Flux pinning in Nb3Sn containing artificial pinning centres: a systematic study

Abstract

Nowadays plans for next-generation particle accelerators demands for record-breaking high-field superconducting magnets, to be conceived based on state-of-the-art technologies from the base conductor to magnet design.In particular, the CERN Future Circular Collider project (FCC) in its hadron-hadron version (hh-FCC) asks for high-field magnet dipoles. The latter are planned to bend the hadrons trajectory through the about 100 km long ring, so resulting in collision energies in the order of 100 TeV. The dipoles are required to produce a stable 16 T field at 4.2 K, with coils made from a conductor sustaining critical current densities J_c of at least 1500 A/mm2 at the same temperature and field.In this respect, even though the high temperature superconductors (HTS) could represent a solid research investment towards the future, at present Nb3Sn is still considered the best candidate for matching hh-FCC tasks. Even considering Nb3Sn to be to-date industrially established, present market products do not reach yet those high-performance requirements, asking for a step forward in the material development towards its very best possible achievements. It is indeed well known that several aspects of Nb3Sn manufacturing can be improved, being it a hard challenge and often requiring for years of R&D.As a possible solution to the problem, a new Nb3Sn manufacturing technology known as "artificial pinning centres (APC) Nb3Sn" started to catch researchers’ interest in 2014. The idea of introducing a wide scenario of small precipitates in the microstructure (A15) immediately showed great impact on the superconducting performances, resulting e.g. in J_c exceeding the FCC specification by a factor 1.5 and more. Zr or Hf is alloyed in the starting recipe of these specimens, both elements forming oxides during the heat treatment by combining with oxygen. The oxides are nanometric precipitates, which contribute to the overall grain size refinement and eventually (individually) to flux pinning. The idea of the artificial pinning centres is per se not new, while its successful application to multi-filamentary Nb3Snit is: finding a suited method for the internal oxygen supply and a reproducible good quality of wire drawing resulted in a great interest in this technology, which motivated the research contained in this thesis.In this work, a systematic investigation of how the microstructure correlates with the main superconducting performance of several APC-Nb3Sn recipes is presented. Starting from the mono-filamentary (binary) strands to the most advanced ternary versions of these prototype wires, an ad-hoc set of samples heat treated each at 675°C, 685°C, 700°C and 720°C (at Fermilab, US) with different percentages of Zr or Hf alloyed to the starting Nb- or Nb-Ta compound and characterized at the Atominstitut (Vienna, AT). SQUID magnetometry was used to measure the specimens J_c , B_irr and T_c at different temperatures/fields, coupled with resistive measurements at 4.2 K for several samples. This experimental technique was used also for pioneering a sensitive method for investigating the geometry of the A15 filaments, fundamental for the assessment of the layer-Jc.In the magnetometry field range - 1 to 7 T- it is usually also possible to detect the maximum value of the pinning force F_p-max at 4.2 K, which is an important parameter towards the understanding of the flux pinning at the operational temperature. This work shows how the most complete experimental datasets clearly indicate the Jc standards of hh-FCC to be matched and even exceeded by the best samples. The samples were also prepared for electron microscopy (SEM, TEM/STEM, EELS, EFTEM) through which it was possible to determine the grain sizes, the precipitate size and densities as well as Sn compositional gradients along the A15 radii. It was found that the content of Zr/Hf does influence the grain/particle size vs heat-treatment temperature, revealing also a consistent grain refinement against the no-oxygen version of the same recipe.Several aspects of the mixed flux pinning in this wires were investigated through the collected data. Critical temperature distributions were either magnetically probed by using SQUID (bulk) and scanning Hall probe microscopy (locally) or mapped by correlating to the Sn content by using EDX. These data were useful to simulate the local currents, finding that it is possible to assess through them both the co-existing grain-boundary and point-particle pinning contributions. It was found from these analyses and from J_c measurements that point particles play a significant role in the overall pinning, motivating additional modelling work to deepen the understanding of such complex systems.Starting from Dew-Hughes models, fits of the experimental data by using different combinations of summation were produced and discussed. The role of B_irr, of the actively pinning grain-boundary surfaces and the elasticity degrees of the fluxon lattice are discussed. A focus on the efficiencies of the individual pinning mechanisms led eventually to a proposal for modelling the point particle one, which was found to decrease hyperbolically once exceeding the coherence length.