Study On The Epitaxial Growth And Its Mechanism Of REBCO High Temperature Superconductor Bulks

Since the high temperature superconductor (HTSC) was discovered, enormous efforts have been made to improve its properties because of its great potential in electric transmission and magnetic applications. 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 ability of trapping magnetic field. Although many achievements have been gained in the growth of REBCO bulk materials, there are still sereval problems to be solved in this field, such as the long processing time, the reduction of trapped field in the grain boundary area, the contamination induced by seed materials, the up limit of Tmax in the growth process restricted by the seed materials. In this paper, attentions are mainly focused on both the growth orientation control and growth mechanism of REBCO bulks. The main progresses are listed below:1. The formation mechanism of grain boundary in the MSMG YBCO bulk materials.YBCO and SmBCO thin films have been used for the first time as heterogeneous seeds to multiseed successfully the melt growth of bulk YBCO. The use of thin film seeds, which may be prepared with highly controlled orientation, is based on their superheating properties and reduces significantly contamination of the bulk sample induced by the seed material. A variety of grain boundaries were obtained by varying the angle between the seeds. Microstructural observations indicate that the extent of residual melt deposited at the grain boundary decreases with increasing grain boundary contact angle. It is established that the growth front proceeds continuously at the (110)/(110) grain boundary without trapping liquid, which leads to the formation of a clean grain boundary.2. The application of buffer layer in the growth of REBCO bulk materials.The buffer layer has been used, for the first time, in the cold-seeding melt-growth (MG) process for REBCO superconductor bulks. In this modified method, a mini pellet was inserted between the seed and the bulk precursor. Notably, the chemical contamination, from the seed material (either REBCO films or SmBCO/NdBCO crystals), was mostly absorbed by the mini pellet. Thus a uniformly high Tc REBCO bulk was readily gained. Secondly, the higher limits of the maximum processing temperature (Tmax) were evidently raised, which is promisingly beneficial for broadening the growth window and overcoming the self-nucleation in the growth of large-sized bulk. In short, the success of this work is that it protects the bulk from seed-induced contamination, and breaks the limit of Tmax caused by the intrinsic properties of the seed material.The research work presented here has provided some new ideas for the optimization of the orientation control and growth mechanism of REBCO bulk, and offer some supports in the growth of REBCO bulk materials with high superconducting properties.