Preparation of pure and doped MgB₂by the field-assisted sintering technique and superconducting properties

Abstract

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field Hc2 and irreversibility field Hirr when compared with pristine MgB2. The reason for better Hc2 and Hirr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of Hc2 and Hirr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T≥24 K, there is mixed pinning (H≤5 T).