The Tsig Method Of Single Domain Ybco Bulk Preparation, Microstructure And Properties

The top seeded infiltration and growth(TSIG) process is one of the most effective methods for fabricating large YBCO bulk superconductors.It overcomes the easy deformation disadvantage of the samples during the conventional melting growth process.In this process,the liquid source infiltrates into a porous Y₂BaCuO₅(Y211) pre-form above the peritectic temperature,which then react to form YBa₂Cu₃O_7-δ(Y123) during the cooling process.In this paper,a seeded infiltration and growth method was applied to the fabrication of a single-domain Y123 superconductor.We report the studies on the fabrication progress,the preparation of the initial powders,the changes of the process,the relationship between the microstructure and the levitation force.On this basis,we have prepared a diameter of 38mm single-domain YBCO bulk superconductors.The YBa₂Cu₃O_7-δ(Y123),Y₂BaCuO₅(Y211),BaCuO₂(Y011) powders were prepared by sintering and milling process alternatively for several times.The sintering temperature is 920℃,910℃,900℃respectively.The SmBCO crystal as the seed material was prepared by the seeded infiltration and growth process.The effect of the initial liquid source weight ratio to solid Y211 phase on the growth morphology,levitation force and the microstructure of the YBCO bulk superconductor have been investigated by a seeded infiltration and growth process(TSIG).The result shows that when liquid phase to solid phase weight ratio is less than 13:10,it is difficult to get the entire sample of single domain YBCO bulk,and when the quality is equal to 13:10,it is possible to get single domain YBCO bulk.The magnetic levitation force measurement shows that when the two-phase mass ratio is less than or equal to 7:10,it is difficult to detect magnetic levitation force.But when the weight of liquid source increases,the levitation force increases and reaches a largest magnetic levitation force as the weight ratio is of 13:10.The analysis of the microstructure of the samples shows that the absence of the liquid phase leads to the growth of Y211 particles,the increasing of the thickness of the Y123 layer crystal,and result in a decreasement of the levitation force of the sample.It is found that the optimal initial weight ratio is about 13:10 during the preparation of single-domain YBCO bulk superconductors.We have studied the influence of the processing parameters on the properties of single domain YBCO bulk superconductors and found:compared to the Y123 powder,as the liquid source,the mixture of the Y123 and Y035 powders is of advantage to increase the quality of the samples.In order to avoid macro-cracks generation due to the rapid heating rate,a lower heating rate is adopted in this paper.The microstructure of the samples obtained at 1045℃with different dwelling time shows that when the dwelling time is less than 3h,the average particles size of Y211 phase is less than 2μm,similar to the size of the Y211 powder.In order to depress the random nucleation during the preparation of single domain YBCO superconductor bulks,a lower cooling rate is helpful to overcome this problem.Single-domain YBCO bulk superconductors with diameters of 34mm and 38mm with cooling rate of 0.25℃/h and 0.20℃/h respectively.The analysis of the microstructure of the samples showed that the problem of pores,different grain alignment and larger Y211 particles in the YBCO samples should be optimized farther to improve its levitation force.