Reversing the doping paradox: graphene spatial nesting enhances intergranular connectivity while inducing minimal transition temperature degradation in MgB₂ superconductors

Abstract This study systematically investigates the influence of pre-treated graphene doping on the microstructure and superconducting properties of MgB₂, focusing on the effects of pretreatment processes, doping content (graphene and Cu), and sintering temperature. Key findings reveal that graphene pretreatment enhances dispersion homogeneity but has a limited impact on coated boron powders. Low-temperature sintering with trace Cu enables finer grain control, though graphene shifts the dominant reaction mechanism to solid–solid (Mg–B), impeding Mg diffusion and causing incomplete reactions even at high temperatures. System equilibrium requires prolonged low-temperature treatment or optimized doping. Crucially, the 5 wt.% Gr + 5 wt.% Cu sample, sintered at 800 °C, exhibits a decrease in the full width at half maximum of the phonon density of states peak. This arises from the tensile strain induced by high-quality graphene that counters residual stress and lattice distortion from carbon substitution, evidenced by mitigating T _c degradation observed in Raman and T _c measurements. In order to illustrate the results in contradiction to the conventional theory, a primary enhancement mechanism is proposed in the present work which is relevant to the co-growth of MgB₂, Mg–Cu alloy, and amorphous phases on graphene micro-substrates. It is indicated that a dense, interconnected, spatially nested architecture within the MgB₂ matrix is the key point to overcome the doping-induced poor intergranular connectivity and prevent the low-field performance suppression.