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

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<i>T</i>_cdegradation observed in Raman and<i>T</i>_cmeasurements. 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 MgB₂matrix is the key point to overcome the doping-induced poor intergranular connectivity and prevent the low-field performance suppression.