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 graphene (Gr) doping, pre-treatment, Cu co-doping, and sintering temperature on the microstructure and superconducting properties of MgB₂. Key findings reveal that Gr pre-treatment enhances dispersion homogeneity but only minimally affects coated boron powders. Low-temperature sintering with trace Cu facilitates finer grain control; however, Gr shifts the dominant reaction mechanism to the solid-solid (Mg-B) pathway, hindering Mg diffusion and causing incomplete reactions, even at elevated temperatures. System equilibrium necessitates prolonged low-temperature treatment or optimized doping. Crucially, the sample with 5 wt.% Gr + 5 wt.% Cu sintered at 800 °C shows a reduction in the full width at half maximum (FWHM) of the phonon density of states (PDOS) peak. This reduction arises from tensile strain induced by high-quality Gr, which counteracts residual stress and lattice distortion caused by carbon substitution, as indicated by mitigated Tc degradation in Raman and Tc measurements. Contrary to conventional understanding, we propose a primary enhancement mechanism involving the co-growth of MgB₂, Mg-Cu alloy, and amorphous phases on Gr micro-substrates. This co-growth fosters a dense, interconnected, and spatially nested architecture within the MgB₂ matrix, overcoming doping-induced poor intergranular connectivity and preventing low-field performance suppression.