The large single-grains and directionally melt-textured (RE/Y)-Ba-Cu-O high $T_c$ composite superconductors with / without $CeO_2$ additions were investigated by top-seeded melt-growth(TSMG) and zone melt growth(ZMG) process in air atmosphere. The TSMG process is widely used to grow well-oriented large single grain $Yba_2Cu_3O_{7-y}(YBCO)$ pellets which is necessary for bulk applications. In order to achieve high critical current densities in bulk YBCO it is necessary to have a textured microstructure with the high $J_c$ directions aligned in the direction of current travel. Directional solidification techniques are now being widely used to produce bulk YBCO samples with transport critical current densities exceeding $10^4 A/cm^2$ in applied fields of several T. It has been reported that REBCO superconductors(RE : Nd, Sm and Gd) show relatively low transition $T_c$ in air. An important drawback of these materials is, however, that the actual composition is $RE_{1+x}Ba_{2-x}Cu_3O_{7-y}$. The relatively large radii rare earths, particularly Nd, easily substitute into the Ba sites, leading to depressed $T_c′s$ when the solidification is performed in air.
Therefore, it is very difficult to grow melt-textured REBCO superconductors with these RE elements having both high $T_c(＜90K)$ and sharp superconducting transition. The objective of this study is to investigate the method of enhancing superconducting properties in air through the complete substitution of the RE/Y composite ions. For achieving better flux pinning properties in this system, the inclusion size of (Sm/Y)211 should be as small as possible, as well as controlling the (Sm/Y)211 volume and the distribution with the (Sm/Y)123 superconducting matrix. The refinement of the (Sm/Y)211 inclusions might explain the improvement of the superconducting properties.
Thus, there is a need to refine the (Sm/Y)211 particles as much as possible, and to minimize the coarsening of these particles in the melt. For TSMG processing, the (100) plane of the Nd1.8 seed crystal was placed on the top of the (Sm/Y)1.8 pellet. A (001) MgO single crystal was used as the substrate. The (Sm/Y)1.8 crystals were grown at 1025 ℃ after seeding a pellet with a Nd1.8 seed crystal and grew epitaxially from the Nd1.8 seed crystal with rectangular shape in the plane of pellet. It is obvious that the (Sm/Y)1.8 composite single grain can be successfully fabricated with the Nd1.8 seed crystal. Also zone melt growth(ZMG) process having a large temperature gradient(G: 300 ℃/h) was performed with / without $CeO_2$ addition in air atmosphere. In this study, fixed hot zone temperature and growth rate are 1085 ℃ and 3 mm/h, respectively. In both samples, TSMG and ZMG, the uniform (Sm/Y)211 inclusions are trapped within the (Sm/Y)123 matrix. The (Sm/Y)211 inclusions in the additional-type sample have irregular shapes, while in the sample containing a $CeO_2$ addition, the (Sm/Y)211 inclusions have spherical shape. Also, in $CeO_2$ addition sample, the (Sm/Y)211 inclusions size was remarkably reduced and the inclusions finely dispersed within the (Sm/Y)123 superconducting matrix. This refinement of (Sm/Y)211 inclusions is observed for all samples. TEM was performed on a sample annealed at 430 ℃ for 100 h.
After oxygen annealing, many defects are observed in the orthorhombic (Sm/Y)123 matrix, especially around the trapped (Sm/Y)211 inclusions. The (Sm/Y)211 inclusions, spherical in shape, are observed in the (Sm/Y)123 matrix.
The existence of stacking faults and a high density of dislocation in addition to the twin structure are observed. Also, twin and twin boundaries having about 50 nm space can be observed. The high resolution TEM image and selected area electron Diffraction patterns(SAED) of the (Sm/Y)123 / 211 interface viewed from a [001] direction were performed. There are no defects and distorted structures in the vicinity of the interface. A DC magnetization measurement was carried out using a quantum design SQUID magnetometer. In both (Sm/Y)1.8 samples, TSMG and ZMG with / without $CeO_2$ addition, an onset $T_c ＞ 92 K$ and sharp superconducting transition were observed. In both (Sm/Y)1.8 samples, TSMG and ZMG with / without $CeO_2$ addition, the critical current density $J_c$ values are observed $4.7×10^4 A/㎠$ and $1.2×10^5 A/㎠$ in O Tesla at 77 K, respectively.
