The Study On The Precise Tunning Of The Y-ba-cu-o Liquid Supersaturation For Growing YBa₂Cu₃O_7-x Epitaxial Films,Single Crystals And Bulks

The superconductor materials have demonstrated attractive superconducting properties since its discovery,which are fascinating for both fundamental studies and industrial applications.While both the studies and applications rely on the the preparation of high quality crystals.Thus,it is very important to grow high performance and orientation controlled superconductor crystals with large size and superior quality.In this paper,by studying the growth mechanism and adjustinging the growth conditions,we realize the accurate control of the oriention and the improvement of the performance for the REBa₂Cu₃O_7-x?RE123 or REBCO,RE=Y,Sm,Nd?high temteralture crystals,combining with the phase diagram and the concept of supersaturation in the field of growth kinetics.In the liquid phase expitaxy?LPE?process,the fine tuning of the solution supersaturation is realized and the artificial microstructures of oriented YBCO films are achieved,such as the composite epitaxial structure of a-axis oriented grains?a-grains?trapped by c-film,the a/c axis oriented grain boundary with well-defined facet and so on,which are very meaningful for the device developments and current transport applications.With respect to the single crystal growth,the polythermal method is applied to increase the supersaturation of the solution.As a result,the growth rate in air is raised and large size and high quality single crystals are obtained,which could be supplied for the fundermental physics studies.For the bulk of high temperature superconductor growth,by studying the influences of the supersaturation on the film thermal stability,the buried-seeding mode is developed to solve the problem of poor thermal stability in low supersaturation,which are also effective for other functional oxides with higher peritectic temperature.The main progresses are listed below:1.The fine tuning of the supersaturation and the artificial microstructures of oriented YBCO films in liquid phase expitaxy?LPE?After years of concentrated efforts,our laboratory not only well clarified the crystallographic orientation mechanism of Y123 LPE films grown on?110?NdGaO₃?NGO?substrate,but also obtained the corresponding supersaturations for the repeatable and reliable growth of a-and c-films.In our work,we realized the fine tuning of the supersaturation.In other words,ont only the above two extreme supersaturations,but also the intermediate supersaturation could be tuned.Furthermore,the supersaturions were applied to prepare some artificial microstructures of oriented films.Here,the composite epitaxial structure of a-grains trapped by c-film was achieved by employing the intermediate supersaturation and the a/c axis oriented grain boundary with well-defined facet was acquired by using the low supersaturation.In particular,by the control of the initial holding time,combining with the fresh solvent material and the melting time,the solution is more efficient to precisely tune to the intermediate supersaturation.Therefore,we can ont only acquire the low and high supersaturations for a-and c-film respectively,but also tune the intermediate supersaturation for growing a composite epitaxial structure of a-grains trapped by c-film,where the a-grains may serve as flux pinning to improve the superconducting performance.Beside,a novel a/c axis oriented grain boundary with well-defined facet of the YBCO film featuring a single-crystalline nature was successfully grown,differing from the conventional ones with ill-defined facet possessing a polycrystalline characteristic prepared by the prior art deposition techniques.Here,the extremely low supersaturation as an essential prerequisite was achieved and exploited on a?110?NGO substrate with the partly-etched c-axis oriented YBCO film.Basing on the selective growth,the preferred a-axis oriented YBCO film hetero-epitaxially grew on the bare NGO substrate directly,while the c-axis oriented YBCO film homo-epitaxially proceeded on the pre-existing YBCO layer,consequently fabricating a distinct a/c grain boundary,which has great importance for fundamental studies and device applications.2.The polythermal method for the growth of large size and high quality single crystalsRecently,excellent collaborative works were reported derived from deep investigations on superconductivity by exploiting large single crystals of YBCO with good quality,prepared by top-seeded solution-growth?TSSG?.TSSG is a mature and classic way to prepare REBCO single crystals.To overcome the demerit of the low production efficiency in the prior art isothermal method?constant supercooling?,we applied the polythermal method?increasing supercooling?,in which the growing crystal was gradually raised,and simultaneously,the temperature was slowly lowered.We found that increased supercooling played a remarkable role in promoting the crystal growth.A large-sized YBCO single crystal with an a-b plane of 13.8×13.5mm² and a height of 12.3mm in the c-axis direction was obtained via a cooling rate of 0.5K/h in air,gaining a growth rate nearly 2 times faster than that by constant supercooling.Additionally,the suitable supersaturation was also considered to maintain the high speed of crystal proceeding in the liquid metastable region.Finally,the optimal growth mode was proposed for various REBCO materials basing on their characteristics of solubility curves.3.The study on the film thermal stability correlating with the seeding mode and the solution propertyThe top-seeded melt-growth?TSMG?is a common method of preparing large size YBCO bulk crystals.Recently-exploited modified melt-growth?MMG?realizes the high efficiency of growth,high purity of Y123.However,it is unexpectedly found that the thermal stability of theNd123 film-seed applied in MMG has a great reduction due to the change of the liquid property.In the quest of solving this problem,a new top-seeding mode,buried-seeding is developed to strengthen the film thermal stability,leading to remarkable enhancement of endurable maximum processing temperature of over 30°C in low supersaturation.Furthermore,the correlation of the thermal stability of the film-seed in melt-growth with liquid property and top-seeding modes was well clarified.Most importantly,the buried-seeding mode was succeefully applied in this work,opening a promising pathway to grow single crystals effectively for other REBCO oxides possessing higher peritectic temperature than YBCO,as well as for other functional oxides.Our study demonstrates the importance of the supersaturation for the crystal growth and application,which may provide specific guidance for the preparations of artificial microstructures of oriented LPE films,large size single crystals and high performance bulks,as well as the supersaturation fine tuning in other sollutions systems.We hope this work could provide some innovative ideas and pathways for the fundermental studies and device applications.