Iron-based superconductors have attractive properties for high-field applications, but there is a lack of understanding of the effect of grain boundary chemistry on the in-field performance. The near atomic-scale resolution, ppm sensitivity and 3D analysis offered by atom probe tomography make it a powerful tool to investigate the nanoscale structure and chemistry of these defects in fine-grained K-doped BaFe2As2 samples. A computational method to systematically extract and compare the Gibbsian interfacial excess of chemical species across grain boundaries has been explored in this work. The robustness of the method has been tested by evaluating the effects of selected variables on simulated APT datasets. The accuracy and precision of the calculated Gibbsian interfacial excess were found to be stable over a range of analysis conditions: varying grain boundary widths and detection efficiencies, spatial precisions below 1.5 nm, and bin widths between 1.2 and 1.6 nm. For the K-doped BaFe2As2 samples studied, segregation of As, Ba, K and impurities of O, Na, and Sb were found at grain boundaries. The Gibbsian excess values were found to vary widely between different boundaries, showing the complexity of the grain boundary chemistry in this material. Possible links between the observed critical current density (Jc) of these samples and their nano-and micro-structure have also been investigated and discussed. (c) 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

A wide range of technologies rely on permanent magnets, including levitation devices, motors, generators and magnetic separators. Replacing permanent magnets with bulk superconductors will enable a step change in performance by providing an order of magnitude increase in the achievable magnetic field. However, the reliable fabrication of large single grained superconducting materials with a high, homogeneously distributed magnetic trapped field remains a barrier to the widespread application of these materials. Limits to the size and geometry of RE-Ba-Cu-O single grain bulk superconductors could be overcome by developing a reliable process to assemble larger components by fabricating superconducting joints between smaller samples. In this work we propose a mechanism of joint formation in GdBCO-Ag bulk superconductors using a YBCO-Ag intermediate that is based on detailed analysis of the joint interfaces. This improved understanding of the joint formation process provides the knowledge required to fully optimise the fabrication parameters, and to produce joints with improved superconducting and mechanical properties.

Superconducting magnets inside a fusion reactor will experience conditions that aren’t seen anywhere on Earth. Materials scientists Susie Speller and Chris Grovenor are trying to predict how long these components can last in this extreme environment.

MgB2 is a promising candidate for commercial superconducting applications because, as grain boundaries in MgB2 are not weak links, there are fewer limitations on the choice of processing technique compared to high-temperature superconducting (HTS) cuprates. MgB2 bulks are usually manufactured by powder processing techniques followed by a sintering process. After sintering, the impurity phases such as MgO and MgB4 along with porosity are formed which strongly affect the superconducting properties mainly the macroscopic path for supercurrent in MgB2 bulks. Investigation of these microstructural features is essential to improve the superconducting properties of these bulks. In this work, high-resolution laboratory X-ray computed tomography (XCT) has been used to investigate the microstructure of MgB2 bulks in three dimensions. The volume fraction of defects and impurity phases along with the size distribution of pores have been studied using this advanced technique. A comparison has been made between the data extracted from conventional characterization techniques such as XRD and SEM and those obtained from the advanced XCT analysis.

The UKAEA's Spherical Tokamak for Energy Production (STEP) programme aims to demonstrate the ability of low aspect ratio tokamaks to generate net electricity from deuterium-tritium (DT) fusion. As STEP have selected REBCO coated conductor (CC) as the current carrier in most magnet systems, understanding how REBCO CC responds to the energetic particle environment of a compact tokamak is crucial, especially given reported changes to the properties of REBCO following neutron irradiation. The STEP confinement system materials group has developed a plan to thoroughly test and validate the superconducting properties of REBCO under conditions as close as reasonably possible to those within STEP prior to its construction. Here our progress in carrying out this experimental plan is presented, followed by details of experiments still in development.

Energy production by nuclear fusion can be the breakthrough in the decarbonization process, and high temperature superconductors (HTSs) represent a game changer for the design of compact reactors. However, reduced size implies that the superconducting tapes will be exposed to an intense flux of neutrons and of secondary particles while carrying a high current; in order to employ HTS in compact fusion reactors it is therefore crucial to precisely assess the effects of irradiation on HTS tapes at the working conditions. To achieve this goal, researchers from different fields met at the irradiation effects on HTS for (IREF) fusion workshop to discuss all the aspects of this topic. This roadmap paper, that reflects the common view of the participants, aims at condensing the outcome of the intense and thorough discussion that took place during the conference, providing a path for the investigation of irradiation effects in HTS to assess their limits of operation in a fusion radiation environment.

