Development of High Performance MgB2 Tapes and Wires

MgB2 shows no weak coupling of grains, indicating that grain alignment is not essential to obtain large current transfer from one grain to adjacent grain. This is a very big advantage because a simple process, such as powder-in-tube (PIT) method, can be applied for the fabrication of wires and tapes. Here, we report the recent progress of our MgB2 tapes and wires. There are two PIT methods. One is an in situ method, in which a powder mixture of Mg and B is used as a starting material. The other is an ex situ method, in which reacted MgB2 powder is used. Superconducting properties are much sensitive to the quality of the starting powder, the porosity of the MgB2 core and the heat-treatment conditions. Impurity addition to the mixture of starting powder is an effective method to improve J(c) of MgB2 tapes and wires. Among the impurities nano-SiC powder is the most popular additive. SiC addition introduces C substitution for B site in MgB2 and enhances H-c2, thus leading to the increase of J(c) in high magnetic fields. Recently we found that the additions of hydrocarbon such as ethyltoluene (C9H12) is very effective in enhancing J(c) values in spite of smaller amount of C substitution for B site. Unfortunately the PIT methods lead to low mass densities of the MgB2 filaments, similar to 50% of the theoretical value, thus limit J(c) values. One of the effective methods yielding higher MgB2 mass densities is the application of hot pressing. This hot pressing enhances mass densities from similar to 50 to similar to 70%, and much increases J(c) values. Another effective method is the internal Mg diffusion (IMD) process. High mass density MgB2 layer can be obtained by this IMD method, and IMD processed wires show much higher J(c) values than PIT wires.