The Investigation On The Element Substitution And Oriented Growth In The Preparation Of REBa₂Cu₃O_7-? Single Crystals,Bulks And Epitaxial Films

REBa₂Cu₃O_7-??RE123 or REBCO,RE=Y,Sm,Nd,La,et al?superconductor is regarded as a crucial object in the field of superconductivity.Doped single crystals with high-unniformity,films with composite epitaxial microstructures and bulks with high-performance are important to theoretical study and potential applications.However,for element substitution and oriented growth,a variety of problems need to be investigate.For example,the doeped single crystals grown by conventional method are inhomogeneous,LaBCO bulks in which the substitution of RE/Ba occurs readily have not been fabricated,c-axis films wih two kinds of a-axis grains can not been prepared repeatedly or reliably and the applied research for the growth rate anisotropy dominated by growth kinetics is lacking.In this paper,combining the growth theory of REBCO,we conducted the investigation on the element substitution and oriented growth.The specific research content and results are as follows.1.Large-sized doped YBCO single crystals with high quality are necessary for precisely characterizing physical and chemical properties,and have great significance for the study of superconductivity mechanisms.However,the doped single crystals reported so far lacked exellent uniformity.In this thesis,the theoretical reason for the poor uniformity of the chemical doped single crystal grown by the conventional method is analyzed.Uniquely,we proposed a novel approach for the growth of the doped crystal,in which a Fe-added Y₂O₃ crucible was used instead of the conventional Y₂O₃ one.Such a Fe-added Y₂O₃ crucible is able to receive or dissolve Fe ion in equilibrium according to the temperature.In consequence,YBa₂(Cu_1-xFe_x)₃O_7-??Fe-Y123?crystals with good uniformity are achieved.These dopant-added crucibles are broadly applicable for preparing other doped single crystals.2.In the crystallization of most functional oxide materials,the peritectic reaction commonly happens.During the reaction,the low-temperature phase envelopes the high-temperature phase and prevents it from reacting with the liquid,resulting in an incomplete peritectic reaction,which seriously hinders the development of material industry.But there have been few studies on this issue.In our work,an almost complete peritectic reaction is observed in the growth of Fe-Y123 crystals using top-seeded melt-growth.A systematic study of chemical substitution including Co,Ni,Mn,Cr,Zn are performed.The nature of this unconventional phenomenon is clarified that the Fe-doping?Co,Ni,Mn?elevates the melting barrier of Y123 and causes a massive homogeneous nucleation catastrophe.This work provides a new idea for solving the problem of incomplete peritectic reaction and promoting the industrial production of materials.3.In the REBCO system,the theoretical critical temperature of LaBCO is the highest,and the price of La is the lowest,making LaBCO be competitive in the practical application.At present,there are two problems in the preparation of LaBCO bulks.One is that the growth is extremely sensitive to the conditions,and the second is that the substitution of La/Ba element is difficult to suppress.In this thesis,combined with the solubility curve and growth kinetics,the negative effects of excessive supersaturation on the growth of LaBCO bulks are clarified.The supersaturation is reduced by increasing the ratio of Ba to Cu and decreasing the content of La422.As a result,a completely-grown LaBCO bulk was prepared successfully.Meanwhile,we also studied the influence of coolingrate on the substitution of La/Ba and found that a large coolingrate is beneficial to suppress the substitution of La³⁺to Ba²⁺site.This work has accumulated experience for the preparation of large-sized LaBCO with high performance in the future.4.The epitaxial structures of YBCO film are various.It is possible to discover new physical phenomena and develop new application fields from these different epitaxial structures.In this regard,the factitious control of abundant structures is the first to be solved.In this work,we evaluated whether?m and?t_m of the supplementary-solvent-material can be adjusted for a given solution state at controlled fractions.Indeed effectively,we demonstrated that changing?t_m can subtlety manipulate the solution in sub-dividing the intermediate supersaturation region and realized the continuous microstructure transformation of YBCO film.Accroding to the result of this work,YBCO films with various structures can be prepared accurately,tautologically and reliably,which strengthens the foundation for the development of YBCO films.This work can also provide reference for the factitious control of epitaxial structures of other materials.5.In the preparation of YBCO bulks,many growth techniques have been developed to improve performance and increase size,but some techniques are not economical and convenient in actual operation.Utilizing the anisotropy of oriented growth,we creatively developed a novel seeding construction,consisting of a 45°film-seed and a would-be[110]-sided buffer-layer.In consequence,the fast growth was realized on both buffer-layer and main-pellet,the sizable YBCO bulk with large c-growth section and 10%higher levitation force was achieved.This work not only proposes a new way to effectively improve the performance of bulk,but also provides inspiration for the practical application of oriented growth.The thesis provides a new idea for the growth of doped single crystals,lays a foundation for the study of almost complete peritectic reaction,creates conditions for the in-depth study of LaBCO,improves the fine-tuning of supersaturation in the preparation of YBCO epitaxial films,and proposes a novel seeding construction to increase the performance of REBCO bulks.We hope that these new ideas,new thoughts and new approaches are beneficial to the theoretical investigation and practical applications of other functional materials.