Measurement and interpretation of AC susceptibility and loss in high temperature superconductors

The discovery of high temperature oxide superconducting materials (HTSC) generated great interest of the scientific community for many reasons, such as producing materials that would be in the superconducting state at temperatures closer to room temperature. It was soon discovered, however, that before being able to use HTSC's in many applications many materials problems needed to be overcome, like brittleness, the granularity effect and difficulty in pinning flux vortices. For applications of bulk superconductors, effective intergranular coupling and strong intragranular pinning are two issues of major concern. The goal of this thesis is to enhance the understanding of AC losses on HTSC's associated with London magnetic field penetration, granularity and depinning Abrikosov vortices using a powerful technique for studying of AC losses generated in superconductors: AC susceptibility. Each of the types of loss mechanisms mentioned above is associated with a loss peak on the imaginary component of the AC susceptibility.; In this thesis, the AC susceptibility measurement was used to identify the three loss peaks: the intrinsic, the intergranular and the intragranular peaks, and to distinguish between the different types of samples studied here. Each one of the loss peaks was explained in terms of well-known models.; Both polycrystalline and single crystal samples were investigated. The {dollar}rm YBasb2Cusb3Osb7{dollar} polycrystalline samples studied were prepared by three different methods: solid state synthesis (SSS), melt-process melt growth (MPMG) and freeze-dried techniques and were studied as bulk and powder samples. {dollar}rm Lasb{lcub}1.85{rcub}Srsb{lcub}0.15{rcub}CuOsb4{dollar} and {dollar}rm(Bi,Pb)sb2Srsb2Casb2Cusb3Osb{lcub}x{rcub}{dollar} single crystals were studied and their results were compared to the results obtained from the polycrystalline samples. Finally some computer modelling was done to simulate the behavior of the intrinsic peak for the {dollar}rm YBasb2Cusb3Osb7{dollar} SSS sample.