Synergistically enhanced heat conductivity-microwave absorption capabilities of g-C3N4@Fe@C hollow micro-polyhedra via interface and composition modulation

g-C3N4@Fe@C hollow micro-polyhedra (HMPs) were synthesized by a facile salt-template-guided freeze-drying-calcining and pyrolysis route, exhibiting the efficient heat conductivity-microwave absorption but electric insulation properties. [Display omitted] •A salt-template-assisted freeze-drying calcination and pyrolysis strategy for g-C3N4@Fe@C hollow micro-polyhedra.•Modulating the interfaces and compositions of the g-C3N4@Fe@C HMPs.•Investigating the interface- and composition-dependent heat conductivity, electrical conductivity, and EM wave absorption properties.•Revealing the synergistic enhancement mechanism of thermal conductivity and EM wave absorption. Heat-conducting, microwave absorption-electric insulation integrated materials (HCMWAEIIMs) have received considerable attention with wide application of miniaturized and highly integrated modern electronics. Currently, the synergistic enhancement of heat conductivity, microwave absorption, and electrical insulation is restricted by some inevitable contradictions among them. In response to this issue, the g-C3N4@Fe@C HMPs were first synthesized as advanced HCMWAEIIMs via a salt-template-assisted freeze-drying calcination and pyrolysis route. The interfaces and composition of the g-C3N4@Fe@C HMPs can be facilely modulated via controlling just the Fe(CO)5 vol Controlling the interfaces and composition of the g-C3N4@Fe@C HMPs can efficiently adjust their thermal, electrical, and microwave-absorbing properties. Inlaying g-C3N4 HMP surfaces with Fe@C CSNPs can not only improve their permeability and matching capabilities but also create an effective phonon transfer route for the enhancement of the heat conductivity and electrical conductivity. Results show that the g-C3N4@Fe@C HMPs have higher heat conductivity (1.81 W/(m⋅K)), better electric insulation (0.04852 S/m), larger ABW/d values (3.58 ∼ 4.48 GHz/mm), higher absorption (-41.18 ∼ -51.62 dB), and thinner films (1.8 ∼ 1.9 mm) than most of previously reported materials. Overall, our work provides a simple and effective strategy for designing advanced multifunctional fillers with potential applications in modern electronics.