Guest editorial : Materials Letters special issue on hollow sphere structures

Abstract

Nature frequently uses cellular and porous materials for creating load-carrying and weight-optimised structures. Thanks to their cellular design, natural materials such as wood, cork, bones, and honeycombs fulfill structural as well as functional demands. For a long time, the development of artificial cellular materials has been aimed at utilising the outstanding properties of biological materials in technical applications. As an example, the geometry of honeycombs was identically converted into aluminium structures which have been used since the 1960s as cores of lightweight sandwich elements in the aviation and space industry. Nowadays, especially foams made of polymeric materials are widely used in all fields of technology. For example, Styrofoam® and hard polyurethane foams are widely used as packaging materials. Other typical application areas are the fields of heat and sound absorption. During the last years, techniques for foaming metals and metal alloys and for manufacturing novel metallic cellular structures have been developed. Owing to their specific properties, these cellular materials have considerable potential for applications in the future. The combination of specific physical and mechanical properties distinguishes them from traditional dense metals, and applications with multifunctional requirements are of special interest in the context of such cellular metals. Their high stiffness, in conjunction with a very low specific weight, their high gas permeability combined with a high thermal conductivity, can be mentioned as examples. What all these different cellular materials have in common is that their physical properties are not only determined by their cell wall material but significantly by their structure.