Porous materials: Less is more
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ADVANCES IN THE SYNTHESIS, CHARACTERIZATION, AND PROPERTIES OF BULK POROUS MATERIALS This special issue of the Journal of Materials Research contains articles that were accepted in response to an invitation for manuscripts.
Porous materials: Less is more Guest Editors: Paolo Colombo The University of Padova, Padova 35131, Italy and The Pennsylvania State University, University Park, Pennsylvania, U.S.A.
David C. Dunand Northwestern University, Evanston, Illinois, U.S.A.
Vipin Kumar University of Washington, Seattle, Washington, U.S.A.
The presence of porosity in a material was first shown by Robert Hooke (1635–1703) who, in his investigations of the natural world using the newly available microscope, observed that the structure of cork was based on regular hollow units which he termed “cells”, because they reminded him of the cells of a monastery.1 Indeed, the fact that most natural structures are porous is a clear indication that porosity plays a determining role in establishing a well-defined and suitable set of properties under constrained optimization conditions, compatible with bottom-up growth.2 A similar observation can also be made for natural and processed foods, in which the presence of porosity is instrumental in tailoring important characteristics such as the geometric surface area (and therefore the dissolution rate and the intensity of flavors), the elastic modulus and toughness (with directly affects the chewing experience), the permeability (adding the possibility of homogeneously mixing fluid and solid components), and even the ratio between profit and direct cost (most of the food products comprise a high volume fraction of quite inexpensive air. . .).3,4 When looking at applications in virtually every field of technology, from energy to the environment or from health and safety to transportation and electronics (e.g., porous low k dielectrics and heat sinks), we can recognize that many components in simple or complex devices contain some degree of designed porosity, which specifically equips them to deliver a set of required (and sometime contradictory) performances. It is, indeed, the unique combination of features that porous materials possess that significantly extend their properties.5 While unintentional porosity in a material or a part remains detrimental and great efforts are still invested to prevent uncontrolled pores, engineered and tailored porosity is becoming more and more prevalent in advanced materials. Many benefits are derived from the deliberate introduction into a material of voids, pores or cells with controlled DOI: 10.1557/jmr.2013.232 J. Mater. Res., Vol. 28, No. 17, Sep 14, 2013
geometrical parameters.6 The processing procedures affect their morphology and architecture at every length scale, besides characteristics such as surface finish, flaw population, residual porosity in the cell walls and compositional purity, which also strongly influence properties as well as the cost of the component. Therefore, development and innovations in manufacturing are a key factor toward enablin
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