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Among the Iron-Based Superconductors, the 1144 family attracted significant attention due to the intriguing intrinsic properties exhibited by single crystals, such as high critical fields and critical currents. Polycrystalline compounds, however, are influenced by the non-ideal nature of the material. In this work, we report the superconducting properties at high magnetic fields up to 17 T of Ca/K-1144 bulk polycrystalline compounds obtained via a mechanochemically assisted synthesis route. The material exhibits a significant robustness of intrinsic features such as the superconducting transition onset, varying from approximately 35 K in the absence of the field to approximately 33 K at 17 T. Broadening phenomena of both resistive and magnetic transitions testifies however the presence of granularity issues, most likely due to extrinsic factors such as oxidation, porosity and random orientation of the crystallites.

Experiments on quench propagation in high-current, high field High Temperature Superconducting (HTS) Cable-In-Conduit Conductors (CICCs) for fusion applications are forthcoming. Among the conductor designs to be tested, there is the ENEA slotted core proposal. In order to support the design of the samples and plan the diagnostics to be employed, a 1D multi-region thermal-hydraulic and electric model of the samples has been developed using the H4C code. After an experimental electric characterization, the model is applied to the simulation of quench propagation in the samples. The simulations show how current redistributes among the tapes and the slots. Additionally, they show that the quench protection strategy is suitable to prevent too high hot-spot temperatures.

YBa2Cu3O7-x (YBCO) films grown by metal-organic decomposition (MOD) are promising candidates for the production of high performance high temperature superconductors (HTS) tapes. Moreover, artificial pinning centers (APCs) introduction proved to be a powerful tool for the improvement of transport properties in YBCO films. Along with the widely employed pinning strategies, such as the introduction of secondary phases in the YBCO matrix, a less studied approach is the nanostructuring of the YBCO substrate, in order to hinder its growth and to obtain a highly defective superconductive layer. In this work, oxide nanostructures were deposited on crystalline substrates via polymer assisted deposition (PAD), a low cost, highly versatile technique, so as to create nanoparticles that could interfere with epitaxial growth of chemical YBCO films. Preliminary analyses of the superconducting films grown on BaZrO3 (BZO) and ZrO2 (ZO) nanostructures showed that the decorated templates are highly effective in straining the YBCO matrix, leading to a significant improvement in transport properties. In fact, we obtain an increase of the critical current density, at 77 K, for YBCO grown on BZO and ZO decorated substrate of the order of 3 times at 2 T and even of the order of 8 times at 5 T with respect to the YBCO grown on bare substrate. The improvement of the pinning properties can be related to the intrinsic modification of the crystal structure of YBCO film due to the presence of these nanoparticles, in agreement with transmission electron microscopy (TEM) analysis performed.

In this paper, the effect of post synthesis annealing treatments on a Fe(Se,Te) polycrystalline material is evaluated and discussed. The samples have been obtained via melting route. The material has been subjected to a high-temperature annealing treatment, carried out for 45 h at 680 °C. The role of the cooling step was investigated comparing samples obtained after a controlled cooling or after quenching in liquid nitrogen. From a morpho-structural point of view, the annealing treatment improves homogeneity, with respect to pristine samples, and influences secondary phase precipitate morphology. Regarding superconducting properties, a key role of the cooling procedure is assessed: controlled cooling leads in fact to a significant improvement of high field behaviour with respect to the melted material, while quenched samples are characterized by a worsening of the superconducting properties. Despite the overall worsening, however, the quenched samples show evidence of the presence of superconducting phases characterized by a remarkably high critical temperature (Tc > 18 K), observed for these materials only in films or under pressure.

The knowledge of the strain distribution of Nb3Sn filaments in a CICC cross-section is a key parameter in understanding the performance and its evolution when the cable undergoes electromagnetic cyclic loading. This work focuses on the Wind and React (WR1) rectangular CICC sample designed by ENEA for the European DEMO project, that has exhibited a current sharing temperature (Tcs) above 6.9 K at 13 T and 81.7 kA, during its qualification tests in the EDIPO facility at the Swiss Plasma Center. Compared to the critical current tests on the constituting wires, the observed CICC performance corresponded to an effective strain value of −0.55%. This work aims to interpret this result on the basis of the analysis of the strain distribution inside the cable itself. Susceptibility versus temperature of WR1 constituting free standing strands has been measured after etching of the stabilizing copper, and it has been compared with the susceptibility measured on the cable during the test campaign in EDIPO. A well-known and already assessed method has been used, starting from susceptibility measurements, to compute the Tc distribution in the cable cross-section and, subsequently, to infer from that the thermal strain distribution. The obtained results give an indication that the strain distribution in WR1 cable is narrow, compared to the strain distribution calculated with the same method for most ITER TF and CS conductors.

This study is focused on the phase separation phenomena in Fe(Se,Te) materials. In particular, it investigates the effect of a high temperature and long time annealing treatment on morphostructural and superconducting properties of bulk samples. The synthesis of Fe(Se,Te) compounds was carried out through a melting route, and the materials were characterized in their morphostructural and magnetic properties. The obtained compounds possess a high degree of inhomogeneity due to dendritic precipitates of secondary phases in a superconducting tetragonal matrix. The high temperature annealing treatment induces a morphological evolution of the material and leads to the structuration of two distinct superconducting phases.

