Search

The influence of porosity on the trapped field achievable in 22 YBCO bulk superconductors of different sizes has been investigated and evaluated using Spearman's correlation coefficient in order to explore the significance of these critical features in a statistically significant way. The diameter of the single grain YBCO bulk sample correlates strongly with trapped field, and significantly more so than the effect of porosity. Therefore, the direct effect of porosity on the average Jc was investigated as the main aim of this investigation. A correlation was found between the average Jc calculated from trapped field measured at the top and bottom of the single grain and the porosity of the sample. This observation suggests that any method of reducing the porosity may significantly improve Jc and trapped field.

The intrinsic mechanical properties of single‐grain RE‐Ba‐Cu‐O bulk high‐temperature superconductors can be improved by employing a thin‐wall geometry. This is where the samples are melt‐processed with a predefined network of artificial holes to decrease the effective wall thickness. In this study, the tensile strengths of thin‐wall YBCO disks were determined using the Brazilian test at room temperature. Compared with conventional single grain YBCO disks, the thin‐wall YBCO disks displayed an average tensile strength that is 93% higher when the holes were filled with Stycast epoxy resin. This implies a thin‐wall sample should, in theory, be able to sustain a trapped field that is 39% higher without exceeding the mechanical limit of the sample. High‐field magnetization experiments were performed by applying magnetization fields of up to 11.5 T, specifically to break the samples in order to verify the effect of increased mechanical strength (and improved cooling) on the ability of bulk (RE)BCO to trap field successfully. The standard YBCO sample failed when it was magnetized with a field of 10 T at 35 K, suffering permanent damage. As a result, the standard sample could only trap a maximum surface field of 7.6 T without failure. On the other hand, the thin‐wall YBCO sample survived all magnetization cycles, including a maximum magnetization field of 11.5 T at 35 K, demonstrating a greater intrinsic ability to withstand significantly higher electromagnetic stresses. By subsequently field‐cooling the thin‐wall sample with 11 T at 30 K, a surface field of 8.8 T was trapped successfully without requiring any external ring reinforcement.

The practical magnetization of arrays of multiple single grain, bulk high temperature superconductors is essential for practical applications, such as trapped flux rotating machines, magnetic resonance imaging and nuclear magnetic resonance. We report a systematic investigation of the pulsed field magnetization (PFM) of a bulk assembly consisting of two rectangular Y–Ba–Cu–O bulk single grains, in close proximity, at various temperatures. The measurements of the dynamic variation of the magnetic flux density, supported by numerical analysis, reveal that the induced screening currents during the rise of a pulsed field may greatly enhance the flux density in the region of the junction leading to uneven flux penetration and to an increased likelihood of flux jumps in this region. Such coupling between field and current promotes magnetic flux penetration and improves the peak trapped field from 3.01 T for a bulk single grain to 3.11 T for the bulk assembly at 30 K, improving the magnetization efficiency from 80% to 90%. The peak trapped field was further enhanced to 3.39 T and 3.31 T for the single bulk single grain and the bulk assembly, respectively, by employing a two-step multi-pulse PFM process.

Over the last years, advanced experiments under engine-representative conditions have been performed in the Oxford Turbine Research Facility (OTRF) to obtain high-quality aero-thermal data that are essential for developing novel cooling designs and for validating computational models. Thanks to its unique experimental capabilities, the OTRF plays a crucial role in the advancement of HP turbines, which are life limiting components of modern gas turbines due to the harsh high temperature environment. The OTRF continues to be developed for additional capability to achieve more engine-representative conditions. Major upgrades include: (i) a combustor simulator able to produce engine radial and circumferential temperature profiles and inlet swirl simultaneously to study combustor-turbine interaction, (ii) fully film-cooled HP turbine designs and 1.5 stage configurations with a low-turning strut, (iii) novel rotor casing effusion cooling concepts, (iv) an advanced in-shaft data acquisition electronics system. More recently, the enhanced capabilities of additive manufacturing have been used in the facility to investigate the performance of novel blade tips and full annular effusion cooling concepts. This allowed complex cooling features and geometries to be implemented and assessed in the facility. The aero-thermal performance of cooled and uncooled blades for three variants of squealer tip design and a next-generation effusion cooling designs have been recently evaluated.

In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared with those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of finite element numerical modeling carried out either in two dimensions (for the axial configuration) or three dimensions (for the transverse configuration). In the axial configuration, the results show that the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads only to a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e., made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3-D numerical modeling.

