Study Of High Current Capacity MgB₂ Superconductor And Its Flux Pinning Mechanism

The discovery of MgB₂ superconductor with critical temperature?T_c?around 39K in 2001 has attracted much attentions due to its special properties,and became an important part for pratical application rapidly.MgB₂ had stronger intergrain connection and lower cost of raw materials compared with other high-temperature superconductors.Moreover,the critical temperature of MgB₂ allows the use of liquid hydrogen as coolant instead of expensive liquid helium compared with the metallic superconductors.However,the critical current density?J_c?value of MgB₂ rapidly decreases under magnetic field due to its weak intrinsic pinning force,which greatly limits the application of MgB₂.Therefore,the improved J_c performance under magnetic field wass a key factor for MgB₂ application.For this reason,numerous attempts have been paid to introduce effective pinning sites to improve the superconductivity of MgB₂.Carbon doping on B site was an effective process to improve the superconductivity of MgB₂,which has great impact on flux pinning force.Graphene is composed of only a few carbon atoms layers with honeycomb lattice arrangement and exhibits superb strength and excellent electrical conductivity.Graphene doping can enhance the tensile strain effect which is suggested to improve T_c of graphene-MgB₂ composites by increasing the electron-phonon coupling strength of MgB₂.Therefore,graphene wass supposed to be an effective dopant to enhance the superconductivity of MgB₂.However,there were much issues need to be solved:?1?With traditional doping method,the non-uniformly distributed particles restrain the sufficient element substitution and inter-grain connections enhancement,thus reducing the absolute values of the critical current density.?2?Pevious studies were mostly focused on the MgB₂ bulks by magnetization method,which have a gap compared with the transimisson method in pratical application.Considering the scattering structure of graphene,coating method was applied to enhance the uniformity of graphene doping.Furthermore,this paper was based on the traditional PIT process and the new IMD process to prepare MgB₂ wires.The critical current density of MgB₂ wire was largly enhanced through grapheme doping.The phase evolution,microstructure and plux pinning mechanism have been systematically studied and illustrated.The graphene coated B doping was adopted in MgB₂ bulks to study the microstructure,superconductivity and flux pinning force at the first time.The results indicated that coating method could enhance the density,refine the grain and improve the grain boundary pinning force of MgB₂ bulks,moreover,this method could homogenize the graphene doping in MgB₂ and reduce the birdnesting phenomenon of graphene doping.The coating method could enhance the critical current density of MgB₂ from 1.9×10⁵ A/cm² to 2.5×10⁵ A/cm² in 20 K and 0 T,compared with that of other sample.And the superconductivity of MgB₂ prepared by coating method gets obviously improvement in high field compared with that of pure graphene doping bulk.Secondly,considering the different reacting ability and thermodynamic properties of different elements,we designed the co-doping samples with coating method with graphene coated on Si,Ti,Nb and Dy₂O₃ particles.Graphene was successfully and uniformly coated on these particles and the influence of these additives on the superconducting properties of MgB₂ samples was systematically investigated.We found that the superconductivity got obviously enhanced by graphene coated Nb and graphene coated Dy₂O₃ addition with little T_c decrease.The critical current density got improvement over the entire range of magnetic fields?0-5T?for G coated Nb addition and graphene coated Dy₂O₃ samples of 3.2×10⁵ A/cm²compared to pristine MgB₂ of 1.9×10⁵ A/cm² at 20 K and 0 T.However,the J_c performance of graphene coated Nb addition sample was higher than graphene coated Dy₂O₃ addition sample in high field.We conclude that the grain boundary pinning force was the major pinning force in MgB₂ because the addition of graphene coated Nb could enhance the boundary pinning force,Netherness,the graphene coated nanoscale Dy₂O₃ particles could improve the point pinning force.Moreover,the reaction temperature of 800 ^oC may be not high enough for graphene to decompose and enter into the lattice of MgB₂ to substitute B site as pure carbon sources.The J_c performance of graphene coated Si and Ti addition samples were decreased due to the reaction of Si/Ti and Mg/B.At last,we prepared high density graphene doping MgB₂bulks by high energy ball milling process.This process could promote the growth of the phase and refine the grain size of MgB₂.Furthermore,the defect density,the contact area of reaction,upper critical field and flux pinning force were also improved by this process.The oxide layers of Mg powders and the oxygen-containing functional group of graphene were removed by the process.The J_c values reached to6.4×10⁵ A/cm² at 20 K and 0 T by high-energy ball milling process,6 Times higher than the undoped sample.Graphene doping MgB₂ wires were researched based on the results of MgB₂bulks.Considering the Nb-B diffusion,we designed a Ta/Nb/Cu structure wires,and resolved this problem.In order to reduce the AC loss,hot extrusion process was adopted to fabricate MgB₂ long wires.We successfully fabricated 168-filaments MgB₂ wires.However,the critical current values of those wires were lower and couldn't meet the requirement of pratical application.We analyzed that the PIT process had porosity problem due to the reaction between the Mg and B powders and low intergrain connectivity problem because MgB₂ was a hard ceramic.Both of those problems result in a low critical current of MgB₂ wires.The internal Mg diffusion?IMD?process was a promising method for fabricated MgB₂ wires because it produces a high-density MgB₂ layer with great intergrain connectivity.In this work,we conbined the graphene doping and the IMD process.Eventually,we successfully fabricated 100-m long 6+1 filament G-doped MgB₂ wire using an IMD process.The Young modulus reached to 3.0×10¹00 Pa by graphene doping compared to undoped sample of 2.5×10¹00 Pa.This indicated that the packing density and inter-grain connectivity were improved by graphene doping.The I_c value higher than 530 A was obtained at 4.2 K and 4 T,which was already acceptable for application.The G doped wire had higher irreversible strain of 38%in comparison to the undoped one of 30%after 650 ^oC×2 h annealing.This indicated that the mechanical properties of MgB₂wires could be improved by graphene doping.Then graphene coated Nb powders were added as dopants to fabricate IMD wires.We found that the n value reached to48 by graphene coated Nb addition,compared with 19.1 of undoped sample,which indicated that the stability of the wires under field got improved.The I_c values reached to 341 A at 4.2 K and 6 T,3.6 times higher than the undoped sample.The improved J_c performance of graphene doping MgB₂ bulks were shown in the wires by the IMD process.And a soild foundation was laid for the pratical application of graphene doping wires.