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 MgB₂, which was ex-situ synthesized at 350 °C under the H₂ pressure of 3 MPa. Quenching prevented the phase transformation of β-Mg(BH₄)₂→α-Mg(BH₄)₂ upon cooling, and the dominant point-pinning effects originated from the self-generated nano-sized β-Mg(BH₄)₂ particles that were confined within the MgB₂ grains. A semi-coherent relationship was found between MgB₂(100) and Mg(BH₄)₂( 1 ˉ 1 4 ˉ ) planes. Meanwhile, the precipitation-induced expansion of the MgB₂ grains remained at room temperature, leading to an increasing number of contact points as well as grain connectivity. Compared with the un-doped MgB₂, 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.