Advances in the Synthesis, Characterization, and Properties of Bulk Porous Materials

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. 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. 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. . .). 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. 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 geometrical parameters. 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 enabling the fabrication of components possessing the desired porosity features (such as average pore size, fraction, shape, orientation and connectivity, as well as their distribution and gradients). In this respect, scaling-up manufacturing methods from laboratory-size to industrial scale is of particular importance, as it enables the production of porous materials with reproducible properties at costs low enough for one or more mass markets (e.g., Styrofoam used for containers, packaging and insulation). The three-dimensional assemblage of a large number of pores can occur in a variety of ways, leading to materials (in the form of a monolith or as a coating or part of a composite) possessing widely different architectures, from foams to honeycombs, from fiber networks to scaffolds, from connected hollow bodies to syntactic foams and sandwich panels, from bio-inspired structures to microand meso-porous materials and parts possessing hierarchical and/or graded porosity. Porous materials provide a great degree of design flexibility in their use, in relation to their architecture, composition and surface functionalization. Such widely different morphologies, structures and architectures result in a wide range of properties, so that the implementation of novel testing procedures and the development of precise ways for quantifying all the diverse features of a porous structure are required. This also points to the increasingly crucial role played by modeling in enabling the description and prediction of the effect of porosity on selected properties, aided by quantitative morphological characterization tools such as tomography, now available with a sub-micron resolution. Tailored porosity is therefore the defining characteristic that allows us to use the well-known materials science DOI: 10.1557/jmr.2013.232