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, H₂ or C₂H₄, at various flow rates, two yttrium precursors, an oxidant stream with varying degrees of N₂ 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/O₂ ratios of 0.25 or lower, monoclinic Y₂O₃ nanoparticles were obtained, whereas at diluent/O₂ ratios of 1 or higher, cubic Y₂O₃ nanoparticles were obtained. A lower diluent/O₂ 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.