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Prospects for Improving the Critical Current Density of Superconducting Nb3Sn Strands via Optimization of Nb3Sn Fraction, Stoichiometry, and Grain Size

Xu, Xingchen
http://rave.ohiolink.edu/etdc/view?acc_num=osu1461071733

Abstract Details

2016, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Superconducting Nb3Sn strands are the key building blocks of high-field (>10 T) magnets. There are multiple figures of merit defining the quality of a Nb3Sn strand, such as critical current density (Jc), residual resistivity ratio (RRR), and effective subelement size (deff). Among these the high-field Jc is the key factor for the application of Nb3Sn strands in magnets, because the supercurrent-carrying capability of Nb3Sn strands essentially determines the limit of the magnetic field that can be generated in a given magnet design. It is the primary goal of this dissertation to explore ways to improve the high-field non-matrix Jc of Nb3Sn strands without sacrificing other factors (e.g., RRR and deff). A green-state Nb3Sn wire is composed of precursor metals or alloys, which transform to superconducting Nb3Sn phase through a diffusion reaction process during a heat treatment at typically 600-800 °C. The high-field non-matrix Jc of Nb3Sn strands was improved significantly due to the extensive efforts by the scientists of the Nb3Sn community. These improvements in Jc were mainly driven by three aspects: improvement of the subelement design (i.e., optimizing the subelement architecture and the precursor amounts, e.g., enhancing the Nb and Sn fractions in subelements), use of the right type and amount of dopants, and optimization of the heat treatment schedules. However, the improvement in Jc of Nb3Sn strands has plateaued since 2002, with the rod-restack-process (RRP) strands pushing the 4.2 K, 12 T non-matrix Jc to ~3000 A/mm2, and the 15 T value to ~1600 A/mm2. Given the plateau of Jc improvement over the last decade, researchers have been asking this question: how to further improve the high-field Jc of Nb3Sn strands? This is also the question that this dissertation aims to answer. For this goal, a literature review of the factors determining non-Cu Jc of Nb3Sn conductors is given in chapter 1. It is seen that the non-matrix Jc of Nb3Sn strands are mainly determined by three factors: the fraction of current-carrying Nb3Sn phase in a subelement, the irreversibility field Birr which mainly depends on the Nb3Sn phase composition (i.e., Sn content), doping, and strain state, and the flux-line pinning characteristics (including the maximum pinning force Fp,max and the pinning force peak field) which mainly depend on grain size. Then these three factors will be addressed in three chapters. In chapter 3, the question “how to improve Nb3Sn fraction in a subelement” is investigated. A model is developed to predict the phase fractions in reacted strands based on the starting amounts of precursors. In chapter 4, a model is developed to find out what essentially determines the composition of non-stoichiometric compounds formed by diffusion reactions. It can be used as a guide for controlling the Sn content of the Nb3Sn phase formed in Sn source/Nb3Sn/Nb diffusion reaction couples. Finally, chapter 5 demonstrates that the subelement structures can be carefully modified to enable the internal oxidation of Nb-Zr alloy, so that fine ZrO2 precipitates are formed in Nb3Sn; these ZrO2 particles can significantly refine Nb3Sn grain size and improve the high-field Jc.
Mike Sumption (Advisor)
Materials Science

Recommended Citations

Citations

  • Xu, X. (2016). Prospects for Improving the Critical Current Density of Superconducting Nb3Sn Strands via Optimization of Nb3Sn Fraction, Stoichiometry, and Grain Size [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461071733

    APA Style (7th edition)

  • Xu, Xingchen. Prospects for Improving the Critical Current Density of Superconducting Nb3Sn Strands via Optimization of Nb3Sn Fraction, Stoichiometry, and Grain Size. 2016. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1461071733.

    MLA Style (8th edition)

  • Xu, Xingchen. "Prospects for Improving the Critical Current Density of Superconducting Nb3Sn Strands via Optimization of Nb3Sn Fraction, Stoichiometry, and Grain Size." Doctoral dissertation, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461071733

    Chicago Manual of Style (17th edition)

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