Doctor of Philosophy
Institute for Superconducting and Electronic Materials - Faculty of Engineering
Zhang, Yun, Improvement of critical current density in MgB2 by optimizing process parameters and chemical doping, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials - Faculty of Engineering, University of Wollongong, 2009. http://ro.uow.edu.au/theses/3060
The discovery of superconductivity in MgB2 in January 2001 has triggered enormous interest around the world. MgB2 has evolved as a promising superconductor for the next generation of superconductor applications, due to not only its higher critical temperature (Tc = 39 K), low material cost, and good weak-link tolerance, but also its rich multiple-band structure. However, several issues exist that urgently need to be solved, such as its low upper critical field (Hc2) and rapid decrease in critical current density (Jc) under magnetic field compared to Nb-based superconductors. High Jc(H) performance is crucial for the application of this material in the so-called “strong electrical application” field. Therefore, the objective of this thesis is to increase Jc of bulk polycrystalline MgB2 via studying the influence of the boron precursor powder and the nominal Mg/B mixing ratio and efforts to further enhance Jc by doping with two types of sources: carbon sources (such as SiC and sucrose) and oxygen sources (such as TiO2/SiO2 and processing atmospheres with different oxygen content).
The control of the boron powder is one of most promising methods to enhance Jc at high fields without significant decrease of the self-field Jc. The particle size, purity, and form of the starting boron powders play an important role in the superconducting properties of MgB2. It has been proved the precursor B powder is very important to critical current density of MgB2. In terms of the influence of boron precursor on the superconducting properties of MgB2, Jc values for MgB2 made from ball-milled high purity boron powders with crystalline phase are at least two times higher than for a comparable MgB2 sample made from ball-milled amorphous boron powder and a factor of 40 higher than typical values of standard MgB2 samples. The possible mechanism proposed to account for this difference is Hc2 enhancement caused by the increased disorder.
Nano-particle SiC is a very effective dopant to improve the in-field Jc of MgB2. However, Jc at lower field become lower than the undoped one. It would be desirable to maintain a high low-field Jc while improve the high field Jc. In order to solve this issue, the influence of Mg content was investigated in a series of undoped and SiC-doped MgB2 samples with systematic variation of the nominal Mg/B ratio (x:2). It has been found that in the undoped MgB2 samples Jc increases with x to a maximum value at x = 1.1 and then decreases. The higher Jc at x = 1.1 is attributed to the better connectivity caused by smaller amounts of impurities. In the SiC-doped samples, it has been found that the sample with x = 1.15 exhibits the best Jc performance for all fields from 0 – 8.5 T, including the self-field, and its Tc is higher than for the sample with x = 1. The enhancement of Jc is attributed to the improved connectivity and the increased disorder. The optimized Mg/B ratio (1.15:2) also diminishes the interband scattering caused by the Mg or B vacancies and, in turn, increases Tc.
Carbonhydrates such as sucrose can decompose at high emperature. The resulting carbon from the decomposition can be used as carbon source to dope into MgB2. This kind of carbon is very reactive and can be incorporated into MgB2 easier. A comprehensive study of the effects of carbohydrate doping on the superconductivity of MgB2 has been conducted. Doping with sucrose at varying rates and annealing temperatures results in an optimal Jc value in MgB2-xCx at x = 0.2 and 850 oC. At 5 K and 6 T, the x = 0.2 sample shows one order of magnitude improvement compared to pure sample. It has been found that sucrose doping causes a small depression in Tc and high resistivity, while Hc2 performance is improved. The reason for the enhancement of Hc2 is likely to be increased disorder caused by C substitution for B and/or diffusion of C atoms in the MgB2 lattice as interstitial atoms.
TiO2/SiO2 additions lead to improved Jc values. Jc values are the highest at the doping ratio of 10 wt% at 5 K and 20 K, and at the doping ratio of 5 wt% at 30 K, when the sintering temperature is fixed at 750 oC. When the doping ratio is fixed at 5 wt%, the sample with the sintering temperature of 750 oC has the best Jc for 5 K and 20 K, while the sample with the sintering temperature of 850 oC exhibits the highest Jc at 30 K. In addition, it has been found that the addition of TiO2/SiO2 nanoparticles results in a small depression in Tc, while the Hc2 and irreversibility field (Hirr) performances are improved. The enhancement of Hc2 and Hirr can be attributed to the existence of precipitates induced by the TiO2/SiO2 doping.
The effect of processing atmosphere on microstructure and superconducting properties was studied for MgB2 samples made using the in situ reaction technique under argon atmosphere with three different purities, ultra-high, high, and welding grade. The critical temperature, Tc, decreases by 0.5 K while the FWHM, the resistivity and the amount of MgO show an increase in the welding Ar processed sample. The Jc, Hirr and Hc2 for the welding Ar processed sample are improved in comparison with the samples treated in ultra-high and high purity argon. Transmission electron microscope (TEM) examination revealed that the sample processed in welding grade argon possessed small grains, a high density of defects, and larger crystalline strains, which act as effective pinning centres. These results verify the dual reaction model where the MgB2 formation and the reaction between oxygen and precursor take place simultaneously, resulting in an optimal doping effect. The Jc(H) behaviour for samples treated at 800oC for 60 hours in the sealed Fe tube shows little difference among the three different Ar atmospheres. The optimal Jc and flux pinning properties in MgB2 can be achieved by using readily available and economical welding grade Ar as the protective atmosphere instead of using ultra-high or high purity Ar. The effects of the sintering time on the superconductivity of MgB2 with sintering in the welding grade Ar atmosphere have been investigated. The sample sintered for 30 min exhibits the highest Jc at high fields. The reason can be attributed to the improved connectivity and the increased Hc2.
Overall, the works in this thesis are mainly on material process and characterization. I have studied many factors such as precursor powder, process parameters, dopants that have strong effects on Jc. The results are useful for future MgB2 fabrications.
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