Generation Of 2D Flux Pinning Centers In MgB₂ And The Mechanism For The Higher J_c?H?

MgB2 superconductor can operate without the expensive liquid-helium cooling,for its higher superconducting transition temperature?39 K?compared to that of the traditional low temperature superconductors.The drawback of MgB2 lies in its weak intrinsic flux pinning force,which results in the quick drop of critical current density with increasing magnetic field and limit its application.The intrinsic flux pinning centers in MgB2 superconductor are grain boundaries and intrinsic defects.Chemical doping can induce extrinsic flux pinning centers in MgB2.The flux pinning centers can be divided into point pinning centers,surface pinning centers?two-dimensional pinning centers?,and volume pinning centers.The type,size,and distribution of pinning centers influence the flux pinning force,connectivity,and Jc?H?of MgB2.The Point pinning centers possess weak flux pinning force and can only react with one flux lines at one time,the volume pinning centers damage the connectivity of MgB2.Surface flux pinning centers possess strong flux pinning force and can react with lots of flux lines,as well as be transparent for the supercurrents.The agglomeration of flux pinning centers can also damage the connectivity.Generation of homogeneous two-dimensional flux pinning centers in MgB2 promise high Jc?H?of MgB₂?In this paper,carbon coated boron powders?B@C?were used to fabricate MgB2for higher Jc?H?,in which the carbon coating was achieved by the polycondensation of resorcinol and formaldehyde on the boron powders surface,combined with the following high temperature pyrolysis.The microstructure,superconducting transition temperature,in-field critical current density,and the flux pinning mechanism have been systemically investigated,and it has been found that the carbon coating layer causes carbon substitution on boron site,and generates homogeneous two-dimensional?2D?carbon layers acting as 2D flux pinning centers.2D flux pinning centers can enhance the flux pinning force and cause no damage to the connectivity compared to the large impurity phase,thus increase the Jc?H?of MgB2.Then,the above results were compared with the investigations of MgB2 prepared with graphene coated boron powder?B@G?and g-C3N4 coated boron powder?B@CN?.It has been found that all the 2D materials can induce carbon substitution on boron site,and 2D flux pinning centers in MgB2 to increase Jc?H?.Compared to graphene and g-C3N4,the carbon coating layer distributes homogeneously on the boron powders and induce intensive carbon substitution,homogeneous distribution of the 2D pinning centers.The graphene layer in the matrix can texture the grains for better connectivity during the MgB2 fabrication.Beside inducing carbon layer acting as 2D pinning centers,g-C3N4 leads to the formation of BN nano particles in the matrix acting as point pinning centers.Some g-C3N4 agglomerate and remain in the matrix as large impurity phase even after high temperature sintering,which damage the connectivity and decrease the critical current density of MgB2.It is concluded that generating homogeneous 2D flux pinning centers in MgB2through homogeneously coating 2D materials on boron powder can fabricate MgB2with both enhanced flux pinning force and excellent connectivity,which potentiates higher Jc?H?and practical application of MgB2 superconductors.