High temperature superconducting (HTS) maglev technology has attracted considerable attention from researchers around the world. It provides a new direction of development for urban rail and high-speed rail transit due to its considerable advantages of frictionless transportation, low noise, and environmental protection. HTS bulk materials are able to provide a significant performance advantage to maglev technology and are a key component to achieving both levitation and vehicle guidance. HTS bulk samples with enhanced flux pinning characteristics can enable improved curve negotiating ability and lateral stability for the maglev vehicle. This paper reports the results of experiments on the dynamic guidance force of single grain Y–Ba–Cu–O (YBCO) and Gd–Ba–Cu–O (GdBCO) bulk HTS single grains in order to explore their potential for enhanced maglev guidance performance. A customized HTS maglev dynamic measurement system (SCML-03) with a rotating, circular permanent magnet guideway (PMG) was employed to simulate the dynamic operation of the HTS vehicle above the PMG at different translational speeds. It was observed from the experimental results that the GdBCO bulk superconductor is more able to resist the attenuation of the dynamic guidance force compared to YBCO under the same operational conditions. In addition, the GdBCO bulk single grain is more able to return to the original equilibrium position following exposure to an external displacement. It is concluded that the GdBCO bulk single grain offers greater potential than YBCO for practical application in HTS maglev given its enhanced guidance performance. The results of these tests provide an important benchmark for the future design of HTS maglev transportation systems.

The success rate of the growth of single grain Y–Ba–Cu–O (YBCO) bulk superconductors, applied typically as trapped field magnets, by melt processing techniques is often rather low. To ensure that the rare earth elements used in these materials are not wasted, and to improve the economics of production, therefore, an effective way of recycling such “failed” samples is essential for the long-term sustainability of the bulk process. In the present paper, a detailed study of the microstructural and mechanical properties of recycled YBCO samples has been made and their properties compared to those of primary grown samples. Recycled YBCO samples exhibit trapped magnetic fields of ∼ 70 – 80 % of those observed in primary grown samples. Microstructural investigations reveal a significant reduction in the porosity and a simultaneous improvement in the distribution of Y2BaCuO5 second phase inclusions throughout the microstructure of recycled, single grain YBCO samples. This means that, while the superconducting properties of the recycled samples are generally inferior to those of primary grown samples, somewhat surprisingly the mechanical properties such as flexural strength, hardness and tensile strength of the recycled samples can be improved. The recycled YBCO samples studied here exhibited an average flexural strength of 75 MPa, which is almost 50 % higher compared to primary grown YBCO, which has significant implications for their use in practical applications that involve large mechanical forces, such as levitation. Good correlations were observed between the microstructural parameters and measured mechanical properties in both primary grown and recycled YBCO samples.

The production of large (RE)Ba–Cu–O single grains ((RE)BCO), where RE = Y, Gd or Sm, of complex geometries is presently limited by the intrinsic slowness of the grain growth process. Additionally, the shapes achievable using established melt processing are constrained by the small number of post-processing techniques available. These materials are brittle and hard, which makes machining a difficult task and largely eliminates the possibility of producing highly intricate shapes. An alternative to this slow and inflexible growth process would be to join many small single grains to form one large composite grain, connected by high-performance superconducting joints. A reliable joining technique would also overcome the need for the careful and time-consuming post-growth machining processes. In this work we report on the use of single grain YBCO–Ag as an interface medium to achieve superconducting joints between (RE)BCO bulks. This joining technique is relatively quick and does not require tight process parameter control as there is no need to re-grow the interface joining material. We report on six joints produced from samples cut and joined in a variety of orientations. In addition, a joint was produced using bulk YBCO from two independent single grains. The trapped field properties of the resulting joined sample were measured and the microstructure at the joint was examined. We show that this simple but effective joining technique makes it possible to produce multiple composite grains with comparable superconducting properties to those of a single grain of the same size.

Funder: King Abdulaziz City for Science and Technology; doi: http://dx.doi.org/10.13039/501100004919, Bulk high temperature superconductors based on the rare-earth copper oxides can be used effectively as trapped field magnets capable of generating large magnetic fields. The top-seeded infiltration growth (TSIG) processing technique can provide a more homogeneous microstructure and therefore more uniform superconducting properties than samples grown using conventional melt growth processes. In the present investigation, the properties of bulk, single grain superconductors processed by TSIG and magnetised by the pulsed-field magnetisation technique using a copper-wound solenoid have been studied. A trapped field of ∼3 T has been achieved in a 2-step buffer-assisted TSIG-processed Y-Ba-Cu-O (YBCO) sample at 40 K by magnetising the bulk superconductor completely via a single-pulse magnetisation process. Samples were also subjected to pulsed-field magnetisation at 65 K and by conventional field-cooled magnetisation at 77 K for comparison. Good correlation was observed between the microstructures, critical current densities and trapped field performance of bulk samples fabricated by TSIG and magnetised by pulsed-field and field-cooled magnetisation. The homogeneous distribution of Y2BaCuO5 inclusions within the microstructure of bulk YBCO samples fabricated by the 2-step buffer-assisted TSIG process reduces inhomogeneous flux penetration into the interior of the sample. This, in turn, results in a lower temperature rise of the bulk superconductor during the pulsed-field magnetisation process and a more effective and reliable magnetisation process.