Understanding irradiation damage of REBCO is increasingly of interest for compact tokamak fusion reactor designs, as these materials are critical for the proposed magnetic plasma confinement systems. Here commercially sourced samples of REBCO coated conductor are irradiated with 300 keV He+ ions to a damage level of 169 x 10-3 displacements-per-atom, to the point where superconductivity is no longer detectable, meaning these samples correspond to a non-functional end-of-life component in a fusion reactor context. Subsequent analysis of the crystal structure through a combination of x-ray diffraction and x-ray absorption spectroscopy measurements reveals a complex variation away from the as-grown structure. The local structure probed by the spectroscopy measurements is further observed to change as a function of the relative polarisation of the incident x-ray beam, indicating that within this damage regime the structural anisotropy of the REBCO unit cell plays a determining role in where defects accumulate within the material. Here the local structure measurements probing the a-b plane of the system vary significantly less than those probing the c-axis direction following irradiation, mirroring the observed trend in the x-ray diffraction data that the a:b ratio is preserved upon irradiation whilst the absolute values increase, whereas the c-axis parameter expands. These observations highlight the role of oxygen defect formation in driving the degradation of superconductivity within irradiated REBCO. These changes are observed to preferentially accumulate along the c-axis of the material, indicating a possible mechanistic signature of the degradation of the superconducting properties within these systems that are evident using a local structure probe such as extended x-ray absorption fine structure.

Ultra-low resistance joints are a key technology enabling superconducting magnets to operate in persistent mode and to achieve the temporal stability required for nuclear magnetic resonance and magnetic resonance imaging (MRI) applications. High performance superconducting joints are manufactured routinely for Nb–Ti and Nb 3 Sn magnets, but technologies for joining other technological superconductors are still in the early stages of development. Here we report the use of a simple cold pressing and heat treatment procedure to fabricate persistent MgB 2 joints with resistance values <10 −12 Ω between MgB 2 wires that have already undergone the full wire reaction process. Trapped persistent currents of 172 A and 160 A were achieved under self-field and 1 T background field conditions respectively at a temperature of 20 K. This corresponds to a critical current ratio of 78% under these conditions, outperforming previously reported joints using fully reacted MgB 2 wire. These findings are relevant for the development of commercial MRI magnets with MgB 2 wires utilizing react and wind methods.

Mg is used as the tracer element in the Li plating electrode to confirm that pure Li metal can plate in isolated pores near the solid electrolyte surface. This results in an internal pressure buildup and in the spallation of the LLZTO solid electrolyte, initiating the dendritic process.

Bi2Sr2CaCu2O8+x (Bi-2212) multifilamentary wire is the only high-temperature superconductor manufactured in the form of an isotropic round wire, and so offers a number of advantages for the designers of high field magnets. However, for high-field (>25 T), high-stability magnet applications, ultra-low resistance superconducting joints (R < 10(-12) Omega) will be needed to take advantage of the excellent properties of the Bi-2212 wire. This study focuses on the fabrication of compact melt processed joints in small coils of Bi-2212/Ag multifilamentary round wires and the testing of their superconducting performance by inductive resistance measurements. Microstructural analysis is carried out to correlate the microstructure to the superconducting performance of the joints. Our optimized technique led to a reliable process for the preparation of small coils with melt processed joints that occupy very small volumes but can still carry the highest persistent currents reported so far for Bi-2212.

The reactivity of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid electrolytes to form lithio-phobic species such as Li2CO3 on their surface when exposed to trace amounts of H2O and CO2 limits the progress of LLZTO-based solid-state batteries. Various treatments, such as annealing LLZTO within a glovebox or acid etching, aim at removing the surface contaminants, but a comprehensive understanding of the evolving LLZTO surface chemistry during and after these treatments is lacking. Here, glovebox-like H2O and CO2 conditions were recreated in a near ambient pressure X-ray photoelectron spectroscopy chamber to analyze the LLZTO surface under realistic conditions. We find that annealing LLZTO at 600 °C in this atmosphere effectively removes the surface contaminants, but a significant level of contamination reappears upon cooling down. In contrast, HCl(aq) acid etching demonstrates superior Li2CO3 removal and stable surface chemistry post treatment. To avoid air exposure during the acid treatment, an anhydrous HCl solution in diethyl ether was used directly within the glovebox. This novel acid etching strategy delivers the lowest lithium/LLZTO interfacial resistance and the highest critical current density.

