The effect of nano-powder additions on the superconducting properties of MgB₂

To investigate the effect of Ag or SiC nano-powder additions on the superconducting properties of MgB₂, a series of superconducting (Ag)_(x)wt.-%(MgB₂)_(100-x)wt.-%(Ag_x - MgB₂) and (SiC)_(x)wt.-%(MgB₂) _(100-x)wt.-%((SiC)_x - MgB₂)(0 ≤ x ≤ 20), containing Ag and SiC nano-powders, respectively, of different diameters (30 nm and 130 nm), were prepared by a simple solid-state reaction route, cold-pressed into a pellet form and investigated. To maintain the same environment of MgB₂/stainless-steel tapes/wires, Ag_x - MgB₂ and (SiC)_x - MgB₂ pellets made out of the mixed powders were put inside stainless steel tubes and then sintered at 900°C for two hours in Ar atmosphere. Characterization performed included both X-ray diffraction and magnetization. No impurity phase was identified for as-rolled samples. However, both the Ag_x - MgB₂ and (SiC) _x - MgB₂ composite pellets, when sintered, contain various impurity phases. The isothermal magnetizations M(H) of a series of samples were measured at temperatures between 5 and 50 K in fields up to 5 T, using a PPMS-9 (Quantum Design). The optimal amounts of Ag and SiC nano-powder in Ag _x -MgB₂ and (SiC)_x - MgB₂ to obtain the largest flux pinning effect are ∼8 and ∼4 wt.-%, respectively. The "two-step" structures in ZFC M(T) curves of (SiC)_x - MgB₂ were more developed than Ag_x - MgB₂. The best flux pinning centers can be created by adding a suitable size and amount of SiC nano-powder, not too large to increase the decoupling between the MgB ₂ grains.