| The discovery of superconductivity of the simple MgB2 crystalline intermetallic compound in 2001 has aroused the extensive interests of world scientists to investigate the structure of MgB2, the mechanism of superconduction, the synthesis methods and application prospects for its commercial use because it has a series of advantages,such as the higher superconducting transition temperature(Tc=39K), the bigger coherence length(5-6nm),the higher upper critical field,grain boundary weak link,simple structure,low cost and moulding easily.MgB2 is a superconductor material which will may be first large-scale applicated for industry at present.In this dissertation, a modified Mg-diffusion to prepare high packed MgB2 bulks under atmospheric pressure and influence of superconductivity,flux pinning properties of these doped MgB2 bulk samples were studied.Then phase purity, microstructure and superconducting properties were characterized due to different Mg/B stoichiometry for MgB2 bulk,finally bulk samples of MgB2 doped with Pr6O11 nano-powder were prepared using a solid-state reaction route and superconductivity, flux pinning properties were studied in detail.This dissertation is compose of five chapters which were, respectively, abstracted as following:In Chap. 1 ,the discovery and history of development about superconductor and its two basic properties were first introduced, respectively. Then, isotope effects,some important characteristic parameters,flux pinning force and cetre of supercondoctor were significantly discussed.At last,applications of superconductor were described.In Chap.2,the discovery,physical and chemical features of MgB2 superconductor were described briefly.Then,crystal structure,performance features,research status were mainly reviewed and critical current density characteristics,flux pinning properties and application prospect of MgB2 superconductor were discussed. Finally, the problems in fabrication on MgB2 bulks or tapes about MgB2 superconductor were analysed and summarized on the basis of above review.In Chap.3, we utilized a modified Mg-diffusion to prepare high packed MgB2 bulks with the highest density, 1.95 g/cm3 in ambient pressure. The results show the optimal heat-treated temperature is 900℃for 24h. The MgB2 prepared by the optimal conditions have superconducting transition temperature Tc by 38.1K, and critical current density Jc by 0.53 and 0.37 MA/cm2 at 10K, self-field and 20K, self-field, respectively. Additionally, after a small of nano-Pr6O11 or graphite doping, the applicable properties of MgB2 have been improved, significantly.In Chap.4, a series of MgxB2 (x=0.9, 0.95, 1.0, 1.05, 1.10) bulks samples have been prepared by the in-situ solid-state reaction method, and then the effects of Mg/B stoichiometry on superconductivity and flux pinning properties of MgB2 bulks have been detailed discussed. Phase purity, microstructure and superconducting properties have been characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and magnetic properties measurement system (MPMS), respectively. The results show that the sample with a nominal composition, Mg0.95B2 has the highest Jc value among all the samples. In the Mg-deficient MgB2, the degraded grains grow and lack of Mg sites have been responsible to significant enhancement of the Jc, Hirr and Hc2 and the enlargedΔTc, FWHM.In Chap.5, bulk samples of MgB2 doped with 0, 1, 3, 5,10wt. % Pr6O11 nano-powder were prepared using a solid-state reaction route. The lattice constants of Pr6O11-doped MgB2 systematically increase with increasing doping level, indicating a substitution of Pr for Mg in the crystal structure, and consequently, superconducting transition temperature, Tc, of MgB2 is slightly suppressed. At low doping level of 1 wt.% Pr6O11, the critical current density, Jc, and the irreversibility field, Hirr are improved. However, at higher doping levels, Pr6O11-doping seems harmful for the performance of MgB2 in high magnetic fields. The Pr6O11-doping mechanism on Tc and Jc is also discussed.
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