Year

2013

Degree Name

Master of Engineering - Research

Department

Institute for Superconducting and Electronic Materials

Abstract

MgB2 is one candidate of the superconductors for practical application. Most of the applications require the high critical current density (Jc) which means high performance and cost-effective. However the Jc of pristine MgB2 is far from the theoretical depairing current density because of the weak flux pinning and poor grain connectivity. In the work of this Master degree by research, several dopants were tried to enhance the Jc, meanwhile their corresponding flux pinning mechanism were studied.

The first three chapters of this thesis give the introduction, literature review and experimental methods. They cover the research motivation, basic superconducting properties of MgB2, status quo of MgB2 research on enhancement of upper critical field Hc2 and Jc, and various experimental methods which are employed in this work.

The last two chapters, chapter 4 and chapter 5, present the main experimental works of this thesis.

In chapter 4, the effect of graphene oxide (GO) doping on improvement of Jc in MgB2 was systematically studied with emphasis on flux pinning mechanism. It was confirmed that both of low field Jc and high field Jc could be enhanced by GO doping, and the high field Jc was further improved by optimized doping level. The low field and high field Jc improvement are due to the improved the grain connectivity and enhanced flux pinning, respectively. To understand the superior performance of GO doped MgB2, the flux pinning mechanism was studied systematically. In the framework of the collective pinning theory, a B-T phase diagram has been constructed and in the single vortex regime, the transformation of pinning mechanism from transition temperature fluctuation induced pinning (δTc pinning) to mean free path fluctuation induced pinning (δl pinning) is observed in MgB2 by adjusting the GO doping level. Furthermore, in terms of the thermally activated flux flow model the pinning potential in high field (B > 5 T) is enhanced by GO doping.

In chapter 5, Based on the overall improvement of Jc in GO doped MgB2 and excellent high field Jc for nano-SiC doping, the effect of GO and nano-SiC co-doping effect on the superconductivity of MgB2 was systematically examined by powder xray diffraction, transport and magnetization measurement. By co-doping impurity phases Mg2Si and SiC appeared and a axis decreased and c axis remained unchanged. Compared with the un-doped samples, Jc was improved at high field for both of 5 K and 20 K. Compared with the 2wt % GO doped sample, at 20 K the Jc was further enhanced by co-doping but at 5 K the Jc improvement was not obvious as at 20 K. At 20 K, the normalized pinning force showed enhancement at high field compared with un-doped one but decrease compared with 2wt% GO doped ones. According to the thermally activated flux flow model, the pinning potentials of codoped samples were further enhanced at high field.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.