Self-sintering-assisted high intergranular connectivity in ball-milledex situMgB2bulks

To understand the mechanisms leading to higher intergranular connectivity in polycrystalline ex situ MgB2 prepared using a pressure-less self-sintering technique, the influence of the initial particle size of the MgB2 powder was investigated. MgB2 bulks sintered at 900 °C for 24 h using ball-milled powders exhibited a network structure with both qualitatively and quantitatively improved intergranular couplings. The connectivity calculated using normal-state transport measurements reached a high value of ~40%, which is comparable to that of MgB2 bulks fabricated via Mg diffusion or high-pressure-assisted processes. Moreover, the sintering time required to achieve a reasonably high connectivity of 25–30% was less than 1 h. Notably, microstructural analyses confirmed the formation of intergranular necks and grain boundaries during the early stage of sintering. These results suggested self-sintering of the ball-milled MgB2 bulks proceeded much more rapidly than those sintered using an MgB2 powder without ball-milling. Increased intergranular contact points and decreased gap length between grains in green compact are assumed to be the origins for the stimulated self-sintering and corresponding increase in the electrical connectivity.