By enhancing the premixing of fuel and air prior to combustion, recently developed lean-burn combustor systems have led to reduced NOx and particulate emissions in gas turbines. Lean-burn combustor exit flows are typically characterized by nonuniformities in total temperature, or so-called hot-streaks, swirling velocity profiles, and high turbulence intensity. While these systems improve combustor performance, the exiting flow-field presents significant challenges to the aerothermal performance of the downstream turbine. This paper presents the commissioning of a new fully annular lean-burn combustor simulator for use in the Oxford Turbine Research Facility (OTRF), a transonic rotating facility capable of matching nondimensional engine conditions. The combustor simulator can deliver engine-representative turbine inlet conditions featuring swirl and hot-streaks either separately or simultaneously. To the best of our knowledge, this simulator is the first of its kind to be implemented in a rotating turbine test facility.The combustor simulator was experimentally commissioned in two stages. The first stage of commissioning experiments was conducted using a bespoke facility exhausting to atmospheric conditions (Hall and Povey, 2015, “Experimental Study of Non-Reacting Low NOx Combustor Simulator for Scaled Turbine Experiments,” ASME Paper No. GT2015-43530.) and included area surveys of the generated temperature and swirl profiles. The survey data confirmed that the simulator performed as designed, reproducing the key features of a lean-burn combustor. However, due to the hot and cold air mixing process occurring at lower Reynolds number in the facility, there was uncertainty concerning the degree to which the measured temperature profile represented that in OTRF. The second stage of commissioning experiments was conducted with the simulator installed in the OTRF. Measurements of the total temperature field at turbine inlet and of the high-pressure (HP) nozzle guide vane (NGV) loading distributions were obtained and compared to measurements with uniform inlet conditions. The experimental survey results were compared to unsteady numerical predictions of the simulator at both atmospheric and OTRF conditions. A high level of agreement was demonstrated, indicating that the Reynolds number effects associated with the change to OTRF conditions were small. Finally, data from the atmospheric test facility and the OTRF were combined with the numerical predictions to provide an inlet boundary condition for numerical simulation of the test turbine stage. The NGV loading measurements show good agreement with the numerical predictions, providing validation of the stage inlet boundary condition imposed. The successful commissioning of the simulator in the OTRF will enable future experimental studies of lean-burn combustor–turbine interaction.

The finite element method has been used extensively in recent years to solve various problems related to applied superconductivity and provides a useful tool for analysing and predicting experimental results. Based on a recently-developed modelling framework, implemented in the finite element software package COMSOL Multiphysics, investigations on the minimum ring reinforcement required to prevent mechanical failure in bulk high-temperature superconducting magnets have been carried out. Assuming homogeneous Jc (B,T) across the bulk sample irrespective of its dimensions, the maximum magnetic stresses experienced, and the minimum ring thickness required to prevent the hoop and radial stresses from exceeding the tensile strength of the bulk superconductor have been determined for varying values of the Young’s modulus, radius, height and temperature of a representative single-grain Ag-containing Gd-Ba-Cu-O bulk sample. This comprehensive analysis details the influence each of these key parameters has on the magnetic stress and hence their impact on the necessary ring thickness to prevent mechanical failure in any given system, i.e., for any combination of material properties and sample dimensions.

Gas turbine engine efficiency and reliability is generally improved through better understanding and improvements to the design of individual components. The life limiting component of the modern gas turbine is the high pressure (HP) turbine stage due to the arduous environment. Over the last 50 years significant research effort has been focused on advancing blade cooling designs and materials. Due to practical limitations little fundamental research on the turbine system is performed in the operating gas turbine engine. Consequently different types of experimental approaches have been developed over the last 4 decades to study the flow and in particular the heat transfer and cooling in turbines. In general the facilities can be divided into continuous running or short duration and cascade or rotating. Over the last 30 years short duration facilities have dominated the research in the study of turbine heat transfer and cooling. The Oxford Turbine Research Facility (formerly known as the QinetiQ Turbine Test Facility, The Isentropic Light Piston Facility and The Isentropic Light Piston Cascade) is a short duration facility developed and built in the late 1970s and early 1980s for turbine heat transfer and cooling studies. This paper presents the developments and measurements taken on the facility over the last 35 years, including the type of research that has been conducted and, the current capability of the facility.

The magnitude of the maximum trapped magnetic field in a bulk, single-grain superconductor is a key performance figure of merit. This is determined, generally, by the magnitude of the critical current density, J c , and the length scale over which it flows. As with all type-II superconductors, J c is related closely to the microstructure of the superconducting material and, in the case of RE–Ba–Cu–O [(RE)BCO, where RE is a rare-earth element or yttrium] single grains, RE2BaCuO5 (RE-211) inclusions in the superconducting REBa2Cu3O7−δ (RE-123) phase matrix are key microstructural features that act effectively as flux pinning centres. Although the distribution of RE-211 in single-grain bulk superconductors has been studied extensively, the variation of J c within a given sample has been much investigated much less thoroughly. A detailed experimental understanding of the variation of J c in these technologically important materials, therefore, is required given the growing popularity and significance of numerical techniques for modelling the behaviour of type-II bulk superconductors. Here we report a systematic investigation of the correlation between Gd-211 particle density and sample porosity, which are microstructural features, and T c and J c in a Gd–Ba–Cu–O bulk, single grain fabricated using a buffer layer and a supply of additional liquid phase. This was performed by cutting the sample into numerous sub-specimens of approximate dimensions 1.8 × 2.8 × 1.5 mm3. We observe that J c decreases with distance from the seed, although more strongly with distance along the c-axis than along the a–b plane. In contrast to what might be expected given the assumed contribution of RE-211 inclusions to flux pinning, we find no evidence of a clear correlation between the local RE-211 precipitate density and local critical current on a length scale of mm. We observe that the porosity of the sample is a more dominant factor in determining the distribution of J c within a single grain.

