Excess Mg in situ powder addition for enhancing critical current density of ex situ MgB₂

In this work, ex situ MgB₂ bulks were added with (1.5 Mg + 2B) and sintered in an attempt to enhance its intergrain connectivity. The addition was varied within the range of 0–50 wt.%, and the sintering was undertaken at 700 °C, 800 °C, and 1000 °C, respectively, for 1 h. Superconducting critical temperature, T_c of the samples was determined to be around 38 K as shown by the temperature dependence of susceptibility measurement. It was found that critical current density, J_c increased with the increased amount of the addition. J_c was further enhanced to 2 × 10⁴A cm² (0 T, 20 K) as the sintering temperature was raised. The increase of J_c is due to improved grain coupling as a result of in situ formation of MgB_2, which fills the voids and connects the ex situ MgB₂ grains. Additionally, the grain coupling was further strengthened by solid-state self-sintering at higher temperatures. The increment of J_c was accompanied by a narrower width of double-step transition of T_c attributable to a more complete MgB₂ phase formation of the samples as the sintering temperature was increased. Graphical abstract: [Figure not available: see fulltext.]