Functional ceramics of nanocrystallinity by mechanical activation

For the first time, a large number of ceramic perovskites, including oxide-based ferroelectrics, relaxors, and piezoelectrics, have been synthesized by mechanically activating constituent oxides at room temperature. The technological breakthrough has a significant implication that the currently employed multiple steps of phase formation by solid-state reaction at high temperatures can be skipped. This will revolutionize the synthesis and fabrication of many of these oxide-based ferroelectrics and piezoelectrics by employing widely available, low-cost starting materials and by forming the required perovskite phases virtually at room temperature. The activation-synthesized ferroelectrics and piezoelectrics exhibit a nanocrystalline structure that is not shown by the materials prepared via any other processing routes. In several multicomponent perovskite systems, intermediate or transitional phases that have been always observed in the conventional solid-state reaction do not occur during mechanical activation. On the basis of several well-designed experiments in our laboratory, we have established that the occurrence of perovskite nanocrystallites in a highly activated oxide matrix proceeds via a fundamentally different mechanism from that in a thermally activated reaction.