The Influence Of Metal Doping On The Phase Formation And Properties Of Sintered New Type-? Superconducting Materials

MgB₂ and FeSe were found as new type of superconducting materials in 21^th centry.Eventhough they have their specific superconductivity respectively,but both of them have many similarities from the point of view of material frabrication.For example:1)structure is simple,both are binary metal compounds;2)the constituent elements are cheap and low toxic;3)both of them belong to the typical binary system that both components have big different melting points and weak reactivity,which is produced by sintering process primarily.Previous research shows that metal doping has remarkable influence on phase forming during sintering process,sintered microstructure and superconductivity of simple binary superconductor,such like MgB₂ and FeSe.But until now the doping mechanism is still not clear.Sepcailly in recent years the co-doping between metal and other elements were already carried out widely.The mechanism of cross influences also needs to be explored.Based on the above information,our research focused on the metal element doping in new type-II superconducting materials.Main conclusions are drawn below:Combined with phase identification and microstructure observation,it was found that Mg Ni_2.5B₂ impurity phase only formed at the grain boundary of MgB₂,when doping micrometer Ni powder in sintered MgB₂.The Mg Ni_2.5B₂ particles tend to be agglomerated,which is likely to decrease the grain connectivity and reduce the superconducting phase,leading to the decrease of J_c.On the contrary,nano-sized Ni powder has positive influence on the formation of MgB₂ phase and the superconducting properties.The nano-size Ni powder will assist the liquid sintering in Mg-B system,which promotes the formation of MgB₂ superconducting phase.Besides,nano-Ni doping also reduces the gain size of MgB₂ efficiently,forms nano-sized Mg Ni_2.5B₂ phase and more lattice imperfections.The increased amount of grain boundary and defects also become effective flux pinning centers.Based on the mentioned advantages before,the enhancement of J_c was obtained under high magnetic field.Due to the limited J_c improvement of pure Ni to MgB₂ under high magnetic field,the previous research showed the cabon doping could improve J_c under high magnetic field effectively.So basing on the Ni doping we further use carbon doping expecting to introduce band scattering and lattice imperfection to further improve J_c under high magnetic field.Although the coated carbon on B powder hinders the reaction between Mg and B,but the nanosized Ni generates low-temperature liquid with Mg promoting the reaction.So the carbon doping was effectively introduced under the Ni doping to obviously improved the H_irr and J_c under high magnetic field.When Cu was used to substitute Ni as doping element in MgB₂,it was found that Cu doping could promote the low temperature sintering of MgB₂ more effectively than Ni doping.Moreover,the Mg-Cu alloy doped with Cu would not enter the MgB₂matrix and would not deteriorate the intergranular connectivity.However,Cu doping can only increase the critical current density of MgB₂ in low magnetic field.Based on this,we attempt to improve the critical current density of low temperature sintered MgB₂ in full magnetic field by Co-doping of copper and graphene.The results show that graphene can not only introduce effective C doping,but also provide a supporter for MgB₂ nucleation with its network structure.The formation of Mg-Cu liquid phase promotes the uniform nucleation and growth of MgB₂ grains,thus forming compact and uniform fine grains.These factors ultimately make the critical current density of MgB₂ significantly improved under the whole magnetic field.Because graphene can only release a small amount of free C in its edge region,it can not replace B position in MgB₂ effectively,which is still limited for improving critical current density of MgB₂.In order to introduce more C-doping and improve the critical current density of MgB₂,we use C-coated B powder instead of pure B powder as the precursor powder for low temperature sintering of Cu-doped MgB₂.The results show that C-coated B powder can in-situ introduce a large number of uniform and effective C doping into MgB₂ prepared by low temperature sintering.At the same time,the fine grain structure of MgB₂ can be obtained by low temperature sintering of Cu doping,which is beneficial to improving the current carrying capacity of MgB₂.Finally,high-performance MgB₂ superconducting bulk material was prepared.Its critical current density reached 1×10⁵ A/cm² at 20 K and 2 T,which basically met the practical requirements.Considering that MgB₂ is widely used in various fields in the form of wires,we further introduce the above idea of low temperature sintering with Cu doping combined with C doping into the central Mg diffusion(IMD)process to prepare high performance and practical MgB₂ wires.The in-situ experimental monitoring and kinetic analysis showed that the coating of Cu on Mg rod could promote the diffusion of Mg and change the formation kinetic of superconducting layer,which accelerated the formation of completely dense MgB₂ layer in the wire.At 600?,the IMD wire was successfully synthesized with dense MgB₂layer and with lager diameter(1.03mm).The MgB₂ wires by Cu coating activated IMD achieved the J_e 6 times higher than traditional PIT MgB₂ wire sintered at high temperature,and twice higher than the second generation of IMD MgB₂ wire with large diameter(0.83 mm).The J_eperformance is comparable to the IMD MgB₂ wire with smaller diameter(0.55 mm).Based on the research route and methods,further study was addressed on the influence mechanism of metal doping on another new type of superconductor FeSe,which is similar to MgB₂.It was found that Mg doping obviously promoted the formation of FeSe superconducting crystal structure and increased the ratio of superconducting and impurity phase,which improved the superconducting properties of FeSe.The T_c was enhanced 25%up to 12.3 K in comparison with undoped sample.After phase analysis and calculation of crystal lattice,it was found that Mg hardly entered into the FeSe superconducing lattice,but it reacted with FeSe forming Mg Se phase.The Mg Se and FeSe formed nano-layered grain structure,which is the root reason for T_c improvement of Mg-doped FeSe.