Percolative nature of current transport in polycrystalline MgB2 wires

• Malic acid doping of MgB2 strongly enhances the in-field critical current up to 20 K.
• The transport critical current behaviour is explained well by the percolation model.
• The high-field property can be attributed to the impurity scattering and anisotropy.
• In contrast, the pinning force maximum mainly affects the low-field performance.

The percolation model, taking into account the percolative nature of current transport due to anisotropy, is a powerful tool for predicting the field and temperature dependences of critical current in MgB2 superconducting wires. The critical current behaviour measured from MgB2 wires can be obviously explained by only four fitting parameters, the anisotropy parameter, the pinning force maximum, the upper critical field along the ab-plane, and the percolation threshold. Moreover, the temperature dependence of the upper critical field is further explained by the dirty-limit two-gap theory.