The improvement of characteristics of modern superconducting devices could be achieved by changing technologies or utilization of new materials. The electron microscopy (EM), electron diffraction (ED) and microanalysis (MA) investigations of Nb-Sn based superconducting wires used for ITER project demonstrated the severities to form A15 Nb₃Sn structure over all volume of superconducting wires. Similar wires might be used for upgraded Large Hadron Collider (LHC) and Very Large Hadron Collider (VLHC). The Sn content in the large part of the superconducting wires could be less then 20 at% which could lead to the deviation of crystal structure of wires from A15 and consequent drop of electrophysical properties. The superconducting wires were formed by bronze method [1] and Nb₃Sn phase was grown during solid phase reaction of Sn solved in the bronze with Nb inserts. One possible reason of Sn depletion is the diffusion barrier appeared on the Nb/bronze interface at the early stages of superconducting wires formation. It was found that this barrier is simply uniform and slowly growing layer of Nb₃Sn adopted A15 crystal structure. The EDX elemental mapping with the help of SuperX detector (Bruker, US) in Osiris (FEI, US) STEM (Fig.1) unambiguously demonstrated the absence of Sn diffusion along Nb grain boundaries. After A15 Nb₃Sn reached critical thickness the Nb-Sn grains with Sn content less than 18 at% began to form. HAADF STEM study of these Sn depleted grains in C_s probe corrected TITAN 80-300 TEM/STEM (FEI, US) at 300 kV (Fig.2) indicated that these grains contained high density of antisite defects start to form and it was Nb substituted for Sn atoms. That conclusion was made by the estimation of intensities of atomic columns as it was done in [2] and comparison with simulated images (P.Stadelmann’s JEMS software was used for image simulations). To increase Sn content in the grains, the changes of technology were proposed. Another possibility is the utilization of new materials and possible candidates are FeTeX (where X- Se or S) superconductors. The X-ray crystal structural analysis, TEM, HAADF STEM, electron diffraction and microanalysis (TITAN 80-300 TEM/STEM) were applied to the study of FeTeX based single crystals and thin films on different substrates. One of HRSTEM images is presented in Fig.3. It was found that there are uncertainties in Fe1 and Fe2 position in FeTeX compounds and ordering of S atoms. The misfit between the FeTeX films and substrates were released through misfit dislocation or an intermediate layer at the film/substrate interface. Thus the FeTeX films were stress free and critical temperature Tc should not change due to structural modifications.

[1] E.Dergunova, A.Vorobieva, I.Abdyukhanov, K.Mareev, S.Balaev, R.Aliev, A.Shikov, A.Vasiliev, M.Presnyakov, A.Orekhov Physics Procedia (2012) 36, 1510.

[2] S.Van Aert, K.J.Batenburg, M.D.Rossell, R.Erni, G.Van Tendeloo Nature (2011) 470, 374.

Figures:

Fig. 1. HAADF STEM image of the initial stage of A15 Nb3Sn compound formation in a superconducting wire overlaped with elemental maps of Nb, Sn and Cu.

Fig.2. HAADF STEM image of Nb-Sn compound in [001] zone axis with antisite defects. The A15 Nb3Sn crystal structure model together with enlarged image of the specimen are in the inserts. Note the different intensities of the atomic columns, correspondent to Sn atoms.

Fig.3. HAADF STEM image of Fe1.1Te single crystal in [100] zone axis. The Fe(1), Fe(2) and Te atomic columns are shown by arrows.

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