Nano-silver addition to bulk MgTi0.06B2 superconductors: Rietveld phase analysis, microstructure and enhanced flux pinning

Bulk MgTi 0.06 B 2 samples containing 0, 1 and 2 wt% nano-silver (denoted Ag-0, Ag-1 and Ag-2) were prepared by in-situ solid-state reaction at 775 °C for 2 h under vacuum, following 30 min of planetary milling at 1000 rpm and uniaxial pressing at 205 MPa. Rietveld refinement of the X-ray diffraction patterns gives MgB 2 as the majority phase in every sample, with the refined MgB 2 lattice parameters unchanged across the series to within 0.0022 Å in a (0.07%) and 0.0009 Å in c (0.03%), and silver accommodated as a distinct cubic Ag 3 Mg phase at 1.05 wt% in Ag-1 and 2.20 wt% in Ag-2, confirming that silver does not substitute into the MgB 2 lattice. Silver addition raises the relative density of the sintered compact from 50.9% to 67.7% and 67.3% of theoretical. Magnetisation loops measured to ±5 T give, at 5 K and 1 T, a Bean-model critical current density of 9.2 × 10 3 A cm −2 for the undoped sample, rising to 1.65 × 10 4 and 1.92 × 10 4 A cm −2 at 1 and 2 wt% Ag — gains of 78% and 108%. The gain increases with applied field, reaching 105% and 197% at 4 T. The maximum bulk pinning force at 5 K rises from 0.13 to 0.28 GN m −3 and its position shifts from 2.08 to 2.26 T, while the irreversibility field at 15 K increases from 4.20 to 4.81 T. Resistive measurements show the mid-point critical temperature falling from 38.4 to 37.8 K and the transition broadening from 0.66 to 1.23 K. Normalised pinning-force curves for all three samples collapse onto a single function peaking at h = H/H irr ≈ 0.42, indicating that silver raises the density of effective pinning sites and improves intergranular connectivity without altering the dominant pinning mechanism.