Bifunctional materials possessing both high electrical conductivity and thermal conductivity hold promise for integrating electromagnetic wave (EMW) absorption with thermal management capabilities, thereby addressing signal crosstalk and heat accumulation issues in integrated electronic devices. However, the opposing effects of percolation phenomena on thermal conduction and microwave absorption hinder the integration of these properties. Herein, diospyros cauliflora-shaped CF@PDA@Fe₃O₄ (MCF) was synthesized via a solvothermal method. The introduced heterogeneous interfaces enhance EMW absorption while impeding charge transport between adjacent carbon fibers (CFs), thereby suppressing percolation effects. Subsequently, magnetic-field-induced alignment of MCFs constructs thermally conductive pathways along the temperature gradient direction. By streamlining heat transfer routes and reducing filler-matrix interfaces, thermal conductivity is significantly enhanced. When the mass fraction of MCF is 20 wt.%, the composite achieves an effective absorption bandwidth of 4.2 GHz and a minimal reflection loss of −49.77 dB, while its thermal conductivity increases by 400% compared to pure polydimethylsiloxane (PDMS). This study proposes a synergistic strategy to concurrently enhance thermal conductivity and EMW absorption in composites, offering a new pathway for developing electronic packaging materials with efficient heat dissipation and broadband EMW absorption.

Understanding the microstructure-property relationship from the microscopic and macroscopic perspectives, instead of semi-empirical rules, can facilitate the design of microcosmic morphology to adjust the impedance matching and dielectric loss of the carbon-based materials, which are still lacking so far. In this study, a clear correlation between microstructure and conduction loss was revealed in agarose-derived carbon using a facile salt-etching strategy, in which ferric nitrate acted more as a morphology modifier for bulky carbon rather than a component regulator. Specifically, with the increasing amount of ferric nitrate, the original smooth bulky carbon was etched with caves, which gradually enlarged in size and depth and thus thinned in wall, and eventually transformed into a three-dimensional (3D) interconnected cellular structure, accompanied by a gradual increase in conductivity. Benefiting from the optimal impedance matching and strong conduction loss originating from the unique 3D cellular structure of agarose-derived carbon, AF-3 exhibited super-wide and strong absorption with an effective absorption bandwidth of 7.28 GHz (10.32–17.60 GHz, 2.9 mm) and a minimum reflection loss of −46.6 dB (15.6 GHz, 2.5 mm). This study establishes the relationship between microstructure, dielectric properties, and loss mechanism in carbon-based materials and also provides a new insight into the fine modulation of EMW-absorbing properties from morphological design.