MgB_2 pinning properties

Title: MgB_2 pinning properties ofAuthor: Yang YeDegree granted by: Southwest Jiaotong UniversityKeywords: MgB_2;; flux pinning;; superconductivitySummary:Text include the following aspects:The first chapter briefly describes the MgB_2 superconducting materials research background, analysis of the main problems currently proposed objectives of this study and research. The second chapter reviews the basic elements of superconductivity and MgB_2 superconductors discovery process, highlighting the MgB_2 pinning properties of superconductors as well as other related properties, and detailed review of the MgB_2 Research chemical doping. Chapter III describes the preparation of this article MgB_2 for the three preparation methods superconducting materials: Neodymium Magnets solid-phase reaction, diffusion and chemical solution method, and a brief characterization of the relevant measuring instruments.Chapter IV of the elemental Al and carbon nanotube doping on MgB_2 separate superconductivity and pinning behavior. In a separate study found that Al-doped, Al can replace Mg atoms into the lattice MgB_2 among the c-axis lattice parameters decrease. When the Al doping greater than 10%, MgB_2 lattice with phase separation of the situation, both in the material may have two alloys exist. On the other hand, Al doping significantly inhibited MgB_2 of superconductivity, as Al miscellaneous increases, the superconducting transition temperature decreases linearly. Meanwhile Al-doped samples H_ (c2), J_c other properties have also been inhibited. However, the pinning force curve from the analysis of results showed that doping does not change the system of http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets pinning, grain boundary pinning system is still the main pinning effect.Analysis of individual CNTs doped MgB_2 crystal structure and superconductivity effects, we found that carbon nanotubes can be doped to a certain extent, improve the MgB_2 superconductor critical current density, but when the doping is too high, doped samples Low-field performance under the carrier to be inhibited. On the other hand, prepared by solid state reaction of the sample holes more, the actual density of only about 50% of theoretical density, and thus improve its performance is still a large space, with spread is prepared for possible ways to solve this problem .In a system, but also a detailed study of carbon nanotubes doped with changes in family samples δT_c and δl pinning behavior characteristics. MgB_2 found in our samples, δT_c and δl pinning behavior of two kinds exist, and with the increase in the amount of carbon nanotubes doped δl pinning gradually increase the proportion obtained experimental results and theoretical analysis to be good together, the results also shows that carbon nanotubes can be doped not only the introduction of the sample point to the role of pinning and the other due to C doping-induced enhancement of δl pinning, can greatly improve the pinning effect of samples. However, in the case of high temperatures, due to temperature and other factors, even in highly doped samples near the transition temperature region, δT_c pinning still showed a larger role.In the diffusion of carbon nanotubes doped superconducting materials MgB_2 study, prepared from a high density of the superconducting bulk, and diffusion method can effectively reduce the sample the presence of impurity phase of MgO, while greatly improved sample under the low-field critical current density. But the connection is too tight grain boundaries but weakened MgB_2 superconductor grain boundary pinning effect resulting in high field pinning force rapid decline, it was necessary to optimize the preparation conditions for the introduction of more effective pinning centers to improve their performance under high field . Carbon nanotubes doped samples in the two groups have shown to improve high field pinning force capabilities, especially the amount of 0.5% doped samples, in the whole range of magnetic field has a better performance.Chapter V of the Al / C and Ti / C while doping on MgB_2 superconductivity and pinning properties.Samples of MgB_2 both Al and C doping was found, Al substitute Mg B and C, two alternative role can occur, and when the role of electron doping rather when, Al doping on the superconducting transition temperature (T_c) inhibition role to be more obvious, on the other hand it can slow the Al-doped C doping on T_c impact. In the superconducting critical current density effects, a small amount of Al-doped superconductors can improve MgB_2 under the low-field critical current density, C doping can significantly improve performance under high field. When the same sample mixed with 1% Al and 1% C when J_c improvements to achieve optimal results, the 0-7T are shown within the sample compared to pure MgB_2 critical current density performance. Pinning force performance analysis