C. L. H. Thieme

Improved high field performance of Nb-Al powder metallurgy processed superconducting wires

Improved overall critical current densities Jc’s were achieved with powder metallurgy processed Nb‐A1 which combined reduced powder sizes and increased nominal areal reductions R. Increased Jc values were obtained for a variety of different heat treatments. For a Nb‐8 wt. % A1 wire with R=3.4×105 a very short treatment at 1100 °C followed by a 750 °C treatment gave (at 4.2 K)Jc>104 A/cm2 at 19 T; a 900 °C treatment followed by a 750 °C treatment gave Jc=104 A/cm2 at 18 T and, at 2 K, Jc was greater than 104 A/cm2 at 20 T; and a treatment of 800 °C for 8 h gave (at 4.2 K)Jc=104 A/cm2 at 17.5 T.

Nb-Al powder metallurgy processed multifilamentary wire

Powder metallurgy processed Nb-Al wire with overall critical current densities, J c , at 4.2K up to 104A/cm2at 19T has been investigated in more detail, Kramer plots and directly measured H c2 values of samples with different heat treatments show an increase in H c2 at 4.2K up to 24.5T. Test coils, using long lengths of wire, and tested in fields up to 15T, show J c values equal to those of short samples. Multiple strand hydrostatic small scale extrusions were made. A number of third element additions including B, Mg, Co, Cr and Ni in fine powder form were incorporated in the P/M processing but these did not improve J c . ac losses were measured and used to determine J c at 10w fields. The high field ac losses are lower than that for any In Situ or powder processed Nb 3 Sn wires.

Nb/sub 3/Al wire produced by powder metallurgy and rapid quenching from high temperatures

Powder-metallurgy-processed Nb-25-at.% Al wires were annealed at temperatures just below the melting temperature. Depending on anneal conditions, the entire Nb-Al part of the wire could be rapidly quenched as a metastable Al Nb(Al) solid solution with an Al concentration exceeding 23 at.%. This A2 phase is sufficiently ductile to be bent without incurring damage. A second anneal at 950 degrees C converted the A2 into A15. J/sub c/ was 10/sup 4/ A/cm/sup 2/ at 22 T, and T/sub c/ was 17.8 K. The rapid quenching of Nb-Al powder metallurgy processed wire is a possible alternative to obtain improved high-field properties. >

Scaleup of powder metallurgy processed Nb-Al multifilamentary wire

Powder metallurgy processed Nb-Al superconductIng wires were fabricated from billets up to 45 mm o.d. with nominal areal reduction ratios, R, up to 2 × 105, Nb powder sizes from 40 to 300 μm from various sources, Al powder sizes from 9 to 75 μm, Al concentrations from 3 to 25 wt % Al and with a wide range of heat treatments. All the compacts used tap density powder in a Cu tube and swaging and/or rod rolling and subsequent wire drawing. Both single strand and bundled wires were made. Overall critical current densities, J c , of 2 × 104A/cm2at 14 T and 104A/cm2at 16 T were achieved for 6 to 8 wt % Al in Nb.

21 Tesla powder metallurgy processed Nb 3 Sn(Ti) using (Nb-1.2 wt%Ti) powders

Powder metallurgy (P/M) processed Nb 3 Sn(Ti) wires were made with alloyed Nb(Nb-1.2wt%Ti) powders using a Cu-45wt% (Nb-1.2wt%Ti) composite. Nominal areal reductions of about 104were used and Sn was introduced by means of a central Sn core. With a Sn-5wt%Cu core a critical current density J c = 104A/cm2at 4.2K was measured at 19.7 tesla. Further additions of Ti were made using Sn-xwt%Ti cores furnished by M. Suenaga of Brookhaven National Laboratory. Values of J c (4.2K) = 104A/cm2at 21.4 tesla were achieved. These values are larger than those obtained with Cu-45wt%Nb P/M processed wires. The effects of prestrain are compared with unalloyed Nb P/M processing. Higher values of J c are obtained in strain-free wires and/or at temperatures below 4.2 K. Electron microprobe measurements of the Ti distribution throughout the single core wires are also presented.

