Growth Speed and Substitution Effects on Alignment and Thermal Transport Properties of Bi-2212 Textured Superconductors

Abstract

Fe-substituted superconducting thin BiSrCaCuO rods with nominal compositions of Bi₂Sr₂Ca₁Cu_2−x Fe _x O_8+δ (x = 0, 0.01, 0.03, 0.05, and 0.1) were fabricated using the laser floating zone technique at two different growth speeds, 15 mm h⁻¹ and 30 mm h⁻¹. The influences of growth speed and Fe substitution on the grain alignment in the rods were evaluated by means of x-ray pole figure studies. The obtained results showed that both applied growth speed and Fe substitution play a crucial role on grain alignment, which is strongly connected with the current-carrying capacity of the rods. It was found that the rods grown at 15 mm h⁻¹ (G15) have stronger orientation than the rods grown at 30 mm h⁻¹ (G30). However, in contrast to the G15 rods, an increased substitution rate improved the orientation of the G30 rods. Another important observation is that the increase on the substitution caused a decrease on the grain size of all the rods. The decrease of critical temperature values of the rods upon substitution was ascribed to both grain size effect and formation of a nonsuperconducting Fe-rich phase detected in scanning electron microscope/energy-dispersive x-ray analyses. The thermal conductivity values of the G15 and G30 rods were found to be in the range of 0.9–1.9 and 1.1–1.18 W m⁻¹ K⁻¹ at 150 K, respectively. The higher values of figure of merit (ZT), at all temperature ranges, were obtained from the highest substituted rods (x = 0.1) for both of the applied growth speeds. In addition, it was observed that the ZT of G30 rods are up to three times higher than that of G15 ones.