The epitaxial growth of Zr-doped CeO2 (CZO) films by chemical solution deposition (CSD) on single crystal substrates such as Al2O3, SrTiO3 and Y2O3-stabilized ZrO2 (YSZ) and their use as buffer layers for Fe(Se,Te) film growth are reported. The growth of 30 nm thin CZO films is mostly dominated by dewetting issues, leading to highly incomplete substrate coverage due to the strong tendency of CZO grains to agglomerate. This effect is less severe on YSZ substrate and, in addition, can be fully controlled by setting the deposition temperature at 950 °C and operating under Ar-H2 reducing atmosphere. These conditions promote the growth of epitaxial and compact CZO films on YSZ, showing a flat surface morphology with root-mean-square roughness around 2.5 nm. Preliminary results on Fe(Se,Te) film deposited by pulsed laser deposition on CZO-buffered YSZ are reported. Although good epitaxial growth is achieved on CZO buffer layer, Fe(Se,Te) films exhibit poor superconducting properties. On the other hand, Fe(Se,Te) films show excellent properties when a seed layer is used, exhibiting a sharp superconducting transition at about 17 K and a critical current density exceeding 1 MA cm-2 at 4.2 K in self field. The main role of the seed layer, consisting of a non-superconducting Fe(Se,Te) film, is to favour the crystal and chemical matching between buffer layer and film, allowing for the growth of high-quality superconducting Fe(Se,Te) at deposition temperature as low as 200 °C. In perspective of applications, this result is a significant step towards process simplification and cost reduction of Fe(Se,Te)-based coated conductor technology development, being the first demonstration of superconducting Fe(Se,Te) films grown on CSD buffer layer.

Superconducting properties of the Fe(Se,Te) system are directly related to its microstructure. However, in the framework of a not well assessed phase diagram, a large spread of chemico-physical and functional properties is reported for compounds characterized by similar overall chemical compositions. This is in particular observed in materials produced by melting routes, where chemical inhomogeneities and multiple phases are commonly obtained. Both composition and morphology of these inhomogeneities seem to play a role in the behaviour of the superconducting phases, pointing out the need to assess the dependence of phase composition and therefore of the material performance on the sample thermal history. In this work, the effect of different post-synthesis annealing treatments is studied. A melting route has been adopted to synthesize polycrystalline samples, subsequently subjected to thermal treatments at different temperatures. Morphology and microstructure were characterized by means of x-ray diffraction and microscopy techniques. The superconducting properties have been evaluated by means of electrical and magnetic measurements. It is observed how microstructure, chemical composition and morphology of the obtained products strictly depend on the annealing temperature. Moreover, a crucial role of the cooling step is evidenced: in fact, a rapid cooling procedure from high temperature leads to the formation of high temperature (Tconset > 19 K) superconducting phases, but the overall bulk superconducting performance is negatively affected. Other annealing treatments can promote instead an enhancement in the critical current at high fields with respect to pristine sample, suggesting that a careful engineering of a post-synthesis thermal step can represent a useful tool to optimize the material properties.

In this work it is shown how SrTiO3 substrates interact with YBa2Cu3O7-δ thin films during film processing. The effect of this interdiffusion on morphology and transport properties is analyzed. The films were deposited by low fluorine Metal Organic Decomposition (MOD) on SrTiO3 (001) single crystals. In general, a sharp c-axis oriented growth and high critical temperature (Tc) were observed in all samples; however, the surface of the otherwise good-quality films is spoiled by the presence of inhomogeneous circular areas of seemingly different appearance and texture. In order to understand the nature of these defects, the substrates and the films were deeply investigated by optical microscopy, scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), contact-stylus surface profilometry (SSP), Atomic Force Microscopy (AFM) and time of flight secondary ion mass spectrometry (ToF-SIMS). The round spots are the manifestation, on the film surface, of ion interdiffusion from the substrate into the YBCO film Spot-free samples can be obtained with different pre-treatments of the SrTiO3 substrates. Although these defects are detrimental for the film local structure, Jc values are not significantly affected, although the defects' influence might become substantial while working on smaller scales.

The quench protection in high-temperature superconducting (HTS) magnets is a well-known issue. Therefore, it is fundamental to have reliable models available for the analysis of the quench propagation in such magnets. The quench propagation in an ENEA HTS cable-in-conduit conductor, developed for fusion applications with the particular reference to an insert for the central solenoid of the Italian Divertor Tokamak Test (DTT) facility, is analyzed here with a new 1-D multiregional thermal-hydraulic and electric model, based on dedicated preliminary analyses and experimental and numerical characterization of the conductor. It is shown that large temperature differences arise in the conductor cross section during the quench propagation. The model is then applied to parametrically assess the effects of delay and fast current-discharge times on the conductor peak temperature, to avoid damaging the HTS. The parametric study shows that both the delay and current discharge time should stay below 0.5 s, to keep the peak temperature below 150 K.

In order to study the effect of pinning in coated conductors, we have used pulsed laser ablation to grow $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-x}$ (YBCO) and $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-x}/\text{BaZrO}_{3}$ (YBCO/BZO) films on metallic templates. Despite the complexity of the structure, after the optimization of the deposition procedure, the YBCO/BZO showed only a moderate reduction of $T_{c}$ with respect to the pure YBCO sample. We use a very high frequency (∼47.7 GHz) dielectric resonator technique in order to induce very short distance vortex shaking, thus probing the steepness of the potential well. We report on measurements of the microwave response of the superconducting structures in terms of the quality factor $Q$ and the frequency resonance $\nu_{0}$ as a function of a moderate dc magnetic field $H$ , at different temperatures $T$ . To keep as much contact as possible to the raw data ( $Q$ and $\nu_{0}$ ), in agreement with a generalized model for vortex dynamics, we determine the pinning parameter $r$ and the upper limit for the creep factor $\chi_{M}$ . Based on the field dependence of the pinning parameters, the vortex pinning regimes were discussed. We find that the improvement of pinning by BZO is much reduced with respect to the effect observed in films grown on single crystals. In particular, the reduction of the critical temperature must be taken into account in order to appreciate the results.

