Optimization of Infiltration and Reaction Process for the Production of Strong MgB $_{2}$ Bulk Magnets

An infiltration and reaction (IR) process can realize a dense MgB
₂
bulk under an ambient pressure, which is preferable for the industrial mass production in cost. However, the trapped field of the IR-processed bulk is commonly rather smaller than that of the high-pressure synthesized bulk, which is caused by the cracks and the thread-like channels of residual Mg preventing the circular supercurrent flow. To obtain the high-quality IR-processed MgB
₂
bulk, we investigated the optimization of heat-treatment condition for the IR-process (sintering temperature, T
_ht
= 700-900 °C and holding time, t
_h
= 1-9 h). A volume fraction of the MgB
₂
phase increased monotonically with increasing Tht and/or th, and the grain-size increased with increasing T
_ht
. As a result, the highest critical current density J
_c
and irreversibility field μ
₀
H
_irr
were achieved for the sample sintered at 700 °C for 9 h. The relation between the superconducting properties and the microscopic structure was also discussed.