Research Progress of Electromagnetic Properties of MgB 2 Induced by Carbon-Containing Materials Addition and Process Techniques

For the high transition temperature ( T c ) and low cost taking both raw materials and fabrication process into account, MgB 2 has been a competitive candidate to replace the conventional NiTi superconductor for high-temperature application in fault current limiters, transformers, motors, magnetic resonance imaging, adiabatic demagnetization refrigerators, generators, etc. The carbon-containing materials addition induced high critical current density ( J c ) is reviewed based on their influences on the upper critical field ( H c2 ), flux pinning force, and connectivity. The doping effects were compared in the overview focusing on SiC, organic dopants, and graphene-related dopants. SiC doping is featured for the high-field critical current density, which is caused by the increased H c2 attributed to the substitution of carbon on boron site and the strong flux pinning force offered by the nanosized secondary phases in the MgB 2 matrix. Organic dopants have the advantage over SiC dopant for their relatively homogeneous distribution in the MgB 2 matrix based on wet mixing of the organics and the raw boron powders. Low doping level of two-dimensional materials can improve the superconducting properties in all measured fields because of the combined advantages of carbon substitution effect and grain connectivity. MgB 2 fabricated with carbon-encapsulated boron also introduces strong flux pinning centers in MgB 2 , which show weak destruction of the connectivity of the MgB 2 grains as reflected by the low-magnetic-supercurrent behavior. High-pressure treatment and diffusion method can fabricate high-density MgB 2 superconductors with better connectivity and increase the J c compared with the in situ and ex situ methods.