Densification of nanostructured titania assisted by a phase transformation

NANOPHASE materials, characterized by an ultrafine grain size, have stimulated much interest in recent years1–11 by virtue of their unusual mechanical, electrical, optical and magnetic properties. Nanophase ceramics are of particular interest because they are more ductile at elevated temperatures than are coarse-grained ceramics11—an important property for the fabrication of ceramic components. Preparing materials that are both dense and fine-grained, however, has proved difficult: the high sintering temperatures generally required to obtain high densities can also lead to exaggerated grain growth, resulting in coarse-grained, nonuni-form materials. Sintering at lower temperatures gives a much finer grain size, but does not in general result in high-density materials. We show here that dense nanostructured titania, with density >99% of the theoretical maximum and an average grain size of less than 60 nm, can be prepared by sintering a titanium oxide sol gel near the anatase–rutile phase transformation temperature (about 600 °C). The increased mobility of the atoms during the phase transformation enhances the sintering rate at lower temperatures, suggesting that this method could be used more generally to produce nanophase materials with near theoretical densities.