Doping Effect Of Y₂Ba₄CuWO_y Phase On Microstructure And Superconducting Transition Temperature In Polycrystalline YBa₂Cu₃O?7-??

Y₁Ba₂Cu₃O_7-??YBCO or Y-123? material is one of the most widely studied high temperature superconductors. It is also a hot topic to study within these years that the non-superconducting phase is mixed into Y-123 material, which can be greatly improved critical current density Jc. Therefore, doping the second non-superconducting phase can improve the performance of Y-123 materials and advance the application value. This paper aims to investigate the effect on the microstructure and superconducting properties of the samples that incorporated Y₂Ba₄CuWO_y?Y-24W1? phase into polycrystalline Y-123 bulk materials. Besides, in this paper we also try to synthesize of Y-123 precursor powder by the citrate pyrolysis technique and explore influence of Y-123 bulk materials transition temperature with the different annealing temperature and times.In the first part of work, we prepared material block polycrystalline Y-123 doped with Y-24W1 phase by solid state reaction method. At the same time, we have made analysis of the crystal structure, microstructure and transition temperature of Y-123 composite material. In the second part, we adopted a new method-the citrate pyrolysis technique to prepare Y-123 precursor powder which processed at different annealing temperature. Finally, we characterized and analysed the key parameters in crystal structure, Tc and Raman spectrum of the Y-123.1. When doping concentration ranges from 0.20 mol% to 3.00 mol%, Y-24W1 phase did not destroy the crystal structure of Y-123 phase which indicates that the incorporation of Y-24W1 is relatively stable chemical properties. The undoped Y-123 bulk samples has larger grain size, pore size and more holes than the sample of doped 0.20 mol % Y-24W1 phase. The morphology of the sample of doped 0.20 mol% Y-24W1 phase obviously get improved, the grain size decreases and the connection greatly increases. When Y-24W1 phase doping concentration is 0.20 mol%, the Tc,onset and Tc, zero decrease a little, which are close to the undoped sample. When the doping concentration is larger, the Tc, onset and Tc, zero of samples decreases obviously. By compared the derivative of resistance and temperature relationship, we draw the conclusion that the Y-24W1 phase can exist in Y-123 grain boundaries and form insula-ting layers. This structure is similar with Josephson junction structure, which is the main reason that the Tc decreases and the width of transition broaden with Y-24W1 phase increasing in Y-123 matrix.2. The Y-123 precursor powder prepared by the citrate pyrolysis technique was treated with different annealing process. The powder was processed between 910?C and 860?C, the XRD pattern of Y-123 samples is orthogonal. When the annealing temperature reached 930?C, the crystal growth direction has changed. Observing the morphology of the fractured core layers of Y-123 sample by SEM, the sample processed at 860 ?C, the grain size is the smallest. It can be found that with the increasing of annealing temperature, the number of holes decreases but the grain size increases. Subsequently, the R-T curves of samples are investigated.The sample processed 12 h at 880?C, 910?C, 880?C for each time has the best metallic behavior. The Tc,onset Tc,zero and transition width of the sample fabricated by citrate pyrolysis technique are close to the sample prepared by solid phase reaction method. At last, from the Raman spectrum of sample processed 12 h at 880?C, 910?C, 880?C for each time, the Raman shift of O?4?-Ag peak of the sample is 502.7 cm-1, which shows that the sample has a well superconductivity.