Electron Microscopy of Hierarchical Materials

Recent innovations in materials chemistry have allowed the preparation of “hierarchical” ceramic and polymer materials that possess features on several different size scales. One of the newest hierarchical materials are ceramics that exhibit a three-dimensional ordered array of half-micron voids. These macroporous structures are synthesized from a liquid ceramic precursor and a polymer colloidal crystal template. This template is extracted by either thermal or chemical methods leaving a structure such as the porous zirconia particle shown in Fig. 1. The final structure of these materials may be thought of as the opposite of opal: the spheres here are the voids. Ordered arrays of dielectric material like these could potentially be used as photonic crystals that interact with visible light. For the microscopist, these materials create new opportunities to study interrelated aspects such as templating, crystallization, and phase transformations. Transmission electron microscopy (TEM) was carried out on a Philips CM30 TEM equipped with a LaB6 filament and operating at 300 kV.