Transmission electron microscopy study of GdBa_2Cu _3O_(7-x) containing nano-sized BaMO_3 (M: Hf, Zr, Sn) rods fabricated by pulsed laser deposition

摘要

Three types of superconducting GdBa_2Cu_3O _(7-x) (GdBCO) layers containing rods of either BaHfO_3 (BHO), BaZrO_3 (BZO), or BaSnO3 (BSO) were fabricated by pulsed laser deposition on Hastelloy substrates with a CeO_2 based textured buffer layer. The critical currents (J _c) values of the GdBCO layers containing those nano-rods are enhanced compared with those of pristine GdBCO layer in high magnetic fields. In order to investigate the relationships between their superconductive properties and their nanostructures, they were characterized in detail by transmission electron microscopy (TEM). TEM is the only method for direct observation of these nano-rods in the GdBCO grains. The GdBCO layers were mainly composed of c-axis oriented GdBCO grains containing numerous nano-sized rods. The crystal orientation relationships between the GdBCO and the nano-rods were as follows; (001)GdBCO//(001)nano-rods and (100)GdBCO//(100) nano-rods. The average diameters of the BHO and the BZO nano-rods were 4.5 and 5.6 nm, respectively. The BSO nano-rods were thicker than other rods. These nano-rods in the central region of the c-axis oriented GdBCO grains were aligned parallel to the c-axis of the GdBCO, while nano-rods in the outer region of the c-axis oriented grains were tilted away from the c-axis. With increase in the thickness of the GdBCO layers, the ratio of the BZO or the BSO nano-rods aligned parallel to the c-axis to those tilted away from the c-axis decreased, so that the J _c-B-θ profiles of the thicker GdBCO layers containing the BZO or the BSO nano-rods became flatter. The BHO nano-rods were homogeneously distributed throughout the GdBCO, and their average length of was less than that of the other nano-rods. The homogeneous distribution and short length of the BHO nano-rods enhanced the J _c values of the GdBCO layers containing them in high magnetic fields. The J _c-B-θ profiles of the GdBCO layers containing the BHO were independent of the layer thickness. From these results, we will discuss about the morphologies and distributions of suitable vortex pinning for applications of GdBCO coated conductor in high magnetic fields.