Tuning magneto dielectric synergy of CoNi composites via confined pyrolysis for microwave absorption.

To address the critical challenges of magnetic particle agglomeration and impedance mismatch in metal-organic framework (MOF)-derived microwave absorbers, this study proposes a synergistic strategy integrating bimetallic doping, hollow structural engineering, and confined pyrolysis. A topological transformation from solid ZIF-67 to hollow ZIF-67/CoNi-LDH was achieved via a Ni2+-induced "etching-coprecipitation" mechanism, followed by the encapsulation of a phenolic resin shell to function as a "confined nanoreactor". The experimental results demonstrate that the robust carbon shell derived from the resin effectively suppresses the Ostwald ripening of metal components, resulting in the uniform dispersion of high-density CoNi alloy nanoparticles within a nitrogen-doped carbon matrix and the formation of a hierarchical "pompon-like" core-shell architecture. Benefiting from optimized impedance matching provided by the hollow interior and a multi-scale synergistic loss mechanism comprising atomic lattice defects, microscopic Maxwell-Wagner-Sillars interfacial polarization, and mesoscopic multiple scattering the composite exhibits superior electromagnetic wave absorption capabilities. With a low filler loading of 15 wt%, the optimal sample obtained at 850 °C achieves a remarkable minimum reflection loss of -55.35 dB at an ultrathin thickness of 1.7 mm. This work validates the efficacy of the confined pyrolysis strategy in regulating magneto-dielectric synergy, offering a scalable pathway for next-generation lightweight microwave absorbers.