Year

2008

Degree Name

Doctor of Philosophy

Department

Institute of Superconducting and Electronic Materials - Faculty of Engineering

Abstract

The effect of the properties of the starting boron powders on the superconducting properties of MgB2 has been studied. Low grade boron powders are attractive because of their low cost, but produced lower surface reactivity and larger particle size than high purity (99%) amorphous boron powder, indicating that the low grade powders cannot be used to achieve the same superconducting properties as those of samples made from pure 99% boron powder. However, the low purity boron powders can be improved by using simple physical and chemical processes, leading to enhanced magnetic critical current density, Jc. In order to get high performance MgB2, it is obviously important to control the phase composition and microstructure of the boron starting powders and the solid state reaction conditions.

Ball milling is an effective method to reduce the boron particle size, so, the effects of ball milling boron powders in different media, such as acetone, ethanol, and toluene, on the superconducting properties of MgB2 needed to be considered and studied. It was observed that toluene was the most effective medium of them all for enhancing Jc. Jc was estimated to be 5 × 103 A cm−2 at 8 T and 5 K for a sample that was ball milled in toluene. This value is much higher than that of the pure MgB2 reference sample that was not ball milled, by a factor of 20. It was considered that ball milling B using toluene leads to smaller MgB2 grains, resulting in enhanced Jc at low operating temperatures and high fields.

MgB2 samples were prepared using as-supplied commercial 96% boron with strong crystalline phase and the same 96% boron (B) after ball milling. The effects of the properties of the starting B powder on the superconductivity were evaluated. It was observed that samples using ball-milled 96% B, in comparison with the reference sample made from the as-supplied 96% B, were characterized by small grain size and enhanced magnetic critical current density (Jc), which reached 2 ×103 A cm-2 at 5 K and 8 T. The improved pinning seen in these samples seems to be caused by enhanced grain boundary pinning at high field. MgB2 samples were also prepared by using 96% boron powder with strong crystalline phase that had been ball milled for various times. Based on Rowell connectivity analysis, when the ball-milling time increased, the connectivity factor, described as the active cross-sectional area fraction (AF), was decreased. This implies that the inter-grain connectivity became worse. These properties could lead to poor Jc in low field. However, the pinning force strength of samples using ball-milled 96% B is larger than that of the reference sample using as-supplied commercial 96% B powder. These results accompany enhanced irreversibility (Hirr) and upper critical fields (Hc2).

Furthermore, the magnetic field dependence of the transport critical current density (Jct) and the grain connectivity of MgB2/Fe wires fabricated from ball-milled boron have been investigated in detail, and strong correlations have been found, as evidenced by differences in grain size, critical transition temperature, and resistivity. It was observed that the samples fabricated by ball milling had relatively small grain sizes, resulting in a weaker field dependence of the Jct in the high field region. On the other hand, the ballmilled boron was associated with poor connectivity between adjacent grains. It is clearly shown that the observed reduction in low field Jct is related to the reduction in the superconducting area fraction that is reflected by the connectivity factor. Even though high temperature sintering could always compensate for the degradation of the Jct in the low field region, the subsequent grain growth in this case was mainly responsible for the degradation of Jct in the high field region. The strong correlation between the grain size and the connectivity can change the field dependence of the Jct, and both these factors are primarily affected by the sintering temperature and by the presence and extent of ball milling.

In the MgB2 field, chemical doping is the most popular way to improve the superconductor properties. It has been reported that significantly enhanced critical current density in MgB2 superconductor could be easily obtained by doping with a hydrocarbon, highly active pyrene (C16H10), while using a sintering temperature as low as 600oC. The processing advantages of the C16H10 additive include production of a highly active carbon (C) source, an increased level of disorder, and the introduction of small grain size, resulting in enhancement of Jc.

Using the same concept, low purity boron powders were used to fabricate pure and submicron-sized carbon sphere doped MgB2 superconductor. The boron powders used showed low reactivity towards MgB2 formation, as compared to high purity (99%) amorphous boron, which might result from the larger grain size and the existence of crystalline boron or boron oxide in the former. However, the samples prepared from this boron powder showed comparable Jc values at 20 K and in low field (<1 T) to those from a sample prepared from 99% amorphous boron. Doping submicron-sized carbon spheres successfully introduced carbon substitution for boron, and so improved the Hc2, Hirr, and in-field Jc properties of the MgB2.

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