SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2

This work describes in detail the simultaneous enhancement of the upper critical field (H-c2) and the critical current density (J(c)) of MgB2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled (dw) carbon nanotubes (CNTs). The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50 T pulsed magnet to determine H-c2(T). We found that the J(c) enhancement is similar in all doped samples at 5 K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range (T >= 20 K); this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects (i.e., several kinds of nanoinclusions). CNTs produce a better C incorporation that is more effective to enhance H-c2 [i.e., dwCNT-doped samples reached a record H-c2(0)similar to 44 T value for bulk MgB2]. All the H-c2(T) curves obtained for different types of doping can be successfully described using a model for a two-gap superconductor in the dirty limit. (C) 2008 American Institute of Physics.