Reaction kinetics of phase formation in Nb-Al powder metallurgy processed wire

The sequence of phases formed in the low-temperature reaction of niobium and aluminium to form the A15 superconductor, Nb/sub 3/Al, was studied in high areal reduction powder metallurgy processed wire samples. The reaction path observed was the same as that reported previously for multilayer Nb/Al thin films. The reaction proceeds without the formation of the sigma phase, Nb/sub 2/Al resulting in a metastable A15 of higher Al concentration. The extent of this metastable extension is estimated to be 22-24 at.% Al, based on the resistive T/sub c/ of 17.0 K observed. The reaction kinetics of the wire samples were examined by scanning heat flux calorimetry, and the activation energy of A15 formation was found to be 3.1 eV. The formation of NbAl/sub 3/ and dissolution of Al into the Nb lattice were both observed as precursor reactions with much lower activation energies. >

Powder metallurgy processed Nb 3 Sn employing extrusion and varying Nb content

Extension of powder metallurgy (P/M) processing of Cu-Nb-Sn is described for small scale industrial uniaxial extrusion ( R \leq 10^{4} ) and for small scale hydrostatic extrusions for areal reductions R = 2000. Successful 2.5 cm o.d. uniaxial P/M processed extrusions at 950 and 250°F were obtained. Model cold hydrostatic extrusions of single tin core and multitin core wire are described. The effect of Nb content for Cu-x wt% Nb, where 36 \leq \times \leq 60 was examined. Increased over-all critical current densities, J c were obtained with increased Nb content up to 50 wt% Nb. For proper comparisons, values of J cm (where the prestress is removed), are given for several compositions. Improvement in J c for T \leq 4.2 K is also presented. Incorporation of Tl in Nb 3 Sn by means of Sn-Ti core processing results in further increases in J c . The low R and large initial powder sizes result in relatively thick Nb fibers which are not completely reacted. The present extrusion P/M processed wires demonstrate several practical approaches for development of high performance materials. Optimization should yield high values of J c at 20 tesla.

Submicron filament multistrand powder metallurgy processed Cu-Nb-Sn wire

Submicron size ultrafine Cu-Nb-Sn superconducting wire has been fabricated by the powder metallurgy process simulating large scale industrial fabrication using the bundling technique. Starting copper and niobium powders ranged from 250 to 500 μm. Both external and tin core processed wires were fabricated with overall current densities of J_{c} \sim 2-3 \times 10^{4} A/cm2at 14 T, demonstrating that both particle size and billet can be scaled up to large scale fabrication.

Practical Processing of A-15 Multifilamentary Superconducting Wire from Powders: Nb3Al and Nb3Sn

Several approaches to fabricating Nb3Al superconducting wires have been published in the last decade. These processes include: the Swiss roll, rapid quenching, mechanical alloying and powder metallurgy processing. This paper will address powder metallurgy (P/M) processing of Nb3Al and Nb-Sn. The recent measurements of critical current densities at high field for P/M processed Nb-Al gave Jc >10 A/cm2 at 19T and 4.2K. Good strain tolerance, low reaction temperatures, and low ac losses at high field all indicate that the P/M process for multifilamentary Nb-Al should lead to practical, high performance, superconducting wires for high fields. Significant improvements in powder metallurgy processing of Cu-Nb-Sn conductors have also been attained with low areal reductions, extrusion processing and with internal Sn cores. Increased Nb content increased Jc more than proportionally, and Ti additives yield very high Jc ‘s at high field. Thus, powder metallurgically processed Nb3Al and Nb3Sn are promising for high field applications.

Intermediate scale industrially processed Nb/sub 3/Sn(Ti) by powder metallurgy

Powder metallurgy processing of Nb/sub 3/Sn(Ti) wire was scaled up to 75-mm-diameter billets, each containing 4 kg of pressed powder composite. Prealloyed Nb-1.5-wt.% Ti powder was used for optimum high-field properties. An initial hot extrusion step provided complete consolidation and excellent integrity of the powder composite. Using the extruded bars, various single and multiple tin core billets were prepared and hydrostatically extruded to 12-mm-diameter bars. Long lengths of internal tin core wire were drawn without any breakage to less than 1 mm diameter. To evaluate the performance of long-length wires, small magnets were fabricated from these wires and tested in background fields of water-cooled Bitter magnets up to 20 T. The processing, performance, and microstructure of these small magnets are discussed. This intermediate-scale process used conventional metal-forming techniques that can be scaled to larger scale industrial processing. >