Magnesium diboride superconductor: thermal stabilization and doping

Author

Andrey V. Shcherbakov, University of Wollongong

Year

2011

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Recommended Citation

Shcherbakov, Andrey V., Magnesium diboride superconductor: thermal stabilization and doping, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2011. https://ro.uow.edu.au/theses/3258

Abstract

The primary aims of this research were to (i) improve the thermal stability of MgB2 wires, and (ii) investigate the effects of different doping materials on the electromagnetic behaviour of MgB₂ in wire form, enabling practical application of this superconductor.

In this work, a pioneering results on the fabrication of Al-stabilized Fe-clad MgB₂ wires have been presented. First, the Al-stabilized Fe-clad MgB2 wires were fabricated using the existing powder in tube method. The possibility of low temperature in-situ MgB₂ formation, as well as the compatibility of the Fe barrier layer and the Al stabilizer at increasing fabrication temperatures of Al/Fe-clad MgB₂ wires have been demonstrated. The range of optimal temperatures (600 - 630^oC) and time of sintering (180 - 30 min) for production of Al-stabilized Fe-clad MgB₂ wires were empirically observed. The main limitations of this method are (i) reduced density of the superconducting core during drawing of relatively soft Al metal, and (ii) degraded connectivity between Al and Fe metals, so that they are not suitable for fabrication of long MgB₂ wires. Thus, a new approach towards fabrication of Al-stabilized MgB₂ wires (namely the hot aluminizing technique) was developed. In this technique, fully optimized and in-situ reacted Fe/MgB₂ wire was immersed in a molten Al bath, resulting in a uniform Al coating of Fe-clad MgB₂ wire. As demonstrated, this approach does not result in any degradation of superconducting properties ofinal Al-stabilized Fe-clad MgB2 wires. However, this enables reduction of the Fe barrier thickness and good connectivity between Fe and Al sheath materials. Furthermore, this approach is easily applicable for simultaneous aluminizing and in-situ formation of MgB2 wires, which can even be extended to fabrication of Al-stabilized Fe/MgB2 coils. Although realization of Al-stabilized MgB2 wires is shown to be possible, future work on stabilization should be focused on increasing the Al thickness, reduction of the Fe barrier layer, and demonstration of the stability effects on Al/MgB₂ wires, which has not been investigated due to limitations of available equipment.

Fe-based doping of MgB₂ wires was studied in order to demonstrate the feasibility of introducing magnetic pinning sites into the MgB₂ system, and to confirm the-oretical studies predicting improvement of current carrying ability of (ferro)magnet/ superconductor hybrid systems. A systematic study of doping materials, which included nano-Fe, home-made Fe₂B, and SiO₂ coated Fe₂B nanoparticles, was conducted, and the effects of the above-mentioned Fe-based dopants on the su-perconducting properties of MgB2 wires were investigated. The SiO₂ coated Fe₂B nanoparticles were employed in order to study the feasibility of insulating the fer- romagnetic inclusions and superconducting matrix, as well as to enabling more ho- mogeneous distribution of ferromagnetic particles in MgB₂ superconducting wires. The severe suppression of critical temperature (T_c), critical current density (J_c), and upper critical field (B_c2) was observed for all magnetic dopants studied with the most detrimental effect coming from nano-Fe doping and to a lesser extent - from SiO₂ coated Fe₂B nanoparticles. The degradation of superconducting prop- erties due to Fe-based dopants was attributed to an increased level of impurity phases in MgB₂ compound, as confirmed by x-ray diffraction studies. Analysis of the electromagnetic properties suggested that the dominant pinning mechanism is the pinning on grain boundaries (as in pure MgB₂ compound), and thus, magnetic pinning was not achieved in the samples studied by introduction of these Fe-based dopants. It is likely that more homogeneous systems (such as thin flms) are required in order to demonstrate the benefits of superconductor/ferromagnet interactions predicted by theoretical investigations.

The effect of polymer C-based dopants on the properties of MgB₂ wires was systematically investigated. Electromagnetic characteristics (B_c2(T) and J_c(B_a), where B_a is an applied magnetic field) of Fe-clad MgB₂ wires with an optimal level of doping material were systematically improved with increasing sintering temperature. The ability of polymer dopants to dissolve in liquid media (water or toluene) and cover B particles by thin layer of amorphous carbon before MgB₂ formation has significant benefits for the fabrication of Fe-clad MgB_2-xC_x wires with en- hanced homogeneity and electromagnetic performance. The analysis of structural and electromagnetic changes in the Fe-clad MgB₂ wires doped with C₂H₆S_i polymer and C₁₂H₂₂O₁₁ materials revealed that more C (compared to nano-C doping) is incorporated into the MgB₂ crystal lattice on B sites during sintering, resulting in (i) some reduction of critical temperature (by about 2 K), but (ii) formation of a larger number of nanoscale structural defects, favoring pinning of magnetic vortices and leading to significant enhancement of the superconducting properties of the final Fe-clad MgB_2-xC_x wires. However, the results of this work indicate an emerging necessity to improve the density of MgB_2-xC_x in the wires in order to reach the superconducting properties obtained in the bulk samples. Another crucial issue is to reduce the level of MgO impurities which is a native impurity formation in any in-situ fabricated MgB₂ compounds, although this level may also be increased by introduction of O-containing compounds (e.g. C₁₂H₂₂O₁₁). As a result, suppressed current carrying ability was observed in C₁₂H₂₂O₁₁ -doped MgB₂ wires compared to the critical current density, Jc(Ba), of MgB2 wires with addition of oxygen free C₂H₆S_i doping material. This observation confirms that, in spite of imperfectly connected MgB₂ grains, flowing current is notably affected by the increased level of insulating MgO impurities that preferentially form at the grain boundaries. Results obtained in this work indicate that oxygen free C₂H₆S_i dopant is to be the most optimal and thus promising for practical application of MgB_2-xC_x wires.