Inter-aggregate mixing in hetero-aggregates formulated in opposed jets fluidized bed

The mixing quality in nanoparticulate systems plays a fundamental role in the functional enhancement of advanced materials. In this study, we evaluate the effectiveness of mixing TiO 2 (rutile) and ZrO 2 (monoclinic) nanopowders using Raman mapping and energy-dispersive X-ray (EDX) spectroscopy within a scanning electron microscope (SEM). Eighteen experiments were carried out in an opposed jet fluidized bed, varying process time, mass ratio, and Laval nozzle back pressure. Raman mapping enabled spatially resolved identification of phases, while SEM/EDX provided high-resolution elemental composition. Both techniques indicated good overall inter-aggregate mixing efficiency, especially at a mass ratio of 1:1, with average Ti atomic fractions close to 0.607. Quantitative comparison showed that Raman micro spectroscopy yielded lower relative deviations from the expected values and required simpler sample preparation, making it a practical choice for assessing mixing homogeneity. Deviations from the expected compositions were more pronounced at other mass ratios (especially 1:2), likely owing to differences in particle size, density, and aggregation tendencies. Finally, in contrast to previous intra-aggregate mixing studies, the current results suggest that inter-aggregate composition tends to stabilize near equimolar proportions regardless of the initial mass ratio, highlighting self-regulating behavior at the macro scale. • Inter-aggregate mixing was successfully evaluated using Raman microspectroscopy and EDX. • Compositional self-stabilization behavior was observed under all operating conditions. • The processing time had a limited influence, suggesting rapid mixing stabilization. • The final system composition is strongly dependent on the mass ratio and pressure.