High-performance carbonaceous absorbers: From heterogeneous absorbents to data-driven metamaterials

Carbon-based materials are a key focus in the advancement of "on-demand" microwave absorbers due to their adjustable electrical conductivity and structure, as well as the presence of surface functional groups and defects that promote various loss mechanisms. Bottom-up strategies to optimize the carbon absorbent phase rely primarily on component hybridization, atomic doping, interface engineering, and the construction of hierarchical structures. However, while these strategies constitute important advancements, they do not extend beyond laboratory settings and remain restricted in scope. Compared to a composite absorber incorporating carbon inclusions within a matrix, greater flexibility in design and property control is achieved, as its adoption has triggered effective medium and homogenization theories for assessing structure-property relations. Metamaterial absorbers are rationally designed composites, resulting from meticulous adjustments in microarchitecture that have recently been accelerated by artificial intelligence (AI)-based algorithms replacing conventional trial-and- error and experimental-based strategies for optimization. These emerging technological routes could also be exploited to add multifunctionality to carbon composite absorbers in actual service environments and to develop the next generation of smart absorbers. This article presents an overview of the achievements, trends, and challenges in these areas from the perspective of composite structures rather than focusing on the individual absorbent phase, a subject that is currently underrepresented in existing literature.