Master of Engineering by Research
Institute for Superconducting and Electronic Materials
Lu, Lin, Enhancement of connectivity and flux pinning in MgB2 superconducting bulks and wires, MERes thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2009. http://ro.uow.edu.au/theses/829
The objective of this work is to improve the connectivity and flux pinning and hence to enhance the critical current density of MgB2 superconductor by the development of following processes and techniques: densification and use of excess Mg addition, substitution and inclusion by doping with nano C, nano-SiC or carbon nanotube (CNT) and BN.
Firstly, a new, direct Mg-diffusion method to synthesize highly dense pure MgB2 bulks has been developed, in which pressed boron bulks are separately packed in sealed iron tubes that have been filled with magnesium powder and then sintered at high temperature over a long period. The influence of the bulk density on the superconducting properties of MgB2 has been investigated through synthesizing bulks with different densities, using a combination of diffusion and the in-situ method. This method is easily applicable to the fabrication of highly dense MgB2 bulks and tape-shaped samples. It was found that the connectivity was significantly improved as the effective area (AF) varied from 0.2 (conventional in-situ) to 0.42 (diffusion), hence the self-field critical current density, Jc, is significantly improved compared with conventional porous MgB2 bulks made by the in-situ method. A sample reacted at 850 °C for 10 hrs exhibited Jc of 1.2 MA/cm2 at 20 K in self-field.
Secondly, a novel artificial pinning centre method has been developed by using thermal strain to induce defects in MgB2 superconductors. Strain engineering has been used previously to modify material properties in ferroelectric, superconducting, and ferromagnetic thin films8. The advantage of strain engineering is that it can achieve unexpected enhancement in certain properties, for example, it can¬ increase the ferroelectric critical temperature, Tc, by 300 to 500C, with a minimum detrimental effect on the intrinsic properties of the material. Strain engineering has been largely applied to materials in thin film form, where the strain is generated as a result of lattice mismatch between the substrate and component film, or between layers in multilayer structures. The residual thermal stress/strain has been observed in dense SiC-MgB2 superconductor composites prepared by the diffusion method. The thermal strain caused by the different thermal expansion coefficients (α) between the MgB2 and SiC phases is responsible for the significant improvement in the critical current, Jc, the irreversibility field, Hirr, and the upper critical field, Hc2, in the SiC-MgB2 composite. In contrast to the common practice of improving the Jc and Hc2 of MgB2 through chemical substitution, SiC-MgB2 composite shows only a small drop in Tc and little increase in resistivity but exhibits a significant improvement over the Jc and Hc2 of conventional MgB2 due to the advantage of residual thermal strains. The present findings open up a new direction for manipulation of material properties through strain engineering in materials in various forms.
Another part of the work in this thesis was on MgB2 bulks and wires that were fabricated by in-situ solid state reaction and the powder-in-tube method, respectively. The effects of excess Mg on the structure and physical properties, such as the lattice parameters, the critical temperature (Tc), the critical current (Jc), the irreversibility field (Hirr), and the upper critical field (Hc2), have been detailed. It was found that Jc, Hirr, and Hc2 were significantly enhanced for Mg excess samples. All these properties were highly sensitive to the processing temperature for the Mg excess samples, while there was only a weak dependence on processing temperature for normal ones. For the bulks, the Tc variation for the 10% Mg excess sample was 1.6 K (36.3 K to 37.9 K) when the sintering temperature was changed from 650oC to 850oC, while it only varied by 0.5 K (37.2 K to 37.7 K) for the normal sample. The low field Jc for the 10% Mg excess samples sintered at 750oC increased by a factor of 3, compared to that for the normal MgB2 sample, while the Hc2 for the 10% Mg excess samples sintered at 650oC reached 8.7 T at 25 K, compared to 6.6 T for the normal sample. Rietveld refinement x-ray diffraction (XRD) analysis showed that the MgO content was reduced in 10% excess Mg samples, leading to an increase in the effective cross section of the superconductor.
MgB2 / Fe wires with 10 at% excess Mg produced by in-situ powder-in-tube processing were compared with normal stoichiometric MgB2 / Fe wires prepared by the same method. It was found that the critical current (Jc) and the irreversibility field (Hirr) were significantly enhanced for MgB2 / Fe wires with excess Mg. The transport Jc for 10 at% Mg excess samples sintered at 800oC, measured in fields up to 14 T, increased by a factor of 2 compared to that for the normal MgB2 wires. The best Jc results for a 10 at% Mg excess sample were obtained by heating the sample for 1 h at 600 ◦C, resulting in Jc for a field of 8 T and a temperature of 10 K that reached 3 × 104 A/cm2. A detailed analysis of the effects of excess Mg on the microstructures, the Jc, and the Hirr of MgB2/Fe wires is presented in this thesis.