Improve The Critical Current Density Of MgB₂ Superconductor

Compared with other Superconducting materials,MgB₂ superconductor shows broad prospects for the preparation and practical application of large-scale and low-cost superconducting wires.However,the degradation rate in critical current density at high magnetic fields is too large,which severely limits the industrial application of MgB₂.The objective of this work is to improve the connectivity and flux pinning and hence to enhance the critical current density of MgB₂ superconductor by the chemical doping/addition.The main reseach works and obtained conclusions are as follows:?1?Two types of graphene[Reduced Graphene Oxide?RGO?and Graphene Nanoplatelets?GNP?]are used as dopant for MgB₂ bulk,and the effects of different amounts of graphene on the crystal structure,microstructure and superconductivity of Mg B₂ bulk are investigated.It is found that the critical current density of MgB₂ could be improved by the two types of graphene doping.,and the optimal doping ratio is obtained.?2?The doping effects of RGO and GNP are summarized and compared,and it is found that the GNP doping shows more advantages:1)there is no generation of hybrid MgB₂C₂;2)the superconducting transition temperature reduces slightly;3)the critical current density is higher at whole magnetic fields;4)the cost of GNP is lower,and the price is only 1/18 of RGO.Therefore,based on the above advantages,the GNP is used as dopant for MgB₂ wire.?3?The effects of different heat treatment temprtature?650?⁹00??on microstructure and superconductivity of Mg B₂ wire doped with 1 wt%GNP are investigated,and the results show that both the microstructure and superconducting properties depend strongly on the heat treatment temperature.700?is found to be the optimal synthesis temperature with the highest critical current density and the strongest magnetic flux pinning,and the main pinning mechanism is grain-boundary surface pinning according to D.Dew-Huhes flux pinning model.?4?In order to avoid the introduction of other impurity elements in doping,the MgB₄ of Mg-B system is used as an additive,and the effects of different amounts of MgB₄ on the crystal structure and superconductivity of Mg B₂ bulk are investigated.It is found that the addition of MgB₄ has little effect on the crystal structure and the superconducting transition temperature of MgB₂ bulk,but the critical current density at high magnetic fields is improved significantly,which is due to the increased point pinning centers of nanoscale MgB₄.This also provides a new way for the choice of additive in the future.?5?Based on the MgB₄ in MgB₂ bulk can provide effective point pinning centers,the higher purity MgB₄ is used as an additive for MgB₂ wire.It is found that the superconducting transition temperature of MgB₂ wire is unchanged,and the critical current density at whole magnetic fields is improved significantly,which is attributed to the improvements in intergrain connectivity and flux pinning.It is worth noting that the critical current density at the low magnetic fields of MgB₄-added MgB₂ wire is increased compared with that of MgB₄-added MgB₂ bulk,which is related to the purity of the additive Mg B₄,the excessive impurity aggregates at the grain boundary and limits the effective current capacity at the low magnetic fields.