Honors Master of Engineering
Institute for Superconducting and Electronic Materials - Faculty of Engineering
Yao, Qiwen, Study of synthesis, structures and superconducting properties of magnesium diboride MgB2 material, M.Eng thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2004. http://ro.uow.edu.au/theses/404
In this thesis, we are mainly discussing element-doped MgB₂thick films and tapes. In Chapter 1, I present some brief discussions about superconductivity and its relevant theories and history. Extensive examples are given for superconductor applications. At Chapter 2, a literature Review of MgB₂superconductor is given. In this chapter, MgB₂’s discovery, particulars and current research are discussed. In Chapter 3, the experimental details and relevant laboratory measurement equipments are introduced; specifically X-ray Diffraction and Scanning Electron Microscopy (SEM) measurements are discussed. From Chapter 4 through Chapter 7, various methods of MgB₂ thick film/short tape fabrication were discussed, as well as the corresponding results. At Chapter 4, we studied the Cu and Nano-SiC doped MgB₂ thick films on Ni substrates processed using a very short-time, in-situ reaction. Pure and doped MgB₂ thick films were fabricated on Ni substrates with varying amounts of elemental Cu and Nano-SiC powders, samples were sintering at 840°C or 900°C for just a few minutes, and then quenched in liquid nitrogen. We found that for films sintered at 900°C, critical current densities Jc were achieved as high as 1.4 × 106 A/cm² at 20 K and 2 T for the pure and SiC added films. Films doped with 5 wt% of Cu powders were observed to have better adherence to the Ni substrate without degradation in Tc, and Jc was found to be slightly decreased, but still remains as high as 7 × 105 A/cm² at 20 K in zero field. It was observed that Jc and irreversibility field increase with an increasing sintering temperature up to 900°C. Furthermore, Nano-SiC addition has significantly improved the irreversibility field compared to un-doped MgB₂ films. In Chapter 5, the effect of nano-Y-ZrO₂ (YSZ) addition on the microstructure and critical current density of MgB₂ superconductors was studied. XRD results showed that YSZ does not react with MgB₂. Furthermore, the transition temperatures of the YSZ Nano-particle doped samples dropped about 3 K compared to the un-doped samples. Samples doped with YSZ powders showed a higher critical current density in low magnetic fields, however the Jc drops faster compared to that made by long time sintering samples. It is proposed that the improved Jc in low fields was due to the enhanced density of the sample, which was caused by the YSZ Nano-particle inclusion. In Chapter 6, I discuss the fabrication, microstructure and critical current density of pure and Cu doped MgB₂ thick films on Cu, Ni and stainless steel substrates by short-time, in-situ reaction. These thick films were prepared using a short time sintering method. Results showed that single MgB₂ phase films can be easily formed in a short period of time (3 minutes) at temperatures above 700°C. Un-doped MgB₂ films were found to be loosely attached to the Ni and stainless steel substrates. However, the MgB₂ with Cu powders addition adhered well to the substrates without serious degradation of Tc and flux pinning. The Jc increased by one order of magnitude and irreversibility field determined from M-H loops also increased when sintering temperature increased from 745°C to 900°C. Jc values in the range of 1-9 x 105 A/cm2 at 15 K have been achieved for both doped and un-doped films sintered at 900°C for a short 3 minutes of time. Finally at Chapter 7, I look at the significant improvement of critical current density in Cu sheathed MgB₂ tapes through Nano-SiC doping and short-time in-situ reaction. We found that Nano-SiC doping samples resulted in a very high critical current density of more than 1 MA per square centimetre in zero field at T ≤ 10 K. It is also found that these Nano-SiC doped MgB₂ /Cu tapes exhibited very small flux jumping at 5 K, indicating a high thermal stability. Such a high Jc field performance together with high thermal stability and large mechanical flexibility makes the Cu-sheathed MgB₂ tapes an attractive candidate for large scale applications.
02Whole.pdf (5499 kB)