A. A. Squitieri

The upper critical field of filamentary Nb3Sn conductors

We have examined the upper critical field of a large and representative set of present multifilamentary Nb3Sn 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 μ0Hc2 detected in the ternary wires are remarkably constant: The highest zero-temperature upper critical fields and zero-fiel...

The microstructure and microchemistry of high critical current Nb/sub 3/Sn strands manufactured by the bronze, internal-Sn and PIT techniques

Recent advances in Nb/sub 3/Sn conductor development have advanced the non-Cu critical current density, J/sub c/, from 2000 A/mm/sup 2/ to almost 3000 A/mm/sup 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/sub 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.

Microchemical and microstructural comparison of high performance Nb/sub 3/Al composites

We have performed a comparison of the microstructures of state of the art Nb/sub 3/Al composites processed using both ordinary RHQT (rapid-heating, quenching and transformation) and TRUQ (transformation-heat-based up-quenching) routes. Cross-sections were examined in the as-quenched, untransformed, and final size strands including Cu-clad strand. Both grain size and microchemistry were examined using a high resolution FESEM in BEI mode, using low accelerating voltage for grain orientation contrast and high voltage for atomic number contrast. The grain size is relatively large in these composites with a Feret diameter of 1300 nm for the TRUQ processed strand (compared with 70 to 160 nm for Nb/sub 3/Sn composites). In the untransformed strand electron backscatter indicated residual chemical inhomogeneity associated with the jelly-roll precursor. In the final strands the variations were much less but longitudinal cross-sections revealed the residual chemical inhomogeneity extending along the strand length. In both the ordinary and TRUQ (Cu and Cu-clad) processed strands a 1 /spl mu/m thick 2-phase reaction layer was revealed on the outside surface of the outer filaments that had an average composition of 10% Al and 90% Nb. D.C. Magnetization measurements at 12 K indicated a 1 T improvement in irreversibility field, H/sup */, for the TRUQ strand compared with ordinary RHQT strands.

The microstructure and critical current density of NB-48 wt.%Ti superconductor with very high alpha-Ti precipitate volume and very high critical current

Superconducting strand has been fabricated with the highest yet recorded 5 T, 4.2 K critical current density for conventionally processed Nb-Ti. Starting from an ingot of high homogeneity Nb-48 wt. % Ti alloy, both mono- and multifilamentary superconductors were fabricated using multiple heat treatment and drawing schedules with very long heat treatments. Using high-resolution FESEM and TEM, the influence of treatment conditions on the microstructure was investigated. With 1000 to 8000 h of total heat treatment time, the volume of precipitate approaches the decomposition limit of the solid solution and the critical current density increases, reaching a maximum >4⋅105 A/cm2 at 5 T, 4.2 K. The amount of the precipitated alpha-phase reaches 25–33 vol. %, and the precipitate thickness is 1–4 nm. In order to create a more ordered multilayer structure in the high precipitate-content strand, the samples were rolled as a final stage of processing. The highest level of current characteristics is reached with this ...

High field flux pinning and the upper critical field of Nb-Ti superconductors

Nb-Ti wires of different composition and metallurgical state have been studied near the upper critical field (H/sub c2/) in an attempt to separate H/sub c2/ the irreversibility field (H*) and the resistively measured transition field (H/sub r/). For optimized multifilamentary Nb-47wt.%Ti wire, we find that H*=10.2 T, H/sub c2/=10.8 T and H/sub r/=11.8 T at 4.2 K. The transitions were studied by extended electric field vs. current density curves, magnetization and small current transport measurements. We found a strong correlation between magnetization, bulk flux pinning (F/sub p/) extrapolation and extended E-J curves for the value of H*, all giving H*=10.2 T. We have also shown that the maximum in H/sub r/ occurs for Nb-44wt.% Ti.

Development of high performance Nb-Ti(Fe) multifilamentary superconductor for the LHC insertion quadrupoles

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/mm/sup 2/ 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 /spl alpha/-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.

Multifilamentary Nb3Sn wires reacted in hydrogen gas

Reaction of Nb3Sn wires in an Incoloy 908® conduit in the presence of oxygen can decrease the conduit’s fracture toughness. This may be avoided by reacting in a reducing atmosphere of 5% hydrogen-95% argon. The effects of the hydrogen reaction on the superconductive properties of bronze-process and internal-tin-process Nb3Sn wires were investigated. Compared with the standard heat treatment in partial vacuum, critical currents decreased in the bronze wire by 12% at 12 T and 14% at 5 T, and in the internal-tin wire by 4% at 12 T and 10% at 8 T. Hysteresis losses over a ±3 T field cycle were reduced by 14% for the bronze wire and by 8% for the internal-tin wire by the hydrogen heat treatment, approximately proportional to the reduction in critical current. The critical temperature was reduced by 0.5 K. There was no significant effect on the residual resistivity ratio of the stabilizing copper.