We have examined the upper critical field of a large and representative set of present multifilamentary Nb 3 Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero-field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90%-99% of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero-temperature critical field that is comparable to the ternary wires. The highest 0 H c2 detected in the ternary wires are remarkably constant: The highest zero-temperature upper critical fields and zero-field critical temperatures fall within 29.5± 0.3 and 17.8± 0.3 K, respectively, independent of the wire layout. The complete field-temperature phase boundary can be described very well with the relatively simple Maki-DeGennes model using a two-parameter fit, independent of composition, strain state, sample layout, or applied critical state criterion.

In powder-in-tube Nb 3 Sn composites, the A15 phase forms between a central tin-rich core and a coaxial Nb tube, thus causing the tin content and superconducting properties to vary with radius across the A15 layer. Since this geometry is also ideal for magnetic characterization of the superconducting properties with the field parallel to the tube axis, a system of concentric shells with varying tin content was used to simulate the superconducting properties, the overall severity of the Sn composition gradient being defined by an index N. Using well-known scaling relationships and property trends developed in an earlier experimental study, the critical current density for each shell was calculated, and from this the magnetic moment of each shell was found. By summing these moments, experimentally measured properties such as pinning-force curves and Kramer plots could be simulated. We found that different tin profiles have only a minor effect on the shape of Kramer plots, but a pronounced effect on the irreversibility fields defined by the extrapolation of Kramer plots. In fact, these extrapolated values H K are very close to a weighted average of the superconducting properties across the layer for all N. The difference between H K and the upper critical field commonly seen in experiments is a direct consequence of the different ways measurements probe the simulated Sn gradients. Sn gradients were found to be significantly deleterious to the critical current density J c , since reductions to both the elementary pinning force and the flux pinning scaling field H K compound the reduction in J c . The simulations show that significant gains in J c of Nb 3 Sn strands might be realized by circumventing strong compositional gradients of tin.

Abstract-Recent advances in Nb 3 Sn conductor development have advanced the non-Cu critical current density, , from 2000 A/mm 2 to almost 3000 A/mm 2 (12 T, 4.2 K). We have quantified a variety of state of the art composites for their microstructures using the fracture/Field Emission Scanning Electron Microscope, FESEM, technique and their microchemistry using Energy Dispersive X-ray Spectroscopy (EDS)/FESEM. The results of the measurements increasingly point to the importance of A15 composition in determining the critical current density as well as grain size. The highest critical current densities, however, are being attained by the internal Sn process which has yet to achieve as high a level of Sn (23-24.5 at.% Sn) in the A15 as for Powder-in-Tube (PIT) in which we measure as high as 25-26 at.% Sn. When Sn diffuses into the Cu stabilizer, it is found to have a great affinity for Nb 3 Sn formation than dissolution into the Cu. A15 forms at the Nb-stabilizer surface with local Cu concentrations within the grains of the stabilizer of less than 0.1 at.% Cu. Elevated levels of Sn, however, were observed at the Cu grain boundaries. Both the quantified variations in composition and the peak levels of Sn indicate that further increases in performance should be expected.

No abstract

Abstract-A development program was initiated in order to develop strand with improved current density at 10.5 T and 1.9 K over existing SSCL designs. The two successful strand designs reported on here both utilized high Fe content Nb-47 wt%Ti alloys to improve the critical current density at high field by 7 %. At 10.5 T and 1.9 K, critical current densities exceeding 1450 A/mm2 were obtained. In this paper we report detailed quantification of the macro-and micro-structures of these strands and correlate these with critical current density measurements at 1.9 K and 4.2 K. The high Fe content significantly reduced the a-Ti precipitate size. The linear relationship between critical current density and precipitate volume found is in agreement with earlier studies. High resolution FESEM electron backscatter contrast suggests a thin layer of high atomic number at grain boundaries.

-A preliminary investigation of a new Nb-Ti-Ta (39wt%Nb, 44wt.%Ti, 17wt.%Ta) alloy has been investigated as a possible material for application at 1.9 K and 10.5 T in the insertion quadrupoles of LHC. 1550 A/mm*, the highest yet reported critical current density at 10.5 T (1.9 K), was achieved in a monofilament of this material. The initial multifilamentary production strand produced a lower 10.5 T (1.9 K) critical current density of 1370 A/mmz. Large variations in precipitate size were produced in the microstructures, which have yet to be fully optimized. Quantitative analysis of the microstructures in a Nb-44 wt.%Ti-15 wt.%Ta alloy reveals a linear relationship between volume % of a-Ti precipitate and critical current density at 5 T and 8 T (4.2 K). The increase in critical current with precipitate volume is less than for Nb-47 wt.%Ti. High resolution FESEM electron backscatter images suggest a high atomic number region adjacent to the grain boundaries after heat treatment.

No abstract