As the only high temperature superconductor with round wire (RW) geometry, Bi2Sr2CaCu2O8+x (Bi-2212) superconducting wire has
the advantages of being multi-filamentary, macroscopically isotropic and twistable. With overpressure (OP) processing techniques recently
developed by our group at the National High Magnetic Field Laboratory (NHMFL), the engineering current density (Je) of Bi-2212 RW can be
dramatically increased. For example, Je of more than 600 A/mm2 (4.2 K and 20 T) is achieved after 100 bar OP processing. With these
intrinsically beneficial properties and recent processing progress, Bi-2212 RW has become very attractive for high field magnet
applications, especially for nuclear magnetic resonance (NMR) magnets and accelerator magnets etc. This thesis summarizes my graduate
study on Bi-2212 solenoids for high field and high homogeneity NMR magnet applications, which mainly includes performance study of Bi-2212
RW insulations, 1 bar and OP processing study of Bi-2212 solenoids, and development of superconducting joints between Bi-2212 RW
conductors. Electrical insulation is one of the key components of Bi-2212 coils to provide sufficient electrical standoff within coil
winding pack. A TiO2/polymer insulation offered by nGimat LLC was systematically investigated by differential thermal analysis (DTA),
thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), dielectric property measurements, and transport critical current
(Ic) property measurements. About 29% of the insulation by weight is polymer. When the Bi-2212 wire is fully heat treated, this decomposes
with slow heating to 400 °C in flowing O2. After the full reaction, we found that the TiO2 did not degrade the critical current
properties, adhered well to the conductor, and provided a breakdown voltage of more than 100 V. A Bi-2212 RW wound solenoid coil was built
using this insulation being offered by nGimat LLC. The coil resistance was constant through coil winding, polymer burn-off and full coil
reaction. The coil was successfully tested at the NHMFL generating 33.8 T combined magnetic field in a 31.2 T background field. Multiple
quenches occurred safely, which also illustrates that the insulation provided sufficient dielectric standoff. For Bi-2212 RW with a
typical as-drawn diameter of 1.0-1.5 mm, this 15 µm thick insulation allows a very high coil packing factor of ~0.74, whereas earlier
alumino-silicate braid insulation only allows packing factors of 0.38-0.48. In addition to the commercial TiO2/polymer insulation, we have
also investigated sol-gel based ceramic coatings through collaboration with Harran University and another TiO2 based insulation coating at
the NHMFL. Since Bi-2212 superconducting coils employ the Wind-and-React (W&R) technology, there are some potential issues in
processing Bi-2212 coils, in particular for coils with a large thermal mass and dense oxide insulation coating. For this study, several
Bi-2212 test solenoids with an outer diameter (OD) of about 90 mm were built and heat treated in 1 bar flowing oxygen with deadweights
applied so as to simulate large coil packs. After the heat treatment (HT), coils were epoxy impregnated and cut. Winding pack was checked
using SEM in terms of conductor geometry and insulation. Some samples were extracted to measure transport critical current Ic and critical
temperature Tc. The results are very promising: test coils presented low creep behavior after standard partial melt HT under mechanical
load, and no Ic degradation was found due to the application of mechanical load, and no inadequate oxygenation issue was seen for thick
coils with ceramic coating on the wire. However, coils were partially electrically shorted after 1 bar HT under mechanical load, and we
believe that increasing insulation coating thickness is necessary. In addition, several small solenoids were manufactured to study OP
processing of Bi-2212 coils. The preliminary results indicate that there are some gaps between turns due to densification of wires (~4%
wire diameter reduction) during 50-100 bar OP processing, and the diameter shrinking of conductors will potentially lead to coil sagging.
So far, we have developed some methods to solve the issue of coil sagging, such as using flexible coil flange to allow smooth sagging of
winding pack during OP processing. We have also investigated electrical joints between Bi-2212 RW conductors, which include resistive
joints and superconducting joints. For resistive Bi-2212 joints, we evaluated conventional diffusion bonding method and soldering method.
In general, the joints (with 42 mm joint length) resistances are below 200 nΩ at 4.2 K and magnetic fields up to 13.5 T, and the effect of
magnetoresistance is clearly present. In addition to resistive joints, we successfully developed a superconducting joint between Bi-2212
RW conductors for persistent current mode (PCM) operations. The joint fabrication procedure is effective and practical, enabling Bi-2212
superconducting joints to be achieved during the standard Bi-2212 HT processing. First, the melting temperatures of Bi-2212 precursor
mixtures with different amounts of Ag additions were investigated by DTA. Then, test joints were fabricated and heat treated in 1 bar
flowing oxygen using the standard Bi-2212 HT schedule. The voltage-current (V-I) properties were measured using the conventional
four-point method at 4.2 K in magnetic fields up to 14 T. A maximum supercurrent of ~850 A was achieved at 4.2 K and self-field. With the
increase of external field, the supercurrent gradually decreased as expected, but a supercurrent of ~450 A was still presented at 4.2 K
and 14 T. Compared with open-ended short samples with identical 1 bar Bi-2212 reaction, we found that the Ic properties of joints did not
degrade. Meanwhile, microstructures of joints were examined by SEM, which clearly presented the formation of a Bi-2212 superconducting
interface between two independent Bi-2212 RW conductors. Furthermore, a Bi-2212 RW closed-loop solenoid with a superconducting joint was
fabricated and fully heat treated in 1 bar flowing oxygen. Using the field decay method, the joint resistance was estimated to be below
5×10-12 Ω at 4.2 K and self-field. / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. / Spring Semester 2016. / March 29, 2016. / Bi-2212, Insulation, Superconducting joint, Superconducting magnet / Includes bibliographical references. / David Larbalestier, Professor Co-Directing Dissertation; Ulf Trociewitz, Professor Co-Directing
Dissertation; Irinel Chiorescu, University Representative; Eric Hellstrom, Committee Member; Wei Guo, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_360331 |
| Contributors | Chen, Peng (authoraut), Larbalestier, D. (David) (professor co-directing dissertation), Trociewitz, Ulf Peter (professor co-directing dissertation), Chiorescu, Irinel (university representative), Hellstrom, Eric (committee member), Guo, Wei (committee member), Florida State University (degree granting institution), FAMU-FSU College of Engineering (degree granting college), Department of Mechanical Engineering (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (152 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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