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[en] Epitaxial titanium diboride thin films have been deposited on sapphire substrates by pulsed laser ablation technique. Structural properties of the films have been studied during the growth by reflection high energy electron diffraction (RHEED) and ex-situ by means of X-ray diffraction techniques; both kinds of measurements indicate a good crystallographic orientation of the TiB2 film both in plane and along the c axis. A flat surface has been observed by atomic force microscopy imaging. Electrical resistivity at room temperature resulted to be five times higher than the value reported for single crystals. The films also resulted to be very stable at high temperature, which is very promising for using this material as a buffer layer in the growth of magnesium diboride thin films
[en] Critical fields of four MgB2 thin films with a normal state resistivity ranging from 5 to 50 μΩ cm and Tc from 29.5 to 38.8 K were measured up to 28 T. Hc2(T) curves present a linear behavior towards low temperatures. Very high critical field values have been found, up to 24 T along the c-axis and 57 T in the basal plane not depending on the normal state resistivity values. In this paper, critical fields will be analyzed taking into account the multiband nature of MgB2; we will show that resistivity and upper critical fields can be ascribed to different scattering mechanisms
[en] We report on the microwave surface resistance of two polycrystalline Mg11B2 samples; one consists of pristine material, the other has been irradiated at very high neutron fluence. It has already been reported that in the strongly irradiated sample the two gaps merge into a single value. The mw surface resistance has been measured in the linear regime as a function of the temperature and the DC magnetic field, at increasing and decreasing fields. The results obtained in the strongly irradiated sample are quite well justified in the framework of a generalized Coffey and Clem model, in which we take into account the field distribution inside the sample due to the critical state. The results obtained in the pristine sample show several anomalies, especially at low temperatures, which cannot be justified in the framework of standard models for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields greater than 0.5Hc2(T) the experimental data can quantitatively be accounted for by the Coffey and Clem model, provided that the upper-critical-field anisotropy is taken into due account. (authors)
[en] In modern Nb_3Sn wires there is a fundamental compromise to be made between optimizing the intrinsic properties associated with the superfluid density in the A15 phase (e.g. T_c, H_c, H_c_2, all of which are composition dependent), maximizing the quantity of A15 that can be formed from a given mixture of Nb, Sn and Cu, minimizing the A15 composition gradients within each sub-element, while at the same time generating a high vortex pinning critical current density, J_c, by maximizing the grain boundary density with the additional constraint of maintaining the RRR of the Cu stabilizer above 100. Here we study these factors in a Ta-alloyed Restacked-Rod-Process (RRP"®) wire with ∼70 μm diameter sub-elements. Consistent with many earlier studies, maximum non-Cu J_c(12 T, 4.2 K) requires preventing A15 grain growth, rather than by optimizing the superfluid density. In wires optimized for 12 T, 4.2 K performance, about 60% of the non-Cu cross-section is A15, 35% residual Cu and Sn core, and only 5% a residual Nb-7.5 wt% Ta diffusion barrier. The specific heat and chemical analyses show that in this 60% A15 fraction there is a wide range of T_c and chemical composition that does diminish for higher heat treatment temperatures, which, however, are impractical because of the strong RRR degradation that occurs when only about 2% of the A15 reaction front breaches the diffusion barrier. As this kind of Nb_3Sn conductor design is being developed for sub-elements about half the present size, it is clear that better barriers are essential to allowing higher temperature reactions with better intrinsic A15 properties. We present here multiple and detailed intrinsic and extrinsic evaluations because we believe that only such broad and quantitative descriptions are capable of accurately tracking the limitations of individual conductor designs where optimization will always be a compromise between inherently conflicting goals. (paper)