GdBa 2 Cu 3 O 7-δ bulk superconductors are a route to higher magnetic fields in rotating machines. Here, we examine the resistance of pulse-field magnetized bulks to the demagnetization fields they may experience in such a system. The bulks were magnetized at 77 K, after which several thousand cycles of AC field were applied. Subsequently, the decay of the trapped field was characterized. We found that the decay per cycle decreases with frequency and is, normalized to the initial trapped field, largest at the edge of the bulk. At 77 K, the reduction in trapped field proved significant (25% in the center for 150 mT (peak) AC field at 6 Hz), however, reducing below 1% when lowering the temperature to 60 K. We explain this observation as being due to increased flux pinning strength at low temperatures. When applying an AC field, we found a temperature rise that increased with applied field amplitude and frequency. However, when applying an AC field of amplitude 45 mT with a frequency of 48 Hz, we found an increase of the bulk temperature of only 100 mK. Therefore, we conclude that the temperature rise within the analyzed AC field frequency and amplitude range does not contribute significantly to the decay of trapped field.

In order to determine the spatial distribution of the flexural strength, three-point bend tests have been performed on bar specimens cut from four YBa2Cu3O7− δ single-grain bulk superconductors. A relatively large spread of the room temperature flexural strength, average and standard deviation of 49.3 MPa and 12.7 MPa, respectively, was observed across the four bulk samples. This is attributed to the systemic microstructural variation introduced by the seeded melt growth. In particular, the strength was shown to be related to the local porosity and Y2BaCuO5 content. To further examine bulk-to-bulk variability, indirect tensile (Brazilian) tests were conducted on twelve 16-mm-diameter bulk superconductors from three production batches. The Weibull modulus for these was calculated to be 8.76, suggesting that, despite the large spread in strength with position within a bulk, the batch processing of bulk superconductors can consistently yield samples with comparable overall mechanical strengths.

In principle, high-temperature superconducting (HTS) flux-pinning maglev has the advantage of self-stable levitation and is therefore a good development prospect as a new rail transit technology. The magnetic field intensity is one of the key factors affecting the levitation performance of single-grain bulk superconductors in the HTS maglev system. To date, however, most researchers have focused on the levitation performance of HTS bulk superconductors in magnetic fields within a permanent magnet guideway (PMG) configuration. The magnitude of the external working magnetic field provided by a PMG is limited to about 0.5 T at 77 K, given that the superconductor usually levitates at a typical working height of 10 mm above the PMG. Therefore, the weak magnetic fields generated by the PMG significantly limits the levitation performance of HTS maglev systems to that of the permanent magnet (PM). In this article, a superconducting magnet test bench was utilized to provide a 0–3 T external magnetic field (B) environment for the study of the levitation force in HTS single grains. The difference in levitation performance between YBaCuO (YBCO) and GdBaCuO-Ag (GdBCO-Ag) bulk superconductors under large, changing magnetic fields was studied systematically. The relationships between levitation force and magnetic field characteristics, including vertical magnetic field component (Bz), transverse magnetic field component (Br), and vertical magnetic field variation (ΔBz) were investigated. The result of variation curves about YBCO and GdBCO bulks' levitation forces under different ΔBz shows that the levitation force of the bulk, single-grain superconductors will reach saturation under a varying ΔBz environment (0.72 and 1.5 T). The values of ΔBz corresponding to the saturation positions are found to be independent of the sizes of the HTS samples. At the same time, GdBCO-Ag exhibits a levitation force that is 112.8% greater than YBCO in B = 3 T magnetic environment. As a result, it is demonstrated that the levitation potential of single-grain HTS bulks can be better exploited by optimizing the distribution of the magnetic field.

REBa 2 Cu 3 O 7-δ bulk superconductors are able to trap large magnetic fields and as a result they have potential to work as strong magnets. In this study, highly textured yttrium, gadolinium, and samarium based cuprate bulk superconductors were prepared using conventional top-seeded melt-textured growth, and magnetized by employing both field-cooling magnetization (FCM) and pulsed-field magnetization (PFM) techniques at 77 K. The same peak value of a trapped field was achieved using FCM and PFM, which means all the bulk samples were fully magnetized by using the PFM method. In addition, the magnitude of the pulsed field required to fully magnetize the bulk samples varied with rare-earth element in the (RE)BCO formulation.

