Manipulating Stoichiometry And Growth Mode In Preparing REBCO Crystals With High Performance

REBa₂Cu₃O_7-y（REBCO）high-temperature-superconductors（HTS）have been widely concerned in the field of science and engineering,due to their unique physical properties and strong practical values.For the progress of basic researches and industrial applications,this thesis focused on the REBCO crystal growth in science and technology,involved the top-seeded melt-textured REBCO bulks,which could stimulate the development of maglev technology,and the top-seeded solution-grown single crystals,which are suitable for the basic physics research.For those REBCO materials prepared by top-seeded melt-growth（TSMG）method,the LRE_1+xBa_2-xCu₃O_7-y（LREBCO,LRE=rare earth element,Sm,Nd,La etc.）bulks with superior performances own broader application prospects.However,problems such as high melting points,substitutions of light rare earth elements for barium elements,intrinsic brittleness and discontinuous growth trends hinder the growth of high performance LREBCO bulks.Additionally,high growth failure rates increase their production costs.With regards to the single crystal growth,sizable and high quality doped YBa₂Cu₃O_7-y（YBCO）single crystals have great significances for the fundamental research in superconductivity.With its unique over-doped status,the Y_1-xCa_xBa₂Cu₃O_7-y system has been drawing much attention from researchers.Whereas,using the traditional pulling method,the intrinsic liquid-creeping problem occurs,which is detrimental to the crystal fabrication.In this thesis,on the basis of REBCO growth theory,investigations on the tuning chemical stoichiometry and growth mode of REBCO system were conducted.By overcoming a series of technical bottlenecks in this field,we achieved innovative results.1.We employed effective thermodynamic and kinetic approaches to grow Nd_1+xBa_2-xCu₃O_7-y（Nd BCO）bulks under an ambient atmosphere.Firstly,we found that during the peritectic reaction process,fast cooling rates reduced the absorption of oxygen in the formation of Nd_1+xBa_2-xCu₃O_7-y,resulting in the decreases of the whole cation values,and therefore,the Nd/Ba substitution effect was suppressed.By enhancing the cooling rate properly,we obtained Nd BCO multi-grains with high T_c（critical transition temperature）.Furthermore,considering the brittle nature and discontinuous growth trend of Nd BCO materials,we optimized the growth step by using a gradual acceleration cooling rate mode.Consequently,combined the cooling-rate-control melt-growth（CRCMG）and the Ba-rich composition-control melt-growth（CCMG）methods,a fully grown Nd BCO single grain（16mm in diameter）with a T_c of 95 K and trapped field of 0.6 T was obtained,which is superior to previously-reported air-processed ones.Secondly,we studied the impact of precursor constitution upon the T_c of Nd BCO crystals for the CCMG approach,and proved that the Ba-rich Nd123 precursor,which was easy to decompose into Ba-rich melt at high temperature,played a decisive role.These controlling means and study results provide pathways for synthesizing other functional oxide materials.2.To effectively improve the mechanical property of Nd BCO materials,the addition of silver into Nd BCO sample（Nd BCO/Ag）is a necessary way.However,the film thermal stability is severely weakened due to the Ag diffusion from the main pellet,which leads to fabrication failure.Here,we systematically studied the relationships among the liquid composition,surface tension and the thermal stability of film seeds.It was proved that the similar chemical composition between the liquid and film lead to a low energy interface and high wettability between these two matters.Hence,Ag was more likely to diffuse into the film-seed,which degraded the film thermal stability.On the basis of this perception,we developed an approach involving film-seed/Ba-rich buffer-layer/main-pellet constructions to strengthen the film thermal stability through wettability modification,and succeeded in raising the T_max（maximum processing temperature in melt-growth）of the film by 15℃.Apart from that,we also focused on the high melting point characteristic of Nd BCO,which makes it hard to find proper seeds.By introducing a novel Mg doped Nd BCO/YBCO/Mg O film seed,we successfully acquired the Nd BCO single grain under a high T_maxup to 1122℃(the highest T_max value ever applied in the growth of Nd BCO),which not only proves the films excellent superheating capability,but also promotes some high-thermal-stability-required seeding technological applications.3.Up to till now,failed bulks with high T_p（peritectic temperature）and high solid fraction are difficult to recycle.To solve this problem,we selected the hard-to-melt failed Sm BCO sample,and developed a novel recycling process,high temperature re-melting plus quenching following by re-growing.We first completely melt down the failed poly-crystalline Sm BCO sample at 1175℃（about 100℃ above its T_p）,and then quickly quenched the sample so that its structure could be transformed.Later,the preprocessed bulk was regrown by a high thermal stability film seed,and became a single-domain bulk with high performance.This method could decrease the cost production of LREBCO bulks and realize a sustainable use of rare earth resources.4.For the growth of Ca-YBCO single crystals,sophisticated pulling technique is a suitable selection.During the process,Y₂O₃ crucibles are traditionally utilized,which,however,possess high wettability with the solution.This results in an unavoidable liquid-creeping problem.Therefore we can only acquire small-sized,low doped and inhomogeneous Y_1-xCa_xBa₂Cu₃O_7-y crystals.To overcome this problem,we employed Ca stabilized Zr O₂（Ca-Zr O₂）crucibles,and succeeded in obtaining a series of homogeneous Y_1-xCa_xBa₂Cu₃O_7-dsingle crystals with acceptable size up to a dimension of 11.2×11×4.8mm³.Using SEM,we found that reaction products were formed at the inner side of these two crystal-grown crucibles.The solution/Ca-Zr O₂ interaction involved the formation of Ba Zr O₃ layers.Due to a high interface energy between Ca-Y-Ba-Cu-O solution and the Ba Zr O₃ layer,the wettability of the Ca-Zr O₂ crucible to the liquid was relatively low.In that situation,the solution almost constantly remained inside the Ca-Zr O₂ crucible.This work not only promotes the superconductivity fundamental researches,but also becomes an applicable approach broadly for producing other doped-crystals.Aiming at preparing for high performance REBCO materials,we used effective ways to control their cation-stoichiometry and crystal growth modes,which are of significance for facilitating fundamental research on superconductivity and promoting the practical applications of superconductors.