The Investigation On The Orientation Control And Thermal Stability In Liquid-phase Epitaxy Of High-Tc Superconductor

The high-Tc superconducting (HTSC) films with high crystalline quality aswell as uniform chemical and physical properties can be achieved via the liquid-phase epitaxy (LPE) technique, which has a promising potential in the applica-tion fields of HTSC tapes and devices. So far, our research works have been fo-cusing on the strict control of orientation in the LPE growth of HTSC films, andthe fabrication of samples with sole orientation in order to fulfill the demandsin HTSC devices and superconducting mechanism research. Meanwhile, thiswork is expected to give favorable guidance to the research of superconduct-ing tapes. In this thesis, many efforts were made to discern the mechanism oforientation transition in the LPE growth of REBa2Cu3Oz (REBCO) materials,further realize the pure orientation growth in REBCO systems, finally achievea general understanding of the orientation transition mechanism in the epitaxyprocessing of REBCO films.After several years of efforts, we have made some progress in the a/c-axisorientation transition, the rigorous control of out-of-plane and in-plane crys-talline alignment in the LPE growth of REBCO materials. 1. The mechanism of a/c-axis orientation transition in the LPE growth ofYBCO films.In the processing of vapor deposition, the orientation of YBa2Cu3Oz(YBCO) thin films is always affected by many parameters, such as substratematerials, deposition temperature, oxygen partial pressure and so on. It is dif-ficult to obtain YBCO thin films with sole orientations, particularly the a-axisoriented films with high quality which are considered to be the necessary com-ponent of the Josephson devices. Although the a-axis oriented YBCO thickfilms have been obtained on the (110) NdGaO3 substrates via LPE method, un-der specified temperature and ?ux composition, the clear mechanism of the a/corientation transition is still an open issue. In order to investigate the growthkinetics related with the out of plane orientation choosing, we performed therigorous orientation control in the LPE growth of YBCO by virtue of adjustingmany growth parameters, and clarified the controversy on the a/c orientationtransition. Furthermore, the experimental facts about the prior nucleation of a-axis oriented YBCO grains even in the c-axis dominant films is found in termsof the study on the initial stage growth. Combining with the theory of"nu-cleation and growth", the competition between a-axis and c-axis growth wascomprehensively analyzed, on which only the growth region characterized byCu-rich ?ux and low supersaturation appropriate for a-axis growth was wellinterpreted.2. Dominant Effect of Oxygen Enhancement on the Crystalline Orientationof YBCO Film prepared by LPE method.The reported growth"window"for a-axis oriented YBCO films is usually narrow, which characterized by high growth temperature vicinal to the peritec-tic temperature (Tp) and relatively Cu rich ?uxes. So far, nobody was able toobtain high-quality a-axis YBCO LPE films by using 3.0:5.0 Ba:Cu ?ux. In-terestingly, the significant effect of oxygen content on the a/c orientation com-petition has been determined: it is considered that the species of incoming ?uxneed much shorter migration to occupy the suitable crystal growth site in thea-axis growth mode. Thus, if we aim to the pure a-axis growth, the surfacemigration should be significantly suppressed by the factor of oxygen content.Consequently, the growth window of a-axis oriented YBCO was extensivelybroadened by introducing the pure oxygen atmosphere. Firstly, the pure a-axisoriented YBCO films with high crystalline quality were obtained in the 3.0:5.0Ba:Cu ?uxes (reported to be inappropriate for the growth of a-axis orientedYBCO). Secondly, the pure oxygen atmosphere provides new thermodynamiccondition for the fabrication of high quality a-axis oriented films, also changesthe previous narrow growth window into a wider one with spacing more than20K.3. The in-plane alignment control by the atmosphere environment in theLPE growth of YBCO films.In addition to the control of out-of-plane crystalline orientation, perfectin-plane texture of YBCO films is also crucial for obtaining superior super-conducting performance. Thus, the study on the thermal stability of the grainswith different in-plane orientations is of great importance. In this work, theLPE growth of the polycrystalline YBCO seed film with eight-fold symmetry(indicates the coexistence of 0o and 45o grains) deposited on (100) MgO sub- strate under a pure oxygen condition was investigated. Surprisingly, it is foundthat the YBCO-LPE films formed under the pure oxygen atmosphere exhibit apure 45o in-plane alignment, which is opposite to the 0o in-plane alignment ofYBCO films grown under the air condition. It implies a new preferential growthphenomenon of in-plane alignment. In such works, firstly we obtained a kindof YBCO-LPE films with distinctively sole 45o in-plane orientation, secondly ageneral comparison comprising interfacial terminal layer, oxygen content, andthe minimum energy cut from different REBCO systems was made for compre-hensive understanding of the in-plane orientation choosing mechanism. On theother hand, the atomic configurations vicinal to the YBCO/MgO interface weresurveyed by high-resolution transmission electron microscopy (HRTEM). Thecorrespondence between different in-plane alignments and terminal atomic lay-ers was discussed, and the interfacial effect on their critical current propertiesis also included.The work presented here enables us to understand the various orientationcompetitions in the LPE growth of REBCO materials. In the future, basedon the understanding of the growth mechanisms, more strict controls on themicrostructures in order to optimize the physical properties are expected. Moreresearch works will be developed to meet a variety of devices demands.