The present work is focused on understanding the role of Ruthenium (Ru) and Yttrium (Y) on the superconducting properties of the A15 Nb3Sn intermetallic compound. Ru and Y shows different solubility limit within the Nb3Sn matrix. Ru and Y-doped Nb3Sn superconducting alloys were prepared through mechanical alloying (MA) route followed by sintering using Field Assisted Sintering Technique (FAST). Analysis by X-ray diffraction demonstrates a significant decrement in crystallite size up to 0.5 wt% for the Ru-doped Nb3Sn followed by a significant increment when doping level reaches 1 wt% Ru. This could be the combined effect of structural disorderliness and Ru dissolution within the Nb3Sn matrix. While a significant increase in the crystallite size is observed for 0.5 wt% Y-doped Nb3Sn and no further change up to 1 wt% Y. This change could be a result of limited solubility of Y within the Nb3Sn matrix. Meanwhile, excessive Y plays an important role in the formation of additional flux pinning centers. The critical current density (Jc) shows a significant increase for all the Y-doped Nb3Sn alloys by a factor of 29 % due to the formation of Y2O3 from the residual Y, which is assumed to serve as additional flux pinning center. The Y2O3 particles was also considered as a possible cause of pinning down of grain boundaries in addition to NbO leading to grain refinement through Zener pinning. Ru-doped Nb3Sn alloys also exhibit an increase in Jc compared to pristine Nb3Sn, but only in concentrations up to 0.5 wt% Ru. At higher applied field, even 1 wt% Ru shows slight increase in Jc than pristine Nb3Sn. Y-doped samples showed superior functional properties.

The UKAEA's Spherical Tokamak for Energy Production (STEP) program aims to demonstrate the ability of a low aspect ratio tokamak to generate net electricity from deuterium-tritium fusion. Specifically, its aim is to deliver a prototype fusion power plant, targeting the 2040s, and a path to the commercial viability of fusion, by engaging with and invigorating relevant industries and the supply chain. STEP will utilize REBCO coated conductors (CCs) as the current carrier in the bulk of its magnets. It has been recognized that neutron irradiation leads to the degradation of REBCO's superconducting properties and that this degradation will limit STEP's availability. However, current knowledge does not cover all the conditions that REBCO CCs will be subjected to while operating in STEP's magnets. Recent preliminary works have shown that these additional service conditions could exacerbate the degradation in REBCO's superconducting properties, and therefore, they each require further investigation. STEP's Confinement Systems' Materials group has developed a plan to characterize the superconducting properties of REBCO under conditions as-close-as-reasonably-possible to those within STEP prior to its construction. The campaign will thoroughly test and validate the choice of REBCO CCs used in the construction of STEP magnets. Here, STEP's current understanding of REBCO and how it is affected by irradiation are presented, followed by the details of experiments designed to develop our knowledge of how REBCO will fare when subjected to fusion neutron irradiation.

Lead-free SnIn solders are promising for superconducting magnet applications. However, their superconducting properties are not as good as lead solders. In order to improve the superconducting performance of the Sn–In solders, researchers have investigated the superconducting properties of ternary systems such as Sn–In–Bi for solder joints. In this study, powders of Pb, Nb, AgCu and grapheme nano pellets in the ratios of 0.5–5 wt% have been added into SnIn (35:65) to investigate their microstructural, thermal and superconducting properties. The added materials enhance the superconducting properties. We find that even low Pb additions show a dramatic improvement in superconducting properties, with an increase in both T c and J c values of up to 6.35 K and 1.47 × 10 4 A m −2 , respectively. This shows that much lower Pb content superconducting solders can be effective and could be used to replace the PbBi solder commonly used with the superconducting properties T c = 8.4 K, H C2 = 1.77 T, H C = 0.0909 T.

