Hard Materials with Tunable Porosity
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Jonah Erlebacher and Ram Seshadri Guest Editors Abstract Porous metals and ceramic materials are of critical importance in catalysis, sensing, and adsorption technologies and exhibit unusual mechanical, magnetic, electrical, and optical properties compared to nonporous bulk materials. Materials with nanoscale porosity often are formed through molecular self-assembly processes that lock in a particular length scale; consider, for instance, the assembly of crystalline mesoporous zeolites with a pore size of 2–50 nm or the evolution of structural domains in block copolymers. Of recent interest has been the identification of general kinetic pattern-forming principles that underlie the formation of mesoporous materials without a locked- in length scale. When materials are kinetically locked out of thermodynamic equilibrium, temperature or chemistry can be used as a “knob” to tune their microstructure and properties. In this issue of the MRS Bulletin, we explore new porous metal and ceramic materials, which we collectively refer to as “hard” materials, formed by pattern-forming instabilities, either in the bulk or at interfaces, and discuss how such nonequilibrium processing can be used to tune porosity and properties. The focus on hard materials here involves thermal, chemical, and electrochemical processing usually not compatible with soft (for example, polymeric) porous materials and generally adds to the rich variety of routes to fabricate porous materials.
The Ins and Outs of Porous Materials Introductory textbooks on porous materials often will present a graph akin to that in Figure 1, in which applications for various kinds of porous media are plotted across the length scale of porosity required for that application.1 Two features of this kind of presentation are lacking. First, for applications with very small length scale porosity, metals or other conductive systems are generally missing. Applications such as electrocatalysis by precious metals, for which high surface area to volume ratios are a must, are not included, despite obvious applicability in important technologies such as energy. The reason for the lack of applications is linked to the thermal processing associated with bulk metals, which intrinsically leads to large structural domains. Low-temperature processing of metals is often used to synthesize nanoparticles, but to make nanoparticles into porous materials involves sintering, again, a thermal process that tends to increase the length scale of porosity well past the nanometer scale. Second, the materials are presented as if they are static, with a burned in, immutable length scale.
Nanoscience has taught us that even hard materials have very dynamic surfaces and interfaces, sometimes at or near room temperature. Diffusional processes can be highly enhanced by nanoscale curvature, which creates large chemical potential gradients by the Gibbs-Thomson (GT) effect. The GT effect describes how the chemical potential of a material with interface curvature r and surface energy γ differs from a bulk mat
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