YBa2Cu3O7−x (YBCO) nanocomposites for wire applications need to operate in a broad range of frequencies, ranging from dc in magnets to GHz in cavities and screenings of future particle accelerators. We have investigated the in-field and angular vortex pinning performance in dc and at 50 GHz of two types of nanocomposites, pulsed laser deposition (PLD) YBCO with mixed Ba2YNbO6 + Ba2YTaO6 (BYNTO) nanorods and chemical solution deposited (CSD) YBCO with BaHfO3 (BHO) nanoparticles (NPs), and the pristine counterpart films, grown on top of single-crystalline substrates. Transport measurements performed up to 9 T between 5 and 77 K show that CSD nanocomposites exhibit a smooth field decay and increased single-to-collective crossover field H* compared to pristine samples, associated to the enhanced isotropic pinning contribution induced by the NPs, while PLD films exhibit unchanged H* and superior critical current densities up to higher irreversibility fields, associated to the anisotropic contribution introduced by the rods. Microwave in-field measurements of the pinning constant kp revealed CSD NCs exhibit a qualitatively similar, but smoother kp(H) than pristine samples, whereas for PLD samples, a growing kp(H) dependence is observed as a result of the increased relevance of the stiffness of the fluxons pinned by nanorods., This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom Research and Training Programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. We acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496), SUMATE project RTI2018-095853-B-C21, cofinanced by the European Regional Development Fund, COST-Nanocohybri CA16218, DWARFS project (MAT2017-83468-R) and Catalan Government with 2017-SGR-1519 and XRE4S.

In the effort to unveil the relations between synthesis conditions, morpho-structural features and superconducting properties of (11)-type iron-based superconductors, this study is focused on FeSe0.5Te0.5 materials obtained from high-temperature melts and different cooling rates. Crystalline and chemical composition are evaluated by means of XRD and SEM analyses, highlighting the formation of complex multiphase structures: the obtained results suggest that high-temperature nucleating phases may play a key role in the observed phase separation. Superconducting properties, evaluated by magnetization measurement and dc electrical resistance in different magnetic field conditions up to 18 T, highlight the low homogeneity of the superconducting phases, reflecting the microstructural observation. © 2002-2011 IEEE.

In this paper, we report on the experimental study of the bending behavior, at 77 K, of a cable-in-conduit conductor composed of a stack of REBa 2 Cu 3 O 7-x (REBCO) tapes inserted into a helical duct formed in the extruded aluminum cylindrical core. The investigation was carried out by powering each single tape individually. By the analysis of the single tape I c dependence as a function of the bending radius, R b , the effect of bending strain as a function of tape position inside the stack was investigated for each tape. The results evidence the good bending strain tolerance of all the tapes, showing the onset of degradation at R b ≈ 0.25 m, ascribed to the slippage occurring among tapes within the stack. Interestingly, the decrease in the intertape contact resistance between neighboring tapes with respect to the straight cable condition reveals that the bending stress results in a beneficial additional transverse load on the stacks, which leads to a more uniform compaction of tape stack. The experimental I c behavior with R b has been explained considering that the current transfer mechanism among tapes might mitigate the degradation of the tape I c due to the bending strain. The identification of the mechanisms acting on individual tapes under cable bending conditions is a relevant achievement in perspective of the interpretation of the electromechanical behavior of the conductor with all tapes powered in parallel.

In this work we report the crystallization of iron chalcogenides compounds starting from a Fe(Se0.5Te0.5) solid pellet. The synthesis process is carried out at 675 degrees C using a molten chloride salts mixture and permitted to obtain well defined and easily separated hexagonal and tetragonal crystals, characterized by Fe-3(Se0.43Te0.57)(4) and Fe1.02Se0.41Te0.59 compositions respectively. The obtained tetragonal compound exhibits superconductivity at 14 K with critical current density of similar to 10(5) A/cm(2) at 5 K.

In this study, chemically decorated surfaces are employed as tools for Artificial Pinning Centres (APC) introduction in superconducting YBa 2 Cu 3 O 7−x (YBCO) films. Self-assembled oxide nanostructures are grown on crystalline substrates via a very versatile and low-cost chemical technique called Polymer Assisted Deposition (PAD). Such decorated templates are used, in a second step, as substrates for YBCO film growth via Metal Organic Decomposition (MOD) or Pulsed Laser Deposition (PLD). The aim of the structures is to produce in the superconducting matrix strain and/or defects that should act as APCs and therefore enhance the transport properties of the samples. The oxides selected for the decoration are commonly used for the introduction of artificial pinning centres in YBCO: BaZrO 3 and ZrO 2 . The decorated surfaces are analyzed via Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). YBCO films have been characterized via SEM, X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), DC resistivity and critical current measurements. The results show that the nanostructures are stable and that they can efficiently introduce strain and stacking faults in the film. This causes an increase in the value of the critical current density J c and better in-field behaviour in all the investigated field range, compared to the analogous film grown on a standard substrate, especially for chemically derived YBCO. Also J c angular dependence measurements, J c (θ), show an isotropic enhancement of the critical current for every angle of application of the field.

Superconducting YBa2Cu3O7-δ films with different amounts of BaZrO3 nanoinclusions were deposited on SrTiO3(001) by low-fluorine chemical solution deposition with the aim of introducing artificial vortex pinning centres. The Zr concentration over the film thickness could be determined by X-ray photoelectron spectroscopy combined with Ar+ ion etching. The Zr/Y ratio has a constant behaviour in the film's bulk. Zr segregation occurs near the surface and Zr diffusion into the substrate is observed near the interface. Conversely, Sr and Ti from the substrate diffuse into the films. Y3d lineshape analysis and X-Ray Diffraction data pointed out that Ti diffusion causes the formation of Y-Ti-O based spurious phases.

