Hierarchical materials: Background and perspectives
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Hierarchical structures of materials: Origins of the idea Starting from steel swords with complex structures to mud and straw-based buildings in Mali to ancient mortars and concrete, efforts to enhance the properties of materials by changing their structures have always been at the center of technological development. Composites, materials consisting of two or more dissimilar constituents with different properties, emerged from such efforts. While the possibilities of composite materials seemed infinite at first, two observations came into focus: (1) the composite constituents have their own, sometimes complex, structures that can in turn be modified, and (2) the potential for improving the properties of composite materials by adding or rearranging reinforcements is often limited—while some properties (e.g., stiffness) can be improved by increasing the volume content of hard reinforcements, other properties (e.g., fracture toughness) degrade. This motivated modifications of structure and properties of composite constituents and their control at several scale levels.1 At the same time, the extraordinary properties of materials found in nature, including wood, nacre, bone, and other biological materials, attracted the interest of the scientific community. It was observed that one of the main sources of such extraordinary properties of biocomposites is their complex hierarchical structure. In his classic paper, Lakes2 summarized
the main ideas of hierarchical materials as a “basis for synthesizing new microstructures, which give rise to enhanced or useful physical properties.” This provided the impetus for the development of new, bioinspired materials based on biomimicry principles.3 With the continuing advances and development of nanotechnology, new possibilities emerged to enable the manufacture of hierarchical materials with constituents modified on the nanoscale.
Biological hierarchical materials: Variety of structures and biomimicking Natural biological materials often demonstrate extraordinary strength, damage resistance, and hardness. For instance, nacreous mollusk shells, which consist of 95% CaCO3 by volume, have double the strength and exhibit a work of fracture that is 3000 times higher than that of monolithic CaCO3.4 Numerous studies have been devoted to the analysis of the sources of such extraordinary properties of biological materials. Several features have been identified, including the staggered brick-and-mortar structure and interlocked platelets of nacre (see Figure 1),5–8 the layered structure with randomly distributed layer thicknesses found in the spicules that provide structural support in certain sea sponges, the functionally graded structures (graded distributions of reinforcement) of bamboo9 and tooth,10 and features of wood such as a cellular multilayered structure and fiber/fibrils with varied distribution
Leon Mishnaevsky Jr., Department of Wind Energy, Technical University of Denmark, Denmark; [email protected] Michael Tsapatsis, Department of Chemical Engineering and Materials Science, University of
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