The Study On Fabrication And Element Doping Of MgB₂ Supercondcuting Materials

Since the discovery of superconductivity in 2001, MgB₂ with a critical temperature (T_c) of 39 K has been believed to be a promising candidate for engineering applications in the temperature range of 20K-30K by GM cryocooler. However, compared with the traditional low temperature superconductors, the critical current density (J_c) of MgB₂ is low. Moreover, MgB₂ often exhibits a rapid decrease in J_c with the increase of magnetic field (B). Therefore, the low critical current density and the poor pinning behavior are the key problems that restrict the application of MgB₂ superconductor. In order to improve the critical current density, the effects of raw materials, fabrication process, elemental doping and substitution on the microstructure and superconducting properties of MgB₂ have been investigated in this thesis.Amorphous carbon doping MgB₂-xCx(X=0.00, 0.05, 0.10, 0.20, 0.30) samples were synthesized by solid state reaction method at 750℃ for 2 hours in argon. The microstructure and superconducting properties of all samples were investigated by X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM) and Physical Property Measurement System (PPMS). The results show that a part of doping carbon goes into MgB₂ crystal and the residue at grain boundary of MgB₂. Ultra fine MgB₂ grains can be found in slight carbon doping sample, indicating that the flux pinning effect can be improved. The J_c value of MgB_1.9C_0.1 sample reaches 8×l0⁴A/cm² at 20K, 3T.Effects of hydrolysis on microstructure and superconducting properties were investigated by XRD, SEM and PPMS. The results show that the hydrolysis process can be found in pure and carbon doped MgB₂ samples. A part of carbon doping residue distributes at MgB₂ grain boundary as second phase particles. The stability of MgB₂ in water is improved by carbon doping. The hydrolysis reaction and speed of MgB₂ was suppressed from grain boundary by carbon doping.We choose the Nb/Cu and Fe/Cu composite tube as sheath material and the in-situ PIT for the technique preparation of MgB₂ wires. The results show that the in-situ PIT technique can be used to produce the certain long length MgB₂/Fe/Cu and...