Study On Superconductivity Of MgB₂ By Co-Doping Of Graphene And Metal

MgB2 is the superconducting material of the simple and stable metal compounds which is found at the highest critical temperature at 39K.Because of its simple structure?composition and easy to make and processing,moreover,the superconductor current is not restricted by grain boundary connectivity,and thus it has a broad application prospects.In this paper,the superconductivity of MgB2 was studied by co-doping of graphene and metal nanoparticles.Ni and Cu were selected as the metal nanoparticles by conventional sintering method and the best co-doping process parameters were determined at the same time.The MgB2 bulks was sintered with graphene and metal nanoparticles by means of powder sintering theory,differential thermal analysis technique and microstructure observation.The main methods of the study were X-ray diffraction,electron microscopy and integrated physical properties measurement system in order to determine the phase formation and microstructure of MgB2 samples and their superconductivity.In addition,the graphene-supported Ni precursor powder was prepared by chemical reduction using graphene as the carrier,and the MgB2 superconducting samples were doped with the precursor to analyze the microstructure and superconductivity.The main contents of this paper and the conclusions are:Graphene and nano-Ni co-doped MgB2 samples were sintered by high temperature differential scanning calorimetry.The effect of co-doping on the formation process of MgB2 phase was monitored meanwhile.In this paper,Mg-Ni is used to form eutectic liquid phase at low temperature.The eutectic liquid phase can accelerate the diffusion process of Mg and B particles,forming a large number of solid solution activation regions.MgB2 can be synthesized in this region under the fast diffusion between Mg and B.So that the eutectic liquid phase can not only accelerate the rate of diffusion between atoms,but also promote the Mg and B reaction with shorten sintering time.The diffraction peak at 60° in the XRD image is shifted,indicating the atomic substitution by doping of the grapheme.The carbon atom will partially replace the B site in the MgB2 lattice,and the substitution will cause the lattice distortion,Forming the magnetic flux pinning center.The main reason for the decrease of the critical current in the high magnetic field is due to the ferromagnetism of Ni particles.Ni was loaded at graphene precursor by chemical reduction method is prepared in this paper.Ni with 10 nm was uniformly distributed on the matrix of graphene by SEM and TEM images without any agglomeration.The low superconductivity of the doped MgB2 is attributed to the less content of Ni and second phase MgNi2 sB2,which could not serve as the flux pinning center.Moreover,the intrinsic of Ni is ferromagnetism,which may lead to worse critical current density at high magnetic field.The effect of different doping amount with graphene and nano Cu on the MgB2 formation process and its superconductivity was also studied.Compared with the samples of MgB2 doped with graphene and nano-Cu in high temperature and low temperature,it is found that the Mg2Cu phase appears in the low temperature,while MgCu2 appears at high temperature.With the higher temperature,Mg is prior to evaporate,leading to the second phase changing from Mg2Cu to MgCu2.lt can also be seen from the SEM image that the MgB2 grains synthesized under the low temperature are smaller with finer grain boundaries,which may serve as effective magnetic flux pinning centers,so the lower critical current density is improved at high magnetic field.After the comparison of Cu and Ni nanoparticles,it can be shown that the ferromagnetic material will seriously reduce the transmission process of the superconducting current,so when the casing material is selected.the ferromagnetic material should be avoided as much as possible.