Cable-in-conduit conductors comprised of twisted stacks of high-temperature superconducting (HTS) tapes constitute a very promising technology by virtue of their easy manufacturing process, flexibility capabilities, and high current densities. In a cable, the current distribution among tapes is one of the key parameters affecting the cable performances. The distribution of current is affected mainly by the self-field configuration (ultimately related to the cable layout) and the termination resistances. In this paper we present a 2D finite element (FE) model, based on the T-A formulation, which computes the magnetic field and current distribution in stacked tapes. This model has been used to describe the experimental V–I results obtained in cables in which different current distributions among tapes are expected. The first case refers to V–I curves of stacks of HTS tapes inserted into ducts formed in the extruded aluminium cylindrical core for a straight cable. The excellent agreement between the experimental findings and the simulation results can be explained in terms of uniform current distribution within the tapes stack, up to the superconducting to normal transition. The second sample, an Al-slotted core Cable-In-Conduit-Conductor, has been bent down to a radius of 0.15 m, and from the measured V–I characteristic of each individual tape, a different tape degradation depending on the tape position within the stack was recorded. The model is able to reconstruct the V–I of the stacks from the characteristic curves of the individual tapes with a satisfactory agreement. The finite element analysis reveals non-uniform current distribution among the tapes, which could expose the cable to a potentially irreversible damage during operation. The proposed FE model constitutes a useful tool for the analysis and predictions of HTS CIC conductor performances and represents a suitable basis for the implementation of more complex models aimed at the design of specific and large applications of this conductor in the next future.

A higherature superconducting (HTS) cable-in-conduit conductor (CICC) suitable for high-field magnet applications and comprised of twisted-stacked coated-conductor tapes arranged around a helically slotted core has been recently proposed and tested, demonstrating full compatibility with existing cabling technologies. To form the desired shape of any coils for high-field magnet applications, any suitable CICC option needs to be bent. For a magnet design, it is then very important to characterize the bending behavior of the CICC and, in particular, to find the smallest bending radius achieved without performance degradation. To this aim, bending tests were carried out on 1-m-long dummy samples of five helical slots in an extruded aluminum core, in which four REBCO tapes and dummy stainless-steel tapes were mounted in each slot. A controlled bending moment has been applied to the HTS CICC samples at room temperature, and each individual superconducting tape has been electrically characterized as a function of bending radius by measuring the critical current and $n$-index values at 77 K in a self-field condition. Results are analyzed and explained with the help of a three-dimensional cable model implemented in ANSYS and analytical calculations. The experimental and numerical results presented in this paper will demonstrate that the twisted-stack slotted-core technology can meet the bending requirements of high-field magnet designs. © 2002-2011 IEEE.

This study investigates chemically decorated surfaces as tools for the improvement of transport properties in superconducting YBa 2 Cu 3 O 7-x films. This approach, still to be thoroughly investigated, makes use of low-cost easily tunable chemical methods to obtain self-assembled oxide nanostructures on a substrate that will serve, in a second step, for the deposition of the superconducting film itself. The structures are supposed to produce in the superconducting matrix a specific amount of strain, which is generally held responsible for the increased transport capacity of variously doped samples. For the growth of the nanostructures, two different methods have been employed: polymer-assisted deposition and metal-organic decomposition (MOD). The main advantages and disadvantages of these two routes are discussed. The oxide chosen for the deposition is one commonly used for artificial pinning in YBa 2 Cu 3 O 7-x , namely Ba 2 YNbO 6 , together with the less common but highly interesting ferromagnetic compound La 0,77 Sr 0,33 MnO 3 . We also show how the density of these nanostructures can be easily controlled, and the necessary requirements for the growth of nanostructures are determined. A variety of crystalline substrates were tested for the deposition of the nanoparticles, such as SrTiO 3 , YSZ, MgO. YBa 2 Cu 3 O 7-x films have been deposited on selected samples via standard low-fluorine MOD and characterized scanning electron microscopy (SEM), x-ray diffraction (XRD), vibrating sample magnetometer (VSM), direct current (DC) resistivity, and critical current measurements. The presence of the nanostructures results, in this case, in a slight increased value of J c , which can be ascribed to the relatively low density of nanoparticles.

Surface faceting is a well-known phenomenon occurring in either metallic or oxide materials. It is due to thermal etching favoured by high-temperature annealing in vacuum, and leads to the formation of steps or facets on the surface. The occurrence of surface faceting on cube-textured metallic substrates for YBCO coated conductor has been only seldom reported and its influence on the epitaxial growth of both buffer and superconducting layers has never been considered. In this paper, we studied the effect of the annealing conditions on the surface morphology of Ni- and Ni-Cu-based cube-textured metallic substrates and the influence of the obtained surface morphology on epitaxial film grown by both physical vapour and chemical solution deposition techniques. We confirm that annealing in reducing atmosphere suppresses both surface faceting and grain boundary grooving. Such a surface is beneficial for epitaxial film growth as it leads to smoother and featureless surfaces. On the other hand, from a microstructural point of view, film deposition on a faceted substrate shows a sharper orientation.

High temperature superconducting tapes based on cuprate compounds are nowadays available on long lengths by several companies. Because high superconducting properties are strongly influenced by the crystallographic orientation of the superconducting film, and because the appropriate crystallographic orientation is obtained through epitaxial film growth, the development of an oriented template is of crucial technological importance. In the rolling assisted biaxially textured substrate (RABiTS) technique, a strong cube texture is obtained by cold rolling fcc metal tape and high temperature annealing, followed by the deposition of one or more buffer layer by either physical vapor deposition (PVD) or chemical solution deposition (CSD). Using the latter approach, it has been recently shown that it is possible to combine substrate recrystallization and buffer layer growth in a single step if the onset of recrystallization occurs at a temperature lower than buffer layer nucleation. In the present contribution, a detailed investigation of biaxially-oriented La2Zr2O7 (LZO) buffer layer grown on cold-rolled Ni-W substrate by combined EBSD, AFM and TEM analyses is reported. It is shown that it is possible to obtain highly {001}〈110〉-textured LZO film directly on cold-rolled Ni-W substrate. Although some small polycrystalline regions appear, this approach leads to a strong suppression of grain boundary grooving typically occurring in RABiT-type substrates. Further, it is shown that, notwithstanding the microstructure of the metal substrate continues to evolve during the conversion heat treatment, the obtained LZO film is well adherent, without cracks or delamination.

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance (Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA. © 2014 IEEE.

