Spray flame synthesis of Y2O3/Al2O3 composite nanoparticles: Effects of precursor volatility and Y/Al ratio on particle morphology and crystal phase

Spray flame synthesis has proven to be a promising method for the high-throughput production of multi-component nanoparticles with atomic-level mixing. However, the fast dynamic of flame synthesis presents significant challenges in precisely controlling the morphology and crystalline phase of the particles, especially for multi components. In this paper, we conduct parametric flame synthesis experiments to identify key factors affecting nanoparticle morphology and crystal phase of Y-Al nanoparticles, focusing on the effects of Y/Al atomic ratio, flame temperature, and precursor volatility. The results indicate that the particle morphology depends on the composition, changing from aggregates with sintering necks to spherical particles without sintering necks and back to aggregates as the Al content increases. This trend is consistent with the change in particle melting points, which initially decrease and then rise again. Thus, the variation in melting point causes changes in sintering time, which in turn affect the particle morphology. The particle crystalline phase gradually deviates from the target phase as the Al content increases. Raising the adiabatic flame temperature from 1551 °C to 2340 °C accelerates the diffusion of Al atoms into the lattice, increasing the target phase proportion in the synthesized particles from 66 % to 99 %. By adjusting the amount of 2-ethylhexanoic acid (EHA) added, the volatility differences between the precursors can be modified. The largest discrepancy in precursor volatility is observed when EHA was added at a 50 % equivalence ratio, resulting in only 6 % of the target phase YAH (YAlO₃, hexagonal type) in the particles. Increasing the EHA addition to 120 % equivalent results in better matching of precursor volatilities, which leads to a significant increase in the target YAH phase to 98 %, demonstrating the critical role of simultaneous evaporation of different component precursors.