Effects of carbon nanotube addition on superconductivity in Y-Ba-Cu-O bulk superconductors

Bulk Y-Ba-Cu-O superconductors have significant potential for engineering applications due to high critical current density, which is attributed to the presence of pinning centers such as Y₂BaCuO₅. The introduction of nano-sized secondary phase is known to act as more effective pinning center than those in micron sizes. The diameter of carbon nanotube (CNT) is close to that of the coherence length of high-temperature superconductors, which is expected to improve the flux pinning performance. We have investigated the effects of CNT addition on the microstructure, superconducting transition temperature (T_c), and critical current density (J_c) of YBa₂Cu₃O_x (Y123) based bulk superconductors. SEM observation showed the distribution of needle-like particles around 100 nm in length in the Y123 matrix for the CNT added samples. The highly porous texture was also observed for the excess addition of CNT. T_c was enhanced from 90.5 K to 91.8 K with increasing CNT addition. It is probable that carbon originated from CNT suppressed oxidation and carrier doping. Jc exhibited the highest value for 0.25 wt% CNT added sample. This suggests that nano-sized needle-like particles act as effective pinning centers. However, a further increase of CNT led to the decline of J_c, which suggests that there is an optimum amount of CNT for the improvement of J_c. The secondary peak was observed for the sample with 1 wt% CNT addition, where CO₃ substitutions with Cu site at the Cu-O chain might induce oxygen vacancies leading to the field induced pinning.