In 2013, the Superconductivity Laboratory at ENEA (the Italian National Agency for New Technologies, Energy, and Economica Developments) started a project aimed at using second generation (2G) High-Temperature Superconductors (HTS) tapes for the manufacture of a cable-in-conduit conductor. The considered layouts are based on a number of HTS tapes, which are arranged as five layered structures of twisted tapes wound on a helically slotted core with an external round jacket, and the conductor is designed to target 20 kA at operating conditions (T = 5 K, B = 15 T; or T = 77 K, self-field). With the commercialization of 2G HTS tapes, several options are now available on the market that can meet the ENEA cable specifications. Thus, for a sound design of the ENEA cable, a measurement campaign was launched aiming to determine the critical current, i.e., Ic, of commercial tapes as a function of an external applied field (up to 12 T) at different temperatures (in the 5-77 K range). Transport measurements were performed at 77 K in the high-field region, whereas the low-field and low-temperature critical currents have been determined from magnetization loops measured with a vibrating sample magnetometer system. Within a COMSOL Multiphysics environment, a 2-D finite-element method (FEM) magnetostatic model has been developed, in which a uniform Ic distribution is assumed in the HTS stacks. Both the experimental and FEM results are discussed in terms of cable performance, showing that current commercial tapes can already meet the ENEA cable specifications at T = 5 K, B = 15 T. © 2014 IEEE.

A MgO/TiN based multilayer structure suitable as buffer for the development of ReBa2Cu3O7-x (RE = rare earth, Y)-based coated conductors has been developed. The epitaxial growth of TiN film on cube textured substrate was widely studied in terms of structural and morphological properties. TiN films were epitaxially grown on Ni-W substrates in a wide range of deposition condition. The TiN films are compact and dense with smooth surfaces. An MgO film deposited on the top of the TiN layer reproduces the characteristics of the underlying film. In order to test the TiN/MgO structure, a YBCO film was deposited using a SrTiO3 film as cap layer. A Cu-based cube textured tape was employed as substrate, i.e., Ni-50%at. Cu-0.5%at. W. The YBCO film presents a major c-axis orientation with a sharp out-of-plane distribution. Zero-resistance critical temperature of about 85 K was obtained. © 2014 IEEE.

A thermal–hydraulic model has been developed for the novel cable-in-conduit conductor (CICC) recently proposed by Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), which foresees the use of second-generation REBCO-based coated conductor tapes stacked into twisted grooves slotted into an Al core. A central hole in the Al core provides a low hydraulic impedance channel for the coolant, whereas direct cooling of the tape stacks is achieved through small channels at the bottom of the grooves and other small passages around the stack. The coolant could be either supercritical helium (SHe), for high-field/high-current applications, as in fusion magnets, or liquid nitrogen (LN2), for self-field/lower current requirements. The issues of the thermal–hydraulic performance and optimization of such a CICC are addressed for the first time in this paper. The 4C code is used to develop an adequate model for this complex 3D problem, taking advantage of the 1D nature of the coolant flow. The model, first calibrated and then validated using the results of thermal–hydraulic tests performed in cryogenic conditions at the Center for Research in Plasma Physics (CRPP), is used to evaluate the flow repartition among the different cooling paths in the CICC for SHe at 4.5 K and for LN2 at 77 K. The model is finally used to perform parametric analyses, varying, e.g., the central hole diameter, as well as the size of the grooves at the bottom of the tapes, within the limits imposed by structural constraints, and to assess the cooling capability of the cable.

A detailed study on YBa(2)Cu(3)O(7-delta) (YBCO) films with BaZrO(3) (BZO) inclusions (YBCO-BZO) deposited on SrTiO(3) substrate with different BZO contents is presented. X ray diffraction, SEM imaging and superconductive critical temperature (T(c)) measurements were performed to evaluate film growth quality. Independently from the BZO content, all YBCO-BZO films showed T(c) values close to that of pure YBCO indicating that the T(c) reduction which is usually observed on YBCO-BZO films can be successfully recovered by changing the deposition conditions. All YBCO-BZO films showed an improved critical current density (J(c)) dependence on magnetic field if compared to the typical J(c)(H) of YBCO, resulting in higher irreversibility fields up to 9 T at 77 K. SEM observation on etched samples suggests a direct link between the defect density measured in YBCO-BZO samples and that calculated from the observed in field performances. The analysis of J(c)(B) as a function of applied magnetic field direction revealed that the pinning due to isotropic pinning centers typical for YBCO is coupled with a c-axis correlated pinning in YBCO-BZO which can be ascribed to BZO induced columnar defects. J(c)(B) recorded at several temperatures down to 10 K revealed matching effect features.

We perform measurements of high-frequency (similar to 48 GHz) microwave surface impedance with an applied magnetic field in YBa(2)Cu(3)O(7-delta) ( YBCO) laser-ablated films with various amounts of BaZrO(3) ( BZO) sub-micrometric inclusions, up to 7 mol % concentration. BZO inclusions are very effective in the reduction of the field-induced surface resistance in our experimentally accessible field range [ 0, 0.8] T. At temperatures low enough, the application of a moderate (similar to 0.2 T) field makes the YBCO/BZO films markedly less dissipative than pure YBCO. This result, examined in the light of the very high measuring frequency ( very small vortex oscillation amplitude) shows that BZO inclusions are even more effective pinning centers than columnar defects. We study the dependence of the vortex parameters ( vortex viscosity, pinning constant) on the BZO concentration. We examine the correlation between the reduction of the microwave dissipation and the areal density of BZO-induced defects. We argue that the very improved performances in a magnetic field are due to individual pinning of vortices on BZO inclusions.

