Improvement of Upper Critical Field and Critical Current Density in Single Walled CNT Doped MgB2/Fe Wires

We evaluated the doping effect of single walled carbon nanotubes (SWCNT) on the phase formation, microstructure, critical current density (J_c), and upper critical field (H_c2) of MgB₂ superconductor. All samples were sintered at temperatures ranging from 650 to 1000degC for 30 min. A typical high sintering temperature (900degC) sample was composed of well-consolidated MgB₂ grain of 100 to 200 nm and nanosized particles, resulting in enhancement of J_c and H_c2. This indicates that lattice defects and microstructural change have occurred as a result of carbon (C) coming from SWCNT. On the other hand, a sample sintered at low temperature (700degC) contained relatively large grains. Specifically, SWCNT doped MgB₂ wire sintered at 900degC exhibited excellent J_c > 10⁴ Acm^-2 up to 8 T at 4.2 K. This result indicates that flux pinning for sample produced at a high sintering temperature are enhanced by SWCNT doping. SWCNT is one of the promising C sources for MgB₂ superconductor with excellent J_c.