In this paper we report the results of an extended study of the effect of sugar doping on the structural and electromagnetic properties of MgB_2 superconductors. High values of the upper critical field (B_c2)of 36 T and the irreversibility field (B_irr) of 27T have been estimated at the temperature of 5K in a bulk MgB_2 sample with the addition of 10 wt% of sugar.The critical current density (J_c(B_a)) of sugar-doped samples has been significantly improved in the high field region. The value of transport J_c has reached as high as 10^8 Am^-2 at 10T and 5K for Fe-sheathed sugar-doped MgB_2 wire. The analysis of the pinning mechanism in the samples investigated indicated that dominant vortex pinning occurs on the surface type of pinning defects, such as grain boundaries, dislocations, stacking faults etc, for both pure and doped MgB_2. In sugar-doped samples, pinning is governed by numerous crystal lattice defects, which appear in MgB_2 grains as a result of crystal lattice distortion caused by carbon substitution for boron and nano-inclusions. The drastically improved superconducting properties of sugar-doped samples are also attributed to the highly homogeneous distribution and enhanced reactivity of this dopant with host Mg and B powders. The result of this work suggest that sugar is the optimal source of carbon for doping MgB_2 superconductor, especially for application at high magnetic fields.

Physics