The widespread use of ceramic (RE)Ba 2 Cu 3 O 7 bulk superconductors (RE-123), where RE = Y, Gd, or Sm, is generally hindered by their poor mechanical properties. While a large number of techniques can be used to improve the mechanical properties of conventional ceramic materials, many of these are incompatible with the growth of single grain, bulk RE-123 superconductors using the top seeded melt growth (TSMG) process. Complications arise due to the need to minimize the degradation of the superconducting properties and produce a single-grained sample. Nonetheless, the addition of Ag to RE-Ba-Cu-O [(RE)BCO] precursor powder has been demonstrated to be effective in improving the mechanical properties of single grain bulk superconductors fabricated by TSMG without deleterious effects on performance. Although large single grains of GdBa 2 Cu 3 O 7 -Ag and SmBa 2 Cu 3 O 7 -Ag have been successfully and reliably grown, it has proven more difficult to grow large single grains of YBa 2 Cu 3 O 7 -Ag. We recently demonstrated the growth of single grain YBa 2 Cu 3 O 7 -Ag bulk superconductors that exhibit relatively good superconducting properties. In this work, we report the flexural stress at a number of locations within YBa 2 Cu 3 O 7 single grains grown by TSMG with and without additional liquid phase and with silver addition. In addition, we have compared the distribution of the failure stress with the pore and silver distribution.

Single domain YBCO bulk superconductors were prepared using a conventional top-seeded melt growth (TSMG) technique. Artificial holes were introduced to the green sample prior to thermal processing using a bespoke “spiked” mould. Me- chanical properties such as elastic modulus, Vickers hardness, compressive strength and tensile strength were measured and compared to the properties of a standard bulk. The presence of the holes the bulk microstructure was observed to limit porosity and lower the concentration of macro-cracks in the bulk microstruc- ture, resulting in significantly enhanced mechanical properties of the bulk single grains. The elastic modulus of the perforated bulks was observed to exhibit an increase of more than 45% compared to the standard samples. Compressive and tensile strengths were also improved significantly in the samples containing artificial holes. Observed differences in Vickers hardness, on the other hand, were negligible. This could be due by the fact that the hard- ness is measured on a small surface area of the single grain sample, where the effect of lower porosity and lower concentration of macro-cracks is less relevant. The introduction of artificial holes to the bulk, single grain microstructure appears to be a very prom- ising technology for the production of melt-textured bulk super- conductors with enhanced mechanical properties.

In this work, a series of melt processed Y Ba Cu O (YBCO) single-grains have been fabricated by the top-seeded melt growth (TSMG) technique. The melt processing is accepted widely as an effective way to grow bulk, single grain YBCO superconductors; however, this process is extremely complex and every step can affect the final properties of prepared bulk. Therefore, the impact of precursor powder preparation and growth conditions was studied for the first time. Cost-effective in-house made powder and commercially available precursor powders were employed and samples were grown employing top seeded melt growth, following the isothermal and the under-cooling growth techniques. The bulk microstructure including Y2BaCuO5 (Y-211) particle size and distribution, superconducting properties (Tc, Jc) and field trapping potential were investigated. The cost-effective high performance batch processing methodology was optimized. The fabricated YBCO bulks (diameter of 28 mm) exhibited average trapped field of 0.85 T at 77 K. Furthermore, other possibilities to achieve advancement in processing (RE)BCO bulks, are proposed.

The magnetic field trapping capability of a bulk superconductor is essentially determined by the critical current density, J c (B, T), of the material. With state-of-the-art bulk (RE)BCO (where RE = rare earth or Y) materials it is clear that trapped fields of over 20 T are potentially achievable. However, the large Lorentz forces, F L = J × B , that develop during magnetisation of the sample lead to large mechanical stresses that can result in mechanical failure. The radial forces are tensile and the resulting stresses are not resisted well because of the brittle ceramic nature of (RE)BCO materials. Where fields of more than 17 T have been achieved, the samples were reinforced mechanically using resin impregnation and carbon-fibre wrapping or shrink-fit stainless steel. In this paper, two-dimensional (2D) axisymmetric and three-dimensional (3D) finite-element models based on the H -formulation, implemented in the commercial finite element software package COMSOL Multiphysics, are used to provide a comprehensive picture of the mechanical stresses in bulk superconductor magnets with and without mechanical reinforcement during field-cooled magnetisation. The chosen modelling framework couples together electromagnetic, thermal and structural mechanics models, and is extremely flexible in allowing the inclusion of various magnetisation processes and conditions, as well as detailed and realistic properties of the materials involved. The 2D model - a faster route to parametric optimisation - is firstly used to investigate the influence of the ramp rate of the applied field and any heat generated in the bulk. Finally, the 3D model is used to investigate the influence of inhomogeneous J c (B, T) properties around the ab-plane of the bulk superconductor on the developed mechanical stress.

