Effective Pinning Potential Of The Melt Textured YBa₂Cu₃O_7-x Sample

Due to the high transition temperature and the quite high critical current density, the YBa₂Cu₃O_7-d (YBCO or Y123) superconductor can be applied for many potential aspects, such as superconducting magnetic suspension, superconducting motor and superconducting flywheel. This dissertation gives a brief introduction of the developed history and the latest investigation on the effective pinning energy of the MTG YBCO, and the significance and main content of this thesis; tells of the research material structure, synthesize technique and physical properties; expatiate on the effective pinning energy of the MTG YBCO, analysis and determining of the three parameters (the glassy exponent, the critical current density, pinning energy) of effective pinning potential (U_eff).1. By magnetic relaxation method we obtained the glassy exponentμ(T, H) of the melt textured Yba₂Cu₃O_7-x sample. The results have a similar behavior compared with ones using the method of magnetic field sweep, and are compatible with the model of collective pinning theory.2. In collective pinning theory, effective pinning potential, U_eff , is a key parameter in describing thermally activated flux motion, but it is a function of the exponentμ, scale energy U₀ and critical current density J_C, under the base of Griessen's and our group's methods of obtaining J_C andμ, respectively. Determining U₀ is a troublesome problem. In this paper, we introduced a new way of determining pinning energy U₀, thus, the complex problem of getting U_eff has been solved successfully.3. According to the collective pinning model, we determine the critical current density J_C for melt textured YBa₂Cu₃O_7-x bulk by using magnetic hystersis loops (MHLs) in a vibrating sample magnetometer (VSM), and only observe the fishtail effect (peak effect) at T=77K and 83K. Further investigation shows that the fishtail effect is the competitive result of the glassy exponentμand the pinning energy U₀, which are varying with magnetic field and temperature. We also observe that the fishtail effect shift towards higher fields as temperature decreased.