Study On Mgb₂ Pinning Mechanism And Pinning Typy

Citric acid doping for MgB₂ can generate chemically active carbon atoms during the sintering process, which would be easily enter the MgB₂ structure. In this thesis the citric acid doping for superconductor MgB₂ has been studied by analyzing the effect on the critical current density, irreversible field and reduced pinning force. Doping with carbon nano-tubes and aluminum have also been studied. By using the flux pinning force models proposed by D. Dew-Hughes. the experimental data of the doped samples were fitted to theoretical equations. The flux pinning types in the citric acid doped samples have been studied by fitting the experimental data to formula from different models.The thesis includes:Chapter 1, the introduction. The background of this topic is explained, and the issues of current researches are analyzed. The purpose of our research is presented.Chapter 2. The superconductivity and important properties of MgB₂ is introduced.Chapter 3. In this chapter, the effects of doping of citric acid on the critical current density, irreversible field and flux pinning behavior are studied. From the analysis of the critical current density and the irreversible field. one can find out that the citric acid doping can reduce the decreasing rate of the critical current density under high field. and also can increase the irreversible field at low temperatures, thus improving the superconducting capability of MgB₂. Fitting the data to the conventional scaling theory of flux pinning force,the doped and undoped samples could not fit the grain boundary pinning model well, but gradually close to the grain boundary pinning model with increasing doping content.Chapter 4. In this chapter the carbon nano-tubes ?CNTs? doped MgB₂ samples are studied. It is found that certain amount of CNTs can reduce the decreasing rate of the critical current density under high field. However with increasing doping content, the irreversible field decreased. By fitting the data to the reduced pinning force formula, one can see the 3%doped sample showed better pinning force behavior but not the highest critical current density.Chapter 5. In this chapter the aluminum doped MgB₂ samples are studied. It is found that only the low concentration of aluminum doping can improve the critical current density at high field. Otherwise the critical current density may decrease with increasing doping content. Aluminum doping can decrease both critical temperature and irreversible field, and it also makes the flux pinning close to the grain boundary pinning.Chapter 6. In this chapter the types of flux pinning of citric acid doped MgB₂ samples are analyzed. By using the collective pinning theory, and the fitting to the B_sb-t and J_c-t relations, it is found that in the undoped sample, ?T_c pinning only contains 46%, much smaller than other reports. With increasing doping content, ?T_c pinning contributes even smaller. From the fitting results, all samples including the undoped, ?l pinning is the main pinning type in the MgB₂ samples.