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Among Iron-Based Superconductors (IBS), the development of superconducting films of FeSe1−xTex [Fe(Se,Te)] is looked at with increasing interest due to the high performance in high magnetic fields and low temperatures. In this work, we report on the optimization of the deposition conditions of Fe(Se,Te) films grown on single-crystal CaF2 substrates by pulsed laser deposition (PLD) technique. The deposition of a Fe(Se,Te) bilayer is exploited to control the stoichiometry and the epitaxy simultaneously. The Fe(Se,Te) top layer is deposited at a temperature as low as 250 °C, using a film of the same material deposited at a higher temperature as a seed layer. The seed layer's structural properties and morphology have been studied by varying the deposition temperature between 350 and 470 °C. Seed layers with sharp out-of-plane orientation and smooth surfaces have been obtained at deposition temperatures of about 400 °C. Fe(Se,Te) bilayer (top +seed layer) shows enhanced superconducting properties compared to single-layer films, with a zero-resistance critical temperature of about 19 K and a critical current density above 0.1 MAcm−2 up to 9 T and 4.2 K. These results demonstrate the importance of the fine control of micro-structural and morphological properties of the seed layer to unlock high-performance Fe(Se,Te) films.

Among Iron Based superconductors (IBSC), the compounds belonging to the 1144 family are characterized by the A1AE1Fe4As4 chemical composition (A = Alkaline, AE = Alkaline-Earth) and considered stoichiometric. In recent experiments, we obtained 1144 samples characterized by different levels of alkali and alkaline earth metals substitutions and demonstrated how the lattice distortion induced by such variation in the chemical composition plays a fundamental role on critical temperature values. In this work, we investigate the possibility to obtain Ca/K-1144 compounds substituted with Rare-Earth (RE) elements. We produced polycrystalline samples in which RE ions partially replace Ca ions in the 1144 phase. The lattice structure is consistently affected by the substituent element inducing a contraction of the c-axis. Multiple substitution of RE and A or AE elements was also attempted, showing that appropriate combinations allow simultaneous inclusion of different elements in the structure. For compounds that include RE elements, the critical temperature of the samples appears to follow to the same trend with respect to the distortion of the crystal lattice induced by A and AE substitutions.

Although Fe(Se,Te) film shows limited superconducting properties compared to REBCO in terms of critical temperature and critical current density, it represents a potential low-cost alternative to REBCO-based coated conductor applications at low temperature and high field condition. In fact, due to the low deposition temperature and absence of oxygen, materials constraints are less strict and Fe(Se,Te) coated conductor can be realized using a very simplified architecture. A further process simplification is the use of cube-textured substrate combined with a single buffer layer by chemical solution deposition (CSD), which may provide a cheap, vacuum-free route for oriented template fabrication. Zr-doped CeO2/Ni-W templates were realized by CSD and successfully tested with epitaxial Fe(Se,Te) film growth by pulsed laser deposition showing good superconducting properties. Further, a detailed analysis before and after buffer layer deposition using optical, atomic, and scanning electron microscopy, electron backscattered diffraction and Raman and X-ray photoelectron spectroscopy was carried out to relate Zr-doped CeO2 film quality to Ni-W microstructure, morphology, and composition. Results show that 30-nm-thick Zr-doped CeO2 film is effective to prevent Ni diffusion. Further, buffer-layer quality is mainly dependent on the characteristics of individual Ni-W grains.

Ca/K-1144 compounds constitute promising materials to be exploited for the fabrication of Iron Based Superconductors (IBSC) wires via the Powder in Tube (PIT) method thanks to the high critical currents observed in single crystals coupled to the simple and robust chemical composition. The production of Ca/K-1144 wires has been however hindered by the reactivity of Ca with Ag, the common choice for sheath materials in IBSC wires. In our recent work, we demonstrated the potentiality of composite Cu/Ta sheaths to be adopted for these kinds of applications. In this work, we show the effect of the variation of sintering temperature and mechanical processing on the morpho-structural and superconducting properties of wires processed through this combination of materials. While critical currents are still to be improved, compared to state of the art 122-Ag wires, with evident margins for what concerns both the synthesis step and the wire production process, the results show how sintering temperature can be raised up to 900 °C. The intrinsic fragility of pure Ta observed during the wire processing is proposed to be mitigated by tailoring the sheath dimensions or the Ta barrier chemical composition.

In recent years, iron-based superconductors have attracted attention due to their interesting electromagnetic properties that open wide application prospects. Among these, FeSe is particularly studied since it is both a superconductor with a bulk critical transition temperature, Tc, of 8 K and a semiconductor used in solar cells. For the design of an iron-based superconductor, the preparation of larger samples is envisaged, and the possibility of maintaining low manufacturing costs is extremely appealing. One possible technique suitable for this purpose is electrodeposition, which combines inexpensive equipment to the easy scaling towards larger/longer samples. In this work, we study the possibility of using electrodeposition as a method for FeSe film growth. We start by analyzing the electrochemical behaviour of the precursor solution via cyclic voltammetry before attempting electrodeposition. We find out that the main drawback of this technique is the microstructure of the deposited film, which is made mostly of unconnected grains. This issue does not rule out this technique for the preparation of superconducting thin films, but it has to be addressed in view of a potential application of this technique to the preparation of an iron-based coated conductor.

