Composition-Dependent Phase Partitioning and Nanoscale Mixing in Rapidly Condensed U/Al and U/Al/Fe Vapors

Rapid cooling of multicomponent vapors produces oxide particles through coupled oxidation, nucleation, condensation, and aggregation occurring on millisecond time scales. In uranium-bearing systems, these pathways remain poorly understood when redox-active and refractory elements condense together. Here, we isolate the effect of initial composition by comparing binary uranium/aluminum (U/Al) and ternary uranium/aluminum/iron (U/Al/Fe) vapors condensed under a fixed temperature profile in a plasma flow reactor (PFR). Oxygen for oxide formation was supplied through the aqueous nitrate precursors introduced into the argon plasma. Transmission electron microscopy (TEM), electron diffraction, elemental mapping, and image-based particle statistics were used to relate particle morphology, crystal structure, elemental partitioning, and projected particle size. Binary U/Al condensates formed predominantly α-UO3 and η-Al2O3 as small, weakly aggregated particles. Increasing Fe relative to Al shifted the condensates toward larger aggregates containing UO2, η-Al2O3, Fe3O4, and mixed U–Fe oxides. Correlated high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) measurements show that U and Fe are spatially associated within particle aggregates, including mixed U–Fe oxide crystallites, whereas Al preferentially partitions into spatially distinct crystalline and amorphous Al-rich regions. These results demonstrate that initial composition controls both phase selection and nanoscale chemical partitioning during rapid condensation, providing particle-scale constraints for models of nonequilibrium multicomponent oxide formation.