Under the EU LEMOCTEC programme, the Oxford Turbine Research Facility (OTRF) was upgraded to include a modern 1½ stage, high-pressure turbine with film cooled high-pressure guide vanes (HPVs) and low-turning intermediate pressure vanes (IPVs). The facility has also been upgraded to include a third-generation engine-representative combustor temperature and swirl simulator at inlet, allowing the study of turbine interactions with inlet conditions representative of a modern lean burn combustor. This paper presents the aerodynamic and mechanical design of the LEMCOTEC high-pressure turbine and its integration and commissioning in the OTRF. Test data with uniform inlet flow is presented, acting as a baseline to assess the performance benefit of optimising the turbine design for a non-uniform combustor exit flow-field. Measurement techniques are discussed, and experimental data is compared to pre-test design CFD results from both the Rolls-Royce HYDRA code and the commercial CFX code.

A robust and reliable in-situ magnetization method is essential for exploiting the outstanding magnetic flux trapping ability of bulk superconductors in practical applications. We report a 4.8 T peak trapped magnetic field, B T, achieved at 30 K in a 36 mm diameter GdBa2Cu3O7-δ –Ag bulk superconductor using pulsed field magnetization (PFM). To realize this, we have developed a reliable two-step multi-pulse PFM process based on understanding and exploiting the avalanche-like flux jump phenomenon observed in these materials. The magnitude of the applied pulsed magnetic field (B a) necessary to trap 4.8 T was merely 5.29 T, corresponding to a remarkable magnetization efficiency (B T/B a) of 90%.

© 2016 American Chemical Society.Single grain, rare earth-barium-copper oxide [(RE)BCO] bulk superconductors, fabricated either individually or assembled in large or complicated geometries, have a significant potential for a variety of potential engineering applications. Unfortunately, (RE)BCO single grains have intrinsically very low growth rates, which limits the sample size that may be achieved in a practical, top seeded melt growth process. As a result, a melt process based on the use of two or more seeds (so-called multiseeding) to control the nucleation and subsequent growth of bulk (RE)BCO superconductors has been developed to fabricate larger samples and to reduce the time taken for the melt process. However, the formation of regions that contain non-superconducting phases at grain boundaries has emerged as an unavoidable consequence of this process. This leads to the multiseeded sample behaving as if it is composed of multiple, singly seeded regions. In this work we have examined the factors that lead to the accumulation of non-superconducting phases at grain boundaries in multiseeded (RE)BCO bulk samples. We have studied the microstructure and superconducting properties of a number of samples fabricated by the multiseeded process to explore how the severity of this problem can be reduced significantly, if not eliminated completely. We conclude that, by employing the techniques described, multiseeding is a practical approach to the processing of large high performance superconducting bulk samples for engineering applications.

The critical-current density Jc of three Y-Ba-Cu-O (YBCO) discs, each cut from the upper section of a melt-processed single grain, has been determined as a function of the depth from the top (seeded) and bottom surfaces of the sample by a modified version of the inductive third-harmonic technique proposed originally by Mawatari et al. It is shown that local Jc in the vicinity of the bottom surface of the sample is lower than that in the vicinity of the top surface for thicker discs and there are reduced effect of surface barrier and intrinsic pinning and important surface damage. The technique employed is recommended as a tool for detecting the imperfection within superconducting structure in bulk YBCO.

Large, single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials. Large, single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials.

We acknowledge the Engineering and Physical Sciences Research Council (EPSRC grant ref. EP/K02910X/1) for financial support.

The characterization of the critical current density of bulk high-temperature superconductors is typically performed using magnetometry, which involves numerous assumptions, including, significantly, that $J_{{c}}$ within the sample is uniform. Unfortunately, magnetometry is particularly challenging to apply where a local measurement of $J_{{c}}$ across a feature, such as a grain boundary, is desired. Although transport measurements appear to be an attractive alternative to magnetization, it is extremely challenging to reduce the cross-sectional area of a bulk sample sufficiently to achieve a sufficiently low critical current that can be generated by a practical current source. In the work described here, we present a technique that enables transport measurements to be performed on sections of bulk superconductors. Metallographic techniques and resin reinforcement were used to create an I-shaped sample of bulk superconductor from a section of Gd–Ba–Cu–O containing 15 wt % Ag2O. The resulting superconducting track had a cross-sectional area of 0.44 mm2. The sample was found to support a critical current of 110 A using a field criterion in the narrowed track region of 1 $\mu\text{V cm}^{-1}$ . We conclude, therefore, that it is possible to measure critical current densities in excess of $2.5 \times 10^{8}\ \text{A m}^{-2}$ in sections of a bulk superconductor.

