Fabrication and Superconducting Properties of Diffusion Processed 7-Core MgB2 Wires

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Powder-In-Tube(PIT) method is the most popular method to fabricate MgB2 wires and tapes. However, PIT processed tapes and wires show relatively low Jc values due to the low density and, hence, low connectivity of MgB2 cores. The internal Mg diffusion (IMD) process is one of the effective methods to obtain high density MgB2. In this paper we report the heat treatment temperature dependence of superconducting properties of IMD processed 7-core MgB2 wires. A pure Mg rod with a diameter of 2 mm was placed at the center of a Ta tube with an outer diameter of 6 mm and inner diameter of 3.5 mm, and space between the Mg rod and the Ta tube was filled with B powder or B-SiC mixed powder. The composite was successfully cold worked into 1.3 mm wire at room temperature without any breakage. Seven pieces of the mono-core wires were bundled and inserted into a Cu-Ni tube and then, cold worked into 1.3 mm wire. The wires were heat treated at 590~800°C for 1 h under an Ar gas atmosphere. At the temperature of 640°C B layer was almost completely reacted with Mg to form MgB2. X-ray diffraction analysis indicated that the major phase in the reacted layer is MgB2. SEM analysis of the heat treated wire clearly indicated that the density of MgB2 layer in the wire was higher than that of a PIT processed wire. Transport Jc values (calculated for the reacted layer) reached 9.9×102 A/mm2 in 10 T at 4.2 K and 1.3×103 A/mm2 in 3 T at 20 K for the SiC added wire heat treated at 640°C. These Jc values are much higher than those of usual PIT processed wires. These high Jc values can be attributed to the high density MgB2 layer obtained by this diffusion method.