Influence Of Ball Milling On The Sintering Process And Superconductivity Of Graphite-doped MgB₂

Since the discovery of MgB₂superconductor in2001,it is extremely popularbecause of simple synthesis process and low cost. But it cannot meet the needs of theapplication because of worse carrying current capacity. Most of the researchers paidtheir attention to the issue how to improve the critical current density. Althoughcarbon doping has a positive effect on the improvement of the current-carryingcapacity in previous study, the effective doping content remains limited in the sampleprepared by conventional doping methods. Since ball milling is a typical technique forthe grain refinement and the enhancement of original powder activity, we expected toimprove the C doping level by this technique. On the other hand, graphite has aspecial electronic structure, and has the functions of grain refinement and thecontributions for sintering. Therefore, graphite was used as the carbon source in ourstudy. First, B and graphite milled together were studied. Based on this, Mg powderswere also mixed with B and graphite using ball milling technique. Finally, the criticalcurrent density was significantly improved by controlling the content and size of MgO.The conclusions were given as follows.However for B+3%C or Mg+B+3%C system, the ball milling can refinecrystalline grain and improve sintering density. In the meantime, ball milling couldpromote C doping to a great extent. More carbon entrance lattice and replace B, sothis cause lattice distortion. This can support effective pinning enter for MgB₂.Ball milling origin powder gives great influence on sintering process. ForB+3%C system, there is a layer B₂O₃, resulting in a limitation for the solid-solidreaction, as well as the acceleration of liquid-solid reaction. For Mg+B+3%C system,Mg powders were also ball milled with B and graphite aiming at promoting thesolid-solid reaction of Mg and B by refining grain particle and improving thechemical activity. Additionly, ball milling drop the temperature beginning reaction ofMg with B. MgO with relatively large particle size distributed at the grain boundaryin the sample sintered from ball milled B and graphite, which would weaken theintercrystalline connectivity. However, lots of homogenously distributed MgOnanoimpurities are observed in the MgB₂grain and can serve as effective pinningcenter for Mg+B+3%C system. The superconducting properties of the samples were measured by the physicalproperty measurement system. The critical current density at high field wassignificantly enhanced, although it was slightly decreased at low field in Mg+B+3%Cmilling system. By identifying the phase composition and observing themicrostructure, it was believed that the superconducting properties would beimproved owing to the effective C doping level, low crystallinity, controlling the sizeand distribution of MgO.