A potential route to synthesize imporous MgB2 bulks by pretreatment of B powder at ambient pressure

Imporous MgB₂ bulks with a density of 1.82 g cm^-3 have been synthesized by pretreatment of B powder in an in situ solid-state reaction at ambient pressure. The results show that the MgB₂ with B powder pretreatment has a significant improvement in J_c, B_irr, microstructure and intergranular coupling, but with no decrease of T_c. At 20 K and 4 T, the J_c is enhanced by 4 times by the pretreatment of the B powder. For the B pretreated MgB₂, the B_irr at 20 K reaches 5 T and J_c at 10 K and 6 T reaches 2200 A cm^-2, compared to 4.2 T and 670 A cm^-2 for the B-not-pretreated MgB₂. It is argued that the small amounts of highly dispersed carbon in B powder may enhance the mobility of B particles during the reaction of B and Mg, which avoids the formation of voids in the positions of Mg particles.