Development and Characterization of High Strength Nb₃Sn Superconductor

• Other Authors: Chen, Jingping, 1970- (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Mechanical engineering
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-3855

Out of the two most used materials (Nb3Sn and NbTi) for superconducting magnets, the Nb3Sn type conductor has been shown to have higher critical properties and capable to generate fields higher than 8 tesla. However, the superconducting properties of Nb3Sn are highly strain sensitive and mechanically weak. To fully utilize the high critical current of Nb3Sn conductor, strengthened Nb3Sn wires are required. In this research a review and comparison of various approaches to strengthen Nb3Sn supperconductors have been carried out. The dispersion strengthening approach was selected as a major strengthening method. Here, a detailed study on the fabrication and characterization of alumina nanoparticle dispersion strengthened Nb3Sn wires is presented. Both the mechanical and superconducting properties were concurrently improved by using this method. To rationalize and select the strengthening approach, stress-strain analysis was carried out. The calculation of the final strength of the composite conductors considers (1) Thermal strain due to the cooling from high temperature to 4 K and, (2) Internal stress/strain due to the shape and distribution of the Nb3Sn filaments in the conductors. A 1D model was developed to calculate the thermal prestrain and predict the performance of composite wire. The results from 1D model were helpful for understanding the influence of thermal stress on mechanical properties. Furthermore, a 3D model was also developed based on the basic Micromechanics theory. The 3D model provided more insights on the relationship between internal stress-strain state and external loads, such as temperature drop and applied tensile or compressive stresses. Because Nb3Sn's performances are highly strain sensitive, especially for high field applications, the study of stress-strain state has significant meanings for improving fabrication process and optimizing the design of the Nb3Sn superconductors. Based on the calculations and literature survey, the fabrication and strengthening approaches were selected and optimized. Modified cable-in-tube (MCIT) process was used as the major fabrication method, because this process has characteristics of simplicity and short fabrication cycle. Moreover, it is capable of dealing with high strength materials and varied designs. Therefore this process is suitable for developing new composite materials and optimizing process parameters. Nanoparticle strengthened Cu (DSC) was used as a precursor. The formability of DSC/Nb combination and DSC/Sn combination were carefully investigated. The bonding between Cu and Cu was also investigated in order to avoid defects related to incomplete bonding. Heat treatment conditions were also studied systematically. Based on all these preliminary investigations, alumina nanoparticle reinforced Nb3Sn wire was fabricated using MCIT process. Microstructure characterization, tensile tests and critical current ~ strain measurements were carried out to compare the newly developed Nb3Sn and the regular wire fabricated by exactly the same procedures. The experimental results were consistent with the calculation. The developed wire showed improvement in both the critical current and mechanical strength. === A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester, 2006. === November 1, 2006. === Superconductor, High Strength, Critical Current, Nb3Sn === Includes bibliographical references. === Ke Han, Professor Co-Directing Dissertation; Peter N. Kalu, Professor Co-Directing Dissertation; Chuck Zhang, Outside Committee Member; Steven W. Van Sciver, Committee Member; Justin Schwartz, Committee Member.