The Effect of Ta and Ti Additions on the Strain Sensitivity of Bulk Niobium-Tin

The effect of tantalum and titanium additions on the composition, the superconducting properties, and their sensitivity to strain of bulk Nb3Sn is investigated. Using heat capacity analysis and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), it is found that the binary Nb3Sn bulk and Nb3Sn bulk with added titanium and tantalum consist of stoichiometric Nb3Sn and niobium(-oxide). Furthermore, it is found that the niobium-to-tin ratio decreases in the presence of tantalum and increases in the presence of titanium, which suggests that tantalum is replacing niobium and titanium is replacing tin in the A15 crystal structure. Using a 10% resistivity criterion, it is observed that the critical magnetic field of unstrained binary bulk is 26.7 T, while the presence of tantalum and titanium raises the critical magnetic field to 29.3 and 30.1 T, respectively. The curves of normalized critical magnetic field as function of strain of all three samples nearly overlap, a strong indication that the variation in strain sensitivity observed in wires is not caused by the titanium and tantalum additions. Understanding the effect of additions on the composition, superconducting properties, and strain sensitivity of Nb3Sn is important for optimizing Nb3Sn conductor technolog

An experimental and computational study of strain sensitivity in superconducting Nb3Sn

The superconducting properties of Nb3Sn, a material that is commonly used in high-field magnet applications, are strongly reduced when the material is deformed. The sensitivity to strain is problematic in high-field magnet applications where thermal contraction differences between materials and Lorentz forces during operation may result in a significant reduction in the performance of the Nb3Sn conductor and thus of the application itself. In order to understand the strain sensitivity, an experimental and a computational investigation are combined into a comprehensive and validated model that explains the strain sensitivity of superconducting Nb3Sn. Binary intermetallic A15 Nb-Sn thin films with various compositions were fabricated and characterized in terms of composition and morphology. The resistivity and critical current density of these thin films, as well as the resistivity of bulk samples, were measured as a function of temperature, magnetic field, longitudinal strain, and transverse strain. From the measurements, the strain dependence of the superconducting properties was parameterized. The validity of the previously published MAG description of the temperature, magnetic field, and strain dependent critical current density Jc is demonstrated, which relates the strain sensitivity of the Jc of Nb3Sn to the strain dependent critical temperature Tc and upper critical magnetic field Hc2. Two other commonly used descriptions are found consistent with this description despite being presented in a different form. A computational model is presented which combines ab-initio calculations with microscopic theory, and the results are shown to reproduce the experimentally observed, normal-state-resistivity-dependent martensitic transformation, critical temperature, and upper critical magnetic field. It is shown that the relatively large strain sensitivity of Tc and Hc2 is a result of a strain-induced distortion of the niobium chains, which is referred to as sublattice distortion. This calculation result is experimentally validated with measurements of the normal state resistivity, Tc, and Hc2 as a function of strain on Nb-Sn thin films, bulk samples, and high-Jc conductors. The lower strain sensitivity of Nb3Al and the much lower strain sensitivity of bcc Nb and Nb-Ti are explained in terms of a weaker and a non-existent sublattice distortion, respectively

Fundamental origin of the large impact of strain on superconducting Nb3Sn

Superconductors can carry very high current densities without resistive loss. This makes them highly suitable for the construction of compact high field magnets of several tens of tesla. However, with increasing current density and magnetic field come high Lorentz loads and high strain levels, and it is empirically known that superconducting properties are affected by strain. The superconducting properties of the most commonly used high field material, Nb3Sn, are particularly badly affected by strain. Here we demonstrate that strain causes significant sub-lattice distortion in the A15 lattice structure of Nb3Sn, and show how this leads to the relatively large reduction of its superconducting properties. The changes are found to be primarily due to changes in the electron density of states, with a lesser contribution due to changes in the phonon spectrum. The amount of sub-lattice distortion further depends on crystal orientation. These findings suggest that it is possible to mitigate the reduction of the current carrying capacity by strain through crystal alignment and sub-lattice stabilization. This would enable superconducting magnets to reach a significantly higher magnetic field, and ease their construction by simplifying react-and-wind coil fabrication

Characterization of High Current RRP(R) Wires as a Function of Magnetic Field, Temperature and Strain

A new instrument for the characterization of superconducting materials as a function of Magnetic Field, Temperature and Strain, was designed, constructed and tested at Lawrence Berkeley National Laboratory (LBNL). A U-shaped bending spring was selected, since that design has proven to enable accurate characterizations of a multitude of superconducting materials for more than a decade. The new device is validated though measurements on very high current Rod Restack Processed (RRP) Internal-Tin (IT) wires, for which we will present initial results, including parameterizations of the superconducting phase boundaries and comparisons with other wire types. Accurate parametrization of modern high magnetic field conductors is important for the analysis of the performance of magnet systems

