Study On The Thermal Stability, Melting Mechanism And Orientation Control Of YBCO Films

Changes from solid to liquid states are common phenomena. The scientific study onmelting of solids can date back over at least a century. The thermal stability of materials,closely related to melting, is a major concern for both fundamental research and practicalapplications of materials. In recent years, it has been reported that superheating of solids canbe achieved by suppressing the heterogeneous nucleation of melt at surfaces. Thisunconventional property provides new insight into the study of melting. More recently,significant progress has been made to achieve superheating. A number of experimentalevidences have clearly proved that YBCO/MgO thin films with free surfaces possess a highsuperheating property, with a superheating of about60K, which is much higher than thatexpected for metals. Such materials are suitable object to investigate the superheating origin,due to the mature and facile preparation process as well as the high thermal stability. They arealso potential materials for wide applications in technology, such as in seeding REBCOmaterials. On the other hand, due to its large anisotropy, YBCO with different orientationsshow large dissimilarity in physical properties. As a result, it is important to study therelationship between film orientation and growth condition, for precisely controlling the filmmicrostructure.In this thesis, our research is focusing on two aspects: melting behaviors and thesuperheating origin of YBCO thin films from both the microscopic and macroscopic points ofview; growth condition related orientation transition mechanism in liquid phase epitaxy. Themain results achieved are listed below.1. Substrate effect on thermal stability of YBCO thin films Systematic experiments were performed by in-situ observation of the melting of YBCOfilms, deposited on MgO, LaAlO3(LAO), and SrTiO3(STO) substrates. The influence offilm/substrate interface structure on the thermal stability has been investigated. Remarkably,the superheating phenomenon was identified to exist in all YBCO thin films, providingexperimental evidence of a universal superheating mode. On the other hand, films depositedon different substrates show different thermal stabilities, with a descending order,YBCO/LAO> YBCO/STO> YBCO/MgO. Distinctively, YBCO/LAO films were found topossess the highest level of superheating, over100K, mainly attributed to the lattice-matcheffect of LAO substrate, i.e., its superior lattice fit with Y123delaying the Y123dissolving.Moreover, instead of a decisive factor, the substrate wettability by the liquid was found toplay an important part in melting growth. The poor substrate wettability gives rise to a liquidmigration from the melting front, which suppresses the melting growth. In addition, theelement doping from the substrate may have a potential effect on the thermal stability of thinfilms, but not a main factor. In brief, this work shed novel lights on the criterions ofsuperheating, for comparing thermal stability of thin films which possess peritecticdecomposition with a low-energy free surface. More importantly, the understanding isadvantageous to improve or explore new film/substrate constructions with higher thermalstability.2. Phase selection in high superheating stateSince Ostwald first mentioned an anomalous phase transition, i.e., the metastable phasewith a smaller free-energy barrier was able to nucleate prior to the stable phase, many effortshave been made on this unconventional phase transition. However, most of these studiesconcentrated on deeply undercooled melts. By applying the high superheating capability ofYBCO thin films, phase transformation behaviors was studied in such a superheating state byemploying HTOM. It was found that from the highly superheated YBCO/LAO film, anunexpected metastable phase (Y200) nucleated with the orientation of Y200[100]//LAO [110], prior to the stable phase (Y211). In other words, a non-equilibrium phase transition inthe high superheating region was observed for the first time (+L’). In contrast, onlyY211appeared from the superheated YBCO films on both STO and MgO. Based on theanalysis of phase diagram and nucleation competition concept, both high superheatingcapability of the film and low interface energy of the second phase with substrates were foundto be responsible for this unconventional phase transition. In brief, the research interests ofphase competition was extended to the high superheating state, which is favored for studyingthe phase relations and exploring possible metastable phases. Similar to the metastable phasetransition in the deep undercooling state, such extension carries great theoretical and practicalsignificance.3. Growth kinetics of second phase from peritectic melting of oxide filmsAs a reverse process of crystallization, melting kinetics of solids is of significantimportance. By high temperature optical microscopy, the peritectic melting evolutions ofYBCO thin films with various microstructures were in-situ investigated. A variety of formingbehaviors of Y211grains were observed. These distinctions were mainly attributed to twofactors, film microstructure as an internal factor and the driving force as an external condition.More distinctively, analogous to the classical crystal growth kinetics, we derived the peritecticmelting kinetics of thin films relating to the formation of the second phase. The correlationamong film microstructures, driving force and melting behaviors was well established, whichprovide a significant perspective to understand peritectic melting of film materials. Accordingto this research, the thermal stability of the thin films can be enhanced by improving themicrostructure at the film/substrate interface.4. Surface superheating and melting of YBCO at atomic scalesIt is well known that surfaces play important roles in inducing melting. However, to ourknowledge, most investigations on surface melting were limited to substances of simplestructures. With molecular-dynamics simulations, the surface melting of YBa2Cu3O7(YBCO) was investigated at atomic scales. We found that the melting kinetics and the thermal stabilitywere significantly orientation dependent. YBCO crystal with a (001) free surface displayed alayer-by-layer melting mode with a flat melting front, while YBCO with a (100) free surfaceexhibited a continuous one with a wavy melting front. Remarkably, the superheatingphenomenon of (001) YBCO can be attributed to the high stability of the Ba-O layer, whichacts as a barrier suppressing melting propagation. Furthermore, the large diffusivity of oxygenatoms within the CuO2-Y-CuO2layer should be responsible for the continuous melting modeof (100) YBCO, resulting in a complete melting. By extending the Cahn’s wetting theory tothe case of superheating, a modified equation was established to describe the correlationbetween the thickness of the liquid layer and temperatures. This study gives both simulationevidences and microscopic understanding for superheating origin of (001) YBCO. Based onthis understanding, higher thermal stability of REBCO films can be expected by controllingthe surface layers. The orientation dependent melting modes provide hints for other functionaloxides with multi-layer structures.5. Mechanism of orientation transition for YBCO films by liquid phase epitaxyDue to the enormous crystallographic anisotropy of YBCO materials, films with differentorientations are for dissimilar applications. Therefore, the precise control of preferentialorientation in the growth process is required. For comprehensive understanding, theout-of-plane orientation transition for YBCO films grown on (110) NdGaO3(NGO) substrate,using liquid phase epitaxy (LPE), was systematically investigated via changing fluxcomposition, processing temperature and oxygen partial pressure. It is found that LPE filmscould grow, remarkably, in a wide temperature range between24K above and25K below theperitectic temperature. The unpredicted c-oriented films formed at the temperatures above Tp,is deduced to be attributed to the etch-back behavior. With decreasing temperature, the YBCOfilms characteristically experienced the double transition of c-axis orientedâ†'a/c-axis mixedâ†'c-axis oriented in air, and a single evolution of the c-axisâ†'a-axis in the pure oxygen atmosphere. By combining supersaturation with the NGO etch-back, and solute diffusion, acomprehensive mechanism of the film orientation transition was clarified, which provide newideas and theoretic basis for precise control of film microstructure.The research work presented here has made significant progress in the superheatingorigin, the surface melting and orientation control of YBCO materials. The understanding hasa guiding significance to both science and techniques. Further efforts can be made on thebasis of the work in this thesis, such as to explore novel film/substrate structures with higherthermal stability, to realize commercialized production of more materials by employinguniversal seeds with excellent superheating capacity, to realize the precise control of filmmicrostructure by LPE method and so on.