The superconducting properties, melting temperatures and crystal growth rates of single grain, RE–Ba–Cu–O [(RE)BCO] bulk superconductors (where RE = a rare earth element or yttrium) decrease with the RE-element sequence of Nd, Sm, Eu, Gd, Dy and Y. The mechanical properties of these technologically important materials, on the other hand, however, improve in the same sequence. Consequently, one promising approach for optimising the balance between mechanical and superconducting properties of bulk (RE)BCO superconductors, or for adjusting growth rate, is the use of combinations of different rare earth elements. In this study, we explore combinations of Gd and Y in the formation of (Gd–Y)–Ba–Cu–O single grains. We describe the optimisation of the growth process for this multi-RE element system and use optical and scanning electron microscopy to study the microstructure of both non-superconducting (Gd–Y)2BaCuO5 [(Y–Gd)-211] phase inclusions and the (Y–Gd)Ba2Cu3O7-δ [(Y–Gd)-123] phase matrix itself. We demonstrate that (Gd–Y)–Ba–Cu–O single grains can be fabricated reliably and that they exhibit reasonably good superconducting properties. We observe that there is an increase in RE-211 particle size in this mixed rare earth system, which, ultimately, limits sample performance, and conclude that this may be a general disadvantage of this approach to the synthesis of single grains for high field engineering applications.

Bulk (RE)BCO, where RE is a rare-earth element or yttrium, superconductors fabricated in the form of rings are potentially useful for a variety of solenoidal-type applications, such as small, high field nuclear magnetic resonance and electromagnetic undulators. It is anticipated that the practical exploitation of these technologically important materials will involve pulse field magnetization (PFM) and, consequently, it is important to understand the behavior of ring-shaped samples subjected to the PFM process. Macroscopic flux jumps were observed in PFM experiments on ring-shaped bulk samples when the peak applied field reaches a threshold magnitude, similar to behavior reported previously in cylindrical samples. Magnetic flux jumps inward when the thermal instability is triggered, however it subsequently flows outwards from the sample, resulting in a relatively low trapped field. This behavior is attributed to a variety of effects, including the inhomogeneity of the material, which may lead to the formation of localized hot spots during the PFM process. In order to further elucidate this phenomena, the properties of a structure consisting of a bulk superconducting ring with a cylindrical superconductor core were studied. We observe that, although a flux jump occurs consistently in the ring, a critical state is established at the boundary of the ring-shaped sample and the core. We provide a detailed account of these experimental observations and provide an explanation in terms of the current understanding of the PFM process.

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ~ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-{\delta} bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique., Comment: Accepted revised version for Superconductor Science and Technology

The widespread use of melt processed, single grain (RE)–Ba–Cu–O bulk superconductors [(RE)BCO], where RE = Y, Gd or Sm, is limited predominantly by the poor mechanical properties of these inherently brittle, ceramic-like materials. The high density of flaws, such as cracks and voids, within the single grain microstructure leads directly to a low fracture toughness. As a result, the Lorentz forces, generated when these materials carry current in the presence of a large magnetic field, create stresses sufficiently large to cause brittle failure. The addition of Ba–Cu–O (liquid phase) and Ag to the precursor composition prior to melt-growth has been demonstrated to be effective in improving the mechanical properties of these technologically important materials. In this work, we characterise the mechanical failure of single grain YBCO bulk superconductors in terms of a Weibull statistical distribution. In addition, differences in fracture mechanisms have been studied to provide a better understanding of how the provision of additional liquid phase and silver produces YBCO single grains with better resulting mechanical properties and how these can be improved further.

The flux-pinning landscape in type-II superconductors determines the response of the flux line lattice to changing magnetic fields. Typically, the flux vortex behaviour is hysteretic and well described within the framework of the Bean critical-state model and its extensions. However, if the changing magnetic field does not move the flux vortices from their pinning sites, their response remains linear and reversible. The vortex displacement, then, is characterised by the Campbell penetration depth, which itself is related directly to the effective size of the pinning potential. Here, we present measurements of the Campbell penetration depth (and the effective size of the pinning potential) as a function of magnetic field in a single-grain bulk GdBa2Cu3O 7 − δ superconductor using a pick-up coil method. Hence, the hysteretic losses, which take into account the reversible vortex movement, are established.

The levitation force between a single grain YBCO cylindrical bulk superconductor and a permanent magnet is investigated. The force measured both in zero field cooling and field cooling condition in the temperature range 20 K - 80 K is reported. The dependence of peak levitation force and power dissipation on the temperature is analyzed and the saturation of the peak force with increasing the critical current of the material is pointed out. A numerical analysis is also carried out for understanding the electrodynamics of the levitation mechanism. Good reproduction of the measured data is obtained and relevant observed phenomena, such as hysteresis of the levitation cycle at high temperature and saturation of the peak force at low temperature, are explained by means of the model. Characterization of the material in terms of critical current density in the field range 0 T - 0.5 T is also obtained by means of the numerical model by means of the fitting of the measured data.

