Natural wood-based metamaterials for highly efficient microwave absorption

Abstract While wood substances that contain water have shown a dielectric effect, they have never been directly proposed as a microwave absorption material to reduce artificial electromagnetic pollution. In this study, the microwave absorption efficiency of wood that contains water was investigated based on its dielectric properties in the microwave frequency range to optimize the parameters of wood unit cells, including the orientation, moisture content (MC) and thickness. Subsequently, inspired by the design concept of electromagnetic metamaterials, single-layer wood unit cells containing moisture were directly integrated into double epoxy resin layers. This procedure formed a natural wood-based metamaterial (NWM) with a simple periodic Sandwich structure without the need for further chemical modification and/or high energy consuming processes. Such design strategies allowed the NWMs to overcome the limitation of inherent dielectric properties of natural wood and present significantly enhanced microwave absorption performance, as well as different absorption behavior. NWMs at a MC ≤ 70% displayed a selective absorption mode, while NWMs at MC ≥ 85% showed a broadband absorption mode. Both absorption modes could achieve the peak absorptivity > 98%. Particularly, the NWM in the broadband absorption mode possessed an effective absorption (absorptivity > 90%) bandwidth of 9.04 GHz, which was 7.6 times that of natural wood (1.19 GHz). The NWMs performed well when irradiated by microwaves with different angles and directions. Also, the thickness of NWMs was only 7 mm, allowing easy incorporation of the materials in engineering designs. The use of sustainable materials, impressive performance, high stability, practical thickness, and the facile and cost-effective production technique demonstrated NWMs with great potential in designing green buildings.