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영어

Cryogenic milling which is a combined process of low-temperature treatment and mechanical milling was applied to fabricate high critical current density (Jc)MgB2 bulk superconductors. Liquid nitrogen was used as a coolant, and no solvent or lubricant was used. Spherical Mg (6-12 μm, 99.9 % purity) and plate-like B powder (~ 1 μm, 97 % purity) were milled simultaneously for various time periods (0, 2, 4, 6 h) at a rotating speed of 500 rpm using ZrO2 balls. The (Mg+2B) powders milled were pressed into pellets and heat-treated at 700°C for 1 h in flowing argon. The use of cryomilled powders as raw materials promoted the formation reaction of superconducting MgB2, reduced the grain size of MgB2, and suppressed the formation of impurity MgO. The superconducting critical temperature (Tc) of MgB2 was not influenced as the milling time (t) increased up to 6 h. Meanwhile, the critical current density (Jc) of MgB2 increased significantly when t increased to 4 h. When t increased further to 6 h, however, Jc decreased. The Jc enhancement of MgB2 by cryogenic milling is attributed to the formation of the fine grain MgB2 and a suppression of the MgO formation.