The initial conclusion is that superconducting and mechanical properties can be potentially improved through compositing or combining the rare-earth elements in growing (RE)BCO single grains, although the melt process needs to be controlled more carefully.

5. A novel method based on either the use of raw oxides or the addition of RE2O3 (instead of RE-211) to grow large, single grains directly has been developed. The method allows the reliable and economic growth of single grains, even from a combination of rare-earth elements.

6. Three-point bend tests have been performed on bar specimens cut from four Y-Ba-Cu-O single-grain bulk superconductors in order to determine the spatial distribution of the flexural strength. A relatively large spread in the flexural strength at room temperature, with average and standard deviation values of 49.3 MPa and 12.7 MPa respectively, was observed across the four bulk samples. Indirect tensile (Brazilian) tests were conducted on twelve bulk superconductors of diameter16 mm from three production batches in order to further examine bulk-to-bulk variability. The Weibull modulus for these samples was calculated to be 8.76, suggesting that, despite the large spread in strength with position within a bulk grain, the batch processing of bulk superconductors can consistently yield samples with comparable overall mechanical strengths to Al2O3 engineering ceramic. This implies that, despite the large scatter of strength with position within the bulk single grain, the mechanical failure of a bulk superconductor as a whole can be predicted accurately, suggesting that these materials are sufficiently reliable mechanically for use as trapped-field magnets.

7. Several techniques have been adopted for improving the mechanical properties of the bulk materials, including fibre enforcement and the introduction of a metal sandwich between layers of individual large grain samples.

8. We have systematically investigated macroscopic flux jumps in single grain GdBa2Cu3O7-d-Ag (GdBCO-Ag) bulk superconductors with diameters of up to 30 mm when subjected to pulsed magnetic fields. The applied pulsed field required to trigger the instability, or flux jump field, Bj, was determined experimentally and found to increase with decreasing temperature, as anticipated. An extended instability criterion based on a 2D axisymmetric model has been used to predict Bj at various temperatures and the results are in good agreement with experiment. A peak trapped field of 4.1 T at the surface and 5.3 T between a stack of two GdBCO-Ag bulk superconductors has been achieved at 30 K by means of an optimized two-step pulse sequence with the assistance of the flux jumps, which is extremely promising for potential applications of these technologically important materials.
Exploitation Route Shaping the magnetic field to achieve the best levitation force at 77 K would help the scientific and technological community understand that engineering the dimensions of a sample and the size of any inter-grain structure can shape the magnetic field, so that extremely large single grains may not be needed for practical applications.
The effect of the size and aspect ratio of the single grain (RE)BCO on its superconducting properties, trapped field and total flux will be of significant potential value to researchers in the field of applications. These data are important for assessing the effect of stacking samples together, which is one of the effective ways to overcome the size limitation of single grain superconductors.
The successful incorporation of Ag into single grain, RE-Ba-Cu-O bulk superconductors will help improve the mechanical properties of the (RE)BCO single grains, which are effectively layered ceramics. Researchers interested in improving mechanical properties for applications will benefit from these techniques.
Sectors Energy,Healthcare,Transport

Description We have found that although single grain samples exhibit the highest trapped field, a larger levitation force arises from multi-seeded samples and a close-packed array of samples due to their higher magnetic moment originating from an induced Eddy current. Therefore, multi-seeded or the close packed, assembled composites of (RE)BCO bulk samples are considered at this early stage of the research to be the preferred form of bulk superconducting material for magnetic levitation applications. Researchers with an interest in applications, such as flying wheels, levitation platforms, Maglev systems and a variety of prototype demonstration devices can learn from the results of this research, in particular, which will enable shaping of magnetic field for better applied performance. We have also found that from extensive experiments that the aspect ratio of YBCO single grains pays an important role on determining the key applied properties, such as trapped field at the immediate surface and in the space between individual single grains. This observation has been extended to a stack of thin bulk samples, which exhibit improved trapped field in the resultant composite bulk compared to a monolithic bulk superconductor of the same diameter. Furthermore, we have demonstrated that this approach can be combined with the use of a tailored arrangement of smaller samples to fine-tune the resultant trapped flux profile to generate a flatter, more uniform field profile. This is of particular significance for a variety of applications, such as MRI based on bulk superconductors.
First Year Of Impact 2017
Sector Energy,Healthcare,Transport
Impact Types Societal

Title Data supporting "Flux jumps in bulk superconductors caused by pulsed magnetic fields" 
Description Original experimental DATA reported in paper "Flux jumps in bulk superconductors caused by pulsed magnetic fields" 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Title Research data supporting "A Novel Pre-sintering method for Growing Y-Ba-Cu-O Superconducting Single Grains from Raw Oxides" 
Description The raw data is extracted from XRD, trapped fields setup and levitation force measurement facilities. Data is all plotted in Origin. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Title Research data supporting "Quantification of the Level of Samarium/Barium Substitution in the Ag-Sm1+xBa2-xCu3O7-? System" 
Description Data for a to-be-published journal paper 'Quantification of the Level of Samarium/Barium Substitution in the Ag-Sm1+xBa2-xCu3O7-? System' 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Title Research data supporting: The effect of size and aspect ratio on the trapped field properties of single grain, Y-Ba-Cu-O bulk superconductors 
Description Bulk, single grain (RE)Ba2Cu3O7-? [(RE)BCO, where RE is a rare earth element or yttrium] high temperature superconductors (HTS) exhibit much potential for use in a variety of engineering applications due to their ability to trap large magnetic fields; the fields trapped can be up to 10 times greater than those generated by conventional magnets. Limitations on the maximum single grain which can be grown are a major obstacle to the further development of these materials. Indeed, multiple samples are often required to achieve the required superconducting properties in applications. The geometry of bulk (RE)BCO single grains samples play an important role in determining the superconducting properties of a given arrangement. In order to gain a better understanding of the full applications potential of bulk single grain superconductors three, relatively long, cylindrical YBCO single grains of different diameters were fabricated and their trapped field and total trapped flux measured at 77 K as a function of sample height. The effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established model. Conclusions based on the trapped field measurements on a variety of single grain samples are presented in this study and the possibilities of using assemblies of smaller samples for engineering devices, in particular, are discussed. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
Description Boeing-Cambridge collaboration 
Organisation Boeing
Country United States 
Sector Private 
PI Contribution The contribution was part of an on-going collaboration on the development of bulk superconductors for applications. We have transferred processing technology to Boeing and have provided bulk superconductors for incorporating in a flywheel energy storage device by Boeing.
Collaborator Contribution Boeing developed the flywheel system that incorporates our samples.
Impact The further development pf the Boeing energy storage flywheel.
Description Siemens 
Organisation Siemens AG
Department Siemens plc, Keele
Country United Kingdom 
Sector Private 
PI Contribution We provide samples for Siemens to do some measurements using their equipment.
Collaborator Contribution Siemens is interested in the applications of bulk materials in motors. Our research results will provide better understanding.
Impact multi-disciplinary
Start Year 2016