Research On Thermal Stability And Its Application Of Rebco Superconductors And Liquid Phase Epitaxial Growth Of Fete_1-xSe_x Film

The thermal stability of thin films has been studied for decades as a basisfor industrial applications and for fundamental scientific research. Since thesuperheating phenomenon of YBa2Cu3O7-x(Y-123) was observed in the liquidphase epitaxial growth of Nd1+xBa2-xCu3O7-x(Nd-123) thick film, many workshave been reported on high thermal stabilities of RE1+xBa2-xCu3O7-x(RE-123)thin films. The study on this novel property of REBCO films is important forapplications and for obtaining a fundamental understanding of melting. Assuitable seeds, RE-123thin films are widely employed in the liquid phaseepitaxial (LPE) growth of RE-123thick films, growth of single crystals andmelt-textured (MT) bulks.On the other hand, iron based superconductors have gained a large amountof interest recently. Understanding these new superconductors requireshigh-quality epitaxial films to explore intrinsic electromagnetic properties andevaluate device applications. So far, extensive researches have been carried outto prepare iron-based crystals and thin films. However, liquid phase epitaxialfilm of these materials is rarely reported. By the liquid phase epitaxial growth(LPE), films with high crystalline quality, uniform composition and controllableorientation can be achieved. Due to the fast deposition rate as well as low cost ofLPE growth, it is a promising technique to prepare iron-based epitaxial films. In this thesis, the difference of thermal stability between REBCO thin filmswas investigated. Under an assumption of quasi-equilibrium, a simplified modelfor the peritectic melting of RE-123was suggested, which clarifies themechanism of different thermal stabilities between REBCO thin films. Inaddition, we have prepared FeTe_1-xSe_xthick film by LPE growth for the firsttime. The main progresses are listed below:1. The thermal stability of REBCO thin films and its superheating in MTgrowthSo far, many studies on the thermal stability of RE-123films have beenperformed in connection with the quality and orientation of the films.Differences in thermal stabilities between REBCO thin films under variousatmospheres may directly correlated to the nature of the film. We investigatedthe thermal stabilities of NdBCO thin films under varied atmospheres and thedifference of thermal stabilities between NdBCO and YBCO thin films. Wefound that the diversity in the melting growth process of thin films dominatedthe thermal stability of these films. The melting rate of RE-123is proportional tothe growth rate of the RE-211phase and the ratio of the concentration differencebetween RE-211growth front and RE-123melting border. Owing to thesimilarity among RE-123materials, we suppose this model might be universallyapplicable for the peritectic melting of thin films beyond superconductors.During the melt textured growth process, the adsorption of Ba-Cu-O liquid bythe pellet was believed a reasonable factor for the high thermal stability forREBCO thin films. The study on the influence of thin film microstructure on itsthermal stability showed that the employment of YBCO buffer layer improvesthe in-plane alignment of NdBCO thin film and eventually enhances its thermalstability in MT process. In addition, the mechanism of epitaxial coarsening of YBCO grains with different in-plane alignments on MgO substrate was studied.It was found that the coarsening process dominated by45°grain in the MTprocess is the main reason for the difference of the preferrd in-plane orientationbetween the LPE and MT growth.2. Application of REBCO thin film in MT growth and large sized bulkgrowthREBCO superconductors are promising materials for wide applications. Thesize of the bulk material is the main restraining factor for these applications. Toovercome the problem of the over high supersaturation, a Tmax2procedure wasintroduced in the MT growth. It was found that a reasonable temperature and anappropriate holding time of Tmax2decrease the supersaturation and thereforeavoid the homogeneous nucleation. Additonally, an Ag doped NdBCO bulkmaterial with a diameter of38mm, which is a world-record size for NdBCObulk superconductor, was achieved by Oxygen Control Melt growth (OCMG)technique using an Tmax2temperature profile. The low growth rate and narrowgrowth window are main limits for large sized REBCO bulk growth. By anemployment of pure oxygen pressure as the growth atmosphere, we reported asuccess in enhanced growth rate of the melt-textured (MT) YBCO bulk. Theenhanced growth rate can be interpreted by both increased solubility anddiffusion coefficient of yttrium in the Ba–Cu–O liquid under high oxygen partialpressure. The most important advantage of this technique is that the enhancedgrowth rate of Y123gives less opportunity for self-nucleation in the liquid,which takes place beyond the growth window and the embryonic period.3. FeTe_1-xSe_xcrystal and epitaxial film growthWe successfully prepared FeTe_1-xSe_xcrystal by melt growth method. Thephase formation of the crystal is determined by X-Ray differaction (XRD) measurement. The temperature and field dependence of the magnetization showthat the Tc onset value of the FeTe_1-xSe_xcrystal is about12.5K. Using LPEgrowth technique, we fabricated FeTe_1-xSe_xepitaxial thick film on (00l)-orientedLAO substrate. Optical micrographs and SEM images show that FeTe_1-xSe_xgrains on the substrate are in a quadrate shape and align along the same direction.The out-plane orientation of the film confirmed by XRD measurement is (00l)FeTe_1-xSe_x//(00l)LAO. The actual composition of the film was determined byenergy dispersive x-ray detector (EDX), the result shows that the film is uniformin the composition, and the actual composition is very close to the nominal one.The work presented here enables us a comprehensive understanding of thethermal stability and the melting process of REBCO thin films, further to realizethe thermal stability control to fulfill the requirement of the bulk growth. Inaddition, with the pioneering work of LPE growth of FeTe_1-xSe_xfilm, a preciselycontrol of film orientation and microstructure can be expected, which will meetthe needs of practical application and fundamental research.