Effect Of Graphene-Metal Co-doping On The Microstructure And Superconductivity Of FeSe

LaO1-xFxFeAs,an iron based superconductor,was discovered in 2008,whose superconductivity could not be explained by BCS(Bardeen Cooper Schieffer)theory,which caused a sensation in the field of condensed matter physics.In the large family of iron-based superconductors,iron-selenium superconductors are not as toxic as iron-arsenic superconductors,which are relatively safe,simple in structure and easy to prepare.Therefore,they are favored by many researchers.At the same time,due to the simple layered structure of iron-selenium superconductors,it has a great research prospect to reveal the superconductivity principle,so it can provide theoretical guidance for further research on iron-arsenic superconductors.At present work,we chose the "11" type iron-selenium superconductor as the research object,and prepared the FeSe bulks polycrystalline superconductor by two-step sintering.The main methods of the study were differential thermal analysis X-ray diffraction,electron microscopy and integrated physical properties measurement system in order to determine the phase formation and microstructure of FeSe samples and their superconductivity.The influence of graphene doping and the co-doping of graphene and metal Nb(micron)on the microstructure and superconductivity of FeSe superconductor was investigated.The main contents and conclusions of this paper are as follows:The results show that the first step of sintering doped graphene can effectively improve the superconductivity of FeSe superconductor.Graphene can provide a useful load for the nucleation of FeSe,making the layered ?-FeSe superconducting phase thin,refining the grains and increasing the grain boundary pinning center,thereby increasing the critical current density(Jc)at high magnetic fields.At the same time,graphene doping can also improve the connectivity between the grains of FeSe superconductors,thus increasing the critical current density(Jc)at low magnetic fields.In addition,when the molar ratio of graphene doping was 0.05,the superconducting performance of sample Fe1.01eG0.05 reached the optimal.Compared with the undoped graphene sample Fe1.01Se,the critical transition temperature(Tc)of Fe1.01SeG0.05 was significantly increased to 10.57k,an increase of 15.3%.Under zero magnetic field,the critical current density(Jc)of sample Fe1.01SeG0.05 reached 5×103 A/cm2.The experiment further explored the influence of co-doping of graphene and metal Nb(micron)on the superconducting performance of FeSe superconductor.The study shows that the doping of metal Nb could further improve the connectivity between grains.Under zero magnetic field,the critical current density of Fe1.01SeG0.05Nb0.04(2)superconductor reached 1.4×104A/cm2,which is 2.8 times higher than that of the sample Fe1.01SeG0.05 only doped with graphene.In addition,the critical current density of FeSe superconductor under high magnetic field was also significantly increased,which indicated that Nb doping could also refine grains.Since Nb replaced Fe bit,the lattice distortion was caused,and the pinning center was introduced,thus increasing the critical current density(Jc)of FeSe superconductor under high magnetic field.At the same time,through the TEM analysis of bulk Fe1.01SeG0.05Nb0.04(2),it was found that the replacement of Fe bit in the ?-FeSe phase by Nb atoms would lead to the formation of FeNb2Se4 ternary phase.