Synthesis and applications of layered structural composites inspired by nacre

Abstract

Nacre is a typical inorganic/organic composite produced by some mollusks as an inner shell layer of their armor systems to protect themselves from predators and other mechanical forces, which exhibits excellent mechanical performance, including high strength and high toughness at the same time. Nacre can also make up the outer coating of pearl to make which high iridescence. Nacre is composed of large fraction of inorganic aragonite calcium carbonate (95vol%) and small fraction of organic biopolymers (5vol%) with highly ordered hierarchical arrangement which is usually called as “brick and mortar” structure. Although nacre mostly consists of a fragile mineral, its toughness is three orders of magnitude higher than that of calcium carbonate in the form of aragonite. The excellent mechanical properties of nacre are highly related to its unique hierarchical structure. In this review, the hierarchical structure of nacre and its strengthening and toughening mechanisms are described from different length scale in detail. Lots of researches have demonstrated that mechanisms such as plastic deformation at the crack tip, crack deflection, crack blunting, pull-out at micro scale and nanoasperities on the tablet surface, mineral bridges, viscoplastic deformation of the organic material at nano scale greatly dissipate energy to make natural nacre stronger and tougher. Besides, recent advances in nacre-inspired artificial materials are summarized here based on the classification of inorganic building blocks including ceramic, clay, graphene and calcium carbonate. Ceramics are always used as the inorganic building blocks in artificial nacre owing to its high strength, high stiffness, low density and low cost, which can improve the application potential of such materials. Clay is chosen here due to its wide distribution and original plate structure, which makes artificial nacre easier to synthesis. Graphene show extremely high strength and modulus, up to 130 and 1000 GPa respectively, thus graphene is chosen to fabricate artificial nacre to obtain ultra-high performance layer composites. Calcium carbonate is the same component as the inorganic plate in nacre, so calcium carbonate based artificial nacre can not only mimic brick and mortar structure of nacre, but also mimic the components of nacre, which could fully take advantages of nacre. All the artificial nacre mentioned above show excellent mechanical properties, including high strength (up to 500 MPa) and toughness (up to 35 MPa m1/2) at the same time. What is more, the applications of nacre-inspired materials, such as light weight and high strength, fire retardant, gas barrier, sensors and supercapacitors are discussed. Owing to the ordered hierarchical structure of nacre-inspired materials, most of such materials show higher performance than the same materials with no nacre-like inner structure, which could satisfy the demand of industrial manufacture. Finally, the advantages and challenges during the design and fabrication of nacre-inspired artificial materials are presented for the improvement of new generation nacre-inspired artificial materials. Although countless nacre-inspired artificial materials have been successfully synthesized and the theoretical model of nacre is relatively mature, there still are some challenges in promoting the nacre-inspired artificial materials’ performance, such as how to improve the fraction of inorganic component in artificial materials with ordered inner brick and mortar structure up to 95vol%, just like the natural nacre; how to mimic the nacre form multi-scale; how to enhance the interface effect in artificial materials; how to fabricate large scale nacre-inspired materials under control and so on. It is clear that continuous researches of nacre and nacre-inspired materials are important for both scientists and engineers.