Green Strategy for\na Large-Format, Superhard, and\nInsulated Electromagnetic Wave Absorber Inspired by a Natural Feature\nof a Conch Shell

Due to the intensification of electromagnetic pollution\nand energy\nshortages, there is an urgent need for multifunctional composites\nthat can absorb electromagnetic waves and provide insulation. However,\ndeveloping low-cost electromagnetic wave-absorbing composites that\nare lightweight, high strength, heat-insulating, and large-format\nfor special environments remains challenging. Inspired by the conch\nshell, this article proposes a green strategy of hydration recrystallization\nself-assembly. Highly biologically active hydroxyapatite (HAP) was\nused to lock in free water to prevent porous carbon fibers from absorbing\na large amount of water. Meanwhile, HAP underwent ion exchange and\nrecombined with hydrated crystals of magnesium oxychloride to form\na gelatinous HAP-5 phase crystal. The cementitious HAP-5 phase crystal\nwas interwoven and interlocked with the support skeleton carbon fibers\nand metal Ni powder to form conch shell composites (Bio-CSC) with\nmultiple interfaces via electrostatic adsorption and metal complexation.\nThis strategy utilized inorganic substances as bridges to uniformly\ndisperse conductive materials such as carbon fibers to construct a\nconductive network with an enriched interface polarization. The prepared\nBio-CSC was composed of multiple heterogeneous interfaces and was\nlightweight and high strength, with a specific strength increase of\n300%. It also provided excellent thermal insulation and electromagnetic\nwave absorption. Its thermal conductivity was 0.071 W·m<sup>–1</sup>·k<sup>–1</sup>, and the lowest RL<sub>min</sub> value\nof −21.88 dB, with a matching thickness of only 1.2 mm. The\ncomposites in this study overcame the limitations of traditional absorption\nmaterials such as high magnetism and single function and may be used\nin fields such as building energy conservation and electromagnetic\nsafety.