Materials characterisation of Nb 3 Sn superconductors

1. Materials characterisation of Nb3Sn superconductors NED steering committee, 30.10.07

2. Contents • Nucleation and growth of Nb3Sn in multifilament superconductors • Other phase transformations during the reaction heat treatment (prior to Nb3Sn formation) • Mechanical properties of reacted Nb3Sn strands • Strain induced degradation of superconducting properties • Coatings of Nb3Sn strands for Rutherford cables NED steering committee, 30.10.07

3. 1) Nucleation and growth of Nb3Sn in multifilament composite conductors • How can we increase the flux pinning force in Nb3Sn strands? • What determines the Nb3Sn grain size in the fully reacted strand? • Why are the Nb3Sn grains in PIT strands larger than in the best IT strands (about 180 nm vs. 120 nm), despite the higher reaction temperature of the IT strand? • Is increased Nb cold work prior to the reaction heat treatment beneficial? • Can the grain boundaries be modified in order to increase the specific pinning force? NED steering committee, 30.10.07

4. Nb versus Nb3Sn grain size Transmission Electron Microscope (TEM) images of non reacted Nb-Ta (left image) and reacted (Nb-Ta)3Sn in the PIT B215 strand.TEM images courtesy M. Cantoni, EPFL NED steering committee, 30.10.07

5. 2) Phase transformations during the reaction heat treatment of Nb3Sn strands • Phase transformations prior to the Nb3Sn formation can influence the superconducting properties of the fully reacted strand. • What are the most stable phases at different temperatures? • What are the kinetics of the different phase transformations? • Phase transformations can be studied by synchrotron diffraction during in-situ reaction heat treatment. Superconductor Science and Technology 20, (2007) L55-L58 NED steering committee, 30.10.07

6. Why is Nb6Sn5 formed in the PIT strand but no (or only very little) Nb6Sn5 is formed in IT strands? Nb6Sn5 Nb3Sn Formation of Nb6Sn5 and Nb3Sn in the PIT B215 strand during isothermal 620 °C heat treatment. SEM/EDX analysis courtesy of G. Arnau, TS-MME. NED steering committee, 30.10.07

7. Is the dissolution of Nb filaments and formation of the ternary Cu-Nb-Sn phase in IT Nb3Sn strands with high Sn content inevitable? SEM/EDX analysis of an IT strand after 10 h isothermal 410 °C heat treatment. Part of the Nb filaments are dissolved and a ternary Cu-Nb-Sn phase, containing approximately 20 at.% Nb and 19 at.% Sn, has formed. SEM/EDX analysis courtesy of G. Arnau, TS-MME. NED steering committee, 30.10.07

8. Phase transformations and void growth in Internal Tin strands with low Sn content Applied Physics Letters, 90, 132510, (2007) NED steering committee, 30.10.07

9. 3) Mechanical strand properties • In the frame of NED we have elaborated the mechanical properties of the pre-cursor materials in a non-reacted Internal Tin strand at room temperature.* The results were needed as input for Finite Element (FE) modelling of the strand deformation during cabling. • FE simulations of the strand behavior in the magnet under operating conditions require as input the mechanical properties of the different phases in reacted Nb3Sn strands at 4.2 K. *“Tensile properties of the individual phases in un-reacted multifilament Nb3Sn wires”, proceedings of the ICMC-06, Prague, (2006) NED steering committee, 30.10.07

10. Stress-strain curve of the fully reacted PIT B215 strand at 4.2 K Stress-strain measurements are possible in the temperature range 4.2-100K (Uni Geneva) and from room temperature up to 1000 K (BAM). Courtesy of B. Seeber, Uni Geneva NED steering committee, 30.10.07

11. 4) Strain induced degradation of superconducting strand properties • Obtain a better understanding of the mechanisms for reversible and irreversible degradation of the superconducting properties in order to make Nb3Sn strands less strain sensitive. • What is the influence of the Nb3Sn strand design and processing? NED steering committee, 30.10.07

12. Diffraction measurements during in-situ tensile tests • “Strain gauges” for measuring the elastic strain in the different strand phases in axial and transverse direction. • Correlate lattice parameter variations as a function of axial/transverse wire stress with the Ic axial/transverse stress dependence. • Internal stress state • Load transfer • Experimental cross check of FE simulations Applied Physics Letters, 91(4), 042503, (2007) Cu, Nb–Ta, and Nb–Ta3Sn lattice constant in axial and transverse direction as a function of uniaxial tensile stress. NED steering committee, 30.10.07

13. 5) Coatings for Nb3Sn strands in Rutherford cables • Un-coated Nb3Sn strands in Rutherford type cables “sinter” together during the reaction heat treatment. • What kind of coating can avoid this effect and maintain the needed interstrand contact resistance? O 1s O-KLL Cu-LMM Al 2s Al 2p X-ray photelectron spectroscopy (XPS) survey spectrum of a Nb3Sn strand matrix with 0.5 m thick Al coating after 1 h-700 °C heat treatment under vacuum. The detected Al is entirely in the form of Al2O3. Courtesy M. Taborelli, TS-MME NED steering committee, 30.10.07