In this paper we study the feasibility of Fe(Se, Te) coated conductors on a simple rolling-assisted biaxially textured substrate (RABiTS) template. Starting from commercially available Ni-5 at% W tapes which show an out-of-plane orientation of about 7° and an in-plane orientation around 5°, we realized a RABiTS template for Fe(Se, Te) coated conductor fabrication by depositing CeO2 thin films on the metallic tape. The oxide buffer layers, deposited via pulsed laser ablation, exhibit an out-of-plane and an in-plane orientation suitable for Fe(Se, Te) deposition and act as a chemical barrier against Ni diffusion. Fe(Se, Te) thin films deposited on such a simple template show a superconducting transition at 16 K and very high upper critical field values with a ΔT c,0 of only 3 K in 18 T. The transport critical current reaches values of 1.7 × 105 A cm-2 at 4.2 K in self-field and is reduced by less than one order of magnitude up to 18 T, remaining isotropic with respect to the magnetic field direction.

Electric joints are an essential part of every current carrying device. In the field of high temperature superconducting applications, quality of a contact between superconducting tapes and electric terminations is crucial for the device performance. When connecting tapes to each other, soldered contacts are commonly used. Recent reports have demonstrated that the resistance of this contact depends on the pressure applied during a soldering process, and the soldering temperature. When superconducting tapes are used in cables or in other applications, they have to be connected to copper terminations, e.g., to a current lead. Thus, tape-to-copper contacts are important. In superconducting cables, tapes are stacked together having a resistive electrical contact from tape to tape. The resistance of this contact depends on the pressure applied on the stack due to, for example, jacketing of the cable or during cool down. In this paper, the dependence of resistance between tapes on the applied pressure is reported. In this contribution two kinds of contacts, soldered ones and mechanical pressed, are presented and discussed. They are realized, respectively, between two superconducting ReBCO tapes from SuperPower, Inc. (SPI) and SuNAM, which were used for manufacturing HTS cable, and between superconducting tapes and copper.

La2Zr2O7 (LZO) film grown by the chemical solution deposition (CSD) technique is a widely studied buffer layer for YBa2Cu3O7-x film growth on a rolling-assisted biaxially textured (RABiT) substrate. A biaxially textured LZO film is epitaxially grown on a cube-textured metal substrate. In a RABiT substrate, cube texture is obtained as primary recrystallization of heavily cold-rolled fcc metals and is normally developed before CSD film deposition to obtain an oriented film growth. In this paper, we show that a biaxially textured LZO film can be grown by depositing the precursor solution directly on a cold-rolled fcc metal substrate and performing a single heat treatment. In fact, we show that if the recrystallization temperature of the metallic substrate is lower than the nucleation temperature of the film, a biaxially textured film is obtained as a result of transient cube texture transfer. Biaxially textured LZO films grown on cold-rolled substrates are compared with films grown on recrystallized substrates. In particular, the growth of LZO on either pure-Cu- or Ni-based substrate was studied. Compared with the LZO film grown on a cube-textured substrate, LZO grown on a cold-rolled substrate shows similar structural properties and morphology. Moreover, films are well adherent without cracks or delamination, suggesting that grain boundary migration in the metallic substrate has no detrimental influence on the film. In some cases, secondary recrystallization of the substrate occurs, without affecting the quality of the LZO film. © 2016 IEEE.

The proposed study investigates the aging of two precursor solutions used for the chemical solution deposition of YBa2Cu3O7-x (YBCO) and composite YBCO films with nanostructured inclusions of BaZrO3 (YBCO-BZO). The stability of precursor solutions plays a crucial role in determining the crystalline quality and the superconducting properties of YBCO films. Although low- or free-fluorine solutions are promising in order to improve the synthesis route and to reduce the environmental impact, the poor stability of the solution is the major drawback of the low-fluorine precursors since they degrade faster (4-6 weeks) than other precursor solutions for superconducting layers (e.g., trifluoroacetates solutions are stable up to 6 months) or buffer layers (stable for several months). Therefore, a deeper comprehension of the degradation mechanisms is fundamental to properly optimize the composition of the precursor solution. The current work studies two precursor solutions (for YBCO and YBCO-BZO) that are complex systems, in which different interactions need to be considered: most of all, the nature of ligands (acetate, propionate, ammonia) that complex each metal and influence not only the solution stability but also the quality of the deposited film. Thus, information obtained from different analyses is correlated with morphological, structural, and superconducting properties of the deposited film in order to understand the underlying chemical mechanisms of aging (salts precipitation, oxidation, polymerization) and its effects. © 2002-2011 IEEE.

The transport properties of nanocomposite YBa 2 Cu 3 O 7-x (YBCO) films grown by metal-organic decomposition (MOD) and prepared with a low-fluorine-coating solution on SrTiO 3 single-crystal substrates were studied with dc and microwave measurement techniques. As demonstrated in previous studies, the combination of these two techniques is a powerful tool for vortex pinning investigation. The BaZrO 3 inclusions (BZO) as artificial pinning centers were introduced into the YBCO matrix by the excess of corresponding Ba and Zr precursor salts to the initial YBCO precursor solution up to 5 mol%. The effect of BZO was clearly observed in magnetic field dependence relations of critical current density (J c ) and the complex surface impedance, i.e., ΔZ, at fixed temperature and magnetic field parallel to the c-axis. The result shows that BZO particles mainly act as isotropic pinning centers. The short-range vortex dynamics was probed by the ~47.7-GHz microwave surface impedance technique. The derived vortex parameters r and k p were sensitive to the presence of nanoparticles, indicating the effect of BZO on pinning. The joint field dependence relations of J c and k p help in elucidating the pinning regime in our nanocomposite MOD YBCO/BZO films.

In this paper, we present a detailed investigation of the self-field transport properties of an ionic liquid gated ultra-thin YBa$_2$Cu$_3$O$_{7-x}$ film. From the high temperature dynamic of the resistivity ($> 220 \textrm{ K}$) different scenarios pertaining to the interaction between the liquid and the thin film are proposed. From the low temperature evolution of $J_{\textrm{c}}$ and $T_{\textrm{c}}$ a comparison between the behavior of our system and the standard properties of YBCO is drawn.

