ABSTRACT A novel reactive sintering method was developed to produce dense ceramics with nanoscale composite microstructures. This method exploits the variable valence state of cerium ions. Compounds that contain cerium cations can undergo oxidation–reduction (redox) reactions depending on the oxygen activity of the processing environment. The perovskite cerium(III) aluminate, CeAlO 3 , undergoes a eutectoid transformation in oxidizing conditions to form nanoscale lamella of cerium(IV) dioxide, CeO 2 , and Al 2 O 3 . Unlike a typical eutectoid reaction, the redox activity of cerium provides an additional lever of control (oxygen partial pressure) over the onset and kinetics of the eutectoid transformation. Multiple processes that affect the final sintered microstructure (densification, coarsening, and volume expansion) of a ceramic can negatively interact and result in porosity or coarsening. By controlling the oxygen atmosphere during sintering, the rates of these competing processes can be optimized to yield a fine‐featured and dense product. Testing indicates that finer CeO 2 /Al 2 O 3 composite microstructures exhibit improved hardness, and microscopy results indicate that these nano‐features introduce crack deflection mechanisms. This reactive sintering method provides a tool to produce bulk ceramics with near‐theoretical densities and nanoscale microstructures, a method that may be generalized to other oxide systems that undergo redox‐activated eutectoid transformations.
Maier et al. (Wed,) studied this question.