The microstructure, mechanical and superconducting properties of (RE)BCO bulk single grains

High-temperature superconducting RE-Ba-Cu-O bulk single grains [(RE)BCO, where RE = Y, Gd, Eu, Sm or Nd] show a wide variability in their mechanical and magnetic flux trapping properties. These variations attribute primarily to variations in the microstructure of (RE)BCO bulk materials. While the influence of secondary (RE)₂ BaCuO₅ (RE-211) particles has been investigated extensively, less is known about the effect of cracks, pores and solidified liquid phase on the mechanical and superconducting properties of (RE)BCO bulk single grains. This thesis initially investigates the correlation between porosity and the critical current density *J_c*. The porosity was determined from optical tracking microscope images, and *J_c* was calculated from the maximum measured trapped field in 22 different sized YBCO bulk superconductors. A correlation between porosity and *J_c* was established by evaluating the experimental results statistically using Spearman’s correlation coefficient. The direct correlation between porosity and *J_c* was investigated in one representative YBCO bulk material using 3D X-ray computer tomography (XCT) and a superconducting quantum interference device (SQUID). It was concluded that on the micrometre scale, *J_c* is impacted by both Y-211 particles and porosity. Subsequently, decreasing porosity can significantly enhance *J_c* and trapped field. The microstructure also significantly impacts the mechanical properties of (RE)BCO bulk materials. Mechanical failure limits the maximum magnetic field to which these single grains can be exposed at low temperature. Subsequently, their mechanical properties limit indirectly the ability of (RE)BCO bulk superconductors to trap high magnetic fields. The second part of this thesis investigates the impact of the microstructure on the mechanical and superconducting properties of 11 partially oxygenated YBCO, 11 oxygenated YBCO and 10 oxygenated YBCO(Ag) bulk single grains using Brazilian testing. The mechanical failure in YBCO bulk samples is influenced mainly by pores and pre-oxygenation cracks but not by cracks introduced due to the tetragonal to orthorhombic phase transition during the post-melt processing oxygenation process. The increased mechanical properties of YBCO(Ag) bulk materials come with a significant decrease in trapped field, originating from secondary grain growth and the accumulation of liquid phase in these materials. Finally, the mechanical and flux trapping properties of 19 EuBCO(Ag) and 20 thin-wall EuBCO(Ag) bulk materials were investigated and compared to their YBCO equivalents. On average, the trapped field and the tensile strength of the EuBCO(Ag) bulk, single grain samples were superior to YBCO bulk samples. Despite the superior properties of EuBCO(Ag) bulk materials, they do not enhance performance compared to YBCO bulk samples when exposed to high magnetic fields at low temperature.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Baumann, Josef

Advisor dc:contributor.advisor

• Cardwell, David

• artificial holes
• bulk superconductor
• critical current density
• EuBCO
• EuBCO(Ag)
• EuBCO/Ag
• mechanical properties
• microstructure
• porosity
• reinforcement
• single grain
• single grain bulk superconductors
• superconductor
• tensile strength
• thin-wall EuBCO
• thin-wall EuBCO(Ag)
• thin-wall EuBCO/Ag
• trapped field
• YBCO
• YBCO(Ag)
• YBCO/Ag

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/b97d3cd6-348f-4fc3-aa29-00f9ca52a16b/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0003-3996-749X
0000-0002-2020-2131

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/370653