MgB₂ 超電導線材の高 J_c 化 ―MgB₂ コアの充填率向上と不純物添加―

抄録

Two important factors that strongly affect J_c of the MgB₂ wires are impurity addition and the density of the MgB₂ core. Carbon or carbon-containing impurity addition brings about carbon substitution for the boron site of MgB₂ and increases Bc2 of the MgB₂ wires and hence, the J_c values at high magnetic fields. The most popular additive, SiC, brings about the formation of impurities, such as Mg2Si, which act as barriers to the superconducting current. The additions of hydrocarbons, such as coronene (C₂₄H₁₂) and anthracene (C₁₄H₁₀), are more effective in increasing the J_cvalues of the MgB₂ wires. Another interesting hydrocarbon additives are ethyltoluene (C₉H₁₂) and xylene (C₈H₁₀). Additions of these impurities also increase the J_c values of the MgB₂ wires although the amount of carbon substitution for the boron site is much less than those of the coronene and anthracene additions. The densification of the MgB₂ core is realized by the application of high-pressure treatment, such as hot uniaxial pressing, hot isostatic pressing and cold pressing before a heat treatment. Mechanical alloying and mechanical milling of boron and magnesium powder mixture using high energy ball milling are also effective to increase the density of the MgB₂ core and the J_c values. The use of nano-meter size magnesium powder accelerates the reaction of magnesium and boron powder, improves the connectivity of the MgB₂ core and increases the J_c values. Internal magnesium diffusion (IMD) method is also effective in increasing the density of reacted the MgB₂ layer and the J_c values. A small MgB₂ coil was fabricated using a 60-mlong IMD processed 7-filamentary MgB₂/Cu wire and was successfully excited with a reasonably high coil Ic, i.e., about 60 % of Ic obtained in a short sample at 4.2 K in a backup field up to 10 T.