The formation of cubic and monoclinic Y2O3 nanoparticles in a gas-phase flame process

Synthesis of phase-pure cubic and monoclinic Y2O3 nanoparticles (16-90 nm) was achieved in a gas-phase flame process. The effect of process parameters on the crystal structure of the Y2O3 nanoparticles was systematically investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fuel gases, H-2 or C2H4, at various flow rates, two yttrium precursors, an oxidant stream with varying degrees of N-2 or Ar dilution and different burner diameters were used to examine the effects of chemical atmosphere, flame temperature, residence time and precursor concentration on the crystal structure of the synthesized Y2O3 nanoparticles. Regardless of the other process parameters, at diluent/O-2 ratios of 0.25 or lower, monoclinic Y2O3 nanoparticles were obtained, whereas at diluent/O-2 ratios of 1 or higher, cubic Y2O3 nanoparticles were obtained. A lower diluent/O-2 ratio was related to higher flame temperature. Thermodynamic analyses suggest that high temperatures likely favour the formation of monoclinic Y2O3 nanoparticles in this flame process.