In the growing field of higherature superconductor (HTS) applications, soldered tape-copper joints are of significant importance. This is because the resistance of the joint often plays a decisive role in the design of a superconducting device, such as current leads or current terminations of cables. A common requirement to the joint is as-low-as-possible resistance and reliable reproducibility, which is crucial for achieving a homogeneous current distribution being independent within each HTS tape placed in a superconducting device. From previous studies, it is known that resistance of HTS tape-copper joint can vary for different tapes and solders. This resistance can be considered as a sum of several contributions: REBCO-layer-copper-laminating-layer interface resistance in HTS tape, copper-laminating-layer-solder and solder-copper interface resistances, and solder and copper resistances. Here, we present current status of investigations on tape-copper joints and trials to estimate different contributions in their resistance value. © 2002-2011 IEEE.

The deposition and characterization of La2Zr2O7 thin films deposited by metal-organic decomposition method on both single crystal SrTiO3 and cube textured Ni-5 at.%W substrates are presented. Metal acetylacetonates in propionic acid solution was used. The results showed that LZO films are epitaxially grown with smooth surface with rms roughness around 2 nm. YBa2Cu3O7-δ films, deposited by pulsed laser deposition technique on LZO buffer layers, showed critical temperature of 90 K and critical current density in self magnetic field Jc = 1.6 and 13 MA/cm2 at 77 K and 4.2 K, respectively. A better Jc retention in magnetic field for YBCO films deposited on LZO/STO than YBCO on bare STO is observed indicating a rather strong vortex pinning as confirmed by microwave measurements.

We present a study of the anisotropic vortex parameters as obtained from measurements of the microwave complex resistivity in the vortex state with a tilted applied magnetic field in YBa2Cu3O7-x thin films with BaZrO3 nanorods. We present the angular dependence of the vortex viscosity $\eta$, the pinning constant k_p and the upper limit for the creep factor \chi_M. We show that the directional effect of the nanorods is absent in \eta, which is dictated by the mass anisotropy \gamma. By contrast, pinning-mediated properties are strongly affected by the nanorods. It is significant that the pinning and creep affected by the nanorods is detectable also at our very high operating frequency, which implies very short-range displacements of the vortices from their equilibrium position., Comment: Proceedings of VORTEX VIII Conference, to be published in Physica C

The addition of artificially induced defects improves the pinning efficiency of magnetic flux vortices in YBa2Cu3O7-delta (YBCO) films. In YBCO films by pulsed laser ablation (PLD) technique this goal can be pursued by the introduction of epitaxial second phase nanoinclusions in the YBCO matrix. In the present work, the effect of BaZrO3 (BZO) inclusions in stoichiometric YBCO films on transport properties has been studied. Composite targets obtained by addition of BZO powder in molar percents ranging from 2.5 to 7 mol.% have been prepared with BZO mean grain size of about I mu m. Structural and morphological characterizations of the films obtained by PLD from these targets have been carried out through X-ray diffractometry and Scanning Electron Microscopy (SEM) analysis. De electrical transport measurements at 77 K have revealed a better magnetic field retention of the critical current density, J(c) being at 3 T the 14.6% and the 16.4% of the zero field values for YBCO-BZO 5% and 7%, respectively. The complex surface impedance has been measured by means of a dielectric resonator at a high microwave frequency (50 GHz). In order to identify the potentially strong pinning induced by the inclusions, the field-dependent complex resistivity has been determined as a function of temperature in a moderate magnetic field (B < 1 T). Very high depinning frequency are estimated in YBCO films with BZO inclusions indicating extremely high vortex pinning and steep potential wells.

A YBa2Cu3O7-δ (YBCO) film grown by metal organic decomposition (MOD) is a promising candidate in the realization of high-performance superconducting wires with high critical current density. Moreover, artificial pinning centers (APCs) inclusions improve the current transport performance of the films in both high magnetic fields and temperatures. © 2015 IEEE.

ReBa2Cu3O7-x(ReBCO{Re} =Y or rare earth) coated conductors are currently available as ribbon-shaped conductors. The high aspect ratio of such conductors is often regarded as a limitation for many electrotechnical applications due to the anisotropy of the electrical and mechanical properties and high ac losses. In this view, a method for the development of cube-textured, low aspect ratio wire substrate for ReBCO coated conductor is presented. The method is derived from the conventional cold-rolling and recrystallization process and modified to obtain a cube-textured wire with low aspect ratio rectangular cross section. To show the feasibility of the method, pure copper was used as raw material. Cube-textured copper substrate with a square cross section and a thickness around 0.5 mm was realized by longitudinally cutting a heavily cold-rolled thick tape. All 4 sides of the wire show a strong cube texture development. Cube texture development is comparable with the one obtained in standard tapes, with a fraction of cube-oriented area above 95% and a full width at half maximum of (002) omega-scan ranging from 6.5 ° to 7.5 ° . The quality of the obtained surfaces was tested through the epitaxial growth of MgO/Pd bilayer. Preliminary results indicate that all 4 sides of the wire are equally available for epitaxial film growth and lead to a comparable film quality. © 2015 IEEE.

Measurements of anisotropic transport properties (dc and high-frequency regime) of driven vortex matter in YBa2Cu3O 7-x with elongated strong-pinning sites (c-axis aligned, self-assembled BaZrO3 nanorods) are used to demonstrate that the effective-mass angular scaling takes place only in intrinsic physical quantities (flux-flow resistivity), and not in pinning-related Labusch parameter and critical currents. Comparison of the dynamics at different time scales shows evidence for a transition of the vortex matter toward a Mott phase, driven by the presence of nanorods. The strong pinning in dc arises partially from a dynamic effect. © 2013 Author(s).

