Properties of Carbon Aerogels and Their Organic Precursors

Aerogels are sol-gel derived porous solids with structural properties, such as porosity, pore size, pore and solid phase connectivity that can be tailored over a wide range to provide unique material properties for different fields of applications, such as filters and adsorbers, catalyst supports, electrodes for electrical energy storage, and materials for lightweight construction or thermal insulation. In this context, carbon aerogels and their organic precursor represent an important class of aerogels with very different physical properties at similar structural characteristics. This is due to the different intrinsic properties of the respective backbone components: At given meso- and macrostructure carbon aerogels are characterized by high thermal and electrical conductivity, significant mechanical brittleness, high porosity of the backbone phase related to micropores (<2 nm), and thus specific surface areas up to about 2000 m(2)/g. In contrast, the respective organic precursors exhibit very small electrical conductivities and a significantly reduced heat transfer via the aerogel backbone phase; they furthermore may be mechanically more flexible and are limited to specific surface areas below 1000 m(2)/g. The chapter provides an overview over typical structural and physical properties of carbon aerogels and their precursors. Understanding structure-property relationships and optimizing aerogels for different applications requires reliable characterization techniques. The review article addresses different characterization techniques as well as the problem of artifacts upon structural characterization; the latter is due to the unique combination of small pore sizes and large porosities characteristic for aerogels. With that in mind, the article alternative, in part even more powerful approaches.