| MgB2 has been regarded as a possible substitute for Nb-based conventional superconductors due to its high critical temperature(Tc),low material cost and weak-link flee behaviour.MgB2 is believed to be a promising candidate for practical applications,especially for MRI magnets in the temperature range of 20-25 K.Unfortunately,the applications of MgB2 are hampered by the degradation of Jc in high magnetic fields,which is mainly attributed to the lack of flux pinning centers and low upper critical field(Hc2).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. An overview of the research on MgB2 superconductor during the last eight years is provided in chapter 1.The development of investigation on MgB2 has been reviewed in Chapter 2 and the main contents are focus on the superconductivity and other physical properties of MgB2,including crystal and electronic structure,critical current density and up critical field.Afterwords,the current research states of chemical doping on MgB2 have been introduced.Chapter 3 has discussed the experimental details as well as the characterization approaches concerned in this project.In Chapter 4,we investigated the effect of two different heat treatment tequeniqures-quenching and preheating on the superconductivity and microstructure of MgB2.It is revealed that quenching can diminish the size of MgB2 grains and improve the critical current density.To a certain extent,preheating will increase the density of the samples and enhance the flux pinning.In that case,the sample with a preheating at 600℃for one hour,followed by a sintering at 750℃for half an hour and then quench presented the best performance-- Jc reaches 0.586×106A/cm2 at 10K in self-field,the irrecersibility filed higher than 7T,the maximum volume pinning force is up to 0.8T at 20K.The influence of the Carbon and Aluminum doping in MgB2 superconductor was presented in Chapter 5.It was found that transition temperature suppressed more severely doping with Al than with C.The doping effect of C is depressed by Al doping when both element are doped simultaneity.The doping effect of citric acid on MgB2 is studied in the first segment of Chapter 6. It was observed that the Jc of MgB2 is significantly enhanced by doping while the critical temperature,Tc,is suppressed as well as carbon doping.The best values of Jc is obtained in 15%doped sample at 10K,4T,which is 1.2×104 A/cm2.The analogous results are obtained in samples doped with sorbic acid.In summary,carbohydrate doping results in a small depression in Tc but significantly increases the C substitution level,reduces the impurities,and hence improves Jc,Hirr,and Hc2 performance.Carbohydrates are cheap,abundant,and readily available.This finding opens a new direction for the manufacture of nano-doped materials using the carbohydrate solution route,which solves the agglomeration problem,avoids the use of expensive nano-additives,and achieves improved performance properties.
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