Research On The Microstructure Control Of REBCO Liquid-phase Epitaxy Film And The Thermal Stability Of YBCO Thin Film

Due to its large anisotropy, the superconducting properties of High Tc-superconductor strongly depend on its microstructure, especially the grain boundary structure. The successful preparation of samples in different forms is the key factor to the potential applications and the mechanism research of superconductivity. To obtain high quality materials, it is necessary to precisely control the microstructure. Meanwhile, those researches promote the development of materials science as well. Among all the techniques, advantages such as low cost, high growth rate and high quality can be achieved by liquid-phase epitaxy. Our study has been focusing on the preparation of REBa2Cu3O7-δ(REBCO, RE=rare earth) film with uniform composition and controllable microstructure. In this paper, the growth orientation control of SmBa2Cu3O7-δ(SmBCO), especially the influence of solid solution property, is discussed. The variation of a-c grain boundaries with growth conditions and the growth mechanism is systematically studied. Fine distribution of a-c grain boundary within a region of 1mm scale is achieved. These results may provide generic guidance for the deposition of coated conductors. A general mechanism of crystallographic axis orientation transition of REBCO LPE film is suggested. Comparison of the thermal stability of YBa2Cu3O7-δ(YBCO,Y123) thin films with different in-plane orientations is present. A success in melt-textured YBCO growth by superheating YBCO thin films is reported, which opens a new gate for the seeding of REBCO bulk. The main progresses are listed below:1. The a-c orientation control of SmBCO LPE filmSmBCO has higher superconducting transition temperature (Tc) and better critical current density (Jc) in high magnetic fields than YBCO. Till now, either by vapor phase deposition or LPE, the growth of pure a-axis SmBCO films has not yet been reported. On the basis of our former experience on YBCO, pure a-axis orientated SmBCO films are grown by combination of flux composition control and 1 atm oxygen atmosphere. The mutual substitution of Sm3+ and Ba2+ could suppress the kinetic barrier, which leads to an a-axis growth, by reducing the required minimum diffusion length for c-axis growth. EDX results in the Y+Sm mixed system support this deduction. Take the growth mode into consider, the supersaturation in melt can easily exceed the a-axis growth limits of SmBCO in the LPE process. The high supersaturation also results in high growth rate of nuclei, making it difficult to isolate pure a-phase growth from the a-c mixed one.2. The control of a-c grain boundary and suggestion of growth orientation transition mechanismWe present systematical study of different kinds of a/c mixed growth results in SmBCO and Y-Sm mixed system by liquid phase epitaxy. The a-c grain boundary structure changes with the supersaturation. Based on the growth competition between a-axis grain and c-axis grain under different supersaturation, we clarify the formation mechanism of different a-c grain boundary. Under certain conditions, a-axis grains disperse in the c-axis matrix with uniform in-plane orientation and fine grain boundaries. This kind of microstructure may not suppress the supercurrent and act as pinning centers. On the basis of our experiments and large amount of published paper, we establish a general mechanism of crystallographic axis orientation transition of REBCO LPE film. When the supersaturation is very low, thermodynamic factors dominate the orientation. With a better thermal stability and lower interface energy, c-axis grains can grow under this condition but a-axis grains can't. Therefore, c-axis oriented film presents. When the supersaturation is large enough to support a-axis grain growth, the kinetic factors govern the orientation. On one hand, at relatively low supersaturation, the atoms may easily escape the growth surface and go back to the flux. Therefore, the diffusion distance of atoms along the growth surface is short, leading to the a-axis growth; on the other hand, high supersaturation supports long diffusion length, resulting in the c-axis growth again. This theory can explain all the conflicting experimental results. A better understanding of growth orientation transition mechanism can be expected.3. In-plane orientation effect on the melting behaviour of YBCO thin film and the application of superheating YBCO thin filmThe thermal stability of low dimensional materials is strongly microstructure-dependent. A fine in-plane alignment, which indicates a semi-coherent interface at the film/substrate interface, could be an obstacle against melting/decomposion. However, poor in-plane alignment, which indicates the existence of defects such as dislocations and grain boundaries, could be an excess energy source that encourages melting/decomposition at the interface. Taking advantage of the superheating property of YBCO thin film, YBCO/MgO thin film is used to grow YBCO MT bulk superconductor successfully. The YBCO seed-film survived after a 1.5h long period of time in the high processing temperature due to its intrinsic superheating property. The idiographic growth condition, which strengthened the stability of seed film, was discussed. Our result provide a new choice of seed for the growth of REBCO MT bulk with high peritectic temperature, such as SmBCO or NdBCO.The work presented here enables us to have a better understanding of orientation controlling factors in the LPE process of REBCO film. With this guidance, more precisely control of film orientation or microstructure in both LPE and VPD methods can be expected. The study of thin film thermal stability provides not only feedback for the thin film preparation but new chioce for MT bulk seeding as well.