also confirmed this point, under the low-field pinning force reaches its maximum before the Al-doped samples and the samples were doped pinning force strength must be greater than the pure sample, but after the magnetic field increases Co-doped sample is shown the best performance.Finally, an advantage of measuring ac susceptibility and dc magnetization method to study the Ti, C co-doped for MgB_2 flux pinning behavior, found that Ti, C-doped superconductors MgB_2 significantly improved critical current density, and in the field under the overall performance is superior to single-doped single-doped Ti and C results. By ac susceptibility measurements, we analyzed the doping on the pinning potential impact. Which MgB_2 and Ti, C co-doped samples were obtained the following relation U (T, B, j) = U_0 (?) And U (T, B, j) = U_0 (?), This result also that Ti, C doping significantly increased MgB_2 pinning properties of materials.Chapter VI of the citric acid doping on the first MgB_2 superconductivity and critical current density. The study found that citric acid can effectively improve the MgB_2 doped superconductors in high field critical current density (J_c), but with generally similar to the C-doped, citric acid doping will lead to the superconducting transition temperature (T_c) decreased, so citric acid at high temperatures increased doping J_c not very obvious. Critical current density and pinning force performance analysis shows that the citric acid doping can effectively improve the MgB_2 in the high field performance. In the 10K, 4T 15% under the optimal doped sample obtained results, the critical current density reached 1.2 × 10 ~ 4A/cm ~ (-2), far higher than the no-doped sample. But when the doping amount increased further to 20% instead of the critical current density has decreased, and thus the optimal doping citric acid may be between 15.20%.Second, the results for sorbic acid, also conducted a similar analysis. Relative to the citric acid, sorbic acid is relatively high carbon content, same doping in the case of more MgB_2 C was introduced into the system, thus demonstrating the high current-carrying performance. In order to better analyze doping MgB_2 pinning properties, the use of traditional pinning force scaling theory of public f_pαh ~ p (1-h) ~ q for two different doping samples reduced pinning force curves were fitted and found that doping makes the fitting results toward the result of grain boundary pinning close. We speculate that in MgB_2 system, the critical current density by the pinning force in addition to the effects of anisotropic grains may also be other factors, and sorbic acid and citric acid doping introduced by the C doping may reduce such impact. In the results of magnetic measurements, this effect is not a good use of percolation model to be explained that in order to better analyze experimental results, on the basis of the percolation model has been amended and supplemented, a nail bar scale of the proposed force formula and the experimental nature of co-closer.On the other hand, citric acid and sorbic acid doped composite result of doping with the chemical solution method with the traditional solid state reaction are more wears its own merits. Because nanoparticles have reunion performance, when using doped solid-state reaction method, the role of agglomeration of the nanoparticles may not be a good dopant dispersed in the system, leading to local over-doped case. Doped with a chemical solution method can effectively solve this problem, when the volatile chemical solution after precipitation of dopants in the raw B powder wrapped in a layer formed on the surface membrane, thus facilitating the doping reaction. Thus in the citric acid-doped samples, we get a better uniformity of the doping samples, moderate doping, all samples have a narrow superconducting transition, further reflecting the superconducting properties of samples with good .Chapter VII of the iron nanowires, nano-iron and ferrocene three kinds of magnetic substances MgB_2 doping effects. Found that iron doping on MgB_2 superconductivity mainly due to the inhibition of strong activity of Fe and B reactions caused by the nano-iron on T_c only showed a strong inhibitory effect, while the nano-iron and the role of ferrocene is relatively small. Iron-doped nanowires increased MgB_2 superconductors critical current density (J_c), the reason may be due to the iron-doped nanowires can be high in the end provide additional magnetic pinning force, increasing the sample under the high field pinning properties. Ferrocene doping on J_c also has an enhancement effect, but in the ferrocene-doped samples, the pinning force curve around the peak to shift the direction of h = 0.17, and high field (h = 0.75) a weak second peak exists, which may be due to the magnetic properties of ferrocene-doped pinning caused. However, this view still requires further experiments to verify.Degree Year: 2009