The introduction of artificial pinning sites in YBa(2)Cu(3)O(7-delta) (YBCO) epitaxial thin films has been obtained by pulsed laser deposition (PLD) technique from YBCO-BaZrO(3) (BZO) composite targets with BZO concentration ranging from 2.5 to 7 mol %. The typical critical temperature, T(c), drop observed in YBCO-secondary phase films grown by PLD has been successfully recovered by increasing the deposition temperature. Transport properties analyses revealed improved pinning force values for YBCO-BZO films for BZO concentration above 5 mol %. These films exhibited an anomalous dual peak structure which has been ascribed to the presence of two pinning contributions. The crossover field, B(m), separating these two pinning regimes has been observed to be temperature independent suggesting the occurrence of the matching field effect. The measured value of B(m) agreed with the matching field value, B(Phi)=Kn(d)Phi(0), as evaluated from transmission electron microscopy investigations. The mark of this phenomenon is retained down to 10 K, making YBCO-BZO more performing than pristine YBCO films at applied magnetic fields below B(m) in all the temperature range inspected. On the contrary, in the high magnetic field range (above B(m)), the effectiveness of correlated pinning is progressively reduced on lowering the temperature suggesting that at low temperatures the dominant pinning contribution arises from isotropic pinning centers. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3477451]

Varying the thickness of the Pd layer, the role of the Pd films deposited on the cube-textured Ni-based substrates, was assessed in order to investigate the issues related to substrate oxidation phenomena and their impact on the quality of the buffer layer structures. Using a low temperature annealing, 600 °C, a threshold for the Pd film thickness, beyond which the Pd layer hinders the oxides formation on the substrate, can be defined. When the Pd-Ni interdiffusion is complete, oxide phases of Ni and of Ni-W coexist in the substrate. Besides, at temperatures as high as 800°C, the NiWO4 formation is favoured as the thickness of the Pd layer increases. Despite of these phenomena, development of a buffer layer template suitable for YBCO growth are reported with a thickness for the Pd layer as low as 20 nm.

In the present paper, we analyze the role of in situ grown BaZrO3 (BZO) inclusions in YBa2Cu3O7−x (YBCO) thin films prepared by chemical solution deposition using a low fluorine coating solution, on the field angle dependence of the critical current density, J c (𝜃), data using the vortex path model. In order to form a coherent picture on the BZO doping influence on the pinning properties of the YBCO matrix, detailed structural analyses performed by X-ray diffraction techniques and microstructural evaluation by transmission electron microscopy are also presented. The evaluation of different contributions to the overall, J c , permitted us to prove the effectiveness of the BZO inclusions acting as isotropic pinning centers, reflected in a uniform component of high relative value with respect to other components. For the studied 10 mol % BZO doping concentration, a threefold increase in the critical current density, J c , of the YBCO host is measured, in self-field at 77 K, corresponding to a value of J c =2.9MA/cm2, whereas a factor 10 is measured at 1 T (J c =0.35 MA/cm2).

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films, where sub-micrometric BaZrO$_3$ particles have been incorporated. As a function of the BaZrO$_3$ content, we observe that the absolute losses slightly decrease up to a BaZrO$_3$ content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently, in that they are not monotonic with increasing BaZrO$_3$ concentration: at small concentration (2.5%) the field-induced losses increase, but large reduction of the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO$_3$ concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance we estimate vortex pinning-related parameters. We find that BaZrO$_3$ inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO$_3$ content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, thus leaving room for further improvements., Comment: 7 pages, 7 figures

MgO films are widely used as the first buffer layer in IBAD-YBCO coated conductors, whereas there are only a few examples of its use in RABiTS YBCO coated conductors. Nevertheless MgO, though challenging in terms of lattice parameter and epitaxy, can effectively passivate the metal substrate thus enabling the use of Ni-Cu-based alloy tapes. Such substrates develop a very sharp cube texture and at the same time can be safely handled when their thickness is as low as 40 μm. Epitaxial MgO films were deposited by e-beam evaporation at 400 °C on biaxially textured metal substrates. A transient Pd layer as thin as 10 nm was used to alleviate the lattice mismatch between MgO and the substrate and to enhance film adhesion. Due to the reduced diffusion of oxygen in MgO, this template is effective against substrate oxidation even in the extreme conditions typical of the YBCO growth. On such a template, La0.66Sr0.33MnO3 (LSMO) was epitaxially deposited by PLD. Moreover, BaZrO3 (BZO) film was successfully deposited by CSD on MgO single crystal and is expected to epitaxially grow also on MgO film.

We performed an extensive characterization of Co-doped polycrystalline 122 iron pnictide samples, Ba(Fe1-xCox)2As2 with x=0.07, 0.09, 0.1, 0.12, by means of high-field dc magnetization and ac 1st and 3rd harmonic susceptibility response measurements. From M(H) loops through the Bean model formula both the magnetic field and T dependences of the Jc are derived. From the 3rd harmonic response of the ac susceptibility as a function of the dc magnetic field up to 10 T recorded at different temperatures, we determined the irreversibility line, using the onset criterion. From these analyses, 7% and 9% of Co-doped samples showed the best Jc performances.

The interface between the cube textured Ni-W substrate, both bare and Pd-buffered, and the CeO2/YSZ buffer layer structure-required to growth YBCO coated conductors-samples have been subjected to different oxidizing conditions and have been investigated with different techniques: X-ray diffraction (XRD), X-ray absorption and Auger electron spectroscopy. Due to the Ni-Pd interdiffusion, the presence of a Pd over layer significantly modifies the substrate surface composition and the substrate oxidation mechanism. Depending on the Pd layer thickness, the formation of NiO can be completely prevented while the formation of ternary oxides and of W oxide is favored. Moreover, the Pd layer affects the extension of interface between the substrate and the CeO2 layer and the portion of uncontaminated CeO2.

We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency ($\nu=$47.7 GHz) in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films grown on SrTiO$_3$ substrates, as a consequence of the introduction of sub-micrometric BaZrO$_3$ particles. The field increase of the surface resistance is smaller by a factor of $\sim$3 in the film with BaZrO$_3$ inclusions, while the zero-field properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO$_3$ inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film., Comment: RevTeX; 4 pages, 3 figures; submitted to Applied Physics Letters