A nacre-inspired structural material with thermochromic properties and mechanical robustness by atomic-level design.

Advanced structural materials are often required to exhibit a combination of light weight, high strength and superior toughness. Biomimetic strategies hold promise for achieving these seemingly conflicting mechanical properties simultaneously. However, current biomimetic structural materials lack active fire-warning and passive flame-retardant functionalities, which poses risks for their application in fire-prone scenarios. Herein, we present a nacre-mimetic alumina-cyanate resin composite (NAC) that has a combination of mechanical robustness with thermochromic and flame-retardant properties. Through controlled atomic doping, chromium atoms are incorporated into alumina microplatelets, forming solid-solution assembly units that exhibit reversible thermochromism and a solid-solution-strengthening effect. The bioinspired 'brick-and-mortar' structure endows the NAC with high strength (∼290.1 MPa) and fracture toughness (∼11.1 MPa m^1/2). Coupled with a machine-learning-based image-recognition system, the NAC leverages its thermochromic properties to deliver a rapid fire warning within 9 s at 250°C, which is significantly faster than traditional electronic fire alarms. Its layered structure effectively impedes oxygen flow, achieving an oxygen-limiting index of 50%, and thus ensuring excellent flame-retardant performance. This design delays the combustion peak and reduces the heat-release value, thereby enhancing the flame-retardant performance. This work demonstrates the effective integration of a structural and functional design for active early fire warning and passive flame retardancy, paving the way for structural materials in advanced fire-warning systems in challenging environments.