| Magnesium diboride, MgB2, as a new magnet-grade superconducting material, is a promising candidate to the application of medical Magnetic Resonance Imaging (MRI) magnet operating at 20-30K and 1-3T and the high-field magnet working at 4.2K and 10-25T. However, due to the lack of effective flux pinning centers and weak intergranular-connection, the poor Jc performance has been thought the key that obstructs the development of MgB2—based superconducting materials and related techniques. The work aims to improve Jc of MgB2 superconducting materials by studying the ways that fabricate the high dense MgB2 superconductor and the properties of co-doped MgB2 tapes with different kinds of dopants. The main research works and obtained conclusions are as follows:(1) A novel Mg-diffusion way has been utilized to prepare the MgB2 superconductor in this dissertation. It is found that the MgB2 bulks prepared by this method hold a series of advantages, for example, high density, little MgO impurities, high Jc, well uniformity, fine grains and so on. We have also researched the MgB2 superconductor with graphite doping by this method. The results show that the Jc of graphite doped MgB2 at 5 K,7T reaches 2x104 A/cm2 as well as the other carbon source doping, like as nano-carbon, carbon nanotube, and nano-diamond. For the 2 wt.% graphite doped MgB2, at 10K and self-field, the Jc is 5.8x105 A/cm2, which is significantly higher than the in-situ MgB2. Furthermore, similarly, the effects of nano-Pr6O11 particles doping on superconducting properties of MgB2 have been studied. It is found nano-Pr6O11 doping can accelerate the diffusion of Mg to boron and enhance the reaction of Mg and boron, and consequently, the Jc and Hirr of MgB2 superconductor increase.(2) The MgB2 bulks with both imporous microstructure and nano-sized grains have been successfully fabricated by the common solid-state reaction way at ambient pressure with a boron pretreatment process. Firstly, the effects of the Mg/B stoichiometric ratio on superconducting performance of MgB2 have been discussed. The results suggest the samples, Mg0.90～0.95B2 have the best Jc and flux pinning performance. Secondly, we have systematically researched the influence of rare-earth oxides doping on Jc and flux pinning of MgB2 superconductor. It is found different from the other rare-earth element doping, after nano-Pr6O11 doping, the Pr atoms can slightly substitute for the Mg atoms at MgB2 lattices, which cause the a small of Tc-decrease, and the improvement of Jc and Hirr on MgB2 superconductor. Otherwise, the reason has been discussed that why does the ferromagnetic transition elements doping significantly depress the superconductivity of MgB2, but the strong paramagnetic rare-earth elements doping have little effect on superconductivity of MgB2. At last, by pre-treating the amorphous boron powder and precisely controlling the remnant toluene content, the imporous MgB2 superconductor with fine grains have been prepared successfully, which have a density of 1.8～1.9g/cm3, the Hirr of 5.1T at 20K and Jc more than 1000A/cm2 at 10K and 7T.(3) The cooperative doping effects of Ti and nano-SiC on superconducting properties of in-situ Fe-sheathed MgB2 tapes have been studied firstly. The results suggest that the Ti doping can significantly improve the transport Jc of pure,5 mol% SiC-added and 10 mol% SiC-added MgB2 tapes. For the pure MgB2 tape, the Jc wil be the best while the content of Ti-added is 10wt.%. Improvement of Jc at Ti doping mainly takes a part in the lower magnetic fields. Compared to the only SiC-added tapes, the Ti+SiC co-doping enhances the transport Jc of MgB2 by a factor of 50～100%. For example, the 10mol% SiC doped tape with heat treatment of 660℃/1h has a Jc of 4150 A/cm2 at 4.2K and 10T as well as the Jc of 7885A/cm2 for the 5wt.%Ti+10mol%SiC co-doped tape.(4) By studying the effects of the different organic compounds co-doping, it is found that Jc of MgB2 tape can be further improved by two (or more than two) kinds of dopants adding. Firstly, the results suggest that the ethylbenzene assisted mixing can significantly improve the transport Jc of the pure, Ti-doped, SiC-doped and Ti+SiC co-doped MgB2 tapes. And then, Ti, Malic Acid, SiC, C9H11NO and two of them as dopants have been added into MgB2 tapes, it is found two kinds of dopants co-adding would be more effective on enhancing Jc of MgB2 than one of them added. For example, at 4.2K,10T, the Jc of the 10wt.% Malic Acid and 5mol%SiC doped MgB2 tapes with heat treatment of 660℃/1h are 7900 and 9900 A/cm2, but it is 14100A/cm2 for the 10wt.%Malic Acid+5mol%SiC co-doped tape. Similarly, for the 10wt.% Malic Acid and 4wt.% C9H11NO doped tapes with heat treatment of 600℃/4h, the Jc are 11300 and 9200A/cm2, and Jc of the 10wt.%Malic Acid+4wt.% C9H11NO is 14900A/cm2 with the same heat treatment condition. At last, we have researched properties of different organic compounds co-doped MgB2 tapes as well as as raw material, and the similar results have been obtained.
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