Research On The Epitaxial Growth And Microstructure Control Of Rebco High Temperature Superconductors

Since the high temperature superconductor (HTSC) was discovered in 1987, enormous efforts have been made to improve its properties because of its great potential in electric transmission and magnetic applications. Due to its large anisotropy and quantum flux penetration mechanism, the superconducting properties strongly depend on the crystal structure, especially the microstructure of HTSC sample. Hence, in order to obtain the HTSC materials with high performance, it is of importance to further understand the mechanism of growth orientation and microstructure control. By optimizing the microstructure, the size and distribution of flux pinning sites could be improved, the number of weak link will decrease and the critical current density be enhanced. Among all types of HTSC materials, the RE1Ba2Cu3O7 (REBCO,RE123, RE=Y,Sm, Nd, Gd etc.) bulk materials have attracted much attention because of its high critical current density under high magnetic field and the ability of trapping magnetic field. While scientific attempts are now concentrating to develop second generation superconducting wires or tapes, the bulk materials are meanwhile on the way to practical applications. On the other hand, there are still several problems to be solved in the growth of REBCO bulks, such as the lack of proper seed, control of flux pinning sites as well as the formation of solid solution inside the material. In this paper, attentions are mainly focused on both the growth orientation control and microstructure optimization of REBCO bulks. The main progresses are listed below:1. Study of thermal stability of NdBCO superheating film and its application as seed in bulk growthIn pervious study, a superheating phenomenon was realized on YBCO thin films. Since the NdBCO film has similar structure as YBCO, it is reasonable to suggest that NdBCO film could also exhibit superheating property. By using NdBCO film as seed, we have successfully grown the homogeneous NdBCO bulk via melt-texture method. During the experiment, NdBCO film seed maintained the crystal structure at a temperature higher than its Tp for 1.5h due to superheating property. It is also realized that the in-plane orientation also has influence on the superheating of film. NdBCO film with higher crystallinity possesses a better thermal stability in the MT process. The development of NdBCO film seed makes it possible to grow large SmBCO and NdBCO single grain bulks. What is more, since the Tp of NdBCO is higher than that of any other REBCO materials, it is further suggested that NdBCO film seed could be used in the growth of all kinds of REBCO bulks.2. In-situ observation of phase epitaxial relationships in Sm-Ba-Cu-O system.Phase epitaxial growth of Sm-Ba-Cu-O phases on the (010) surface of Sm2BaCu05 (Sm211) whiskers was in situ investigated under various thermal conditions by means of the high temperature optical microscope. At 1100℃, needle-shaped phase epitaxially grew on the Sm211 substrate. The EDS and XRD analyses identified this phase as Sm2Ba4Cu2O9 (Sm242). It is suggested that the metastable existence of Sm242 phase on Sm211 substrate is due to the formation of epitaxial interface and small size of Sm242. During a slow cooling down to 1050℃, two kinds of c-oriented Sm123 grains epitaxially grew on the Sm211 substrate. They were denoted as 0°grains and 45°grains. During a further reheating up to 1085℃, it was found that the 0°grains preferentially dissolved already at 1055℃, indicating that the 45°grains had a higher thermal stability than the 0°ones, mainly due to a better lattice matching. Finally, a possible epitaxial correlation between Sm123 and Sm242 phases was proposed and supported by lattice matching calculations. These studies help us to further understand the formation of epitaxial relationship under various thermal equilibrium conditions, which could be useful in the improvement of flux pinning sites inside the REBCO bulks.3. Influence of Ba-rich Sm242 phase addition in Sm123 bulk growth.By adding a novel Ba-rich Sm242 compound in the precursor material, high performance Sm123 bulk superconductors were successfully grown in air by melt-textured (MT) growth. Highly-c-oriented SmBCO and NdBCO thin films with a superheating property were effectively utilized as seeds to induce an epitaxy nucleation and growth. A primary study on the effect of the Sm242 addition on superconducting properties was carried out, proving that the critical transition temperature (Tc) can be substantially improved. It was found that, with the addition of 10 mol% Sm242, a high Tc of over 94 K with a sharp transition (△Tc less than 1 K) can be achieved in the air-processed Sm123 bulk, which is the higher than any other reported air-processed Sm123 bulks. Moreover, a relatively strong peak effect in the Jc-H curve is present at the intermediate field range, which could be explained by the formation of a compositional fluctuation within the Sm123 matrix. An improvement in the particle size and distribution of Sm211 in the Sm123 matrix was also observed.The research work presented here has provided some new ideas for the optimization of both the orientation and microstructure control of REBCO bulk. In the future, it is hoped that the growth of large-sized REBCO bulk with high performance could be achieved. More research work should be developed to meet the demand of various applications.