Design and characterisation of MgB2 bulk superconducting magnets containing Y2O3 and Mg additions

Thesis

The aim of this thesis was to investigate new processing methods and powder treatments (prior to consolidation) in order to manufacture MgB2 bulk specimens with improved superconducting properties. The initial idea was to transpose a successful concept used for improving the mechanical properties of steels and superalloys to superconductors. In the 1970s, a new concept, known as Oxide Dispersion Strengthening (ODS), was explored to improve the mechanical properties at high temperature of nickel superalloys. The process aimed to create a fine dispersion of stable oxide (usually Y2O3) precipitates in a metallic matrix. In structural alloys, these precipitates effectively pin the dislocations and grain boundaries, leading to improved high temperature strength. The main objective of this work was to investigate if the ODS concept can be successfully applied to MgB2 bulk superconductors. In this case, the nano precipitates need to act as effective pinning centres for magnetic flux vortices to improve the superconducting properties, in contrast with conventional ODS materials in which they pin dislocations. The first chapter starts with an introduction on superconductivity, followed by a brief review of MgB2 literature. The concept of sintering is then introduced, followed by a description of the Field Assisted Sintering Technique (FAST). Finally, articles studying the manufacture of MgB2 bulks by FAST are discussed, before introducing relevant papers on the main types of MgB2 modifications: carbon doping, oxide and metallic additions and ball milling. The second chapter details the processing equipment and manufacturing routes used to produce the specimens studied in this work. It also introduces the different characterisation techniques and data analysis performed to extract material parameters. The three results and discussion chapters are presented in order of increasing complexity to gradually build up understanding of the relationship between processing parameters, microstructure and superconducting properties. Chapter 3 is dedicated to the characterisation of MgB2 bulks made from unmodified powder and sintered by FAST at temperatures ranging from 900 to 1200 °C. Chapter 4 first introduces the ODS process developed in this work and the analyses performed on ODS powders, then discusses the consolidation of these powders and the characterisation of ODS MgB2 bulks. Chapter 5 explores a new liquid phase/reactive sintering process I developed to improve the densification of MgB2 at lower processing temperatures. The process is first described, followed by the characterisation of Mg-MgB2 bulks made from MgB2 powders containing Mg additions. Chapter 5 ends on the concept of Mg-Y2O3-MgB2 composite bulks and investigates if these two additions can work in synergy to produce high performance composite superconductors.

Authors: Matthews, G

• DOI: 10.5287/ora-nbdj9e20j
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Y2O3; ODS superconductors; microstructure; superconductors; MgB2; bulk superconductors; Mg additions; liquid phase sintering; artificial pinning centres
• Subjects: Superconducting magnets; Superconductors; Ceramic superconductors; Materials science