Research On Microstructure Control And Mechanical Properties Of MgB₂ Matrix Composites

The lack of effective flux pinning center in MgB₂ superconductor restricts its application in the field of high current carrying.In order to solve this problem,a simple and effective way is to improve the current carrying performance of MgB₂ by chemical doping.Many kinds of carbon materials with different structures have become one of the research hotspots at home and abroad.Graphene,carbon nanotubes and carbon compounds can be doped with carbon as additives.However,according to the current reports,the literature mostly uses one-dimensional and two-dimensional carbon materials as MgB₂ additives,but there are few reports on the doping of three-dimensional carbon materials.Based on this,in order to improve the grain connectivity of MgB₂ and to introduce an effective flux pinning center,three dimensional carbon（3D-C）materials were synthesized by spray drying method from the perspective of morphology design,and two kinds of three-dimensional carbon composites were synthesized by freeze drying method[three dimensional carbon loaded cuprous selenide（3D-C@Cu₂Se）、Three dimensional carbon loaded nickel（3D-C@Ni）],respectively study their action mechanism on MgB₂.The research methods mainly include differential thermal analysis,X-ray diffraction,scanning electron microscopy and magnetic measurement system,which are used to determine the phase composition,micro morphology and superconducting properties of the composite samples.As a practical functional material,the research on the mechanical properties of MgB₂ is also very important.In this paper,the indentation mechanics of MgB₂ is studied with the help of nano indentation instrument.The relationship between the mechanical properties and micro characteristics of MgB₂ is established through the load displacement curve recorded by the instrument in real time;In addition,graphene itself has excellent mechanical properties.This paper attempts to improve the mechanical properties of MgB₂ by doping graphene.The results show that 3D-C can release more active C after its structure is destroyed at high temperature,and can enter the MgB₂ lattice to replace the B-site.Finally,effective carbon doping is formed,which significantly improves the critical current density（J_c）of MgB₂ in high field.The impurity scattering caused by carbon doping increases the irreversible field value(H_irr)of the sample,in which 4.8 T of the sample with doping molar amount of 0.1 is significantly higher than 4.1 T of pure MgB₂.Doping 3D-C@Cu₂Se or 3D-C@Ni improves the grain connectivity of MgB₂,which is due to the reduction of Mg volatilization and the increase of sample density under the synergistic effect of 3D-C providing nucleation sites and Cu₂Se or Ni activating phase formation by low temperature sintering.According to the flux pinning model,it is determined that there are both grain boundary pinning and point pinning in the samples.It is the effective introduction of pinning particles that further enhances the flux pinning ability of MgB₂.Due to the Ni in 3D-C@Ni is ferromagnetic,which makes it not obvious to improve the superconductivity of MgB₂.Doping 3D-C@Cu₂Se significantly improves the J_c of the sample in the whole magnetic field range.When MgB₂ resists the external force of the indenter,the nonuniformity of its microstructure makes the reproducibility of indentation data worse.The hardness of MgB₂shows size effect,that is,the hardness decreases gradually with the increase of indentation depth.The creep rate sensitivity index increases with the increase of indentation depth,which is the result of the acceleration of dislocation slip velocity.Graphene is easy to agglomerate after doping,which is not evenly distributed in the sample.Therefore,graphene can not be used as an effective reinforcing phase to improve the resistance of MgB₂ microstructure to external force.