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_{\text{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_{\text{c2}}, thus leading to the increase of J_{\text{c}} in high magnetic fields. Recently we found that the additions of hydrocarbon such as ethyltoluene (C₉H₁₂) is very effective in enhancing J_{\text{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, {\sim}50% of the theoretical value, thus limit J_{\text{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 {\sim}50 to {\sim}70%, and much increases J_{\text{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_{\text{c}} values than PIT wires. ©2012

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