Year

2006

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The objective of this work is to further enhance the critical current density of the MgB2 superconductor by doping with the two carbon sources: multiwalled carbon nanotube (CNT) and nano carbon. The work in this thesis concentrates on the fabrication and characterization on the CNT and nano C doped MgB2 with main objective being the enhancement of the critical current density in the high magnetic field. Consequently, introducing effective pinning centres in the form of dopants to enhance the flux pinning will be the main task of this project.

In this project, the effect of carbon doping MgB2 with carbon nanotubes and nano C on transition temperature, lattice parameters, critical current density and flux pinning for MgB2-xCx with x = 0, 0.05, 0.1, 0.2 and 0.3 under the various condition was studied. Both types of doping showed excellent Jc compared to the pure MgB2, with significant enhancement observed at higher temperature. Magnetic Jc(H) was enhanced by a factor of 72 at 5K for a field 8T and a factor of 33 at 20K for a field of 5T for nano C bulk samples, respectively. On the other hand, Jc(H) of CNT samples was enhanced by a factor of 26 and 13 under the equivalent conditions. In high field, transport Jc of magnitude 2122 A/cm2 and 3821 A/cm2 was observed at 4.2K and 12T for CNT and nano C doped MgB2. These results indicate that flux pinning was enhanced by the boron substitution for carbon with increasing processing temperature. However, it was found that the lattice distortion and optimum doping level is different in the CNT and nano C samples which is due to the reactivity of the carbon source, resulting in different carbon substitution rate. Due to better reactivity and homogenous mixing of nano C, nano C doped MgB2 resulted in better improvement in magnetic and transport Jc(H), as compared to CNT doped MgB2. This is mainly because CNT fibres with high aspect ratio tend to entangle, which suppressed the reactivity.

The depression of Tc, which is caused by the boron substitution for carbon, increases with increasing the doping level, processing temperature and duration for both types of carbon doping. By controlling the extent of the substitution and inclusion of carbon, we can achieve the optimal improvement of critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in Tc. In addition, the values of Hc2 and Hirr are higher for CNT doped samples than for the pure MgB2 at the same value of T/Tc. The morphology of the CNT doped MgB2 is similar to that of nano C doped MgB2, but different from the pure MgB2. The microstructure exhibits noticeable nanoparticles with size around 10-20nm, which are believed to be MgO and MgB2.

Magnetization measurements indicate a change in the critical current density with the length of nanotube and not with its outside diameter. This is due to longer nanotubes tending to entangle with each other, preventing their homogenous mixing with MgB2 and dispersion. Low intensity ultrasonication, as a method of dispersion of CNT’s into precursor magnesium and boron powder, was introduced to improve homogeneity of mixing of CNT’s with the MgB2 matrix. Ultrasonication of CNT doped MgB2 resulted in a significant enhancement in the field dependence of critical current density, while avoiding the side-effects that would occur at higher processing temperatures.

Carbon nanotubes (CNT’s) have unusual electrical, mechanical and thermal properties. The elongated CNT’s induce anisotropy in Jc in relation to the direction of applied field in MgB2/Fe wires and the value of Jc for the carbon nanotube-doped wires is insensitive to heating rates. We believe that by taking the extraordinary electrical, mechanical and thermal properties of CNT’s, the mechanical properties and thermal stability of CNT doped wire will be substantially improved. Studies on these properties are underway.

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