[en] PIT and RRP® Nb3Sn strands are being developed for high field accelerator magnet upgrades for the high luminosity LHC. Here we report a quantitative study of the shape and position of PIT filaments and RRP® sub-elements after rolling lengths of unreacted PIT and RRP® round wires to simulate cabling deformation. In the as-drawn condition, filament shape distortion occurs preferentially in the outer ring filaments. By contrast, rolling induces non-uniform shear bands that generate greater distortion of inner ring filaments. By making a full digitization of the shapes of all filaments, we find that a critical distortion occurs for thickness reductions between 10% and 20% when filament shapes in inner filament rings heavily degrade, especially in the vicinity of the strong 45° shear bands imposed by the rolling. It is well known that maintaining diffusion barrier integrity is vital to retaining adequate RRR in the stabilizing copper needed for magnet stability. Diffusion barrier breaks occur preferentially in these distorted inner filaments and drive local Sn leakage during reaction, increasing RRR degradation. (paper)
[en] Powder-in-tube (PIT) Nb_3Sn wires are competing with Restacked-Rod-Process (RRP"®) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP"® and a lower non-Cu J_c (12 T, 4.2 K) (2500–2700 A mm"−"2 versus 2900–3000 A mm"−"2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ∼25% is made up of ∼1–2 μm sized grains (typically ∼10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-T_c grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with T_c <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer J_c and high specific grain boundary pinning force, Q_G_B. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance. (paper)
[en] We report on the development of multifilamentary Nb3Sn superconductors by a versatile powder-in-tube technique (PIT) that demonstrates a simple pathway to a strand with a higher density of flux-pinning sites that has the potential to increase critical current density beyond present levels. The approach uses internal oxidation of Zr-alloyed Nb tubes to produce Zr oxide particles within the Nb3Sn layer that act as a dispersion of artificial pinning centres (APCs). In this design, SnO2 powder is mixed with Cu5Sn4 powder within the PIT core that supplies the Sn for the A15 reaction with Nb1Zr filament tubes. Initial results show an average grain size of ~38 nm in the A15 layer, compared to the 90–130 nm of typical APC-free high-Jc strands made by conventional PIT or Internal Sn processing. Furthermore, there is a shift in the peak of the pinning force curve from H/H irr of ~0.2 to ~0.3 and the pinning force curves can be deconvoluted into grain boundary and point-pinning components, the point-pinning contribution dominating for the APC Nb-1wt%Zr strands.
[en] We report specific heat measurements on neutron-irradiated MgB2 samples, for which the critical temperature is lowered to 8.7 K, but the superconducting transition remains extremely sharp, indicative of a defect structure extremely homogeneous. Our results evidence the presence of two superconducting gaps in the temperature range above 21 K, while single-gap superconductivity is well established as a bulk property, not associated with local disorder fluctuations, when Tc decreases to 11 K
[en] The effect of α-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature T c and the upper critical field H c2, and properties more of interest for applications, like the critical current density J c and the related pinning landscape. The irradiation-induced suppression of the film T c is significantly smaller than on a similarly damaged single crystal. Moreover H c2 behaves differently, depending on the field orientation: for H//c the H c2 slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature H c2, the effects on the J c(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 to 6 T after an irradiation of 2 × 1015 cm−2. On the contrary, J c(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects (PD) acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the J c(H//ab) performance is not related to a decrease of the J c anisotropy as found in other superconductors. Instead it is due to the presence of PD that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure. (paper)
[en] We study the critical current density Jc of a series of MgB2 epitaxial thin films with disorder introduced in a controlled way by neutron irradiation. We find that the in-field Jc is significantly increased for an intermediate neutron fluence of ∼7.6 x 1017 cm-2, reaching ∼1.8 x 105 A cm-2 at μ0H = 4 T, which is larger by more than one order of magnitude than the critical current density of the pristine sample. Moreover, in the same irradiated sample, Jc(μ0H = 9 T) is still as large as 1.1 x 104 A cm-2. By transmission electron microscopy, we find evidence of the presence of amorphous regions created by irradiation whose average size matches the coherence length ξab. Analysis of the shape of the critical current density and pinning force curves indicates that such regions with a suppressed superconducting order parameter behave as effective point-like pinning centers for magnetic flux lines.