| High sensitivity of the critical current density (Jc) to external magnetic field has limited the performance of MgB2 superconductors in practical applications. Attempts to accomplish this have invoked the introduction of numerous techniques including chemical doping, irradiation, and various thermomechanical processing techniques, chemical doping is found to be the most effective way for improving the critical current density of MgB2 superconductors, carbon doping is the best among them. In this paper, we mainly studied the effects of ultrasonic B powders using different organic solvents and glucose doping on the superconductivity, the pinning bahavior and microstructure of MgB2 in depth, moreover, we also investigated the influence of two different heat treatment conditions on the crystal structure and superconductivity of glucose doped samples.In Chapter 1, the development, the basic characteristics, and the classification of superconducting materials are briefly introduced, In addition, including the structure, the superconducting properties, and the research progress of MgB2 superconductors and so on. The aims as well as the main contents of the present dissertation are also explained. Then the experimental details as well as the characterization approaches concerned in this project have been discussed in Chapter 2.In Chapter 3, we have investigated the effect of ultrasonic B powders using different organic solvents on the superconductivity and microstructure of MgB2. It is revealed that the full reaction of raw materials and the improvement of grain structure could benefit from ultrasound B powder, resulting in the enhancement of the flux pinning and high field current-carrying capacity.The effects of glucose (C6H12O6) doping on the superconducting properties and microstructure of MgB2 bulk have been firstly researched in Chapter 4. Then glucose-doped MgB2 samples at different heat treatment conditions have been prepared by chemical solution method. From the results of glucose doping, we found that the lattice parameter a and the Tc decrease with increase of the doping level. However, the Jc is significantly enhanced and the Jc(B) properties are improved at low temperature, which is attributed to the lattice distortion due to the C substitution for boron. Microstructure analysis indicates that MgB2 grains can be greatly refined by appropriate amount of C6H12O6 doping, forming a highly compacted network structure, which may be the main reason for the enhancement of the flux pinning. From the results of heat treatment, we found that high-temperature sintering is benefit to the increase of the proportion of carbon substitution for boron, so that the flux pinning properties of doped samples is improved. On the other land, it also increases the diffusion rate of Mg to B, So this would make the distribution of the impurity and defect more uniformly. Therefore, we believe that the method of combining the three, grain refinement, carbon substitution of boron and the uniform distribution of impurities, defects etc, is an effective way to improve the pinning properties of the MgB2 samples.In Chapter 5, the flux pinning behavior of MgB2 samples doped with the different proportion of glucose is fitting with the expression of fp=Abp(1-b)q which is often used in conventional flux pinning theory. It is found that the reduced pinning force curve drifts to the grain boundary pinning mechanism with increase of the doping level. Furthermore, the temperature dependence of critical current density, Jc(t), at particular field within the single vortex regime is analyzed. AδTc pinning mechanism is found to be mainly responsible in pure MgB2 sample, whileδl pinning is dominant with little effect ofδTc pinning for glucose doped samples. It is observed that the contribution ofδl pinning enhances with increasing C6H12O6 addition level. And the effect on the pinning mechanism almost gets saturated when the doping level is higher than 10 wt%. |
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