Preparation And Superconducting Properties Of Doped MgB₂ Using Different Carbon Sources

The critical current density J_cis the crucial parameter for realizing the industrial application of MgB₂superconductor.Improving J_cof MgB₂under field at high temperature（10～20 K）has always been the focus of scientific researchers.Carbon doping has been proved to be the most convenient and effective way to improve the J_cunder high field.In addition,grain boundaries are believed to act as strong flux pinning centers indicating the high J_cvalue can be achieved in the nano-structured MgB₂.Based on the above two points,C₆H₆,nano-Si C and g-C₃N₄were selected as carbon sources,and nanocrystalline carbon doped MgB₂bulks have been synthesized successfully by combining the high energy ball milling and high temperature and high pressure techniques in this thesis.The effects of carbon source doping on the microstructure and superconducting properties of MgB₂have been discussed and analyzed systematically.The related contents and conclusions were as follows:C₆H₆,Mg powders and B powders were mixed and then milled for the times ranging from 5 to 90 h.The milled powders were sintered into C₆H₆doped MgB₂bulks under ambient pressure and at temperatures ranging from 650℃to 850℃.The effects of different doping levels of C₆H₆on the microstructures of MgB₂were studied,and pinning mechanisms were analyzed.It turns out that the value of J_cstill remains at 10⁵A/cm²orders of magnitude at 5.5 T and 10 K,3.8 T and 15 K,1.9 T and 20 K,respectively.Grain boundary pinning is dominant under the medium and high field.Nano-Si C doped MgB₂bulks have been prepared at the temperatures ranging from650℃to 850℃ for 1～5 h under ambient pressure using nano-Si C,Mg and B powders as raw materials.Then the bulks were ball milled for 60～200 h to obtain nanocrystalline powders.Finally,the nanocrystalline powders were sintered at the temperature ranging from 600℃to 900℃ under the pressure from 3 to 5 GPa to obtain nanocrystalline Si C doped MgB₂bulks.The microstructures,superconducting properties and pinning mechanisms were analyzed.The results indicate that the nanocrystalline Si C-doped bulk with an average grain size of 25 nm prepared at 5 GPa and 900℃ exhibites J_cof 10⁵A/cm²at zero field and at 5～20 K,which is due to high densification and good grain connectivity of the samples.Under the medium and high magnetic field,flux pinning mechanism is mainly from grain boundary pinning at the temperatures of 10 K and 15 K.Similar preparation methods were used to obtain the nanocrystalline g-C₃N₄doped MgB₂bulks.The g-C₃N₄doped bulk with an average grain size of 30 nm prepared at 3GPa and 800℃ exhibites that the J_cis as high as 10⁵A/cm²at 15 K and 3.7 T compared to Si C-doped MgB₂.At 20 K,J_cremains at 10⁵A/cm²orders of magnitude under higher field than that of Si C doped MgB₂.There are many pinning mechanisms for nanocrystalline g-C₃N₄doped MgB₂bulks under the medium and high magnetic field.