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

Synthesis of phase-pure cubic and monoclinic Y 2 O 3 nanoparticles (16-90nm) was achieved in a gas-phase flame process. The effect of process parameters on the crystal structure of the Y 2 O 3 nanoparticles was systematically investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fuel gases, H 2 or C 2 H 4 , 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 Y 2 O 3 nanoparticles. Regardless of the other process parameters, at diluent/O 2 ratios of 0.25 or lower, monoclinic Y 2 O 3 nanoparticles were obtained, whereas at diluent/O 2 ratios of 1 or higher, cubic Y 2 O 3 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 Y 2 O 3 nanoparticles in this flame process.