Cellular Solids
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Cellular Solids L.J. Gibson, Guest Editor Abstract This brief article describes the content of this issue of MRS Bulletin on Cellular Solids. Cork, wood, sponge, and bone are all examples of cellular solids in nature. Engineered honeycombs and foams are now made from polymers, metals, ceramics, and glasses, and their structure gives them unique properties that can be exploited in a variety of applications. The articles in this issue provide an overview of the fabrication, structure, properties, and applications of such porous solids as cellular ceramics, aluminum and other metallic foams, and scaffolds for tissue engineering, as well as discussions of techniques for understanding, modeling, and measuring their behavior and properties. Keywords: cellular solids, foams, honeycombs.
Many materials have a cellular structure: an assembly of prismatic or polyhedral cells with solid edges and faces packed together to fill space. Cork, wood, sponge, and trabecular bone are all examples of cellular solids in nature (Figure 1). Engineered honeycombs and foams are now made from polymers, metals, ceramics, and glasses (Figure 2). Their cellular structure gives them unique properties that are exploited in a variety of applications. Their light weight makes them attractive for the cores of structural sandwich panels in products ranging from downhill skis to lightweight building panels. In compression, cellular solids can withstand large strains at nearly constant stress, allowing them to absorb the kinetic energy of an impact without generating high peak stresses. For this reason, they are often used in energy-absorption devices such as helmets and automobile bumpers. Closed-cell foams can be made with low-conductivity gases that remain trapped inside the cells, making these foams excellent materials for thermal insulation. Open-cell metallic foams, with their high thermal conductivity and interconnected pores allowing fluid flow, are used in heat-exchange devices. The interconnected porosity of opencell foams is also exploited in their use as filters. Porous scaffolds used in tissue engineering can be considered open-cell foams; their interconnected porosity is essential for cells to penetrate the scaffold and migrate through it. The structure of cellular solids has been studied since the 1660s, when Robert Hooke examined a section of cork in his microscope1 and first used the term “cell” to describe its structure. Sir William Thomson
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(later Lord Kelvin) identified the spacefilling unit cell that minimizes surface area per unit volume as a tetrakaidecahedron with slightly curved faces.2 Recently, Weaire and Phelan3 identified a unit cell of even lower surface area per unit volume, composed of six 14-sided cells and two 12-sided cells. Both are described in Kraynik’s article on foam structure in this issue of MRS Bulletin. Today, computer software for generating foam structures and minimizing their surface energy4 makes detailed
descriptions of the structure of cellular solids possible, as Kraynik reports. The mechanical
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