Green Strategy for a Large-Format, Superhard, and Insulated Electromagnetic Wave Absorber Inspired by a Natural Feature of a Conch Shell

Due to the intensification of electromagnetic pollution and energy shortages, there is an urgent need for multifunctional composites that can absorb electromagnetic waves and provide insulation. However, developing low-cost electromagnetic wave-absorbing composites that are lightweight, high strength, heat-insulating, and large-format for special environments remains challenging. Inspired by the conch shell, this article proposes a green strategy of hydration recrystallization self-assembly. Highly biologically active hydroxyapatite (HAP) was used to lock in free water to prevent porous carbon fibers from absorbing a large amount of water. Meanwhile, HAP underwent ion exchange and recombined with hydrated crystals of magnesium oxychloride to form a gelatinous HAP-5 phase crystal. The cementitious HAP-5 phase crystal was interwoven and interlocked with the support skeleton carbon fibers and metal Ni powder to form conch shell composites (Bio-CSC) with multiple interfaces via electrostatic adsorption and metal complexation. This strategy utilized inorganic substances as bridges to uniformly disperse conductive materials such as carbon fibers to construct a conductive network with an enriched interface polarization. The prepared Bio-CSC was composed of multiple heterogeneous interfaces and was lightweight and high strength, with a specific strength increase of 300%. It also provided excellent thermal insulation and electromagnetic wave absorption. Its thermal conductivity was 0.071 W·m^-1·k^-1, and the lowest RL_min value of -21.88 dB, with a matching thickness of only 1.2 mm. The composites in this study overcame the limitations of traditional absorption materials such as high magnetism and single function and may be used in fields such as building energy conservation and electromagnetic safety.