Mechanically milled bulk nanocrystalline Nb(_3)Sn with improved superconducting properties in high magnetic fields

Abstract

The upper critical field of binary Nb(_3)Sn has been increased by mechanical milling and Hot Isostatic Pressing (HIP'ing) pre-alloyed powder to form bulk nanocrystalline samples. Extensive investigation of milling parameters allowed optimization of sample yield, purity and microstructure. X-Ray Diffraction (XRD) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) data show that milling can produce amorphous Nb(_3)Sn whilst maintaining low contamination levels (< 0.4 wt. %). Nb(_3)Sn powder milled under an argon atmosphere using specially-manufactured niobium milling tools has been consolidated in a HIP at high pressure (2000 Bar) and a range of temperatures (450 - 850 С). ICP-MS analysis allowed restoration of the milled Nb(_3)Sn stoichiometry prior to powder consolidation. In-field variable-temperature resistivity and magnetic susceptibility measurements of these disordered high-purity bulk Nb(_3)Sn samples are presented. The critical temperature, Tс, of Nb(_3)Sn is depressed by milling, but the recovery in Tc with HIP temperature coincides with a systematic reduction in microstructural disorder. The relationship between upper critical field, B(_C2)(^0.5P)(0), and HIP temperature reaches a peak of 31.7 土 0.4 т at 700 С, ֊6 т higher than typical values for binary Nb(_3)Sn. The very high normal-state resistivity (200 ± 27 μΩ cm at 20 K) and low Scherrer grain size (70 nm) imply that this increase is due to a disordered nanocrystalline microstructure. To facilitate the investigation of the properties of technological Nb(_3)Sn over a large range of applied strain (-1 to +0.3 %), a procedure for attaching superconducting wires to Tİ-6A1-4V helical strain springs is also presented.