Technologies based on High Temperature Superconductors (HTS) are evolving rapidly toward maturity. Within the magnetic confinement fusion environment, several projects are demonstrating the possibility to integrate HTS in the coil systems. With respect to Low Temperature Superconducting (LTS) technologies, HTS could allow extending the operating space of fusion coils, either at higher temperatures, or at higher magnetic field levels, and in any case with larger operating margins. Different perspectives and development strategies are proposed, depending on whether HTS is considered a technology to completely substitute LTS, or to integrate and extend its performance range. A fundamental common requirement is the assessment of the layout, the feasibility, and performance demonstration of high-current conductors. Starting from the results achieved with the Al-slotted core cable-in-conduit conductor, and with a view on existing concepts for standard copper and aluminum cables, we have designed a new HTS sector-cable concept, to allow a flexible conductor design and a robust industrial processing. Several trials have been carried out, to verify the manufacturing approach, using either Al- or Cu-based stabilizers. Prototype sub-cables have been characterized at 77 K and self-field, as a necessary step toward the final target of a CICC operating stably with 60 kA at 4.2 K and 18 T, that is presently of interest for the EU-DEMO Central Solenoid Coil.

Among IBSC superconductors, the 1144 family (AAEFe4As4 with A = alkali metal, AE = alkaline-earth metal), has attracted considerable interest in recent years because of their propensity to form lattice defects that positively influence the flux pinning properties. Extensive research is underway to optimize this class of materials for the low-temperature high-field regime, both in the form of single crystal and polycrystalline powder. The latter is of particular interest because the 1144 material has been shown to be suitable for wire production through the easy and well-assessed Powder In Tube (PIT) process. In our recent experiments, it has been shown that the 1144 structure can be tailored to obtain a doped compound with a different chemical formula (AxAE1-x)(AEyA1-y)Fe4As4, that is characterized by unaltered critical temperature values despite the high level of substitution up to 15 at.%. In order to unveil the influence of the double substitution on the grain boundary and pinning properties of doped samples, we recently started an extensive characterization campaign whose preliminary results are presented here. Both structural and morphological properties of the compound are not affected by the simultaneous doping with 10 at.% Na and 5 at.% Ba, as revealed by XRD and SEM analyses. On the contrary, the study of magnetic properties shows that although there is no improvement in transport properties, the aliovalent substitution induces a clear change in the in-field behavior which deserves further investigations.

Iron based superconducting wires (IBSCs) produced by the Powder in Tube (PIT) method rely on the use of silver sheaths as chemical buffer between the outer metal and the superconducting core. The adoption of silver entails however some limitations, such as the viable temperature range when coupled with copper, and the incompatibility with calcium-based IBSCs already at 600°C, driving the research towards other wires architecture. Taking inspiration from the low temperature superconductors field, we decided to evaluate the adoption of tantalum as diffusion barrier in a layered Cu/Ta architecture, choosing a Ca/K-1144 IBSC as case study considering the high reactivity issues already reported in the case of silver sheaths for this compound. Squared wires were produced through a groove rolling lamination process coupled with a thermal treatment at 800°C. The microstructural analyses show the absence of interdiffusion between the different parts of the wire, and the magnetic characterization shows performance in line with similar polycrystalline manufacts, with margin of enhancement to be pursued via the optimization of the mechanical process and other experimental variables. The reported results suggest thus the effectiveness of tantalum as diffusion barrier for Ca/K-1144 PIT wires.

Silver free Ti-43at.%Cu (Ti-Cu) eutectic alloy have been developed for the brazing of Be and Cu99.32Cr0.6Zr0.08 (CuCrZr). The structural and microstructural properties of the as obtained alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The analysis have revealed that the Cu-Ti eutectic alloy have a fine and homogeneous eutectic structure with the phase composition corresponding to the Ti-Cu phase diagram. These observations are consistent with the thermal properties of the alloys determined by DTA-TG analysis. The role of Mn on the eutectic structure, as well on the wetability properties was also studied. The effect of Mn is to produce a less coarse eutectic structure and to decrease the wetability angle. The mechanical testing of CuCrZr/Ti-Cu/CuCrZr junctions have demonstrated that these alloys are suitable for the brazing of Be and CuCrZr. The shear strength of the CuCrZr/TiCuMn/ CuCrZr increases with Mn concentration reaching the value of about 280 MPa for 2at% Mn.

The realization of YBCO-based coated conductors with high critical current density involves the deposition of highly biaxially textured YBCO films. The use of epitaxial growth shifts this requirement to the template used for YBCO deposition. In the rolling-assisted biaxially textured substrate (RABiTS) approach, an appropriately oriented template is provided by a cube-textured substrate. The development of a cube texture is the result of recrystallization occurring in a heavily deformed tape, which is activated by high-temperature annealing. In the case of Ni-based alloys, thermal treatment at temperatures ranging from 900 to 1150?°C for at least 30 min is commonly used. The determination of the minimum conditions for thermal treatments in terms of temperature-time involved in the recrystallization process is therefore of practical interest.

The toughness mechanisms of epoxy-based nanocomposites reinforced by double-walled carbon nanotubes (DWCNT) are investigated using Raman microscopy mapping. Maps of the residual strain of the carbon nanotubes were generated for the fracture surfaces obtained after tensile loading. By imaging through different microscope objectives, Raman mapping allowed to determine the distribution of the nanotube residual strain, indicating tension or compression, in different sampling volumes near the fracture surface. The G′ band with its high sensitivity to nanotube diameter variations, and not overlapping with the characteristic epoxy Raman lines, was selected for the mapping. Results provide a quantitative analysis of the nanotube bridging and pull-out mechanisms identified by scanning electron microscopy (SEM) inspection of the fracture surfaces.