Study On The Fabrication And Superconducting Properties Of Single Domain YBCO Bulks

RE-Ba-Cu-O?REBCO,RE denotes rare earth elements?has shown significant potential of bulk superconductors for engineering permanent magnet applications such as superconducting magnetic bearings and flywheel energy storage systems,due to their distinguished abilities of high critical temperature?T_c?,auto-stable levitation performance and trapping larger magnetic field at boiling point of liquid nitrogen?77 K?than iron-based permanent magnets.Meanwhile,YBCO bulks have become the most appealing candidate materials for the industrial application due to the preparation is simple,reliable and low cost.For further enhancement of the trapped magnetic field of the YBCO bulk superconductors,it is important to increase the critical current density?J_c?and/or the size of the single-grain bulk.However,there are some problems in YBCO bulks grown by the top-seeded melt-textured growth?TSMTG?technique,such as: the macrosegregation of Y₂ BaCu O₅?Y211?second phase particles and the weak links between the multi-domain areas due to the excluded impurity phases at grain boundaries.It is key to solve these problems for further improving the field-trapping ability of YBCO bulks.The aim in this thesis is to further improve the trapped field of single seeded and multi-seeded YBCO bulks by solving the problems existed in TSMTG mentioned above.The main research contents and results achieved are listed as follows:1.The influence of Y211 phase content on the growth of YBCO bulks.The thermal analysis test and quenching growth were performed on the precursor powders with different Y211 phase content.It was found that the crystallization temperature of YBCO melt decreased with Y211 phase content increased and the growth rate increased with Y211 phase content increased.The results established the foundation for the study of the influence of precursor pellets arrangements on the growth and trapped field of YBCO bulks in the next chapter.2.The influence of layered precursor pellets on the growth and trapped field of YBCO bulks.The layered YBCO bulks were grown from layered precursor pellets.The growth morphology,superconducting properties and microstructural observations of the bulks have been studied and compared.The layered precursor pellets slow down the saturation of Y³⁺ cations along the c-axis growth front,allowing a sufficient growth along the a-b plane.That is why the layered YBCO bulks possessed a larger c-growth sector.The trapped field peak value of 22 mm layered YBCO bulk at 77 K is up to 0.8 T,nearly 29 % higher than standard YBCO bulk.Microstructural observations revealed that the layered precursor pellets inhibited the accumulation of Y211 phase particles,leading to a homogeneous distribution of Y211 phase particles and a decrease in microcracks.The relatively low viscosity of the layered precursor pellets also makes the pore density decrease.3.The high performance YBCO bulks grown from the optimized precursor pellets.Based on the results in previous chapter,combined with the solute diffusion growth model and trapping/pushing theory,the modified YBCO bulks were grown from the optimized precursor pellets.The experimental results showed that the modified YBCO bulk exhibited a homogeneous distribution of Y211 phase particles and a significant improvement of trapped field.The trapped field values of 25 mm and 34 mm modified YBCO bulks at 77 K were up to 0.91 T?maximum 0.96 T?and 1.2 T?maximum 1.28 T?respectively.4.The influence of Y211 buffer on the weak links at the grain boundary between multi-domain areas.The multi-seeded YBCO bulks were grown by adding an Y211 buffer between the SmBCO seeds and precursor pellets.The trapped field distribution and microstructural observations at the grain boundary were investigated.It was found that the Y211 buffer process was effective in reducing the excluded impurity phases at the grain boundaries and the weak links were suppressed to a great extent,which ultimately grew the multi-seeded YBCO bulks with single peak trapped field distributions.