Investigation Of Flux Pinning In MgB₂

The main contents of thesis are organized as follows:In Chapter 1, the background of MgB₂ is introduced, where the problems of the recent work are revealed. The aims as well as the main contents of the present dissertation are also explained. The development of investigation on MgB₂ has been reviewed in Chapter2 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, the doping effect of Al on superconductivity of MgB₂ is studied as well as CNTs doping. In the section of Al doping, it is found that c-axis cell parameter is reduced by substitution of Al for Mg in the lattice. The phase separation is observed in samples doped at level higher than 10%. On the other hand the superconductivity of MgB₂ is suppressed severly by Al doping, while the mechanism of flux pinning in doped samples is not affected.The critical current, J_c, is ehanced by CNTs doping at low level, while depressed at high doping level. The density of samples prepared by solid state reaction is only 50% of theoretical values. It may be the key problem to improve the J_c in MgB₂ furtherly. Theδl andδT_c pinning properties of MgB₂ doped with CNTs is investigated and it is observed that the main pinning mechanism transforme fromδT_c toδl pinning with increasing CNTs doping.The density of MgB₂ is increased for samples prepared by diffusion methed. Less MgO impurity phase and better grain connection are also observed in these samples; however the behaviour of J_c is suppressed in high field range due to the weakening of grain boundaries.In Chapter 5, the cooperation doping effect of Al/C and Ti/C on superconductivity of MgB₂ is investigated. The observed decrease of T_c of samples could be attributed to the band filling effect and interband scattering. The similar behavior of H_irr versus contension carbon was observed in the both series of samples. It was found that transition temperature is suppressed more severely doping with aluminium than with carbon. The doping effect of carbon is depressed by aluminium doping when both elements are doped simultaneity.In the last section of Chapter 5, flux pinning behavior of carbon and titanium concurrently doped MgB₂ alloys has been studied by ac susceptibilityand dc magnetization measurements. It is found that critical current density and irreversibilityfield of MgB₂ have been significantly improved by doping C and Ti concurrently, sharply contrasted to thesituation of C-only-doped or Ti-only-doped MgB₂ samples. AC susceptibility measurement reveals thatthe dependence of the pinning potential on the dc applied field of Mg_0.95Ti_0.05B_1.95C_0.05 has been determinedto be U(B_dc)∝B_dc^-1 compared to that of MgB₂ U(B_dc)∝B_dc^-1.5. As to the U(J) behavior, a relationshipof U(J)∝J^-0.17 is found fitting well for Mg_0.95Ti_0.05B_1.95C_0.05 with respect to U(J)∝J^-0.21 for MgB₂. All theresults reveal a strong enhancement of the high field pinning potential in C and Ti co-doped MgB₂.The doping effect of citric acid on MgB₂ is studied in the first segment of Chapter6. It was observed that the J_c of MgB₂ is significantly enhanced by doping while the critical temperature, T_c, is suppressed as well as carbon doping. The best values of J_c 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. The fluxpinning behaviors is fitted with expression of f_p∝h^p(1-h)^q which is often usedin conventional flux pinning theory, and it is found that the values of f_p decreasemuch rapidly than the values of surface pinning theory in the high field range. However, the experimental data could not be explained by the percolation theory which is proposed by Eisterer because our values of J_c are obtained from magnetichysteresis loop. In order to explain the result, a new method of fitting f_p isproposed. It is found that the behavior of f_p in high field range is mainly influenceby the anisotropy of irreversibility field of MgB₂.In Chapter 7, Mg_1-xFe_xB₂ superconductor was prepared by in situ solid state reaction to study the effects of nano-iron doping on the superconductivity of MgB₂. Two kinds of nano-iron sources have been used: nano-iron particles and nano-iron wires. It was found that crystal structure was not affected by nano-iron doping for either form of dopant source, while T_c, J_c, and H_m were severely suppressed by iron nano-particles doping. In contrast, with iron nano-wires doping, T_c of the MgB₂ superconductors were not changed remarkedly, while their J_c and H_irr were slightly enhanced at all the temperature ranges investigated in this work. Under 4 T field, the sample doped with 1% wt iron nano-wires reached the highest J_c of 1.1×10⁴ A/cm² at 10 K and 2.2×10² A/cm² at 20 K, respectively. It is argued that the Fe nano-wires may be introduced into MgB₂ as external pinning centers in high field region.In addition, ferrocene is also used to dope in the MgB₂. In this situation, the results may be consider as the codoping of Fe and C, because ferrocene is decomposed into iron and carbon at high temperature. It is deteced that the J_c of doped samples is increased and this may be due to the doping of carbon in MgB₂.