Commercial fusion power plants will require strong magnetic fields that can only be achieved using state-of-the-art high-temperature superconductors in the form of REBa 2 Cu 3 O 7−δ -coated conductors. In operation in a fusion machine, the magnet windings will be exposed to fast neutrons that are known to adversely affect the superconducting properties of REBa 2 Cu 3 O 7−δ compounds. However, very little is known about how these materials will perform when they are irradiated at cryogenic temperatures. Here, we use a bespoke in situ test rig to show that helium ion irradiation produces a similar degradation in properties regardless of temperature, but room-temperature annealing leads to substantial recovery in the properties of cold-irradiated samples. We also report the first attempt at measuring the superconducting properties while the ion beam is incident on the sample, showing that the current that the superconductor can sustain is reduced by a factor of three when the beam is on. Impact statement REBa 2 Cu 3 O 7−δ high-temperature superconductors are an enabling technology for plasma confinement magnets in compact commercial fusion power plants, owing to their ability to carry very high current densities when processed as quasi-single crystals in the form of coated conductors. In service in a fusion device, the magnet windings will be exposed to a flux of fast neutrons that will induce structural damage that will adversely affect the superconducting performance, but very little data are currently available on the effect of irradiation at the cryogenic temperatures relevant for superconducting magnets. Moreover, even room-temperature annealing substantially affects superconducting properties after irradiation, so to obtain key technical data for fusion magnet designers, it is important to measure these properties in situ , under irradiation. This work shows that for the first time, it is important to consider how energetic particles directly influence superconductivity during irradiation because we observe a reduction in zero-resistance current by a factor of as much as three when an ion beam is incident on the sample. Although neutrons will not interact with the material in the same way as charged ions, primary knock-on ions from neutron damage are expected to have a similar effect to the He + ions used in our study. Graphical abstract

Understanding the effects of high energy neutron damage on REBa 2 Cu 3 O 7 − δ (REBCO) coated conductor is of vital importance for the design of the magnetic confinement systems for compact nuclear fusion power plants. However, neutron irradiation campaigns can only be carried out in a few facilities, and the experiments are very slow and expensive partly because the samples become radioactive. Ion irradiation provides an easily accessible alternative route to studying the effects of radiation on high temperature superconductors, which not only increases the volume of technical data that can be obtained but also enables more complex experiments such as in situ cryogenic irradiation. The question is, does ion damage offer a good proxy for neutrons? Here we use high energy resolution fluorescence detected x-ray absorption spectroscopy to probe the effects of fast neutron irradiation on the local environment around the copper ions in the REBCO layer of coated conductor tapes. We find that the spectral changes are similar to those induced by helium ion irradiation, suggesting that both projectiles produce the same types of structural defect in the REBCO lattice, although there is some evidence of an additional type of defect present in the sample heavily damaged by He + ion irradiation. It is also shown that the linear degradation of superconducting transition temperature (T c ) of coated conductors with the calculated number of displacements per atom occurs at the same rate for neutrons and helium ions. Together these results provide new evidence suggesting that helium ions can emulate neutron point defect damage in REBCO high temperature superconductor reasonably well, increasing confidence that helium ions could be used as a useful proxy for neutrons in future experiments.

The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (J (c)) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic J (c) by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic J (c) by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature.

Atom probe tomography (APT) has been used to study the effect of fast neutron irradiation on the local chemistry of Nb 3 Sn samples. Two RRP ® wires doped with 2 at% Ti were analysed, one in the as-received condition and the other irradiated to a neutron fluence ( E > 0.1 MeV) of 2.82 × 10 22 m −2 in the TRIGA-II reactor. The irradiated sample had a reduced T c , an increase in F p , a shift in the peak of the F p curve suggesting the introduction of secondary point pinning, and an increase in the estimated scaling field B *. APT analysis has shown that polycrystalline Nb 3 Sn has three distinct regions of composition, near stoichiometry Nb 3 Sn (low Nb), regions with a higher Nb content than expected in equilibrium Nb 3 Sn (high Nb) and grain boundaries. The summed composition of these three regions lies within the Nb 3 Sn phase for both the as-received and irradiated samples. The distinct regions of high Nb Nb 3 Sn demonstrate incomplete diffusion in the as-received sample, and the reduction in volume of these high Nb regions after irradiation implies significant radiation induced diffusion has occurred. The presence of other features in the atomic-scale chemistry, such as the extent of Cu segregation at grain boundaries, three types of dislocation array, and unreacted Nb nanoparticles, are compared between samples.

Abstract This chapter introduces the idea that superconductivity emerges because it is the most energetically favourable state of the material. The concepts of phase diagrams and phase transitions are explored using water as an example of a familiar system. What happens to a superconductor when it is placed in a magnetic field is discussed and compared with other types of magnetic materials, culminating in explaining the difference between type I and type II superconductivity in terms of the energetics of the system. The important concept that type II superconductors usually contain magnetic flux lines is introduced. An “Under the Lens” section focusses on the idea of spontaneous change, showing how the second law of thermodynamics is related to the definition of Gibb’s free energy. A “Wider View” section discusses why phase transitions are so important in materials science, using the deposition of thin films and the processing of materials as examples.