Enhancement Of Critical Current Density In MgB₂ Bulk Superconductors By Chemical Doping

The newly discovered high-temperature superconductor MgB₂ has attracted considerable interest from theoretical and experimental points of view since MgB₂ has achieved a record high superconducting transition temperature T_c in the conventional intermetallic superconductors. An overview of the research on MgB₂ superconductor during the last five years is provided in chapter 1. The discussion leads to a conclusion that though great enhancement of the upper critical field has been achieved in MgB₂, surpassing the properties of NbTi, the applications of MgB₂ are hampered by the degradation of J_c in high magnetic fields, which is mainly attributed to the lack of flux pinning centers in superconductors. A fine and well-bonded particle structure of superconducting phase with introduced fine nano-size second-phase particles is pursued to order to improve the flux pinning properties of MgB₂. The work in this thesis concentrates on the influence of chemical doping on the superconducting and normal properties of MgB₂ superconductor with an emphasis on its flux pinning behavior.In chapter 2, we have systematically reviewed the important progress since the discovery of MgB₂. We first illustrate the basic normal amd superconducting properties of MgB₂. Flux pinning properties of MgB₂ and methods to improve the properties of MgB₂ have been discussed. It is found that high superconducting phase content, fine particle structure of MgB₂ grains, high density and good grain connectivity, few microcrack and second phase content are helpful to increase of critical current density of MgB₂. The upper critical field and its anisotropy are the most important and determining parameters for the field of zero resistivity in polycrystalline MgB₂, in contrast to high temperature superconductors, where the irreversibility field is related to the pinning properties. The two gap superconductivity in MgB₂ provides the distinctive flexibility to increase the upper critical field H_c2 to a much greater extent than in on-gap superconductors. This may be achieved not only by the usual decrease of electronic mean free path, as in low-T_c superconductors, but also by optimizing the relative weight of the σ and Ï€ scattering rates through either B-site substitution, which mostly affect the in-plane σ scattering, or Mg-site substitution, which mostly increase the out-of-plane Ï€ scattering.Bulk samples of MgB₂ were prepared with 0, 3, 5, and 10 wt% Bi₂Sr₂CaCu₂O₈ (Bi-2212) particles, added using a simple solid-state reaction route in order toinvestigate the effect of inclusions of a material with higher Tc than the superconducting matrix. The density, diamagnetic signal, and critical current density, Jc, of the samples change significantly with the doping level. It is found that Jc is significantly enhanced by the Bi-2212 addition. Microstructural analysis indicates that a small amount of Bi-2212 is decomposed into G12O and other impurity phases while a significant amount of unreacted Bi-2212 particles has been prevailed in MgB2 matrix and act as effective pinning centers for vortices. The enhanced pinning force is mainly attributable to these highly dispersed inclusions inserted in the MgB2 grains. Despite of the effectiveness of the high Tc inclusions in increasing superconducting critical currents in our experiment, our results seem to demonstrate the superiority of attractive centers over repulsive ones. A pinning mechanism is proposed to account for the contribution of this type of pinning centers in MgB2 superconductors.Several bulks of MgB2 doped with Ti or /and C were prepared by an in-situ reaction method to determine the combined effect of carbon and Ti doping and probe the detailed mechanism. The magnetization measurement showed that Mgo.95Tio.05B1.95Cb.05 sample reveal improved flux pinning than MgB1.95C0.05 sample at H<50000 Oe and 20 K, indicating that carbon and Ti are largely cooperative to improve the critical current-density-field JC(H) behavior in our sintered process. No impurity line of TiC was detected in XRD patterns. Moreover, overlap of (100) peaks of MgB1.95C0.05 and Mg0.95Ti0.05B1.95C0.05 showed that Ti doping did not reduce the amount of carbon in MgB2. Microstructures observed in scanning electron microscopy indicate that the addition of Ti eliminated porosity present in the carbon-doped MgB2 pellet resulting in an improved intergrain connectivity and an increase of effective current pass. Further, MgB2 doped with C plus Ti, which mainly consists of spherical grains about 200-300 nm in size, showed higher grain homogeneity than carbon-doped sample, suggesting that Ti doping into MgBi.xCx has played an important role to uniform grains.