Removal of MgO and enhancement of critical current density in urea-doped MgB₂ bulks by melting impregnation method

Bulk MgB₂ samples with 2 wt% urea have been prepared by melting impregnation method at 800 °C. Based on the advantage of MgO dissolution from urea, the size of MgO particles was significantly reduced (~10 nm) by the melting impregnation method to realize more efficient pinning effect than conventional urea doping. As the content of such MgO pinning centers inevitably decreased to be less than 2.3 vol%, the critical current density of the melting impregnated sample was enhanced (5.0 × 10³ A cm⁻², 20 K and 3 T) over the entire field, in contrast with the un-doped sample. This is on the other hand attributed to the C substitution for B and nano-sized amorphous regions within the MgB₂ grains, which brought extra pinning effects. Instead of dissolving the edge of the MgB₂ grains, the dispersed urea by impregnation method mainly corroded the interior of the MgB₂ grains to introduce such amorphous defects for excellent superconducting properties.