Doctor of Philosophy
Institute for Superconducting and Electronic Materials Faculty of Engineering
Motaman, Ashkan, Current limiting mechanism in MgB2 wire, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials Faculty of Engineering, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4352
Since discovery of the MgB2 as a superconductor; Lots of efforts have been taken by different researchers in worldwide to make this compound applicable for the electrical application.
Valuable superconducting properties of this compound such as but not limited to its high Tc, intrinsically ―weak-link‖ free grain boundaries, rich multipleband structure and its low fabrication cost, led to consideration of this compound by researcher as a strong substitute for nowadays commercial Nb- Ti superconductor which is more expensive than MgB2.
The work presented in this thesis is focused on current limiting mechanisms in MgB2 superconducting wires.
In the literature review chapter, we discussed on MgB2 obstacles towards industrialization and electronic applications as well as undertaken protective measures to win over the existence obstacles. MgB2 main obstacles in regards to electrical application lie on its rapid drop in critical current density (Jc) under applied magnetic field, and low upper critical field (Hc2), which exclude this compound from many industrial applications where a high Jc under high magnetic field is required.
A comprehensive study of the effects of structural imperfections in MgB2 superconducting wire has been conducted in the chapter followed by literature review. As the sintering time and temperature change the MgB2 structural imperfection therefore, sintering condition plays vital role in final superconducting properties of the MgB2. Lower sintering temperature increases the MgB2 structural imperfections due to increase in MgB2 lattice disorder and leads to higher impurity scattering between the π and σ bands of MgB2, resulting in a larger upper critical field. From the other side, lower sintering temperature results in smaller MgB2 grain size which improves the pinning forces, and thereby, enhances the critical current density. In spite of using the low sintering temperature, Voids and porosities still exist in the MgB2 structure and suppress the critical current density.
After comprehensive study on the structure of stoichiometric compound of the MgB2 wire, we continued our research on the MgB2 wires treated with 10 wt.% malic acid (C4H6O5), as this sample showed the best performance result so far among the other MgB2 doped wires. State of art investigation has been executed on the pinning mechanism of the MgB2 wires, treated with 10 wt.% malic acid (C4H6O5), at different sintering temperatures. Our investigation revealed that flux pinning is dominated by point and correlated pinning at lower and higher magnetic fields, respectively, for the carbon-doped samples sintered at both 700 and 900◦C. from the other side, the δl pinning is dominant at lower operating temperatures, and δTc pinning starts to be dominant close to Tc, This means that spatial variation in the charge carrier mean free path is mainly responsible for the flux pinning mechanism in the malic doped MgB2 wires sintered at 700 and 900°C.
We have also analyzed the pinning mechanism of the MgB2 wire, 10% malic doped and un-doped, based on the percolation model. The critical current behavior measured from MgB2 wires can be obviously explained by only four fitting parameters, the anisotropy parameter, the pinning force maximum, the upper critical field along the ab-plane, and the percolation threshold. Moreover, the temperature dependence of the upper critical field is further explained by the dirty-limit two-gap theory. With the carbon dopant, there was a clear decrease in the anisotropy parameter, resulting in an increased high-field critical current density. In contrast, the pinning force maximum is found to be main factor affecting the low-field critical current density.
And finally in last chapter, we analyzed the correlation between the critical current density (Jc) and the n-value extracted from the electric field versus current density characteristic. The power-law relationship (m) between the Jc and the n-value, n ∝ Jcm, represents a critical index, which is strongly dependent on operating temperatures.
MgB2 conductors still can have an impact in Magnetic Resonance Imaging (MRI) applications for the following reasons: (1) respectable properties for low to mid-field magnets operating at temperatures as high as 20 K; (2) wire manufacturing no more difficult than Nb-Ti wire manufacturing; (3) Mg and B are lower cost raw materials than Nb and Ti; (4) MgB2 is 1/3 the density of Nb-Ti, so that the same kg of raw material will yield three times the piece length; (4) faster charging rate compared to Nb-Ti based magnet; and (5) the high critical temperature offers a larger thermal margin than the 9 K of Nb-Ti.