Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Mustapic, Mislav, Enhancement of MgB2 superconductor by magnetic nanoparticle doping, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3822
Magnesium diboride superconductor is easy and cheap to produce, and it can be operated at temperatures that can be maintained by inexpensive cryocoolers, requiring only electrical power input. In order to make it competitive with classical superconductors, its performance in high magnetic field needs to be improved. This is done by improvement of vortex pinning through including non-superconducting nano-defects. Theoretical work shows that magnetic nanoparticles could give better vortex pinning than the non-magnetic ones. Existing experimental reports on this topic are patchy and inconclusive. This work presents a systematic study on the preparation of various magnetic nanoparticles, their incorporation into MgB2, and mechanisms for improvement of current carrying capabilities for the most successful nanoparticles.
A number of different nanoparticles were prepared and tried: carbon coated Co, carbon coated Fe, carbon coated Ni, CuFe2O4, Fe2B, Co2B, NiCoB, Fe3O4, CoB, SiO2 coated Fe2B, Co2B, and commercial Eu2O3 and Dy2O3. Most of them resulted in marginal improvement of MgB2, or even in degradation of its properties. The most successful results were obtained with NiCoB nanoparticles 5 nm in size. They are the focus of this project.
The best NiCoB nanoparticles were prepared by the wet method of chemical reduction of metallic salts, which yielded nanoparticles small enough to be successful pinning centres in MgB2.The success of this method is due to the medium level alkaline environment with surfactants which supported their growth. The nanoparticles were clearly superparamagnetic. Optimization of the preparation procedure for nano-NiCoB doped MgB2 gave the best performance for 2.5 wt% NiCoB doping, the use of amorphous 500 nm-size boron with micron-sized Mg, and heat treatment at 850℃ for 30 minutes.
Detailed analysis of the improvement of critical current density, Jc, by this doping revealed that the connectivity between superconducting crystals and the vortex pinning were both improved as a result of this procedure. The connectivity improved due to refinement of crystals and, at the same time, due to removal of MgO from the crystals due to the reaction between Mg and the dopant nanoparticles.
The vortex pinning improved only at low temperatures. High temperature heat-treatment was required to obtain the vortex pinning. A reaction between the NiCoB nanoparticles and Mg led to formation of Mg2Ni and Co2Mg nanoparticles that were incorporated into the MgB2 matrix, where they acted as pinning centres. Competition of NiCoB for Mg with MgO was the most likely cause of the decrease in MgO content in the samples, leading to better connectivity. These new nanoparticles did not have a spontaneous magnetic moment.
No effects of magnetic pinning were observed in this project, although a large number of different magnetic nanoparticles were used as dopants. Therefore, this project casts doubt on the effectiveness of magnetic nanoparticles as a means of improvement of vortex pinning in MgB2 through the interaction of magnetic vortices with the magnetic moment of nanoparticles.