A new carbon source MgB2C2 for the synthesis of carbon-doped MgB2 materials

One of the major problems of superconducting MgB2 materials for extensive application is a large decrease in Jc with an increase of magnetic field. Carbon substitution for the boron site of MgB2 has been well known to reduce this problem and numerous carbon containing compounds have been proposed as carbon sources thus far. However, the actual carbon substitution level, which is dependent on carbon source materials and sintering conditions, tends to be lower than the nominal one. In addition, impurity phases or hydrogen gas generate from most of the carbon sources during heat-treatment except for pure carbon and B4C. In this paper, MgB2C2 is proposed as a promising new carbon source for preparation of carbon-doped MgB2. Although synthesis of MgB2C2 has been considered to be difficult through a simple solid state reaction under an ambient pressure, we found that this phase can be obtained from powder mixture of nano-carbon (carbon black), boron and magnesium by sintering in a sealed stainless tube. The carbon-doped MgB2 polycrystalline bulks using MgB2C2 as a carbon source contained almost the same carbon level as nominal one and exhibited improved Jc characteristics in magnetic field.