© Published under licence by IOP Publishing Ltd. The large scale use of (RE)Ba2Cu3O7 bulk superconductors, where RE=Y, Gd, Sm, is, in part, limited by the relatively poor mechanical properties of these inherently brittle ceramic materials. It is reported that alloying of (RE)Ba2Cu3O7 with silver enables a significant improvement in the mechanical strength of bulk, single grain samples without any detrimental effect on their superconducting properties. However, due to the complexity and number of inter-related variables involved in the top seeded melt growth (TSMG) process, the growth of large single grains is difficult and the addition of silver makes it even more difficult to achieve successful growth reliably. The key processing variables in the TSMG process include the times and temperatures of the stages within the heating profile, which can be derived from the growth rate during the growth process. To date, the growth rate of the YBa2Cu3O7-Ag system has not been reported in detail and it is this lacuna that we have sought to address. In this work we measure the growth rate of the YBCO-Ag system using a method based on continuous cooling and isothermal holding (CCIH). We have determined the growth rate by measuring the side length of the crystallised region for a number of samples for specified isothermal hold temperatures and periods. This has enabled the growth rate to be modelled and from this an optimized heating profile for the successful growth of YBCO-Ag single grains to be derived.

Pulsed field magnetisation (PFM) is the most practical method for magnetising bulk superconducting materials as trapped field magnets (TFMs), but the record trapped field achieved by PFM to date is still significantly less than the true trapped field capability of these materials. In this paper, a flexible numerical modelling technique based on the finite element method is used to provide a comprehensive and realistic picture of multi-pulse PFM, which has been shown to be effective in increasing the trapped field/flux over a single pulse. Firstly, the maximum trapped field capability of a representative sample is determined using two types of numerical model simulating field-cooling (FC) and zero-field-cooling (ZFC) magnetisation. Next, various sets of magnetic field pulses are applied to the bulk to analyse multi-pulse PFM. An increase in the trapped field can be achieved after a 2nd pulse and to do so an increased amplitude of applied field is required to maximize the trapped field fully. The numerical analysis shows that this occurs in subsequent pulses because it is more difficult for the magnetic flux to penetrate the sample and there is a lower temperature rise.

Superconducting MgB2 has been synthesized successfully by a modified infiltration and growth (IG) technique. The ambient pressure technique is relatively simple and scalable to complex shaped bulks. The extent of MgB 2 phase formation has been found to be influenced strongly by the IG process time and/or temperature, and this is found to reflect in the X-ray diffraction patterns, magnetization measurements, and microhardness. Scanning electron microscopy images show a bimodal particle size distribution with 20–50 nm sized fine precipitates in the inter particle region. A critical current density of 400 $\mbox{kA}\cdot\mbox{cm}^{-2}$ was measured at 5 K.

Large, single-grain bulk high-temperature superconductors (HTS) can trap magnetic fields over 17 T below 30 K and up to 3 T at 77 K, and have significant potential to replace permanent magnets, the fields from which are limited to significantly less than 2 T. Therefore, bulk HTS samples are ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. In electric machines, in particular, the higher flux density improves the power density of the machine, resulting in smaller, lighter devices. However, in a real electric machine environment, bulk HTS samples can be exposed to AC magnetic field fluctuations that can affect the distribution of the supercurrent in the material and attenuate the trapped field, leading to a reduction in the magnetic loading of the machine, and in some cases, full demagnetisation. In this paper, the variation of trapped field with the frequency and magnitude of an external time-varying magnetic field is analysed numerically, and the mechanisms of the attenuation of the trapped field in HTS bulks are investigated using a two-dimensional (2D) axisymmetric finite-element model based on the H-formulation, considering both the electromagnetic and thermal behaviour of the bulk sample.

In this paper single domain, bulk SmBCO samples have been fabricated successfully in air in a top-seeded melt growth (TSMG) process using a conventional chamber furnace, in addition to samples doped with ZrO 2 . To improve the reliability of seeding, Sm-123 + Sm-211 mixed-powder buffers were used to increase the success of the SmBCO single grain growth process. SmBCO single grains of diameter as large as 20 mm and 10 mm in thickness with and without ZrO 2 were fabricated successfully using Sm-123 + Sm 2 BaCuO 5 (Sm-211) mixed-powder buffer layers. The geometric configurations of the buffers were also optimized as part of this study to further increase the success of the single grain growth process. Superconducting properties of J C and T C of the specimens under the buffer layer have also been investigated.

The multi-seeding process has the potential to enlarge the sample size of (RE)BCO (where RE = rare earth or Y) single-grain, bulk superconductors with improved fabrication speed, and in previous studies on multi-seeding, a significant improvement was made in the alignment of the seeds in such samples using a novel bridge-seeding technique. In this paper, we report the experimental measurements of the pulsed field magnetization (PFM) of a 0°-0° bridge-seeded Y-Ba-Cu-O (YBCO) sample. The PFM is carried out using a solenoid coil, as well as a split coil arrangement with an iron yoke, at temperature of 65, 40 and 20 K, and the resultant trapped fields and magnetic flux dynamics for these two PFM techniques are compared. It is shown that such bridge-seeded bulk YBCO can be fabricated that performs as a bulk magnet with trapped fields comparable to or better than standard, single-seeded high-$J_\text{c}$ samples, with the potential of enlarging the sample size. Furthermore, the split coil arrangement with an iron yoke is useful to enhance the trapped field and has a positive effect on the maximum temperature rise in the sample, which increases at lower temperatures and seriously impacts the achievable trapped field from PFM.