Second-phase segregation and micro strain/lattice parameter dependent transition temperature in polycrystalline MgB₂

Un-doped, metal-doped, and carbon-doped MgB₂ samples were prepared by furnace cooling and quenching to investigate the second phase behavior and the resultant critical current density J _c performance under different heat treatment processes, which is infrequently mentioned, and to explore the strain/lattice parameter dependence of the superconducting transition temperature. To release the residual stress, quenching induced second-phase segregation in these MgB₂ samples shows a negative effect on the J _c. Nevertheless, the dislocations and the lattice distortion assisted the enhancement of the high-field J _c in the un-doped and metal-doped MgB₂ samples, which indicated that quenching could be technically applied for the fabrication of metal-sheathed MgB₂ wires and tapes to obtain excellent J _c. After evaluating the micro strain and the lattice parameters' (c and a for hexagonal lattice) variation, a dome was observed in the illustration of the strain/lattice parameter c/a dependence of T _c, which differed from the reported linear relation in previous work. This suggests that the c/a ratio and the strain may be the predominant parameters for scaling of the superconducting dome width in the superconducting phase diagram of MgB₂.