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 highdensity MgB_2 superconductors with better connectivity and increase the J_c compared with the in situ and ex situ methods.