High critical current density of MgB₂ bulk superconductor doped with Ti and sintered at ambient pressure

Ti-doped MgB₂ superconductors with different doping levels were prepared by solid-state reaction at ambient pressure. The density, diamagnetic signal, and J_c of the samples change significantly with the doping level, with the best result achieved at cursive chi = 0.1. At 5 K, the J_c reaches 2 × 10⁶ A/cm² in the self-field and 5 × 10⁴ A/cm² in 5 T. At 20 K, the J_c is still as high as 1.3 × 10⁶ A/cm² in the self-field and 9.4 × 10⁴ A/cm² in 2 T. It is observed that partial melting occurs in the Ti-doped samples, resulting in an excellent grain connection and extremely high density. In addition, some fine particles (with sizes from 10 to 100 nm) of the second phases induced by Ti doping are distributed in the MgB₂ matrix, and this may play an important role in flux pinning enhancement.