Honeycomb sandwich structures (HSSs) are excellent candidates for light and efficient microwave-absorbing materials. In this work, we design an HSS using SiO2 fiber-reinforced epoxy resin (SiO2f/ER) composites as both the top and bottom layers to improve the impedance matching with free space. Target dielectric properties of the honeycomb and coated lossy material of the HSS were calculated based on the multilayer transmission line theory, metal backplane model, and homogenization theory. In addition, the interface effect between the SiO2f/ER and honeycomb of the HSS was discussed theoretically, experimentally, and numerically, indicating a 1–4% contribution of microwave absorption resulting from the interface. By analyzing the equivalent resistance, equivalent capacitance, as well as equivalent inductance, the enhanced microwave absorption of HSS is attributed to the formation of the interfacial transition zone, which benefits both impedance matching and electromagnetic loss.
Split-ring resonators are excellent left-handed metamaterials for significant electromagnetic coupling behavior. In this work, a split-ring resonator prepared with Ni-doped zeolitic imidazolate framework-67/epoxy resin (ZIF-67/ER) was embedded in the top layer to optimize microwave absorption efficiency in the 2–4 GHz frequency band. The Ni-doped ZIF-67/epoxy resin served as the bottom layer to improve microwave absorption efficiency in the 4–8 GHz frequency band. Honeycomb with a conductive carbon black coating served as the middle layer to generate electromagnetic loss for the overall frequency band. Based on the composite structure integration technology, RL < −10 dB was realized under the oblique incidence of 0–70 degrees. Both simulation and experiments indicate that a split-ring resonator made of lossy material can be an effective strategy to broaden the effective absorption bandwidth and increase the corresponding structure’s insensitivity to polarization and the incidence angle of microwave.
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