Study of MgO formation and structural defects in in-situ processed MgB2/Fe wires
We fabricated MgB2/Fe wire by a powder-in-tube (PIT) technique, using an in situ process. All wire samples were sintered for 30 min at different sintering temperatures ranging from 650 to 1000 °C. We found strong correlations among crystallinity, critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and microstructures for all MgB2/Fe wires. We observed that the sample with the lowest sintering temperature, ~ 650 °C, had a larger lattice strain, Jc, change in resistivity ∆ρ(ρ300 K - ρ40 K), Hirr, and Hc2, but a lower density and residual resistivity ratio (RRR). Based on the relationships between all these superconducting and microstructure parameters, grain boundaries are likely to be acting as the predominant pinning centers for MgB2, so grain growth of MgB2 corresponds to a reduction of effective pinning. It should be noted that changes in the MgO fraction within the MgB2 matrix were almost independent of the sintering temperature. This indicates that most MgO may be coming from the starting material.