The superconducting properties, composition and microstructure of nanocrystalline superconductors for high magnetic field applications

Abstract

We report on the fabrication and properties of state-of-the-art nanocrystalline superconductors with 0 ≤ x ≤ 1. High-energy ball milling was used to produce disordered nanocrystalline powders from binary compounds and from pure elemental powders. A detailed analysis of powders milled for up to 30 h was performed using X-ray diffraction, differential scanning calorimetry and AC magnetic moment measurements. Powders milled for 6 h (bcc) and 20 h (amorphous) were consolidated using a hot isostatic press (HIP) operating at 2000 atm in a temperature range from 600 to 1200 . The role of isochronal post-HIP heat treatments was investigated for those materials HIP’ed at 600 and 1200 . For comparison purposes, heat treatments were also performed on milled powders.
Nanocrystalline bulk materials were characterized using a.c. magnetometry, resistivity and SQUID measurements in magnetic fields up to 9 T to measure the critical temperature and the upper critical field. X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray analysis were used to investigate the constituent phases, the distribution of compositions and to estimate the volume phase percent of each phase.
Bulk materials fabricated from milled compounds showed a partial crystallisation recovery after HIP’ing at 600 but subsequent annealing does not further improve the superconducting properties. For material fabricated from elemental powders, after equivalent processing using milling and HIP’ing, the material is more disordered. In contrast to compound precursors, post-HIP annealing produces significant changes in the superconducting properties. In particular, a strong enhancement of and was achieved after a post HIP anneal of nanophase and materials. In this work, bulk material fabricated from milled elemental powders and HIP’ed at 600 produced (A15) in bulk form with record values of ~ 12 K and ~ 13 T. We find that the role of the optimal post HIP anneal of highly disordered material is to enhance the A15 crystal nucleation while minimising the crystallisation of oxides and secondary phases that occurs at higher temperatures and competes with superconductivity.