2012/09/21 by Hiroya Tanaka, Akiyasu Yamamoto, Jun-ichi Shimoyama +2 · 50 citations
Materials Science · Physics and Astronomy · #Boron #Boron and Carbon Nanomaterials Research #Critical current #Crystallite #Grain growth #Grain size #Rare-earth and actinide compounds #Sintering #Superconductivity #Superconductivity in MgB2 and Alloys #cond-mat.supr-con
paper · pdf · doi:10.1088/0953-2048/25/11/115022
published in Superconductor Science and Technology 25(11), 115022 (IOP Publishing) · 21 pages, 9 figures, to be published in Supercond. Sci. Technol
arxiv created 2012/09/21 · openalex publication_date 2012/10/10 · arxiv updated 2015/06/11 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
The transport current carrying capacity of ex situ processed MgB 2 is expected to be greatly enhanced if a strong intergrain connectivity can be realized. Although percolation theory predicts that ex situ MgB 2 samples should have a high connectivity of over 30% due to their high bulk density ( P ≈ 75%), the reported connectivities of ex situ MgB 2 bulks and wires are generally less than 10%. This is presumably because ex situ MgB 2 has a much weaker intergrain connectivity than in situ MgB 2 . It is well known that heat treatment after cold working of ex situ MgB 2 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 MgB 2 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 MgB 2 . Sintered ex situ MgB 2 bulks from a laboratory-made ball-milled powder exhibited a greatly enhanced connectivity of 28%, which is the highest connectivity of pressureless ex situ MgB 2 bulks, wires, and tapes. Surprisingly, grain growth did not occur during long-duration (∼100 h) sintering in the sintered ex situ MgB 2 bulks. This is in marked contrast to in situ processed MgB 2 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 MgB 2 polycrystals without grain growth.