Balancing the trade-off in malic acid-doped MgB2: The competition between secondary phase pinning and porosity-induced degradation

Organic acid doping can enhance the current-carrying capacity of MgB 2 superconductors but may induce cracking and fracture due to microstructural degradation. In this work, MgB 2 bulks with different malic acid doping levels were investigated by combining experimental characterization with cohesive zone model-based RVE simulations to clarify the competition between flux-pinning enhancement and porosity-induced degradation. Increasing malic acid content promoted carbon substitution and lattice distortion, as indicated by the contraction of the MgB 2 lattice parameter a from 3.083 to 3.074 Å and the increase in lattice strain from 0.00202 to 0.00263. The 10 wt% sample exhibited the highest magnetic critical current density among the doped samples, reaching 1.9 × 10 4  A cm -2 at 20 K and 2 T. However, the porosity increased from approximately 6-7% at low and intermediate doping levels to 11% at 10 wt%, leading to stronger stress concentration and intergranular damage. The grain-boundary damage variable D GB decreased to 0.273 at 5 wt% but increased to 0.436 at 10 wt%, indicating a transition from second-phase-assisted crack suppression to porosity-dominated degradation. These findings provide a theoretical basis for managing sintering-induced damage in MgB 2 superconductors, highlighting the balance between performance enhancement and structural integrity during organic doping.