In-situ formation of metal-ceramic microstructures by partial reduction reactions

The formation of metal-ceramic multi-phase microstructures by partial reduction of ternary or higher ceramic oxides has been experimentally investigated in the model system FeMnO and in the more practical system AlNiO by changing the oxygen partial pressure at constant temperature and total pressure. In the FeMnO system the addition of impurities such as A1₂O₃, CaO, Cr₂O₃ and ZrO₂ to the initial, polycrystalline oxide solid solution (Fe_{1 - x}Mn_x)O strongly influences the location of the metal precipitation during reduction. The experimental observations are discussed based on solubility limits and segregation of impurities. In the AlNiO system, depending on the reduction temperature, two different morphologies of nearly pure Ni particles, equiaxed and rod-like, form within an oxide matrix. Equiaxed Ni particles (0.02 to 0.5 μm in diameter) embedded in α-Al₂O₃ were formed at 1350 °C, while rod-like Ni particles (~ 5 μm in length and 0.1 μm in diameter) in a metastable "defect spinel" phase, containing much less Ni than NiAl₂O₄, were formed at 1100 °C. Electron microscopy studies were performed for microstructural characterization, phase identification and chemical analysis.