Confinement of self-generated Mg(BH₄)₂ particles for simultaneously enhanced flux pinning and grain connectivity in polycrystalline H-doped MgB₂

The present study reports a simple quenching method to simultaneously improve flux pinning and grain connectivity in polycrystalline H-doped MgB 2 , which was ex-situ synthesized at 350 °C under the H 2 pressure of 3 MPa. Quenching prevented the phase transformation of β -Mg(BH 4 ) 2 → α -Mg(BH 4 ) 2 upon cooling, and the dominant point-pinning effects originated from the self-generated nano-sized β -Mg(BH 4 ) 2 particles that were confined within the MgB 2 grains. A semi-coherent relationship was found between MgB 2 (100) and Mg(BH 4 ) 2 ( 1 ˉ 1 4 ˉ ) planes. Meanwhile, the precipitation-induced expansion of the MgB 2 grains remained at room temperature, leading to an increasing number of contact points as well as grain connectivity. Compared with the un-doped MgB 2 , the quenched H-doped one exhibited enhanced critical current density over the entire field. These results are expected to guide the design of flux pinning centers for improving the J c performance of standard type II superconductors.