The effects of disorder on the normal state and superconducting properties of Nb3Sn

The effect of disorder on the normal state resistivity and the superconducting properties of Nb3Sn Sn is explored in a combination of ab initio calculations and microscopic theory. The crystal symmetry is calculated to be preferentially tetragonal at a normal state resistivity below 27.0 ±1.4 μcm, and preferentially cubic above this value, which is shown to be consistent with the experimentally observed transition point. The phonon density of states, the Eliashberg spectrum a2 (w)F (w), the electronphonon coupling constant, the characteristic frequency, the critical temperature Tc, and the upper critical magnetic field at 0 K Hc2 (0) are calculated over a large normal state resistivity range and shown to be consistent with experimental observations. The high degree of consistency between the calculation results and experimental observations is a strong indication that the calculation approach utilized here, a combination of ab initio calculations and microscopic theory, is a useful tool for understanding the superconducting and normal state properties of Nb3Sn

Superconductivity in Nb-Sn thin films of stoichiometric and off-stoichiometric compositions

Binary Nb-Sn thin film samples were fabricated and characterized in terms of their composition, morphology, and superconducting properties. Nb-Sn was magnetron-sputtered onto heated R-plane sapphire substrates at 700°C, 800°C, and 900°C, using a custom-built heater assembly. Samples were cut into strips, where each strip has a unique composition. For a subset of the samples, Nb-Sn was selectively etched away at an etching rate of 6 ± 1 nm/s using an aqueous solution of 3 vol.% hydrofluoric and 19 vol.% nitric acid. The sample composition was investigated with a scanning electron microscope with an X-ray energy dispersive spectroscopy detector. Surface and cross-section morphologies were investigated using scanning electron microscopy and scanning transmission electron microscopy, revealing a dense columnar poly-crystalline grain structure. X-ray diffraction measurements indicate a highly textured film that is (100) oriented out-of-plane and random in-plane. The critical temperature Tc (ranging from 9.8 to 17.9 K), critical magnetic field μ0Hc2 (ranging from 12.5 to 31.3 T), residual resistivity ratio (RRR), and normal state resistivity ρ0 were measured and found to be broadly consistent with literature data on bulk Nb3Sn

Optimized and Practical Electrical Joints for CORC type HTS Cables

Within CERN the development of REBCO-CORC (Conductor On Round Core) type cables is pursued in view of possible application in future detector and accelerator magnets. An important issue is the design and qualification of terminations for connecting CORC cables mutually or to bus-bars. A termination design is envisaged that combines a simple manufacturing process with a lowest possible joint terminal resistance in the few nΩ range at 4.2 K, first for a single CORC cable and subsequently for CORC based Cable-in-Conduit Conductors. The investigation concerns the effect of tapering the CORC cable within the joint to form a staircase like geometry, which allows current to pass more directly from the copper joint casing to the inner REBCO layers of the CORC cable. Simulations have shown a substantial decrease in joint resistance at operating current in the case both CORC cable and joint casing are tapered. The CORC cable and new joint were tested at CERN. In this paper, some details of the new joint design, fabrication process, and model are presented and the results are summarized

Analysis of bulk and thin film model samples intended for investigating the strain sensitivity of niobium-tin.

Bulk samples and thin films were fabricated and characterized to determine their suitability for studying the effect of composition and morphology on strain sensitivity. Heat capacity and resistivity data are used to determine the critical temperature distribution. It is found that all bulk samples contain stoichiometric Nb{sub 3}Sn regardless of their nominal Nb to Sn ratio. Furthermore, in bulk samples with Cu additions, a bi-modal distribution of stoichiometric and off-stoichiometric Nb-Sn is found. Thus the nominally off-stoichiometric bulk samples require additional homogenization steps to yield homogeneous off-stoichiometric samples. A binary magnetron-sputtered thin film has the intended off-stoichiometric Nb-Sn phase with a mid-point critical temperature of 16.3 K. This type of sample is a suitable candidate for investigating the strain sensitivity of A15 Nb{sub 1-{beta}}Sn{sub {beta}}, with 0.18 < {beta} < 0.25. The strain sensitivity of Nb-Sn as a function of composition and morphology is important for an in-depth understanding of the strain sensitivity of composite Nb{sub 3}Sn wires