Generalized approach to the microstructure direction in metal oxide ceramics via polymerization-induced phase separation

When three-dimensionally ordered macroporous (3DOM) materials are synthesized in polymeric colloidal crystal templates using a Pechini-type approach, polymerization-induced phase separation (PIPS) can occur. Depending on the reaction conditions, the porous products have a variety of morphologies, including an extended inverse opal structure, bicontinuous networks of 3DOM materials interrupted by extended voids, uniform 3DOM microspheres, sheet structures of templated macroporous oxides, and hollow particles obtained by structural disassembly. In this study, the mechanism underpinning morphology control of 3DOM metal oxides through PIPS is elucidated for Ce_0.5Mg_0.5O_1.5 and CeO₂ systems. The mechanistic information is then applied to synthesize target morphologies for Mn₃O₄ and Fe₂O₃/Fe₃O₄ systems, demonstrating the more general nature of the synthetic approach for aqueous metal precursors that can be complexed with citric acid. The effects of reactant balance, complexation behavior, processing temperature, and template sphere size are related directly to the microstructures obtained. The predominant controlling factor of microstructural evolution in PIPS Pechini precursors is found to be the degree of polymerization of the polyester, which can be controlled through tailoring the reagent imbalance. 3DOM microspheres produced by the method are between 0.5 and 3 μm in size, with polydispersities below 25%.