Strongly connected ex situ MgB2 polycrystalline bulks fabricated by solid-state self-sintering

Abstract

The transport current carrying capacity of ex situ processed MgB2 is expected to be greatly enhanced if a strong intergrain connectivity can be realized. Although percolation theory predicts that ex situ MgB2 samples should have a high connectivity of over 30% due to their high bulk density (P ≈ 75%), the reported connectivities of ex situ MgB2 bulks and wires are generally less than 10%. This is presumably because ex situ MgB2 has a much weaker intergrain connectivity than in situ MgB2. It is well known that heat treatment after cold working of ex situ MgB2 improves the connectivity and the critical current density. However, it is currently unclear whether such heat treatment induces self-sintering that results in the formation of necks, the elimination of pores, and an increase in contact area. In the present study, we investigated the microstructure, normal-state electrical connectivity, and critical current density of ex situ MgB2 polycrystalline bulks prepared by systematically varying the sintering conditions under low pressure. Samples heated at a high temperature of ∼900 °C for a long period showed an increased packing factor, a larger intergrain contact area, and a significantly enhanced electrical connectivity, all of which indicate solid-state self-sintering of MgB2. Sintered ex situ MgB2 bulks from a laboratory-made ball-milled powder exhibited a greatly enhanced connectivity of 28%, which is the highest connectivity of pressureless ex situ MgB2 bulks, wires, and tapes. Surprisingly, grain growth did not occur during long-duration (∼100 h) sintering in the sintered ex situ MgB2 bulks. This is in marked contrast to in situ processed MgB2 samples, for which significant grain growth occurred during heat treatment at ∼900 °C, producing grains that are several tens of times larger than the initial boron grains. Consequently, the critical current density as a function of the external magnetic field at 20 K progressively improved with sintering due to the relatively small grain size and good intergrain connectivity. We thus conclude that solid-state self-sintering is an effective approach for producing strongly connected, dense ex situ MgB2 polycrystals without grain growth.