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Title: The magnetic flux pinning behaviour in high-temperature superconductors
Other Titles: Gao wen chao dao ti zhong de ci tong ding zha xing wei
Authors: Hung, Kwok-chi (洪國智)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 1998
Publisher: [Dept. of Physics and Materials Science], City University of Hong Kong
Subjects: Flux pinning
High temperature superconductors
Magnetic flux
Notes: 1 v. (various pagings) : ill. ; 30 cm.
CityU Call Number: QC611.97.M34 H86 1998
Includes bibliographical references (leaves R1-R10).
Thesis (Ph.D.)--City University of Hong Kong, 1998
Type: Thesis
Abstract: In Chapter 1, a brief history of superconductivity is reviewed. We have introduced the basic distinctive properties of superconductor: zero resistivity and Meissner effect. The differences between type-I and type-II superconductors are then discussed and the applications of superconductors are summarized. Because it is undoubtedly important and useful for type-I1 superconductor from the point of view of applications, Chapter 2 will concentrate on the behaviour of type-II superconductor. In fact, most of the superconductors up to now (including high-Tc superconductors) are non-ideal type-II ones. The specific phenomena for the non-ideal type-II superconductors including flux creep, flux flow, and thermally assisted flux flow (TAFF), have been introduced. Moreover, the critical current density, origin of the irreversibility line, magnetic relaxation, flux pinning and flux pinning centres have been reviewed. We then introduced the Bean critical state model which have been developed to deduce the critical current of the sample. In Chapter 3, we briefly review the flux dynamic theories in type-II superconductors, which explain and interpret the specific phenomena: flux creep, TAFF and irreversibility line. Then, several relationships between effective pinning potential Ueff and current density J proposed based on different assumptions, and also the expressions for the current relaxation, have been reviewed. Moreover, the methods for determination of effective pinning potential by using different measurements including dc magnetic measurement, electrical resistivity measurement and ac magnetic measurement, have been introduced. Chapter 4 has listed the methods of characterization for the high-Tc superconductors. In Chapter 5, the dependence on temperature, both ac and dc magnetic field, frequency of the intergranular component of the ac susceptibility in our granular YBa2(Cu0.9Cd0.l)3O7-δ superconductor has been found. From the study of the temperature dependence of the critical current density, the grain boundary junctions in YBa2(Cu0.9Cd0.l)3O7-δ should be SIS-type (superconductor-insulator-superconductortype). The effective pinning potential is found to be proportional to the inverse power of magnetic field. The current density dependence of the Ueff(T,H,J) is found to obey the inverse power law of the collective pinning theory and this indicates that the intergranular weak link has the collective pinning behaviour. Chapter 6 briefly discusses the synthesis of high-Tc Hg-based superconductors by means of sealed quartz tube techniques that we have successfully fabricated. Then, we will summarize the characteristics of the crystal structures of our samples. The microstructures of pure and Pb-doped Hg-based superconductors have been studied using transmission electron microscope (TEM). Intergrowth structures with different combinations of Hg- 1212, Hg- 1223, Hg- 1234, and Hg- 1245 phases have been observed clearly in our samples. Moreover, the screw dislocations have been observed in our samples. Some other microdefects such as edge dislocations and mixed dislocations are also found in our TEM images. In Chapter 7, by measuring the hysteresis loops, the critical current density Jc is determined. Scaling behaviours of the pinning force density are discussed. By using the short-time annealing technique, the irreversibility line and the flux pinning properties of the Hg-1223 specimen are enhanced due to introducing more lattice defects, such as screw dislocations, intergrowth structures and stacking faults, into the Hg-1223 specimen. Furthermore, it is shown that the temperature dependence of the irreversibility line Hirr exhibits different features within two temperature regions. In the low temperature region, Hi, is proportional to [ l - (T/Tc)²]6.3, while in the high temperature region, Hirr is proportional to [1-(T/Tc)²]1.5. The observed Hirr can be clarified by comparison with the theoretical model. In Chapter 8, various critical state models used to estimate the critical current density Jc of the granular superconductor are studied, and the results of such estimation are compared with the AC susceptibility measurements of the Hg0.69Pb0.31Ba2Ca3Cu4O10+δ sample. By using least-square analyses, it is found that all these critical state models generally agree quite well with the experimental data. The theoretical results of Jc for different critical state models are close one to another in the high temperature range, i.e. for T close to Tc of the superconductor. In Chapter 9, the current density relaxation equation which accounts for the derivative of the ac external field has been formulated. We have found that in very short time window, say 10¯5 - 10¯³ s, the current density of the Hg0.69Pb0.31Ba2Ca3Cu4O10+δ superconductor has nonlogarithmic relaxation characteristic behaviour. This characteristic nicely agrees with the collective pinning model. Furthermore, it is found that the effective attempt time strongly depends on the current density and this current density dependence of the effective attempt time plays very important role in the study of the current relaxation in high-Tc superconductors. In Chapter 10, for the case of strongly nonlinear response, we determine the temperature, magnetic field and current density dependence of the effective pinning potential Ueff(T, H, J) for the Pb-doped Hg-1234 superconductor fabricated by us. It is found that the fast drop of the effective pinning potential with current density is due to the large value of the characteristic exponent µ which depends on the existing types of non-superconducting phases that form the intergrowth structures with the dominant matrix. The characteristic curve E(J), i.e. the electric field E versus the current density J has been obtained from the ac susceptibility technique. To consider the current relaxation due to the presence of giant flux creep, we have studied the temperature dependence of the critical current density Jc for our specimen. On the other hand, for the case of linear response, we have theoretically studied the case of linear response (TAFF regime) in which the ac-field amplitude applied is sufficiently low. By using the ac susceptibility technique, the ac-field amplitude and frequency dependence of the imaginary part of the ac susceptibility X" for the Hg0.69Pb0.31Ba2Ca3Cu4O10+δ (HgPb1234) superconductor have been studied in details. It is found that the crossover ac-field amplitude µ。h。,crossover is ~ 0.05 mT under a dc field of 40 mT for the HgPb-1234 sample. This finding seems to nicely agree with the theoretical prediction for the crossover due to the transition from TAFF to Flux Creep regime. Moreover, the µ。h。,crossover is found to depend on different samples or more generally on the flux pinning properties of the superconductors and also on the strength of the dc magnetic field. When the applied ac-field amplitude > 0.05 mT, the system is brought into the Flux Creep regime. In this regime, it is found that the effective pinning potential for the HgPb-1234 superconductor is proportional to the inverse power p of the current density, that is Ueff ∝ J¯µ with µ = 0.5. Finally, Chapter 11 concludes the thesis.
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